PLANT ENGINEERING
PIPING STUDY
by
SANJAY PRADHAN
ROLTA INDIA LIMITED
CONTENTS Piping Study 1 2
IMPORTANCE AND NECESSITY OF PIPING STUDY ROLE OF PIPING ENGINEER 2.1 Adequacy 2.2 Economy 2.3 Clarity 2.4 Accuracy 3 PROCESS ENGINEERING DATA AND FLOWSHEET ( PFD ) 4 PIPING & INSTRUMENTATION DIAGRAM ( P&ID ) 5 MATERIAL TAKE OFF 5.1 First MTO 5.2 Second MTO 5.3 Third MTO ( Final MTO ) 6 PLOT PLAN (PLANT LAYOUT) AND INITIAL DESIGN 6.1 Introduction 6.2 Steps for Development of a PLOT PLAN 6.3 Importance of PLOT PLAN 6.4 Important Units 7 PIPING MATERIALS SPECIFICATION 8 EQUIPMENT LAYOUT DRAWINGS 9 NOZZLE ORIENTATION 10 PIPING LAYOUT 10.1 Main Service Piping 10.2 ‘IN PLANT’ Piping 11 INSULATION & HEAT TRACING 11.1 Insulation 11.1.1 Preparing an Insulation Specification 11.1.2 Data required to prepare Insulation Specification 11.2 Heat Tracing 12 ISOMETRIC DRAWINGS 13 PIPE SUPPORTS
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-2PIPING STUDY 1
IMPORTANCE AND NECESSITY OF PIPING STUDY
In the Piping Study, the function of the piping design engineer is to apply knowledge of fluid flow, stress analysis, material properties and engineering judgement. Piping Engineer convert the process specifications into drawings and data of the Project or Process Plant. From this drawings and data the required materials can be purchased, fabricated and assembled into piping systems, which perform the process requirements. This function must be fulfilled at the minimum design cost, with close attention to the provision of an economic and satisfactory piping system, which will operate without physical failure or excessive pressure losses for the life of the plant. During the piping design stage, close communication must be maintained with other engineers working on the project. Information on layout, vessels, instrumentation, insulation, purchasing, and erection, must be received, processed, and transmitted by the piping design office. Piping design is the most lengthy and complex part of the entire design procedure and always on the critical path of the project plan. Each pipeline must be treated individually and be put through the universal engineering design assessment to cover: • • • • • • • • • • •
Function (flow, temperature and pressure) Material Specification Locations of Equipments Piping Routing Elevations and Dimensions Item numbers Grid numbers Battery Limits Support Standards Pipe Insulations, Tracings etc. Material Requirement (bill of material, purchase requisitions)
The piping engineer has therefore, considerable responsibility for economic and accurate design. Over the years the design methods and the detail engineering activities in consulting organization, large process and contracting companies for piping design and the techniques have converged towards a procedure for producing simple symbolic data conveying maximum information at minimum cost, which is similar through-out the world.
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The basic elements of this procedure are: • • • • • • •
2
Use of 3D Intelligent and Specification driven Software like PDS Interface of Intelligent Software like PDS to other software like CAESAR-II Rapid data retrieval Standardization of engineering design methods for piping layout and pipe stress analysis, fluid flow and material selection Maximum use of National & Standard Codes Symbolic drawing procedures Standardization of document format for issue of piping information to facilitate initial use and later retrieval of data
ROLL OF PIPING ENGINEER Piping Engineer is responsible for a substantial part of the total project cost. Good progress in piping is critical to completion of the project on time. In addition to the function as a specialist design engineer, the piping engineer must provide considerable amount of information transmittal and design continuity inside the project design organization. His particular concerns must be for the following points: 2.1 Adequacy
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2.2 Economy
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2.3 Clarity
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2.4 Accuracy
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The piping design must be adequate to meet the process specifications and the physical conditions in which the plant is to operate. Design must be optimised and must be cost effective so as to be within project budget. Maximum use must be made of company and National Standards. Standardized formats for data presentation must be used. Data retrieval of previously proven practices must be effective to eliminate unnecessary design work. Much of the piping data is derived from and used by other engineers and must be clear, consistent, and reliable. Standardized formats are of considerable value in this aspect. Details of pipe work and purchasing data must be complete and accurate. Mistakes do not emerge until erection is under way and rectification work is costly and delays project completion.
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3
PROCESS ENGINEERING DATA AND FLOWSHEET ( PFD )
FIG.1 The flowsheet is a simple diagrammatic picture of the plant, which shows the equipment items connected by the essential process control circuits or major process requirements. A typical flowsheet is shown in FIG.1. No attempt is made to show equipment items to scale, in their correct elevations or in their correct relative locations to each other. Simple symbols are used to represent different types of equipment items. The aims of the flowsheet are to define exactly the essential requirements of the process design and to present an easily understood picture of the process stages and controls. Example of a typical flowsheet formats are shown in the next two pages. For the process plants generally the flowsheet is split into number of flowsheets to provide comprehensive data on particular stages of the process. For example number of flowsheets may be produced for following stages: • • • • • • • •
Raw Material Storage Reaction Separation Purification By-Product Recovery Effluent Treatment Product Storage Utilities Page 4 of 48
In the Process Plant it is often necessary to produce both Process and Utility flowsheets. In these cases, the Process flowsheets should be divided by Process Stage as above but it will probably be more convenient to split the Utility flowsheets firstly by Utility and secondly, by Plant area. Care should be taken in numbering and naming equipment items on flowsheets. These numbers and names will define equipment items throughout the life of the plant and must be unambiguous, brief but descriptive. Numbers can be pure number or can be alphanumeric to classify the equipment item. Classifying letters commonly used are: • • • • • • • • •
P T PV C H S F K KOD
Pump Atmospheric Tank Pressure Vessel Column Heat Exchanger Stirrer Furnace or Fired Heater Compressor Knock out drum
And these or any other consistent system can be used successfully. Although the flowsheets are produced outside the piping design section, their style, equipment numbering system, and subdivision all form the basis for important piping design documents. The process and piping engineers must evolve a mutually acceptable form of flowsheet to prevent misunderstandings. The process engineering data are used to supplement the pictorial information of the flowsheet. Details of equipment, operation, control, materials of construction, heat and mass flows, temperatures, pressures, and flow stream composition must all be provided to enable specialist engineers to design the plant equipment. The piping engineer in particular must have specifications of following items to design piping system. • • • • • • • •
Schedule of piping & connecting equipment items Flow rates in piping Flow stream compositions in piping Physical properties of process materials Flow temperatures and pressures Instrumentation and control equipment in pipes Permissible pressure drop in pipes Materials of construction for piping components
Again it is essential for process and piping engineers to develop acceptable standard presentation of these data, in the form of ’Piping & Instrumentation Diagram, (P&ID)’.
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4 PIPING & INSTRUMENTATION DIAGRAM ( P&ID ) The P&ID is the most important document produced by the ‘Process Department’, working in close contact with the project, process and piping engineers. When complete, it will be used as a common data source by all the disciplines like piping, instrument, erection, and operating staff who require a clear, unambiguous statement of the piping design and connections. The symbols used to represent process equipment items are frequently the same as described for the flowsheet. Some of the standard symbols used in the P&ID, are shown in the FIG.2.
FIG.2
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The more pictorial style will promote greater familiarity with the equipment and the designer’s objective of providing maximum information at minimum cost will be achieved. All nozzles on equipment items must be shown in a roughly accurate location whether for piping connections, instruments, vents, drains, or spares. All the nozzles must be given the tag numbers. The pipe work representation is non-scalar and entirely symbolic at this stage. P&ID should show the following details: • • • • • • • •
Which equipment items are connected together or to their pipes or service mains Which piping specification to be used for each pipe and the junction points where two specifications join The pipe size of every pipe The unique identity for every pipe Where valves are required Which valve specification is to be used All ‘in line’ fittings such as sight glasses, strainers, steam traps etc. The location of, ancillary equipment for every ‘in pipe’ instrument
Every pipeline on the plant is shown, together with its installed ‘functional ‘ fittings and valve, e.g. strainers, orifices, etc. Piping fittings such as elbows, flanges, etc. are not shown but reducer should always be shown. All the valves and functional fittings required in each pipe to meet all the above foreseeable condition must be shown and identified in their correct functional location. For example, a control valve must have block valves upstream and downstream. It is no use showing both block valves on one side and hoping that the detail piping designer will realize what is needed. In short, everything needed on plant must be shown in the P&ID and accounted for purchasing and erection stages. Conversely, any item not shown in the P&ID will not be installed on the plant. Parallel to the initial P&ID work, the pipe sizes should be calculated. The process engineer for the main process lines will normally prepare a very rough ‘size’ estimate. But with the final layout available and all pipes shown diagrammatically in the P&ID, the process designer can assign pipe sizes regarded as finalized except for the final check at final detail stage. The process designer should remember that the P&ID covers all stages of plant start-up, operation and emergency and must ensure that the pipe sizes selected cover all these condition adequately. At this stage, the Process Engineer must consider the terminal point for all the pipes shown in the P&ID, so that each individual pipe can be given a unique identity known as line number. In the very simplest case a ‘pipe’ runs from one equipment item to another, with two terminal points, without a branch connection. This simple case presents no difficulties, but in the majority of cases found in practice, common problems arise in case of following cases: • • •
Pipes change specification between equipment items Pipes branch to feed more than one item, e.g. when a pair of pumps feed one vessel Branch pipes are often connected to other pipes, e.g. utility sub-headers feeding several items or a process pipe with a branch to another process pipe to meet a start-up condition Page 7 of 48
• •
Equipment items are bypassed Control valves in pipes have a bypass loop fitted.
In establishing how many pipes exist, some general principles can be stated to distinguish between `pipes’ with more than two terminal points and cases where lines must be split into two or more pipes. • •
•
•
A pipe must have only one specification to ensure that only one type of material is used for fabrication. Hence, if the specification changes, a new pipe must be created at the specification change point Pipes can have more than two terminals, but should run from on plant location to one other location. Those, which connect to two items such as a pair of pumps or heat exchangers, can properly be regarded as one pipe, but a branch run to a separate vessel remote from the main run of the pipe should be treated as a separate pipe and identified as such When a small piece of equipment is bypassed, the inlet pipe can be treated as a pipe with two terminals at the equipment location, one terminal connecting to the equipment item and the other connecting to the block valve. Similarly, the outlet pipe starts with two terminals at the outlet and at the block valve. On larger equipment items where the bypass line may be 20 ft. (6.1m) long or more, it is better to treat the bypass as a separate pipe Bypass loops around control valves or similar equipment in a pipe should be treated as part of the pipe
So far, each pipe has been assigned terminal points, a specification, and a size, but no identity. It is vital that every pipe can be identified by is on unique reference and this is most easily done by an alphanumeric system designed, firstly to split the pipes by function and secondly, to give each pipe in the function groups a number. The combination of `Function/Number’ is unique and identified any pipe from all others. The functional split should separate process pipes from utilities generally and should divide utility fluids. Each group is given an identifying letter as indicated below: Letters
Function
Process pipes: P F PJ PT
General process pipes. Process vents if to flare stack Jacketed pipes Traced pipes
Utilities: HS LS CW TW CA G
High pressure steam and condensate. Low pressure steam and condensate Cooling water Towns water Compressed air Town or natural gas Page 8 of 48
The four features assigned to each pipe, namely: • • • •
Size Functional letter Sequence number Pipe specification
If require other parameters like Insulation etc. will be added in the Line Number. The P&ID now completed and can be released for piping layout, detailing and material take of work. It will be used also by the instrument engineer as a guide to instrument location, by the site engineer as a record of all piping, guide and construction checklist. The importance of the P&ID as a project liaison and record document requires formal approval by the project manager, process engineer, and piping engineer. Strict control of modification procedure is essential to maintain control of all piping work. Virtually all the process `knowhow’ and much engineering technique are recorded on the P&ID and its security controlled to prevent information leakage to unauthorized persons. A section of a completed P&ID is shown in FIG.3 to illustrate the use of the symbols and conventions described and to show the amount of detailed information recorded. In its final form the P&ID contains such a mass of detailed information that an index of some form is required for easy and reliable reference by project staff. This index is provided by a line schedule or line list, which is drawn up in the final stages of P& ID draughting. The line schedule lists all pipes in group order and numerical order and shows against each pipe its start and finish terminal points (in the direction of process flow), design, operating, hydro test conditions etc. The line schedule lists all pipes in group order and numerical order and shows against each pipe its start and finish terminal points (in the direction of process flow), design, operating, hydrotest conditions etc. With the P&ID, line schedule, and piping specifications, layout and detailing of pipe work can be preceded methodically. At this stage Piping Designers using these source documents need have very little knowledge of the process or the project as a whole since a complete basis for their design has been provided.
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FIG.3
5
MATERIAL TAKE OFF MTO is a list of total number of valves, flanges, pipes and other piping elements required for the plant. Each MTO include preparation of BOQ, construction work volume and material requisition. Inquiries are floated immediately after the first MTO and technical bid analysis is carried out to select the suitable vendors. Obtaining the raw material required for pipe work forms a major part of the purchasing effort on the project. The high cost of the material, the lengthy delivery times often needed, the late project stage at which final requisition totals become available, and above all, the sheer variety of types and sizes of materials required, pose difficult problems for the purchase department. Each of these type and size combinations represents an individual component, which must be obtained by the purchase department in the right quantity, at the budgeted price and within the project Programme time scales. Failure to obtain any component will almost certainly result in costly fabrication and erection holdups and may well delay project completion. The purchase department, therefore, needs early and reliable instructions from the design office. The fact that a final statement of piping materials requirements is not available until all the pipes have been detailed does not prevent reasonable estimates of major material requirements being made at early project stages. A very rough estimate can be made when the preliminary layouts, flowsheets & P&IDs are available, by Page 10 of 48
combining data from these drawings with company or personal experience. As later design work expands the basic data so the initial estimate is replaced by an actual bill of material. Although the work of material takeoff is simple, it is time-consuming and repetitive yet requires a high degree of concentration and responsibility if mistakes in addition are to be avoided. Most major companies now operate computer programs for the Material Take Off work. Usually these programs assign a coded term to each type of component/material/size combination e.g. AABBCC Code. All the basic information like lengths, quantities etc. is extracted from the Intelligent 2D PDS-PID or from SPPID and MTO Reports are prepared. A variety of additional outputs can be produced from the basic data to: • • • •
Show variations from earlier estimates Print requisitions or orders Warn the expediting section of overdue material Prepare cost summaries
5.1 First MTO • • •
Gives approximate information for the number of piping elements needed based on preliminary inputs This is developed immediately after the plot plan and first issue of PFDs and P&IDs are ready Information is approximate.
5.2 Second MTO • •
The data for second MTO is obtained from the 3-D model when the modelling is in progress Gives almost accurate information on the number of piping elements
5.3 Third MTO ( Final MTO ) • •
This is issued after the 3-D plant model is ready and finalized. This is the final MTO and must contain accurate information on the number of piping elements.
6 PLOT PLAN (PLANT LAYOUT) AND INITIAL DESIGN 6.1 Introduction A plot plan is a master plan locating each unit/facility within the plot boundary. When the process flowsheets, process data sheets, basic P&IDs, Contour Plans and other required data are released, engineering design can be started on preparation of plot plan activities. The most important task is to establish a rough layout for the plant and for this purpose the basic proportions of the equipment items are estimated from the flowsheets and P&IDs and integrated with the preliminary civil design to produce a number of possible alternative layouts. Selection of the optimum layout is made to Page 11 of 48
suit the plant design and physical constraints of the site and the finally agreed rough layout is converted to a finished, firm design. All relevant requirements of civil, mechanical, instrument, and electrical engineers are embodied in this final layout. Basic drawings can be used for layout trails and recording the final layout. Whichever method is chosen should be governed by considering which provides the best method of communication between all designers and non engineering staff concerned with producing the final layout or using this final layout as the basis for further work. The piping engineer is one of the leading members of the project team producing layouts and layout is a responsibility of the piping section. In the preliminary stage, some of the main factors to be considered are: • • • • • • • • •
Avoidance of excessively lengthy or complex pipe work Balancing cost of placing equipment in structures for gravity low against the cost of pumped flow Relative cost of pumped reflux or pumped cooling water to condensers on tall distillation columns Adequate room for pipe tracks between plants Space for pipe wells in multifloor plants Pressure drop on vacuum or high pressures processes Heat losses on high temperature processes Allowance for weight of piping in structural design Operating elevation requirements for pump suction or thermo siphon reboilers
The intention should be to produce layouts for study, which are all practicable and reasonably economic so that a final layout can be selected. When one rough layout has been chosen for working up to the final plant layout, more detailed attention must be paid to good piping practice and provision made for: • • • • • • • • • •
Headroom under pipes and pipe racks Space for control valve loops Straight lengths of pipe for flow measurement Access to main operating valves and controllers Support for large pipes in plant Anchor points for hot pipes Routing of buried pipes clear of pile caps or other obstructions Main electrical and instrument trunking in plant Clearance between gangways, stairs, ladders, and adjacent pipes Valves and fittings around close coupled items such as pumps, and heat exchanges.
When completed and agreed, the final layout should not be altered. Attempts to modify the layout after this stage (unless these modifications are very minor) can be disastrous in terms of cost, completion time, and in loss of momentum and morale within the project design team.
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6.2 Steps for Development of a PLOT PLAN • • • • • • • • • • • • • • • • • • • • • •
Study the contour map and establish the grid points The N-S and E-W or X/Y grids, the plant north in relation to geographical north should be established The free area along the plot boundaries as per the statutory norms should be established Work out the area requirements for the green belt, vehicle-parking etc. as per the norms The process blocks shall be located in the sequential order of the process flow so that material handling (solid/liquid) is minimized The blocks shall also be arranged considering prevailing wind directions so that flammable gases do not get carried to sources of ignition Storage tank shall be grouped according to process classification Centralized control room shall be located based on annual shutdown philosophy so that hot work shall not affect the operation Two adjacent process units shall be located based on annual shutdown philosophy so that hot work shall no affect the operation Process unit shall be located at a higher ground away from unwanted traffic Process units shall be serviced by peripheral roads for easy approach Utility block shall be kept at a safe area close to the process plants Electrical sub-stations shall be placed at the load centre to minimize cabling Receiving station shall be placed near the supply point Warehouses shall be located close to the material gate to avoid traffic within the process area Flares, furnaces/heaters, cooling towers etc. shall be placed depending on the wind direction Battery limits must be properly defined and provision for further expansion shall be provided Raw water storage shall be placed closer to other water source. Fire and raw water tanks shall be located together and also the water treatment if required. Fire stations shall be away from the hazardous area and nearer to main gate Effluent treatment plant shall be located away from the process and utility areas on the downwind direction. This could be the lower grade level or higher-grade level depending upon the philosophy. Consider recommendations from the statutory authorities for inter unit distances Residential colony shall be located away from the plant and closer to the city limits
6.3 Importance of PLOT PLAN • • • • • • • •
Serves as a guideline for layout drawings Gives the directional location of the plant Gives the co-ordinates and layout of Main Pipe Rack Gives the Mean Sea Level Elevation and Elevations of all the units Gives the exact location of any complex/utility and process unit Gives the locations of all the equipments Battery limits clearly defined Relative position of all the complexes in the plant can be had from the plot plan Page 13 of 48
6.4 Important Units •
Process Units Process Units are separated from each other, from tank farms and the overall site boundaries; located down wind but not uphill from any sources of ignition or concentrations of people, and arranged in a manner such that streamlined flow through the process units is maintained.
•
Boiler and Utility Plants Boiler and Utility Plants are located, centrally to the main users, which they supply, upwind of any equipment processing flammable or hazardous products, and separated from any hazardous area. The main objective of centralizing the boiler and utility plants is to minimize the interconnecting facilities between the boiler and utility plants and their main users. This has the effect of reducing the installed cost of any interconnecting system. Another important factor to be considered when locating the boiler plant is the height of the boiler exhaust stack and its location relative to the prevailing wind, as it is most important that the products of combustion are dispersed without polluting the local community of any of the on site facilities or buildings. Utility plants comprise facilities such as utility water treatment, storage and distribution and storage, instrument air and plant air compression. Since it is not processing hazardous materials, should be grouped together in a single area central to the main users, for the same reasons as for the boiler plant. For the air compression plant it is important that intake air is taken from a safe area where the air is clean and uncontaminated by hazardous, toxic or flammable materials.
•
Electrical Substations and Control Rooms Electrical substations are located upwind of sources of flammable materials. Substations may be located nearer to equipment containing flammable materials provided that one of the options below is selected: • •
Switch gear suitable for the environment, which is likely to be more expensive Pressurize the switchgear room thus eliminate the ingress of flammable materials
It should be noted that any air intake associated with the pressurizing system must be taken from a safe location, which is free from contamination of flammable toxic or hazardous material. When locating the main electrical sub station where the plant is importing, and in some cases exporting power to the national grid, care must be taken to Page 14 of 48
define the sterile area created by the incoming power cable into the sub station switchyard. Advice on the extent of this sterile area must be sought from the project lead electrical engineer. The location of the main electrical substation should be as close to the site boundary and the maximum distance possible from the main processing plant this has the effect of reducing high voltage cabling costs. Control rooms are typically located upwind of any equipment handling toxic or flammable materials and on the periphery of the process unit, which it serves, with its doors facing away from, as well as towards, the operating unit. •
Flare Flare systems are required to dispose of large quantities of flammable gases, especially during an emergency shutdown. Flares are principally of the multi jet ground or elevated type, and as a result will require large sterile areas around them to isolate them from personnel, equipment and property from effects of heat radiation and convection. The size of the sterile area is governed, in the case of the elevated flare by the height of the flare tip above grade. The extent of the sterile area created can be increased or decreased by altering the flare tip elevation. In the case of the ground flare the sterile area of 150 meters radius is not so readily altered as the sterile area is dictated by the method of construction of the flare itself and if the sterile area requirement is severely compromised then the use of an elevated flare must be considered. When locating ground flares care must be taken to insure that the discharge plume does no overheat area or create “no go areas” within the plant with respect to operating personnel. It can be seen therefore that the flare of either type must be located in a remote location within the overall site facilities upwind and to the side of the process facilities and far enough away from the boundary fence so as not to effect the adjacent community.
•
Cooling Towers Water cooling towers are located downwind from all the facilities any sources of ignition, and centres of population. The location of the cooling towers must be such that their plumes do not pollute or create hazards for either the operating company personnel or any adjacent communities. Cooling towers are therefore being located down wind of processing facilities, utility plant, administration buildings etc. and at such a distance not to be effected by flammable, toxic or hazardous material discharges.
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•
Tank Farms Tank farms are located peripherally, and down wind of any sources of ignition or centres of people within the facilities, and at the same time isolated from hazardous areas. The location of the tank farm must also be optimised relative to process units and the loading/unloading facilities. Liquid gas storage tanks must be located in a remote area of the site, without being close to the site boundaries, to minimise the risk of ignition of undetected leakages by uncontrolled ignition sources, such as motor vehicles on the public highways. Also the considerations for locating other tanks must not be ignored, when locating liquid gas storage tanks.
•
Effluent Treatment Plant Effluent treatment plants are located peripherally and downwind from concentrations of people and any sources of ignition and the selected location must also be remote from any site developable boundary. The effluent plan location is also dependent upon the location of the final treated effluent discharge, and the effluent discharge must not contaminate any water intake to the facilities by recirculating the discharged effluent.
•
Administration Area including Workshops & Warehouses The location of the administrating area, workshops and warehousing is peripheral separated from hazardous areas and upwind of any toxic or flammable products or feedstocks, but near to or adjacent to the main entrance for the overall facilities. The administration area is likely to include some of or all of the following facilities: •
Laboratory, canteen, first aid facilities, ambulance house, fire station, gate house, offices and car parking for the total facility
It is most important that non-operating personnel within the administration area etc. do not have to pass through the process operating area as part of their normal day-to-day activities. So this requires that the administration be kept separate from such operating areas, and the converse is true for process operating areas. •
Other Site Layout Considerations Any allowance for future expansion should only be incorporated when instructed to by the operating company.
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7
PIPING MATERIALS SPECIFICATION A piping specification is a document, which contains information on material standards, component standards, and pipe work installation practices to be followed. Pipes designed to a particular specification will follow different routes, but will all be fabricated from the same material and components, and will all have common design features such as fabrication procedures, jointing methods, drain points, and test pressures. Piping specifications are a most important feature of pipe work design, and work to assemble the specifications should start in the piping section as soon as the flowsheet and process engineering data are available. The piping designer analyses the flow conditions and fluids specified in this process data and separates all the individual pipes into groups, which have common conditions of: 1. 2. 3. 4.
Piping material Maximum pressure Maximum temperature Piping fabrication procedure whether socket welded, butt welded or screwed, flanged, etc. 5. Gasket materials 6. Maximum and minimum sizes of pipe By this analysis, the large number of pipes on the plant will be condensed into a small number of groups on typical project with 250 pipes, probably five or six groups would be found adequate and economic. The grouping operations take no account of the diversity of process fluids handled within a group; provided that above are satisfied, the fluid handled is unimportant. A group constructed from carbon steel, for example could handle process fluids such as caustic soda or solvents and utilities such as air, steam or water. Some care and good judgments is needed when defining the conditions of pressure and temperature to avoid uneconomic design. The above mentioned six conditions established for each group constitute a definition of piping design requirements for that group. The piping designer now produces the material, component, and procedural information forming the piping specifications for the groups. The first requirement is to establish the Piping Design Code for the plant: this may be a national standard such as those in common use e.g. ANSI B 31.1 – Code for Power Piping, ANSI B 31.3 – Code for Petrochemical Piping etc. Use of these codes provides a formalized design procedure. Use of these codes as Piping Design Basis got following advantages: •
It is proved by many years of background experience to be adequate and economic and which is accepted by operating, contracting, and insurance companies
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• • •
It is easily understood by any engineer at any time in the plant design or operation. The Basic design assumptions and procedures are set out and ambiguities in design methods eliminated It Provides piping components that are compatible for strength, dimensions, and assembly in ranges of materials suitable for fabrication by welding Specified materials and components by these codes are readily available as stock or standard items from suppliers
Similarly, many useful materials (e.g. glass and plastics) are commonly available for corrosive or hygienic duties but are not covered by codes. In these cases design procedures are usually established by manufacturers to assist the piping designer. From the range of components and materials available within the provisions of the Piping Design Code, the designer now selects the appropriate items, flange ratings, and wall thickness to the National standards laid down in the Code. Any non-Code, company or makers standard items are also selected if required and combined with the National standards to complete the list of permissible materials and components forming the first part of the piping specification. At this stage, sufficient information has been generated to enable bulk piping material take offs to be produced. The piping engineer can now complete the second part of the piping specifications dealing with pipework practice while other engineers are working on the layout and P&ID activities. The practices specified can be divided logically into: • •
Generalized good practice details or company practice to provide one standard detail where several alternatives exist. Typically, this section would cover items such as provision of vent and drain points steam trap assemblies etc. Details related to the particular service covered by the specification, for example, avoiding trapping liquid gases between valves.
Piping specifications should be presented on standard forms suitable for copying and filing, so that all staff using them can find information easily, quickly, and reliably. The standard form has the further advantage that omissions can be seen and queried with the piping designer. Unauthorized creation of new specifications or unauthorized changes to existing specifications must be avoided at all costs. Piping specifications must be identified and indexed so that duplication is prevented and retrievals of past data is made easier. One useful method of indexing is to assign a three-figure number to each specification and to use this number to convey some information on the general class of piping material used. The piping groups already analysed for preparation of piping specifications are used as basic design data. Each group is broken down into the sub-groups having common fluids in addition to other common conditions. So far, the pressure/temperature rating and general valve materials can be defined, but each individual pipe must be examined to decide on the valve function – whether this is simple shut-off, controlled flow, throttle, non-return etc. A duty specification is now available for every valve and the piping designer can decide from personal of plant Page 18 of 48
experience, consultation with valve manufacturers, and price lists as to which valves are to be used. The design element in valve selection is now complete and it remains only to identify each type of valve and present relevant data to other members of the project team. The data so far assembled, enable the piping designer to specify accurately and simply that valves required for any project duty and to call up valves from past projects. For details design and purchasing, however, some final details are needed which must be collected and shown on the valve specification e.g. • • • • •
Flange rating, type and thickness Height from valve centre to hand wheel Hand wheel diameter Trim details Packing type
All the notes on standardization format and responsibility for issue, amendment, control, and indexing made against piping specifications apply to valve specifications also. The piping designer must evolve a style of specification suitable for the needs of his own organization.
8
EQUIPMENT LAYOUT DRAWINGS The first operation is to prepare an accurate, functional drawing of the plant equipment and structures as shown in FIG.4, showing all equipment and major steelwork, platforms, building lines, Etc. Where gangways, access or maintenance space is required, which should be indicated in this basic drawing. The object of this basic drawing is to show the exact location of the Equipment, access, etc. and thus define by difference the space left for the piping. Any information, which does not contribute positively to this function, should not appear at all. Within this space, the relationship of pipe to pipe, and pipes to plant are shown, each successive group of pipes being considered in turn and draughted until all are accounted for and the layout is complete. •
Important points in the development of an Equipment Layout: • • • • • •
Study the basic documents like Process Description, PFDs, P&IDs, Process Data Sheets, Piping Study Layouts (described in the next section), Plot Plan and Mechanical Data Sheets The N-S and E-W or X/Y grids, the plant north in relation to geographical north are marked Number of Equipment Layouts will be prepared depending upon the complete plant is having number of Process units, flours, Utility units, Tank farm area etc. Equipment Layout is generally prepared to the scale of 1:50 Generally one layout will be prepared for one unit, flour or area If the Process unit is so big, then number of layouts are prepared according to the battery limits decided in the Plot Plan Page 19 of 48
• • • • • • •
• • • • • • •
The layout is the plan view, which shows the location of all the equipment coming in that area All the equipment plan view will be drawn to the scale Layout must show clearly the support details of the equipments like number of supports, type of support & support locations Also this drawing shows the large diameter pipelines connecting from equipment to equipment, large diameter nozzles, ducts etc. whose routing is frozen This drawing will never show the nozzle orientation of the equipments Layouts are designed to meet the process requirements While designing the layout, some Standard rules & regulations are considered according e.g. the design basis, relevant codes & standards, local authorities, statutory rules, other relevant rules like IBR, CCE and SMPV Battery limits must be properly defined and provision for further expansion shall be provided Layout includes structural & civil layouts, road layout, pipe rack layout, equipment maintenance space, equipment platforms, ladders, staircases etc. Equipment Layout is the drawing, which is prepared after preparation of plot plan, in which the locations of all the equipments are fixed Final piping layout can only be possible after completion of this drawing These drawings give the directional guidance for the proper and correct piping routing This drawing shows the location of all the equipments with the tag numbers, the pipe rack, package units, road locations, battery limits, North direction (True North & Plant North) It gives all the required dimensions, angles etc.
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FIG.4
9
NOZZLE ORIENTATION Nozzle is usually referred to a flanged connection on a boiler, tank or any static or rotary equipment consisting of a pipe flange, a short neck and welded attachment to the equipment. A short length of pipe, one end of which is welded to the equipment with the other end chamfered for butt-welding is referred as welding nozzle. Following points are important while preparing Nozzle Orientation Drawings • • • •
Generally Drawings are prepared in A4 size There should not be any obstruction in pipe routing. Nozzle orientation is usually done according to pipe routing. Fouling of nozzles, supports, lugs etc. shouldn’t occur Elevation and orientation should be proper such that nozzles can be operated and maintenance can be done easily
Nozzle Orientation Drawing is the plan view of the equipment, shows the following information: • • •
North Direction as per the Plot Plan Elevation of all the Nozzles Foundation Elevation and Elevations of other important levels will be shown Page 21 of 48
• • • • • • •
10
Exact locations of all Nozzles Angle, radius and offset of the Nozzle Length of the nozzles Tag numbers and Sizes All the supports, lifting lugs, name plate also must be shown All the welding seams must be shown Ladders, staircase, handrail orientation must be shown
PIPING LAYOUT When the plant layout, Equipment Layouts, pipe specifications, PFDs, P&IDs are completed, piping layout work can commence. The pipe specifications and process and utility P&IDs provide the designer with information on: • • • • • • •
Terminal points Identity Pipe size Functional fittings required Permissible pressure drop Materials Component standards Any special procedures
The site and plant layout give a picture of the area in which the pipes may run and general good practice imposes further limits on the location and routing of piping. The piping designer must work within these constraints and fulfil the specified conditions for each pipe by applying experience, judgment, and engineering science to produce an effective, economic pipe layout. In laying out any pipe installation the first considerations are the general aspects applying to all piping, followed by consideration of the particular qualities of the given piping systems contained in the layout. The final pipe layout is the last stage of true design in the piping activity; all further work consists only of interpreting the designer’s intention and adding detailed information for the use of craftsmen. The pipe layout may cover both: • •
Main service piping carrying utilities and process materials to various plants on a large works In plant piping which interconnects plant items within buildings structures or individual process areas
9.1 Main service piping Pipes carrying HP and LP steam, condensate, fuel oil, towns water, cooling water, towns gas, compressed air, and effluent, are typical main service pipes, and on some works central distribution of inert gases or some process materials is accomplished by running special service pipes with main service. Such pipes run from central distribution points to all areas of the site, serving existing plant and providing facilities from which new plants can be fed by tapping off or small Page 22 of 48
extensions. Main electrical cables are frequently run with the service pipes to take advantage of pipe support structures or pipe trenches and provision should be made for these cables at design and construction stages. Since the pipes form part of the site materials distribution system, it is logical and usual for them to follow the road network and bring to installed or planned plants, all site services. The piping layout is thus largely settled by general site considerations and considerations of piping practice. Main services may be run on elevated steel pipe bridges or at ground level and it is usual to run the pipes in groups to save space. Individual pipes may be buried for protection against frost or for safety. In designing main service pipe racks, some general principles must be followed whether the pipes run at ground or elevated level: •
•
• • • • • • • • • •
Pipe sizes should allow for increased flow rates required as plans are added to enlarged and space left for the addition of extra pipes. If forward plans for the site are not available, about 30 per cent extra flow should be allowed for, and about 25 per cent extra width left in the pipe rack Standard pipe spacing is maintained to avoid fouling of pipes. Pipes should run with flanges staggered to minimize centre distance spacing. Care is needed in positioning valves to ensure that they and flanged joints, do not foul. This is the type of matter, which if standardized and tabulated, will save considerable time at the layout stage Space should be allocated for electrical cables. This will probably mean adding steel work to carry the cable, which required continuous or frequent support If two tiers of pipes are used to reduce width, always run utilities above process lines so that any spillage from the higher pipes to the lower pipes is harmless As far as practical, pipes with the most frequent off takes to one side of the rack should run at that side of the rack Steam and other hot pipes should be kept to one side of the rack to allow maximum room for expansion bends over the other pipes. Pipes should never run above cables in case of spillage and cables should not be near hot pipes Branch off takes are usually normal to the flow but may be rolled in the vertical plane Where the track changes direction, the pipes should change elevation. This gives an opportunity to change the arrangement of pipes if required Flanged or screwed joints should never be located over roads or walkways or buried under roads. Welded joints are free from this restriction Pipes are normally laid without fall. If a particular pipes requires a fall, it should be run at one side of the track on separate brackets to give the required slope Provision must be made for anchor points to take up expansion forces from hot pipes or reactions at bends in large water pipes Pipe gantries or sleepers should be designed to carry the weight of the insulated pipes full of water, even though some of the large pipes may be carrying gases; at some time hydraulic testing in situ will be carried out and the pipe supports and steelwork must take the weight at test conditions Page 23 of 48
•
•
Clearance should be left around pipes for painting. About 2 in should be left between pipes and support members on pipe bridges. If pipes are laid in a shallow trench about 4 in each side will be needed. Ground level pipes should be about 12” above ground Pipes should not be buried unless a clear-cut advantage can be gained. The main advantages of buried pipes are protection from freezing, fire, or accidental damage and these are valid reasons for burying fire mains or in exceptional cases, cooling water supplies if guaranteed flow under emergency conditions is essential. When pipes are buried, some general precautions should be taken Pipes should be below frost line or be buried to a minimum depth of 2 ft 6 in (0.75) Pipes passing under roads or access ways should be cased in concrete If pipes run near roads or across cables, they must always be below cable level Gas pipes should not be laid within 1 ft (0-3m) of plastic or asbestos potable water pipes Provision must be made for valve chambers constructed in brick or concrete Trench routes must be defined at an early stage in the job so that pipes clear foundations and the trenches can be dug during the main site civil works activity Trench routes should not interfere with construction access Full and accurate record drawings should be made of buried pipes for future use by the works
9.2 `IN PLANT’ Piping •
General The piping layout proper consists of combining the information on size, terminals and fittings, etc. defined by the P&IDs / specification data with the optimum route possible within the plant layout. Better pipe layouts are obviously achieved if proper account has been taken of piping factors in the plant layout as described earlier. The layout of `in plant’ piping is always a compromise, representing the best balance the designer can achieve within constraints set by six factors – cost, operation, safety, maintenance, construction, and aesthetics. The relative importance attached to each of these factors constitutes the general plant design philosophy. Whilst there are, in theory, an infinite number of routes in space between the pipe terminals, the general design philosophy, the need to leave room for other pipes and the space occupied by the components limits the designer’s choice to a few alternatives. Which alternative the designer selects depends on the relative importance of the six constraints identified above and whether the particular pipe being examined can follow the general design philosophy. As each pipe on the P&ID added to the layout, it is ticked off on the line schedule so that a check can be made of any pipes not shown on the piping layout. Page 24 of 48
Usually, the largest, most expensive, or most important pipes, e.g. distillation column vapour line, furnace piping, or main service headers are drawn first so that they can follow the most direct routes and have prior claim on the space available. At later stages, the smaller, less expensive, and less important pipes are drawn around the main pipes until all pipes have been laid out. The operation as described, is simple for one pipe only, but a skilled layout designer is needed on complex plants with many pipes to ensure that a satisfactory, economic pipe arrangement is produced. On many plants, particularly the multi-floor type common in process industries, the pipes run inside and are supported from the plant structure. With a freestanding plant having equipment at or near ground level, there is no steelwork, which can be utilized for pipe supports, and the preferred solution is to provide an elevated structure, or `yard’, running down one side or through the middle of the plant. All the plant utility and interconnecting piping except short nozzle-to-nozzle pipes between adjacent items, runs along the `yard’. This is more elegant and economic and safer than allowing individual pipes and supports to straggle around the plant items. Provision of a pipe yard is common practice in oil and petrochemical plants. The decision to use this system must be made at the plant layout stage when in some cases the yard piping approach results in simple and economic layout of plant and piping. During the development of the piping layout, the designer must keep in mind many points arising from the various constraints and it is useful to establish a checklist of such points as an aide-memoire for guidance of all design staff. A suitable checklist is given below for guidance and will be useful in most circumstances even though every designer could probably add extra points from his own experience or to cover his own company’s requirements. A typical Piping Layout Drawing is shown in the next page, FIG.5.
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FIG.5 •
Checklist •
General o Collaborate with equipment designers to ensure that nozzle positions and orientations are convenient for pipe layout. This is important on large nozzles or where proprietary equipment has alternative nozzle positions and where nozzles required to be specified at the time of purchase o Run Pipes in North-South, East-West and Up-Down directions wherever possible. This will line up pipes with building or structural steel lines and make support easier. Pipes will also run along access walkways and make operation of valves more convenient o Assign in advance, separate elevations for North-South and East-West and Up-Directions whoever possible. This will line up pipes with building or structural steel lines and make support easier. Pipes will also run along access walkways and make operation of valves more convenient o When a pipe changes direction, change the elevation of the pipe to avoid fouling of pipes with each other o Run pipes in groups wherever practicable, so that larger and simpler supports can be provided and many individual hangers avoided Page 26 of 48
o Route pipe groups-particularly containing large pipes-along lines of main steel work so that supports can be taken from main members. Do not forget to advise the structural designer of this before imposing large pipe loads back on main members o Spacing of pipes within groups should be practical minima as noted for main service piping unless certain lines need frequent access o On multi-floor plants, pipe wells should be established so that main service headers and inter-floor piping can pass through floors simply without the need for many small pipe holes. Wells should be near main column and beams to avoid weakening flooring o Pipes in plant should be dimensioned to centrelines to make detail checking easier o If horizontal and vertical pipes run alongside a building wall, run each system at a different distance from the wall so that crossovers can be made without insertion of extra fittings o Collaborate with electrical and instrument engineers to ensure that main cables and trunking are shown and cleared by pipes at earliest stages of pipe layout. Cables, etc. cannot be easily cut and joined if they foul pipes on site-this means pipes would have to be modified, entailing unnecessary extra construction cost o Make provision for flexibility in hot or cold piping by use of loops, bellows, or expansion joints. Flexibility is needed, not only for high temperature or cryogenic systems –short, stiff jacketed line or steam header can be just as much of a problem as a high temperature alloy pipe if flexibility is neglected o Allow extra space for larger size and more complex construction work needed on anchors or variable supports for hot pipes o Allow for thickness of insulation (especially on high temperature pipes) in spacing pipes in relation to plant features o Run and support pipes which are subject to plant-induced vibration separately-piping from compressors, fans, centrifuges, ball mills, and similar equipment comes into this category. Special damping supports or mass added to pipes may be needed and space must be allowed for remedial action on site since it is difficult to forecast extent of vibration and remedial work at he design stage o The possibility of plant expansion must be borne in mind-leave valvedoff connections on main service headers to allow for future needs o Allow for runs of straight pipe needed for some flow measuring devices. Also ensure rotameters are drawn with vertically upward flow. Make provision for supports spaced on centres •
Cost o Pipe routes should be the shortest and simplest possible, consistent with flexibility and clearance requirements. Extra pipe and fittings increases capital cost and creates extra pressure drop o Minimize use of flanges; use only at terminals, valves, and fittings unless maintenance requirements for cleaning, disconnecting, and dismantling override cost. Flanged joints are more expensive in cost and space usage than welded joints Page 27 of 48
o Use pulled bends where possible rather than weld elbows bends are cheaper to fabricate in many cases and have lower pressure drop •
Operation o Establish minimum dimensions of gangways and working areas. Extra width should be allowed for gangways to be used by vehicles. Working area sizes should be agreed with operating staff before pipe layout starts o Valves in regular or emergency use should be conveniently located, e.g. Horizontal spindle valve hand wheel may be 5ft. 0 in. (1.5m.) above the floor, Vertical spindle valve hand wheel may be 3 ft. 6 in. (1.07m.) above the floor, Small horizontal spindle valve hand wheel will be up to 7ft. 0 in. (2.1m.) above the floor, etc. o Valves regularly operated should be reached from working platforms or special local platforms reached from permanent ladders. If unavoidable, valves less than 3 in (75mm) bore, operable by one hand, may be operated infrequently from portable ladders o Valves requiring regular operation, which cannot be reached from platforms, should have chain wheel or extension spindle operators provided. Chain wheel operation can be difficult for large valves and, on outside plants. If chain wheels are used, the chain should hang to within 3 ft. (1 m.) of a platform and be located at one side of any gangway o At least 6 in. (0.15 m.) is needed all round any valve handle or hand wheel to clear the operator’s hands o Valves should not be installed ‘ spindle down ‘, otherwise deposits may accumulate in glands o Valves should not be installed in vertical runs if this can be avoided. Such installation allows a pocket of liquid or condensate to collect and stagnate above the valve, which cannot easily be drained o Consider need for gear or motorized operation of large valves, cocks or high pressure valves o Test points for pressure and temperature should be left on important pipes for use in commissioning. Usually a ½ in union + plug is adequate. For lagged pipes a bull plug, long enough to project through the insulation, should be used o Vent and drain points should be fitted on pipes frequently tested or dismantled. These points should be ½ in nominal bore minimum size and arranged so that they can be rodded to ensure free venting or drainage o Sample points, when needed, should be about 3 ft (approximately 1 m.) above the floor level and must never be above eye level. Check with operating staff the space needed for insertion and removal of sampling vessel o Dead legs of infrequently used pipe where stagnant fluid can accumulate must be avoided. Long term hidden corrosion or blockages can be established in such pipes, which are not apparent until the pipe is used
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o Plants in the open need careful examination of the possibility and consequences of pipes freezing up in winter. Fluids in slow or intermittent flow may freeze or become viscous under low temperature conditions where heat losses through the pipe cannot be made up by the flow of warm fluid. Where freezing or operational difficulties are likely, steam or electric heating should be used to ensure proper operation in all foreseeable winter conditions. It is poor economy to risk a winter shutdown for the sake of saving some small bore steam and condensate pipe and fittings o Steam feeds to plant heating systems should be grouped in operating sets at convenient locations so that the system can be controlled from central points and the operators are not expected to find and operate many small valves in, perhaps, difficult-to-reach placed. These systems usually entail many small bore pipes and valves and it is essential to have clear identification at the operation point of which parts of the system are controlled by each valve so that logical operation and fault finding procedures can be established. Condensate returns grouped and identified so that reliable routine checking of steam trap operation can be assured. Remember that the operator is human too-he will carry out his duties best when they are made easy for him by the designer o All steam trap sets feeding into condensate headers must have a bypass and test point fitted, so that operational checks and maintenance can be carried out safely and the plant can run whilst a defective trap is being repaired o Instruments in piping to be read by operators should be about 5 ft. (1.5m.) above floor level o Where plant conditions are displayed on a local instrument and controlled from a manually operated station, the instrument must be clearly and easily visible from the operating station o Provision should be made for the insertion and removal of temporary strainers during start up, particularly upstream of pumps, compressors, control valves, meters etc. o If flexible hoses are used to wash down or temporary steam, air, or other supplies to the plant, the layout should be based on maximum hose lengths of 50 ft 0 in (15m) and sufficient hose points and hose reels provided to maintain this maximum length o Pump suction pipe work should be laid out for minimum pressure drop, particularly if the pump suction has to lift fluid from below the pump or if volatile fluids are being handled Excessive pressure loss in these cases can cause pumping failure through loss of prime or vapour locking o If reducers are needed in pump suction lines, they should be fitted directly to the pump inlet. The temptation to fit reducers before the pump isolating valve, or strainer to make use of reduced size fittings, should be resisted suction pressure losses increase rapidly as suction line size is reduced o On critical or large volume pumps, a straight length of about three diameter should run from the last valve or fitting to the pump suction to
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smooth the flow into the impeller and allow the pump to operate smoothly and efficiently o All centrifugal pump should be fitted with a pressure gauge on the outlet. Since the volume versus pressure relationship for a pump is easily obtained, the pressure reading give a clear picture of pump performance and during operation o Piping around reciprocating compressors should allow for pulsation dampers at inlet and outlet sides. Dampers are almost always fitted at the outlet side and may be found necessary on the suction side and the layout must be capable of easy modification if necessary o Compressor inlet piping should not contain pockets where condensate can collect and be entrained into the inlet pipes-slugs of liquid entering the compressor can cause severe damage •
Safety o All pipes above floors, platforms, gangways, and stairways must leave at least 7ft. (2.1m.) headroom blow the lowest part of the pipe or any pipefitting o Pipes running across areas not normally designated as access areas should be a minimum centre height of 2 ft 0 in (0.6m) above the floor, so that they do not present a trip hazard o Pipes must not protrude into gangway widths or into working platform areas o Small bore pipes, glass or plastics pipes, and pipes carrying hazardous fluids must be protected if installed alongside a gangway used by vehicles. Barriers or steel shield must be provided o The possibility of hazardous reaction between the contents of pipes and the contents of other nearby pipes or vessels in the even of leakage must be considered. Typical examples are acid and cyanide, water and sodium, volatile flammables and hot surfaces, or water and strong acids. If such combinations exist and lethal conditions can arise from accidental mixtures, one of the systems must be rerouted away from the other o Flanged or screwed joints should not be located over walkways or stairs. This does not apply for welded joints o Pipes carrying non-conducting flammable volatiles must be bonded for electrical continuity and earthed to prevent the accumulation of static electricity charges which can, if arcing o earth near the pipe discharge, cause fie or explosion. Screwed piping using PTFE thread seal tape at the screwed joints must also be bonded-the PTFE tape can effectively insulate line sections from each other o Do not run hot pipes near power cables-any local heating of the cable will reduce its allowable power capacity rating and may damage the cable. Do not run solvent or acid lines over plastic cables o Pipes carrying main services, e.g. cooling water steam, or air through areas of fire hazard, should be fireproofed so that services can be maintained for emergency shutdown and cooling if a fire breaks out
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o Process lines carrying lethal, flammable, or valuable fluids should avoid fire hazard areas if possible. If they must run through such areas, consider fireproofing o Consider effect of fire on non-ferrous, plastics, glass, or lined pipes o Emergency shutdown valves should be placed in a sheltered but visible area from which the operator can escape easily during emergencies o If possible, pipe water-cooled heat exchanges so that the inlet is at the bottom and the outlet at the top. If the cooling water supply fails, then the exchangers will be left full of water to provide a limited reserve of cooling capacity o Make sure safety drench showers and eyewash points are installed on caustic, acid, or solvent plants o If positive isolation of hazardous fluids is required, fit `double block and bleed’ valve arrangements, or line blind valves. Do not rely on this spade plates in the line for such duties, they can corrode away without any external evidence and fail when most needed o Avoid the possibility of trapping liquefied gases between two valves in a line. If both valves are essential, fit a relief valve connected to the plant vent system so that high pressures from liquid expansion or vaporization can be relieved o A relieve valve should always be fitted across the outlet and inlet of a positive displacement pump of compressor so that overpressure caused by downstream blockages does not damage the machine o Relief valves should be installed with inlet axis vertical and discharge axis horizontal o Inlet piping to relief vales should be short, straight and at least the same line size as the inlet flange of the valve. Discharge piping should be larger bore and could require special supports to cope with sudden heating of pipe or sudden fluid reaction forces when the relief valve lifts o Disposal of vented fluids should ensure the following: o Harmless gases (e.g. many aqueous solutions, high b.p. oils) pipe to appropriate plant effluent drain and disposal system. The pipe end must be visible so that discharge can be seen o Harmless liquids (e.g. many aqueous solutions, high b.p. oils) pipe to appropriate plant effluent drain and disposal system. The pipe end must be visible so that discharge can be seen o Hazardous of flammable liquids or gases (e.g. solvents, methane, phosgene) must be piped into a closed vent system equipped with collecting vessels, scrubbing plant, or flare stack to collect or dispose of the fluid o Vents and drains on hazardous or flammable pipe lines should run to a plant disposal system o All closed vent systems must be designed so that the system pressure during operation is not so high as to prevent the opening of limit the flow through any relief valve o Vent lines should be self-draining away from the relief valve to pockets or vent system knockout drum to prevent condensate logging of lines with consequent high back pressure when the relief valve operates
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o Access to emergency exists; fire escapes or access for firefighting must not be obstructed by pipes. •
Maintenance o Pipes should never run below lifting beam installed for plant maintenance o Pipes should not run directly over equipment requiring process cleaning or mechanical attention. Typical items which need such work are: o Heat exchangers, Vessels with full diameter lids – Require cleaning out o Pumps and compressors, Agitates vessels, Centrifuges, Process machinery, evaporators, Small mills – Require mechanical work, possibly require overhead clearance for lifting o Pipes should run above lighting fittings so that obstruction or shadows are prevented and so that routine lamp maintenance is not hindered by the presence of pipes o Do not support pipes off other pipes, particularly in vertical banks on horizontal pipes o Always leave short spool pieces connected to the flanges of any item likely to need removal from the plant (e.g. pumps, vessel lids, etc.). When removal is required, the spool pieces are removed and the item can be moved without major disturbance of pipe work o Control valves, safety valves, and large valves generally should, because of their weight, be supported separately from the pipe in which they are installed o Supports for piping should allow for the removal of valves and fittings without requiring use of temporary supports whilst the fitting is out of the pipe o Supports for piping should allow for the removal of valves and fittings without requiring use of temporary supports whilst the fitting is out of the pipe o Large valves, fittings or control valves should be mounted under or near steelwork from which the item can be lifted for maintenance. If this cannot be done, then consider fixing special hitching points for maintenance use o Control valves should be mounted at least 15 in. (0-38m.) above floor level and with a clear distance below obstructions at least equal to the valve height, to allow for removal of bottom cover and top control gear o Access should be provided to control valves and `inline’ instruments depending on the frequency of maintenance. Suggested means are: o Control valves, motorized valves, and normal instrumentation for level, temperature, pressure, or low-these need fairly limited in situ attention and a fixed ladder is acceptable o Special instruments for infra red, pH, gas chromatography, refractive index, etc. may need frequent careful attention and calibration, and a permanent platform should be provided o Pipes to be cleaned frequently (about once per week) should be provided with flanged rodding out points at changes of direction. Page 32 of 48
About 30 ft. (9 m.) is the maximum that can be rodded effectively from one end; 60 ft (18m) if rodding from both ends o Fine filters should always be provided on compressor suction piping to prevent entry of grit and scale o Piping around reciprocating compressors should be arranged to leave room for maintenance of cylinder heads, valves, etc. Also ensure that withdrawal of pistons, crankshaft or camshaft is not prevented by pipes o Heat exchanger piping running alongside the exchanger shell should clear the edge of the tube sheet flange by about 2ft. 6 in. (0.76m.) to leave access to the bolts and joints •
Construction o Ensure that pipes erected initially do not impede construction access for later plant stages. This may mean that pipes must be laid out to eave one side of a plant open for construction equipment or that buried pips should be re-routed o Large pipes and fittings should be positioned near some means of temporary access and support of simplify construction o Provide vent, drain, and pressure gauge points for hydraulic tests on lines. Inert gas purge connections may be needed on some plants o Where pipes must pass through floor plates or concrete floors, establish position and size of holes needed at the design stage so that pre-cut plates or cast-in holes can be provided. Cutting holes on site is messy and expensive
•
Aesthetics o Route pipes along or perpendicular to building lines, unless process conditions prohibit this o Ensure verticals are vertical and horizontals are horizontal-any small deviation will be exaggerated and unsightly o Support spacing must not allow any visible sag between supports-this gives an untidy appearance in addition to being a cause of logging o Do not run pipes across windows or under roof lights o Although appearance is seldom important, a good pipe layout will `look right’. If appearance is important, have the courage to present a good, functional layout before trying to hide the pipes behind plant or disguising their appearance
The final piping layout is a work of considerable complexity, which is intended to ensure that all pipes will fit into the plant and also to provide the basis from which detailed fabrication and erection drawings can be made. Once the layout is complete, in sections or as a whole, a large amount of detail drawing work can be started and often a considerable number of designers will be engaged on this work to reduce its duration to a practical minimum. Unless the information on the piping designer’s intentions is clearly conveyed to the detailer, much of the drawing office work will be abortive, confusion will result at site and the best pipe layout rendered useless because the fabricators and erectors will not know that the designer expects of them. If good pipe and valve specifications and a clear P&IDs have been Page 33 of 48
prepared, much of the detailed information is easily understandable and the piping designer can, at the layout stage, concentrate on the best means of specifying his pipe routing and arrangement requirements to the detailers.
11
INSULATION & HEAT TRACING 11.1
Insulation In the industrial processes it is necessary to maintain the temperature of fluids, gases, and vapours during transit in pipelines. This means that heat loss through pipe walls and equipment shells must be avoided or minimised by certain mean. To avoid this heat loss pipes and equipments are insulated with insulating materials like mineral wools such as rock, slag, glass, ceramic and hair felt. There are four main reasons for maintaining the temperature of a product in pipeline transit: • • • •
Many products are highly viscous or even solid at ambient temperature, but can be reduced in viscosity or melted by heat to such an extent that free flow is maintained and they can be easily pumped and controlled Other products require to be closely held at temperatures, which will preserve their physical characteristics and avoid separation or a change of chemical or physical state Pipeline heating is sometimes necessary to prevent freezing, particularly in colder parts of the world where freezing of the product or perhaps moisture particles in a gas stream can take place In other instances, pipe lines are heated to maintain a close control on viscosity of the liquid prior to passing through a metering control unit where a variation in density would upset the required accuracy
Typical products, which require heat application in pipe transit, are, resins, polymers, waxes, tar, pitch, asphalt, sulphur, and many foodstuffs Thermal insulating materials (including protection) should have: • • • • •
Resistance to attack by chemicals with which they may come into contact If this is not possible, then the insulation should be provided with a resistant coating or jacket Resistance to moisture sufficient that they do not deteriorate under wet conditions. This is important when operating in the open area. Cover to resist from vibration, mechanical shock, and abrasion, as most insulants Insulations are mechanically weak, at least protection them against damage Characteristics which allow them to be formed, as required, to effectively insulate awkward pipe and fitting shapes
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11.1.1 Preparing an Insulation Specification Because the plant as a hole is usually involved, it is seldom that pipework alone forms a complete specifications. The specification has also to cover all types of equipment, including heat exchangers, boiler drums, dryer, distillation units, reaction vessels, pumps, and other equipment. Similarly it is required to prepare Insulation Specification, which is usually divided into two main sections, 1. Pipes and 2. Equipment. 11.1.2 Data required to prepare Insulation Specification • Pressure and temperature (If steam, saturated or superheated) • Ambient air temperature • Pipe outside diameters (Related to Standard Specifications where possible) • Pipe lengths • Number of bends, valves, special fittings etc. and type of cover required • State any pipe fittings, valves, joints etc. which require periodical removal of lagging • Type of flange insulation required • Type of insulation reinforcement • Protective finishes required, related to possible leakage of oil or corrosive chemicals, ingress of moisture, indoor or outdoor operation, chloride stress corrosion • If there is any fire risk • Vibration or impact effects which will react on the piping • Number of hours running/annum • Number of years which plant will operate • Cost of fuel and cost of heat distribution to point of insulation • Guaranteed efficiency required, Expressed in terms of heat lost • Insulation material to be employed • Provisions for expansion to avoid crack deterioration of the insulation • Protection against moisture `leak through’ at pipe hangers • Any special conditions of insulation application site, i.e. special scaffolding, excessive heights, etc. 11.2
Heat Tracing There are several ways of Heat Tracing to carry out pipe line heating, the more common being as follows: • • • •
Electric surface heating tapes or cables Jacketed pipe lines for use with steam and other heat transfer media External tracer lines-clip-on or welded on Use of heat transfer cements to improve rate of heat conduction
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12
ISOMETRIC DRAWINGS The most effective and almost universal manner of producing all this information is the pictorial dimensioned drawings, commonly termed `The Isometric Drawing’. This type of drawing conveys a two-dimensional picture of a three-dimensional shape by using the isometric draughting conventions of representing vertical directions by vertical line on the drawing and representing horizontals to left and right of the aspect point by lines included at 30° to the drawing sheet horizontal.
FIG.6 Ideally an isometric should show only one pipeline from start to finish and this should be the aim in detail draughting. In practice, a minority of exceptional cases will be found when a pipe is too complex to be clearly represented and must be broken down into two or more simpler drawings; alternatively, some simple lines connected together can best be shown on one combined drawing. In the latter case, however, all the pipes must be to the same specification and, at a specification change point, a new isometric should be started. The need to start and finish isometrics at sensible points in the piping systems highlights the need, already noted, to locate break points between separately numbered pipes in positions where new pipes might be expected to start naturally. Pipes and piping components are represented on the isometric by simple stylised symbols, which are almost self explanatory and are commonly accepted. Some of the commonly used symbols in Isometric Drawings are shown in FIG.6.
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Piping is a single bold line drawn along the pipe axis and typical symbols for other items are shown. The shape of the pipe is shown correctly and all components are correctly located relative to each other in the pipe, but no attempt is made to draw to scale. Indeed, the reverse is true in that scale is completely sacrificed to clarity and any complex portions (e.g., a control valve set) are drawn large enough to be easily read and dimensioned, but long straight runs of pipe are foreshortened. Fittings are drawn without regard to scale-for example, large and small valves may be drawn the same size. A typical Isometric Drawing is shown in FIG.7.
FIG.7 •
Check List for Isometrics •
General o Study the layout checklist given in the previous section 8, for guidance when the layout leaves some freedom in detailing o Show all details with project north in the same direction o Check certified equipment drawings for flange ratings o Decide whether or not to include gasket thickness in dimensioning and stick to the decision throughout the project. Usually gaskets less than 1/16 in. can be neglected; gaskets over 1/16 in. should be included o Ensure pipe terminal co-ordinates and connecting nozzle identities are shown
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o Show P&ID and Piping Layout drawing numbers for reference on every detail drawing o Ensure all continuation points are highlighted and continuation drawing numbers shown o Try to show a reference dimension to a stanchion or floor beam to give the erector a locating point, but do not show excessive pictorial detail of adjacent plant or structures o Indicate flow direction when pipes slope and when non-return valves are fitted o Show special fabrication requirements, e.g. x-ray or heat treatment if required o Draw boldly and simply. If using preprinted isometric grid paper, make sure all drawn lines (including witness lines) are heavier and print clearer than the grid lines o Keep lettering and figures to about 3/16 in (5mm) minimum size and easily readable •
Operation o Look at each pipe individually to check that it can be vented or drained either into another pipe or vessel or by fitting vent and drain points. Never leave undrainable pockets in lines carrying hazardous or corrosive liquids o Make sure that valves and fittings are oriented correctly to flow e.g. o Rotameters vertically upward o Non-return valves work in correct direction of flow o Globe valves have pressure underseat rather than on top o Sight glass faces can be seen by operators o Strainers are installed to makers instructions o Ensure valve hand wheels and instrument faces point towards (but do not foul) Operating areas and put note on detail drawing for erector’s guidance o To prevent solids depositing in glands, try to avoid mounting valves with spindles horizontal or pointing downwards o Try to avoid installing valves in vertical lines-the leg of liquid left in the line above the valve when the valve is closed cannot easily be drained o Put valves in off takes from pipe racks or racks of pipes outside the pipe/supporting steelwork area for easy operation o Leave room for temporary strainers in pipes to trap residues left in the pipes during construction o Check if any special cleanout procedure is required for compressor feed pipes, instrument air or similar lines, and give details on drawings o Avoid air pockets in pump suction lines. Reducers, if used, should be eccentric type o Try to leave 1 ½-3 diameters of straight pipe leading in to pump suction to reduce eddies entering pump. On critical suction duties (e.g., hot, volatile liquids or pumps with large suction lift), increase straight length to maximum amount possible. o If possible provide equal flow patterns to both pumps when installed spare pumps are used Page 38 of 48
o Fit priming connections to centrifugal pumps which draw liquid from tanks blow pump levels-the pump cannot generate suction until the impeller is running in liquid o Consult instrument engineers for details of instrument mounting generally, but some simple rules are: o Put tapping for pressure point at side of pipe rather than top (this forms an air pocket) or bottom (which allows solids to deposit on gauge) o Thermometer pockets (usually ½ in. or ¾ in. nominal bore) should be installed at a change in direction in the pipe either by fitting a tee connection or a socket fitted into an elbow. Obstruction of flow is minimized by this method of installation-for example a ½ inch nominal bore pocket occupies only 29 per cent of the flow area of a 1 1/2inch pipe o Thermometer pockets can cause serious obstruction to flow in lines 2 in nominal bore and below when installed across the pipe, e.g. the ½ in. nominal bore pocket occupies 65 per cent of the flow area of the 1 ½ in. nominal bore pipe. In these cases an enlarged section should be provided o Steam trap piping is important and maker literature should be consulted for detailed guidance – valuable data are provided by reputable manufacturers. In particular, observe the following: o Always provide a strainer before a trap-no trap will work with dirt under its seating o Fit an isolating valve before the trap-otherwise, the plant has to be shut down to maintain it o On duties where good condensate removal is vital, fit a valved bypass around the trap-the plant can then discharge condensate while the trap is being maintained o The discharge from every trap should either be visible (at a tundish or sight glass) or capable of being tested by discharge to atmosphere, otherwise it is impossible to check trap operation o If traps discharge to a pressurized condensate main, an isolating valve should be installed to protect maintenance workers and a non-return valve fitted to prevent condensate blowing back from the pressurized main when the trap is not discharging o Trapping points in pipes should be taken off bottom of the pipes, to prevent condensate being carried over the trapping point •
Safety o Make sure pipes are at least 7 ft. (2.1m.) above platforms or working areas. Do not run pipes at very low levels (say less than 2ft. (0.6m.) above floors or platforms) even although these areas are not designated for operation or access-pipes below this level are a serious trip hazard during maintenance or emergency operations o Position valves to leave at least 6 in. (0-15 m) clear space round hand wheels o On hot or corrosive liquids, avoid joints at eye level or over platforms
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o Check relief valve or bursting disc discharge piping carefully with the designer. General points to note are: o Fit light weatherproof cover over exit to atmosphere to prevent rainwater entering and settling in pockets or on relief devices. This cover must be of light construction to reduce its inertia-it must open quickly and with minimum pressure rise when a relieving device operates o Provide small drain holes at lowest point of atmospheric discharge pipes. Note-Do not do this on flare systems or on sealed vent systems o Discharge pipes entering a common main should make a 45° lateral connection with the main as shown in Fig.14.34, to minimize back pressure from main and to assist flow into the main o Check supports of discharge lines on relief valves from point of view of weight of valve and reactions from sudden flow when device operates suddenly o If screwed lines are used for flammable non-conductive fluids (e.g. hydrocarbon solvents), ensure that PTFE thread seal tape is not called for at screwed joints unless earthing connections are also made •
Maintenance o Fit short spool pieces at nozzles of all equipment to facilitate disconnection of piping during maintenance. Typical items where this feature is useful are Pumps, Compressors, Pulverizers and Crushers, Vessels with flanged lids, Centrifuges etc. o If short spool pieces are used at valves or control valve sets, ensure that the spool is long enough to withdraw bolts easily on the spool side-it is sometimes impossible to withdraw the bolts on the valve side o Do not position flanges where spanner access to bolts is restricted by steelwork, handrails etc. o The lowest point of any part of a pipe should be at least 6 in. (0.15 m.) above floor or platform level. Where a control valve is fitted in the pipe, increase this to 12 in. (0.3m.) to permit removal of bottom covers o Provide drain points on control valve sets handling dangerous fluids so that the pocket of liquid between the block valve and the control valve can be emptied safely o Standardize steam trap sets as far as possible so that rapid repair by replacement techniques can be used
•
Construction o Provide adequate (not excessive) site make-up points or closing lengths o Avoid, if possible, leaving site welds to be made in horizontal lines-on such welds the lower half must be made from below and is more difficult for the welder o Ensure flange or thread ratings match throughout all pipes, particularly at valves, control valves, and vessels. Valve flanges are sometimes made to certain flange tables only. Cast-iron valves are almost always Page 40 of 48
o
o
o
o o o
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flat faced; if raised face flanges are called for by the specification, the valve mating flange must have the raised face machined off to match the valve When pipes are carried over from one isometric to another, check that the break point is at an accessible location. Also ensure continuation notes are shown on drawings When starting new points at a tee from a main pipe, leave a stub about 12 in. (300mm) long projecting from the main or use a welded tee so that the pipes can be joined on site by a butt weld. Do not leave erectors with the job of making difficult stub tee connections on site When small branches are needed on large pipes, provide gusset plates to reinforce the branch against damage during transport and construction Provide temporary strainers in lines to remove construction debris Check hot lines with designer to see if cold pull is called for-if so, specify location and amount on drawing If expansion bellows are fitted in pipes, add note to remind erector to leave the protecting bracer rods in during construction and remove at start-up
PIPE SUPPORTS During the pipe layout stage, supports should be defined for use on the project. These may be company standard or maker’s standard types as found most economic; the important point is that standards be selected. Standard assembly and detail drawings with parts lists for each type should be available in a format which leaves the pipe size to be supported and possible other features (e.g. length of hanger type support) to be specified separately. By this means, a relatively small number of drawings can serve for a large number of supports. Each type of support should be given a short identifier, which may be based on manufacturer’s or company’s standards or perhaps identified specially for the project. Support positions and types should be selected, as the piping layouts are being finalized and completed. To avoid complication, it is not usual to indicate the supports on the pipe layout drawings, but a progressive recording of positions should be done on separate transparencies or on the piping isometrics. Dimensional positioning of type and recording of all supports are important, particularly so if hot pipes subject to expansion are concerned. In this case, a hanger placed a matter of inches out of position can cause stresses, which may split a pipeline during test or at start-up. There is also the question of pipe deflection leading to overstressing and pipe logging if a support is not placed in accordance with its calculated position. A plan of supports need only be a simple type of drawing provided that location dimensions are incorporated. There is no need to indicate supports in detail a small circle plus the support schedule number is all that is required.
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Supporting of pipes should follow the general sequence of layout of piping. This will ensure that important and large pipes are attended to first. A support schedule should be drawn up as the job proceeds to identify types and sizes record and provide a progressive checklist of detailing, fabrication, and erection work. Various types of pipe supports are shown in FIG.8. •
Function of the system support: The mechanical requirements of the piping support system are: •
•
• • •
To carry the weight of the piping filled with water (or other liquid involved) and insulation if used, with an ample safety margin –use a factor of three (= ratio of the load just causing failure of support or hanger to actual load) or the safety factor specified for the project. External loading factors to be considered are the wind loads, the probable weight of ice buildup in cold climates, and seismic shocks in some areas To ensure that the material from which the pipe is made is not stressed beyond a safe limit. In continuous run of the pipe, maximum tensile stress occurs in the pipe cross section at the support. The system of supports should minimize the introduction of twisting forces in the piping due to offset loads on the supports; the method of the cantilevered sections to substantially eliminate torsional forces To allow for draining, holdup of liquid can occur due to pipes sagging between supports. Complete draining is ensured by making adjacent support adequately tilt the pipe To permit thermal expansion and contraction of the piping To withstand and dampen vibrational forces applied to the piping by compressor, pump, etc.
Ideally, each point of support would be at the centre of gravity of an associated length of piping, Carrying out this scheme thru the entire piping system would substantially relieve the system from twisting forces, and would be stressed only vertically Following points should be to considered while supporting the pipes: •
• •
Ideally, each point of the support would be at the centre of gravity of an associated length of piping. Carrying this scheme through the entire piping system would substantially relieve the system from twisting forces, and supports would be stressed only vertically The presence of heavy flanges, valves, etc., in the piping will set the centre of gravity away from the midpoint of the associated length The nature of the conveyed material, the process, and flow requirements determines how much sagging can be accepted. Sagging is reduced by bringing adjacent points of support closer. Pocketing of liquid due to sagging can be eliminated by sloping the line so that the difference in height between the adjacent supports is at least equal to thrice the deflection (sag) at the mid -point, lines which require sloping include blow down headers, pressure relief lines, and some process, condensate and air lines Page 42 of 48
•
•
•
•
Sloped lines on pipe racks: Sloped lines can be carried on bracket attached to the pipe rack stanchions. To obtain the required change in elevation at each bend, the bracket may be attached at the required elevation and the slope obtained by using shoes of different sizes-this method leads to fewer construction problems. Shoes of graded sizes are also the best method for sloping smaller lines on the pipe rack. It is not usual or desirable to hang lines from the pipe rack unless necessary vertical clearances can be maintained. Sloped lines in buildings: Inside a building, both large and small sloped lines can rest on steel brackets, or to be held with hangers. Rods with turnbuckles are used for hangers on lines required to be sloped. Otherwise, drilled flats bar can be used. Supporting pipe made from plastics or glass: Pipes made either from flexible or rigid plastics cannot sustain the same span loads as metal pipe, and requires a greater number of support points. One way to provide support is to lay the pipe upon length of steel channel section or half section of pipe, or suspending it from other steel pipes. The choice of steel section would depend on the span load size and type of plastic pipe.
Design Points: •
General • • • •
•
Design hangers for 2” and larger pipe to permit adjustment after installation If piping is to be connected to equipment, a valve, etc., or piping assembly that will require removal during maintenance, support the piping such that temporary support are not needed Base load calculations for variable-spring and constant load supports on the operating conditions of the piping (do not include the weight of hydrostatic test fluid) If necessary, suspend pipes smaller than 2” nominal size from 4” and larger
Terms used in support • • •
•
Support The weight of piping is usually carried on supports made from structural steel, concrete or wood. Hanger Device, which suspends piping (usually a single line) from structural steel, concrete or wood. Hangers are usually adjustable for height. Anchor A rigid support, which prevents transmission of movement (thermal, vibratory, etc.) along piping. Construction may be from steel plate, brackets, flanges, rods, etc. Attachment of an anchor to pipe should preferably encircle the pipe and be welded all around as this gives a better distribution of stress in the pipe wall. Tie Page 43 of 48
•
An arrangement of one or more rods, bars, etc. to restrain movement of piping. Dummy Leg An extension piece (of pipe or rolled steel section) welded to an elbow in order to support the pipeline.
The following hardware is used where mechanical and/or thermal movement is to be guided: • •
•
•
•
Guide A means of allowing a pipe to move along its length, but not sideways. Shoe A metal piece attached to the underside of a pipe, which rests on supporting steel. Primarily used to reduce wear from sliding for lines subject to movement. Permit insulation to be applied to the pipe. Saddle A welded attachment for pipe requiring insulation, and subject to longitudinal or rolling movement (resulting from temperature changes other than climatic). Saddle may be used with guides. Slide Plate The two plates used in a support are made from or faced with a material of low frictional coefficient to reduce frictional resistance during movement of pipeline to with stand mechanical stress and temperature changes. Plates are often made from graphite blocks, steel plates with a Teflon facing are available and may be welded to steel. Spring hangers or supports Allow variations in the length of the pipe due to change in temperature, and often used for vertical lines. There are two types of spring hanger or support: •
•
•
‘Constant Load’ Hanger The device consists of a coil spring and lever mechanism in a housing. Movement of the piping, within limits, will not change the spring force holding up the piping, thus, no additional forces will be introduced to the piping system. ‘Variable spring’ Hanger, and support These devices consist of a coil spring in a housing. The weight of the piping rest on the spring in compression. The spring permits a limited amount of thermal movement. A variable spring hanger holding up a vertical line will reduce its lifting force as the line expands towards it. Both i.e. variable spring hanger and support place a load on the piping system, where this is undesirable; a constant load hanger can be used instead.
Hydraulic dampener, shock, snubber, or sway suppressor One end of the unit is attached to piping and other to structural steel or concrete. The unit expands or contracts to absorb slow movement of the piping, but rigid to rapid movement. Page 44 of 48
•
Sway brace, or sway arrestor It is essentially a helical spring in a housing, which is fitted, between piping and a rigid structure. Its function is to buffer vibration and sway.
•
Welding to pipe If the applicable code permits, lugs may be welded to pipe for 1. Fixing hangers to structural steel, etc. 2. Attaching to pipe 3. Supporting pipe Welding supports to prelined pipe will usually spoil the lining, and therefore lugs etc. must be welded to the pipe and fittings before the lining/cladded insulation is applied. Welding of supports and lugs to the pipes and vessels to be stress relieved should be done before heat treatment.
Various types of piping supports
Rod
Eyerod
Rod
Turn buckle with rod
Clevis with rod
Spring hanger with eye rod
Clevis with rod
Clevis with rod
Spring hanger with rod
Hanger with lug
FIG.8
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FIG.8 (CONT.)
Clevis with pipe and clamp
Clevis with pipe Channel with weld
Dummy leg
T section
Wye-type shoe
Saddle with roller
Supporting Elbow
Bent flat bar
U bolt
Adjustable support
Welded brackets
Dummy leg
With Slide plates
Steel rod
Drip leg support
FIG.8 Page 46 of 48
FIG.8 (CONT.)
With spring support
Piping support assembly FIG.8
•
Layout consideration to facilitate support: The two cardinal principles in routing lines for economic support, restraints, and bracing are: • •
•
Group pipe lines so as to minimize the numbers of structures needed solely for pipe supports, restraints, or braces. Keep lines located close to possible points of supports, etc. either to grade or to structures, which are to be provided for other purposes.
The significant general considerations affecting the routing of the piping for favourable support may be summarized as follows: 1. The piping system should be self-supporting insofar as practicable and consistent with flexibility requirements. 2. Excess flexibility may make additional supports or restraints necessary to avoid movement and vibration in such amplitude as to arouse personnel apprehension. This situation is apt to occur on vertical lines where only one point of support is needed to sustain the weight. 3. Free movement expansion joint systems involving appreciable unbalanced thrusts from pressure should be avoided unless such forces can be taken can be taken on substantial structure, or at grade. 4. Piping prone to vibrate, such as compressor suction or discharge and driver exhaust lines, should be routed for support independent from other piping and lightly braced structures and buildings. Routing should permit the use of resting or similar supports offering resistance to motion and providing same damping capacity, rather than hanging supports. 5. The pipeline should be sufficient close to the point of support or restraint so that the structural connection can have adequate rigidity and details can be simple and economical. Page 47 of 48
6. Piping from upper connections on vertical vessels is advantageously supported from the vessel to minimize relative movement between supports and piping; hence such piping should be routed and supported close to the connection. 7. Piping in structures should be routed beneath platforms, near major structural members at points at points favourable for added loading, avoid the necessity of making these members heavier. 8. Sufficient space should be allotted so that the proper support assembly details may be accommodated. 9. Access clearance must be provided in order that support fixture parts requiring maintenance can be serviced. Allowable spans for horizontal lines are principally influenced by the need to: 1. Keep stresses within suitable limits. (Instability may be a factor in the case of large thin-walled pipe) 2. Limit deflections (sagging), if necessary for: • • •
Appearance Avoid pockets Avoiding interferences
3. Control natural frequency (usually by limiting the span) so as to avoid undesirable vibration. In most cases, an adequate estimate of the stress is readily obtained from simple beam relationship: S = 1.2(wl2 / Z) Where S = maximum bending stress, (psi) Z = sectional modulus in.3 l = pipe span, ft. w = total unit weight, lb/ft. Deflection under weight effect is generally of secondary importance in piping just, as it is in structures. In fact, some piping designers are inclined to disregard deflection entirely and to consider the process units, however the deflection of the line should be kept within reasonable bounds in order to minimize pocketing and to avoid pocketing and to avoid possible interference in congested areas due to sagging. Appearance, too, will be a factor in many cases. A practical limit for average piping in process units is a deflection on the order of ½ in. to 1 in. For piping in yards or for overland transmission lines a value of 1½ in. or greater is generally acceptable. For power piping a deflection limit as small as 1/8 in. is specified by some designers.
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