Title of Paper
Interpretation of IP15 in Process Plant Design: a Commonsense Approach -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Author 1 AJ Tuckett Company Foster Wheeler Energy Limited Address Foley House 5 Seaward Place Glasgow Scotland G41 1HH e-mail
[email protected] Author Background Andy is Chief Process Engineer Engineer for Foster Wheeler Energy Limited Limited Glasgow Operations. Operations. He has a BSc (Hons) in Chemical Engineering from Loughborough University. He has over 20 years’ experience in process design and engineering of oil and gas projects. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Author 2 I R Calcott Company Foster Wheeler Energy Limited Address Foley House 5 Seaward Place Glasgow Scotland G41 1HH e-mail
[email protected] Author Background Iain is a Principal Process Engineer for Foster Wheeler Energy Limited Glasgow Operations. He holds a BSc (Hons) in Chemical Engineering from Heriot Watt University and has worked for over 17 years in process design and plant operation. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Page: 1
INTRODUCTION Hazardous area classification of process plants is often undertaken using as a basis the Institute of Petroleum Model Code of Safe Practice Part 15 ‘Area Classification Code for Installations Handling Flammable Fluids. This code was re-issued as a second edition in August 2002 to take a more risk-based approach to area classification. As a consequence of this there are now additional considerations which must be taken into account when determining the hazardous radius. This paper summarises how Foster Wheeler has interpreted and applied the revised code on a number of recent projects for the oil and gas industry. The main differences between the second edition of the IP15 code and the first issue can be summarised thus: •
•
•
The second edition is based on an assessment of the risk to operators who work within hazardous areas. In addition it provides an updated, demonstrable methodology for specifying hazardous radius. This is achieved by assigning a level of release for secondary sources, and takes account of the period of time an operator spends exposed to a potential release source, and the probability of ignition of that release. The new IP code is based on dispersion modelling taking into account variables such as pressure and mist/spray formation. As the radius is now based on dispersion modelling of specific release compositions there is no longer a distinction between heavier than air or lighter than air releases. Chapter 5 ‘Classification of Individual Point Sources’ details the methodology for classification of the radius associated with equipment and piping. The difference now is that the release/leak source is now quantified, for example a flange leak is likely to have a diameter of 1 to 6 mm. In addition the diameter of the leak source is related to the level of release, with respect to secondary (Zone 2) releases. As before, the hazardous area is expected to be mainly Zone 2 with localised Zone 1s.
METHODOLOGY FOR HAZARDOUS AREA CLASSIFICATION There are a number of discrete steps that need to be completed in order to arrive at the hazardous area classification for a process plant. These form the basis of the work procedures employed by Foster Wheeler: •
Identify the point sources of release
•
Determine the grade of release and fluid category
•
Establish zone classification
•
Determine hazard radius
•
Preparation of hazardous area drawing
•
Hazardous area classification review
The procedure adopted is represented in a flowchart in Figure 1, each of the above steps are then discussed in more detail below.
Page: 2
Input
Activity
P&IDs Plot Plans Heat & Material Balance
Identify Release
Output
Determine Grade of Release
Amend Design to remove continuous; Primary sources
Determine Fluid Category
Physical Properties Heat & Material Balance
Establish Zone Classification
Ventilation
Level of Release Client’s Operations Staff Input
Probability Pocc
IP15 Plot Plan
Number of Releases Nrange
IP15
Exposure
IP15
Probability of Ignition Pign
IP15/ Air Dispersion
Hazardous Radius
Hazardous Area Classification Drawings
Review
Agreed Hazardous Area Classification
Figure 1
Page: 3
Revise
IDENTIFICATION OF SOURCES OF RELEASE The starting point for any area classification is to determine all the potential sources of release. To do this, it is important that the process design of the facility is well-developed, and that a plant layout is available in at least a conceptual form. Typically, the following key documents are used when determining the sources of release: • • • •
Equipment list Piping and instrument diagrams Heat and material balance Plot plan
In addition, it is important for the process engineer to consider other potential sources of release which may not be shown on the above documents. Examples of these include oily water drain system, drain hubs and vent pipes, vents from balanced bellows relief valves, analyser vents, etc. It is important that all continuous, primary sources and secondary sources which are likely to have an impact on the extent of a Zone 2 outside the plant boundary are identified at an early stage. Small secondary releases within the process plant are less important, as they are unlikely to have an impact on the extent of hazardous zones. DETERMINE THE GRADE OF RELEASE AND FLUID CATEGORY The grade of a source is dependant on the likely frequency of release. The grades are defined in IP15 as, continuous, primary or secondary. Continuous release is considered, as a rule of thumb, for instances where it is likely to be present for a period in excess of 1,000 hours per year, primary releases for between 10 and 1,000 hours per year, and secondary for less than 10 hours. In addition to the above, where releases are present for less than 10 hours but occur on a frequent basis during normal operation, such as regular sampling, they should be considered as primary grades. During design, effort must be made to minimise the number of primary releases as far as is practicable. This can include such measures as installation of closed sampling systems, closed drain systems, and reducing the diameter of vents and drains. The fluid category is defined in IP15 as follows: Fluid Category
Description
A
A flammable liquid that on release, would vaporise rapidly and substantially. This category includes: a) Any liquefied petroleum gas or lighter flammable liquid b) Any flammable liquid at a temperature sufficient to produce on release more than about 40% vol vaporisation with no heat input other than from the surroundings A flammable liquid not in Category A, but at a temperature sufficient for boiling to occur on release A flammable liquid not in categories A or B, but which can, on release, be at a temperature above its flash point, or form a flammable mist or spray A typical methane-rich natural gas Refinery hydrogen o Heavy oils etc with flash point greater than 100 C handled at a temperature below flash point, which will not form a mist or spray on release
B C G (i) G (i) Unclassified
Page: 4
ESTABLISH ZONE CLASSIFICATION For an open, freely-ventilated area, the following Zones apply (depending on the grade of release): •
Continuous: Zone 0
•
Primary: Zone 1
•
Secondary: Zone 2
For enclosed areas, such as an analyser house, compressor shelter, and sumps, reference has to be made to IP15 to determine whether the applicable zone has to be increased to take account of inadequate ventilation. DETERMINE HAZARD RADIUS The method used to determine a hazardous radius is dependant on the grade of release. For primary and continuous releases, where the potential leaks sizes and fluid categories are within the limits of the tables within IP15 section 5 and Annex 6, the hazardous radius can be determined directly from these tables. For cases outside the limits, it is necessary to undertake dispersion modelling to determine the extent of a flammable cloud. In cases where this has been found to be necessary, Foster Wheeler typically use PHAST (Process Hazard Analysis Software Tool) from DNV Software. For a secondary grade release it is first necessary to determine the level of release, and then to determine the hazardous radius either directly from IP15 or from dispersion modelling. The procedure used for determining level of release is described below. LEVEL OF RELEASE The level of release takes account of the period of time that personnel are exposed to a risk, determined from the number of sources of release to which they are likely to be exposed, and the likelihood of ignition. The level of release is either level I, level II or level III and is described as the release frequency. The release frequency is based on personnel risk only. It is the general intent that the level be assigned to an installation rather than determine the level for each individual point source. This can prove to be quite difficult when the boundary of an installation may have varying risks of ignition. The first step in determining the level of release is to establish exposure of personnel. In the IP Code exposure is determined from: Exp= Pocc X Nrange Where Pocc is the probability of an individual being on site within the hazardous area. Nrange is the time weighted average number of release sources which can affect the individual during their time within the hazardous area IP15 provides four values for Pocc: Page: 5
•
•
•
•
0.22: Individual spends 100% of all shifts exposed to a hazardous release 0.13: Individual spends an average of 5 hours per shift exposed to a hazardous release 0.055: Individual spends an average of 2 hours per shift exposed to a hazardous release 0.028: Individual spends an average of 1 hours per shift exposed to a hazardous release
The likely exposure of personnel needs to be discussed with the plant’s operating team, but in practice Foster Wheeler has found that in modern automated plants the exposure of personnel tends to be between 2 and 5 hours, giving rise to a P occ of between 0.13 and 0.055. It is essential that the assumptions made in determining the probability of exposure are well documented and recorded in order to provide an audit trail for later review and to meet HSE requirements. The next step is to determine Nrange. The tables in IP15 give the average number of sources to which an individual will be exposed during his period within the hazardous area, depending on the activity which he is undertaking. Three levels are indicated: low, medium and high. The percentage of time an individual is then exposed to each of these levels then needs to be assessed. It is important that these values are discussed with the client’s operating personnel. The basis for all decisions taken must also be recorded for record and audit purposes. Typical values of time an individual is exposed, used on previous projects are: •
•
•
Low: 20% Medium: 50% High: 30%
It is now possible to determine the exposure from table C2 (in ID15). Depending on the assumptions made we would typically expect to arrive at an exposure of between 1.5 and 2.0. Before determining the level of release it is now necessary to determine the probability of ignition. In table C3 of IP15 the following four strengths of ignition are given: •
•
•
•
Controlled, probability of ignition, 0.003 Weak, probability of ignition, 0.01 Medium, probability of ignition, 0.1 Strong, probability of ignition, 1
By taking account of the proportion of time the most exposed individual will spend exposed to each of the strengths of ignition, a value for Pign can be determined. Using the values of exposure and Pign, the appropriate level of release is determined from Figure C2 (in ID15). Referring to Figure C2, Foster Wheeler has found that in most instances, level I is applicable for process plant, but with certain areas close to strong sources of release exposed to level 2, in practice level 3 has not been encountered. The calculation of release frequency level is required to be made as a minimum for each type of plant area e.g. process area, utility area, offsite area. These calculations are carried out by the process department with input from client (operations/maintenance), and engineering disciplines as necessary.
Page: 6
The output of these calculations should be to provide a release frequency level for all areas of the plant which is subsequently used to establish hazardous radius for releases as per IP Code section 5.4. HAZARDOUS RADIUS For each source of release the hazardous radius can be determined using Chapter 5 and Annex C3 of IP15. The leakage rates of hydrocarbon, and hence the extent of hazardous radius, is dependent on the hole size through which the leak occurs combined with the operating pressure. Guidance is given in IP15 on the size of holes likely to occur for certain credible scenarios. If necessary, air dispersion modelling can be undertaken to determine the extent of hazardous zones. Foster Wheeler process group represents the above information in tabular form on a hazardous release schedule. HAZARDOUS AREA DRAWING The above information can be represented on a hazardous area classification drawing. This drawing is based on the plot plan for the process unit with the hazardous zones overlaid. Drawings are produced in both plan and elevation to give a full three dimensional representation of the hazardous area. HAZARDOUS AREA REVIEW Once the preliminary hazardous area classification drawing has been prepared a formal review is undertaken to confirm that the proposed area classification is in accordance with project requirements, and is endorsed by the various engineering departments and importantly the client/operations representative. The review team should typically be composed of the following personnel; •
•
•
•
•
•
Process engineer Project engineer Electrical engineer Instrument engineer Design safety engineer Client representatives, (including operations and maintenance personnel)
The review will typically be chaired by the Project Engineer and follows a check list to ensure that the area classification drawing has correctly interpreted the information contained in the hazardous release schedule and to gain buy in and agreement on the various assumptions made during the hazardous area classification process, particularly with regard to operator occupancy and ignition sources. All actions arising from the review must be recorded and should be entered into a project design safety action tracking register to ensure an auditable trail to close out exists. Following the review meeting the hazardous area drawing will normally require to be updated to incorporate the actions of the review team before being released for engineering design use.
Page: 7
The approach employed by Foster Wheeler, as detailed above, results in the hazardous area classification of a process plant that follows the recommendations of IP15 and, importantly, can also be audited. By ensuring that the client is involved throughout the process and particularly during acceptance and agreement, it ensures that the design process is understood and accepted by the operator. Opportunities exist to modify the design of a process plant, to remove or downgrade hazardous releases to reduce the zone classification or the extent of a hazardous radius. By undertaking the hazardous area classification in a structured manner, these opportunities can be identified more easily and resolved. The formal hazardous area classification review is an important part of the process as it brings together a range of disciplines, and helps to ensure that the classification is agreed and accepted by all parties concerned, and that it meets project requirements.
REFERENCES Institute of Petroleum, Area Classification Code for Installations Handling Flammable Fluids, Part 15, 2nd Edition August 2002.
Page: 8
