Chapter 7

Pumps Princi ple , Operatio Operatio n and Maintenance

7.0 CENTR CENTRIFUG IFUGAL AL PUMPS PUMPS SEAL S: Sealing The proper selection of a seal is critical to the success of every pump application. For maximum pump reliability, choices must be made between the type of seal and the seal environment. In addition, a sealless pump is an alternative, which would eliminate the need for a dynamic type seal entirely. Sealing Basics There are two basic kinds of seals: static and dynamic. Static seals are employed where no movement occurs at the Juncture to be sealed. Gaskets and O-rings are typical static seals. Dynamic seals are used where surfaces move relative to one another. Dynamic seals are used, for example, where a rotating shaft transmits power through the wall of a tank (Figure 7.1), through the casing of a pump (Figure 7.2), or through the housing of other rotating equipment such as a filter or screen.

Figure 7.1: Cross Section o f Tank and Mixer.

Figure 7.2: 7.2: Typic al Centri Centri fugal Pump.

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A common application of sealing sealing devices is to seal the rotating shaft of a centrifugal pump. To best understand how such a seal functions a quick review of pump fundamentals is in order. In a centrifugal pump, the liquid enters the suction of the pump at the center (eye) of the rotating impeller (Figure 7.3 and Figure 7. 4).

Figure 7.3: 7.3: Centrifugal Pump's Liqui d End.

Figure 7.4: Fluid Flow in Centrifugal Pump.

As the impeller vanes rotate, they transmit motion to the incoming product, which then leaves the impeller, collects in the pump casing, and leaves the pump under pressure through the pump discharge. Discharge pressure will force some product down behind the impeller to the drive shaft, where it attempts to escape along the rotating drive shaft. Pump manufacturers use various design techniques to reduce the pressure of the product trying to escape. Such techniques include: 1) the addition of balance

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holes through the impeller to permit most of the pressure to escape into the suction side of the impeller, or 2) the addition of back pump-out vanes on the back side of the impeller. However, as there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the atmosphere. Such sealing devices are typically either compression packing or end-face mechanical seals. Packi Packi ng Seals: Seals: The most common arrangement for gland packing used in pumps is the solid packed stuffing box shown in (Figure 7.5) In this arrangement, the packing gland is tightened to compress the soft packing rings until leakage is restricted to an acceptably small amount. It is important not to over tighten the packing gland so that all leakage is prevented, as it is this small amount of liquid, which lubricates the packing and so reduces friction and wear. As packing wear eventually permits a greater loss of liquid from the stuffing box, the gland tightness is adjusted to restore the amount of leakage to normal. A replaceable sleeve is often fitted to the shaft in the region of the stuffing box. The sleeve protects the shaft from wear caused caused by the rubbing action of the packing and can be replaced when worn at a much lower cost than having to replace a worn shaft.

Figure 7.5: Solid Packed Stuffing Box.

Although many pumps are used with this simple type of packing arrangement, it should not be used when the pump is operating with suction lift conditions because air may be drawn into the pump through the stuffing box. Causing the pump to lose suction.

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Pumps designed to operate under suction lift conditions use a sealing or injection type of packing arrangement. arrangement. This uses a liquid to help help seal the packing gland and prevent air being drawn into the pump. This sealing liquid comes from either the discharge side of the pump or from an external source, as shown in (Figure 7.6). If the sealing liquid is supplied from the pump, then the stuffing box is said to be internally sealed. If the sealing liquid supplied from an outside outside source, then the stuffing box is said to be externally sealed.

Figure 7.6: Internal and External Gland Sealing.

Pump, which uses a sealing type packing arrangement, must provide some means of of distributing the sealing liquid within the stuffing box. box. This is usually done with a sealing ring cage, often called a lantern ring. Lantern rings are usually made of brass or bronze and are normally positioned mid-way in the stuffing box, with an equal number of packing rings at each side. If the pump is to handle liquids containing sand or grit, then the sealing system should be of the external type. The pressure of the sealing liquid must be greater than the pump suction pressure. A typical packed stuffing box arrangement is shown in (Figure 7.7) It consists of: A) Five rings of packing, B) A lantern ring used for the injection of a lubricating and/or flushing liquid, and C) A gland to hold the packing and maintain the desired compression for a proper seal.

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Figure 7.7: Typical Stuffing Arrangement (description of parts)

The function of packing is to control leakage and not to eliminate it completely. The packing must be lubricated, and a flow from 40 to 60 drops per minute out of the stuffing box must be maintained for proper lubrication. The method of lubricating the packing depends on the nature of the liquid being pumped as well as on the pressure in the stuffing box. When the pump stuffing box pressure is above atmospheric pressure and the liquid is clean and nonabrasive, the pumped liquid itself will lubricate the packing (Figure 7.8).

Figure 7.8: Typical Stuffing Arrangement when Stuffing Box Pressure Pressure is Above Atmospheric Pressure. Pressure.

When the stuffing box pressure is below atmospheric pressure, a lantern ring is employed and lubrication is injected into the stuffing box (Figure 7.9). A bypass line from the pump discharge to the lantern ring connection is normally used providing the pumped liquid is dean.

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Figure 7.9: Typical Stuffing Box Arrangement when Stuffing Box Pressure is Below Atmospheric Pressure.

When pumping slurries or abrasive liquids, it is necessary to inject a dean lubricating liquid from an external source into the lantern ring (Figure 7.10). A flow of from .2 to .5 gpm is desirable and a valve and flowmeter should be used for accurate control. The seal water pressure should be from 10 to 15 psi above the stuffing box pressure, and anything above this will only add to packing wear. The lantern ring Is normally located In the center of the stuffing box. However, for extremely thick slurries like paper stock, it is recommended that the lantern ring be located at the stuffing box throat to prevent stock from contaminating the packing.

Figure 7.10: Typical Stuffing Box Arrangement when Pumping Slurries.

The gland shown in (Figures 7.7 through 7.10) is a quench type gland. Water, oil, or other fluids can be injected into the gland to remove heat from the shaft, thus limiting heat transfer to the bearing frame. This permits the operating temperature of the pump to be higher than the limits of the bearing and lubricant design. The same quench gland can be used to prevent the escape of a toxic or volatile liquid into the air around the pump.

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This is called a smothering gland, with an external liquid simply flushing away the undesirable leakage to a sewer or waste receiver. Today, however, stringent emission standards limit use of packing to non-hazardous water based liquids. This, plus a desire to reduce maintenance costs, has increased preference for mechanical seals. Packin Packin g Materials: The materials commonly used to make packing include cotton, asbestos and flax, which are usually woven or braided to form a continuous length of square section. The lengths of packing are often impregnated with graphite to assist in reducing friction and are available in a range of different section sizes to suit pumps having differing stuffing box dimensions. Packing lengths are often reinforced with wire strands which strengthens the material and helps it to retain its i ts shape.

Figure 7.11: Types of Braided Packing.

Pumps that handle cool water often use non-reinforced cotton or asbestos as packing packing materials. Pumps handling handling liquids at temperatures o over 105 C usually need packing packing with with a reinforced asbestos material because it resists heat and hardening. In addition to the natural packing materials, there is also a range of synthetic and metallic packing materials, which may be more suitable for high temperature applications or for certain types of pumped liquid. Many of the synthetic packing materials are made in the form of a "V" or chevron section and are installed installed with the open part part of the "V" facing the

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liquid being pumped. pumped. In this position, the pressure of the liquid in the pump pump tends to expand the packing and helps it to seat on the shaft. (Table 7.1) gives details of several different types of packing materials and their applications.

FLUID

PACKING

FLUID

PACKING

Clear water hot or cold, Sewage, Slurries, Calcium brine, Neutral liquidsmaximum o temperature 100 C

White asbestos or cotton plaited construction with general service lubricant. Graphited.

Solvents, Alcohols, Fuel oil, Kerosene, Chlorinated o hydrocarbons to 120 C

White asbestos strands, plaited or interwoven construction, impregnated with a solvent – resistant lubricant. Graphited.

Clear hot or cold water, Neutral liquidsmaximum o temperature 200 C.

Special white asbestos plaited construction with a high temperature lubricant. Graphited.

Where metallic packing is preferred for hot or cold water, mild alkalis, mild acids, brine boiler feed service. Maximum o C. temperature 230

Crinkled lead foil sheets with resilient asbestos cor.

wher suction pressure exceeds 3 bar.

Suphuric, Nitric, and other acids. Maximum o temperature 130 C

Blue African asbestos Alkalis and other liquids plaited construction with PH factors above 7 with an acid resisting and for temperature of o lubricant. Graphited. 30 to 130 C

Teflon impregnated white asbestos braid packing.

Alkalis, Caustic soda, Silicate of soda, Salt brine, Sulphat. Maximum o temperature 130 C

Weak or concentrated White asbestos lattice Acids especially those braid construction with a PH factor 4 or with non-soapy less. For temperature lubricant. Graphited. o 30 to 130 C

Teflon impregnated blue asbestos braid packing.

Food products, any liquids where noncontamination is the controlling factor. Maximum o temperature 85 C

First grade asbestos or cotton plaited construction with an edible compound which is colorless, tasteless and odorless. Nongraphited.

Table 7.1: Types of Packing and Applications.

Packing Packing Installation: Installation: It is important that the correct number of packing rings are used when inserting packing into the stuffing stuffing box. This can be determined determined by counting the number of rings previously removed, checking the manufacturer's recommendations or by measuring measuring the depth of stuffing box. box. When using

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the last method, it should be remembered to deduct the width of the lantern ring from the total depth of the stuffing box. The normal procedure for installing woven or braided packing is to insert each ring of packing separately and push it into position with the gland. This procedure is repeated until the stuffing box has been filled. If the packing is not of the lubricated type, the rings should be dipped in oil before insertion. insertion. This will help help to prevent the packing from becoming overheated during initial start-up. When lantern rings are used, it should be carefully noted how many rings of packing should be placed at each side of the lantern ring so that it can be correctly located in the stuffing box to admit the sealing liquid. After the stuffing box has been completely filled, the gland should be evenly tightened down to ensure that the packing is seated properly, but must then be backed off and the fasteners re-tightened only to fingertightness. This enables enables the packing packing to expand when when it becomes becomes heated heated after start-up and so prevents it becoming damaged by friction burning. When re-packing a stuffing box, the condition of the shaft or sleeve should be checked to make sure it has not been damaged by scoring or is excessively worn by the rubbing action against the packing. If excessive leakage occurs after re-packing, it is more likely that the shaft or sleeve is scored or worn, in which case the only remedy is to replace it with a new one. A stuffing box should never be only partly re-packed. A total re-pack job should be done whenever more than one ring r ing of packing is required. If only the outer rings are renewed, they will have extra pressure placed on them because the old internal packing rings will be too worn to be effective. It is normally permitted to add one extra ring of packing to a stuffing box that has a slight leak, but a note should be made when the ring is added so that more rings are not added at a later date. Any further increase increase in the amount of leakage will most likely indicate that a new shaft or sleeve will be needed.

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Mechanical Seals: When leakage of the pumped liquid is not acceptable, packing seals are not suitable and one of the many different types of mechanical seals is used instead. A mechanical seal is a sealing device, which forms a running seal between rotating and stationary parts. They were developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting environmental standards of government regulating agencies and maintenance costs can be lower. Advantages of mechanical seals over conventional packing are as follows: 1. Zero or limited leakage leakage of product product (meet emission regulations.) regulations.) 2. Reduced friction and and power power loss. loss. 3. Elimination of shaft shaft or sleeve wear. 4. Reduced maintenance costs. 5. Ability to seal higher pressures pressures and more corrosive environments. environments. 6. The wide variety of designs allows use of mechanical mechanical seals in almost almost all pump applications. Although they are relatively simple in their construction, mechanical seals have complex design features which involve a high degree of precision in their manufacture. Because of this, maintenance maintenance must be be of a high standard. standard. Mechanical seals are much more expensive than soft packing, but their use is becoming more widespread because of their superior sealing ability and reliability. The Basic Mechanical Seal All mechanical seals are constructed constructed of three basic sets of parts as as shown in (Figure 7.12): 1. A set of primary seal seal faces: one rotary and one stationary…shown stationary…shown in (Figure 7.12) as seal ring and insert. 2. A set of secondary seals seals known as shaft packings packings and insert mountings such as 0-rings, wedges and V-rings. 3. Mechanical seal seal hardware including including gland rings, collars, compression compression rings, pins, springs and bellows.

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Figure 7.12: A Si mp le Mec han ic al Seal .

Principles of Operation: Operation: The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all seals, one face is held stationary in a housing and the other face is i s fixed to, and rotates with, the shaft. One of the faces is usually a non-galling material such as carbongraphite. The other is usually a relatively hard material like silicon-carbide. Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent adhesion of the two faces. The softer face usually has the smaller mating surface and is commonly called the wear nose. There are four main sealing points within an end face mechanical seal (Figure 7.13). The primary seal is at the seal face, Point A. The leakage path at Point B is blocked by either an 0-ring, a V-ring or a wedge. Leakage paths at Points C and D are blocked by gaskets or 0-rings.

Figure 7.13: Sealing Sealing Points for Mechanical Seal

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The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces. In designing seals for the desired leakage, seal life, and energy consumption, the designer must consider how the faces are to be lubricated and select from a number of modes of seal face lubrication. The constructional details of a typical mechanical seal are shown below in (Figure 7.14).

Figure 7.14: 7.14: Typi cal Mechanical Seal Arr angement.

The main components of the mechanical seal are: A. Stationary Seal Ring – This is usually made of tungsten carbide or stainless steel and fits into the seal plate. B. Stationary Seal Ring Seal – This is an O-ring which prevents leakage of liquid between the stationary seal ring and seal plate. C. Rotating Seal Ring – – This is secured to the shaft and is pressed against the stationary seal ring by the combined action of the spring and the pressure of the liquid. The material used is normally carbon. D. Rotating Seal Ring Seal – – This is also an O-ring , which prevents leakage the rotating seal ring and the shaft. E. Spring – – This is a single coil spring which helps to maintain contact pressure between the faces of the t he rotating and stationary seals. F. Thrust Collar – This – This is secured to the shaft by a grub screw and takes the reaction thrust of the spring.

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The position along the shaft where the thrust collar is mounted will control the amount of thrust exerted by the spring upon the sealing faces. G. Seal Seal Plat e – This seals the stuffing box and also houses the stationary seal ring. H. Seal Plate Seal – This may be either an O-ring or a flat gasket which prevents leakage from the joint between stuffing box and seal plate. The seal rings are supplied in matching pairs and have their contact faces lapped to provide a very very smooth surface finish and flatness. The bores have have been machined to a highly precise squareness squareness with the contact contact surfaces. Because of the precision and pairing of seal rings during manufacture, no attempt should be made to exchange them singly or modify them in any way. If one seal ring face shows signs of damage, such as scoring, pitting or cracking, then both seal rings should be replaced with a new pair. When fitting a mechanical seal, great care must be taken to prevent any foreign matter entering the the seal. Special attention should should be given given to the seal seal faces as even the smallest particle of dirt trapped between the seal faces can cause leakage and also damage to them. Mechanical seals are best kept in their wrappers until just before they are to be fitted. This reduces the risk of contamination contamination and accidental accidental damage. Manufacturers fitting instructions should always be observed. To select the best seal design, it's necessary to know as much as possible about the operating conditions and the product to be sealed. Complete information about the product and environment will allow selection of the best seal for the application. Mechanical Seal Types: Mechanical seals can be classified into several types and arrangements: PUSHER: Incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at the seal faces. This feature compensates for seal face wear and wobble due to misalignment. The pusher seals' advantage is that it's inexpensive and commercially available in a wide range of sizes and configurations. Its disadvantage is that ft's ft' s prone to secondary seal hang-up and fretting of the shaft or sleeve.

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Figure 7.15: Pusher Type Mechanical Seal. Seal.

UNBALANCED: They are inexpensive, leak less, and are more stable when subjected to vibration, misalignment, and cavitation. The disadvantage is their relative low pressure limit. If the closing force exerted on the seal faces exceeds the pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.

Figure 7.16: Unbalanced Type Mechanical Seal.

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CONVENTIONAL: Which require setting and alignment of the seal (single, double, tandem) on the shaft or sleeve of the pump. Although setting a mechanical seal is relatively simple, today's emphasis on reducing maintenance costs has increased preference for cartridge seals.

Figure 7.17: Conventional Type Mechanical Seal.

NON-PUSHER: The non-pusher or bellows seal does not have to move along the shaft or sleeve to maintain seal face contact, The main advantages are its ability to handle high and low temperature applications, and does not require a secondary seal (not prone to secondary seal hangup). A disadvantage of this style seal is that its thin bellows cross sections must be upgraded for use in corrosive environments Figure 7.18: Non-pusher Type Mechanical Seal.

BALANCED: Balancing a mechanical seal involves a simple design change, which reduces the hydraulic forces acting to close the seal faces. Balanced seals have higher-pressure limits, lower seal face loading, and generate less heat. This makes them well suited to handle liquids with poor lubricity and high vapor pressures such as light hydrocarbons. Figure 7.13

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CARTRIDGE: Which have the mechanical seal premounted on a sleeve including the gland and fit directly over the special model shaft or shaft sleeve (available single, double, tandem). The major benefit, of course is no requirement for the usual seal setting measurements for their installation. Cartridge seals lower maintenance costs and reduce seal setting errors Mechanical Mechanical Seal Seal Arr angements: SINGLE INSIDE: This is the most common type of mechanical seal. These seals are easily modified to accommodate seal flush plans and can be balanced to withstand high seal environment pressures. Recommended for relatively clear non-corrosive and corrosive liquids with satisfactory' lubricating properties where cost of operation does not exceed that of a double seal. SINGLE OUTSIDE: If an extremely corrosive liquid has good lubricating properties, an outside seal offers an economical alternative to the expensive metal required for an inside seal to resist corrosion. The disadvantage is that it is exposed outside of the pump which makes it vulnerable to damage from impact and hydraulic pressure works to open the seal faces so they have low pressure limits (balanced or unbalanced). Figure 7.20

DOUBLE (DUAL PRESSURIZE PRESSURIZED): D): This arrangement is recommended for liquids that are not compatible with a single mechanical seal (i.e. liquids that are toxic, hazardous, have suspended abrasives, or corrosives which require costly materials). The advantages of the double seal are that it can have five times the life of a single seal in severe environments. Also, the metal inner seal parts are never exposed to the liquid product being pumped, so viscous, abrasive, or thermosetting liquids are easily sealed without a need for expensive metallurgy. In addition, recent testing has shown that double seal life is virtually unaffected by process upset conditions during pump operation. A significant advantage of using a double seal over a single seal. The final decision between choosing a double or single seal comes down to the initial cost to purchase the seal, cost of operation of the seal, and environmental and user plant emission standards for leakage from seals.

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Figure 7.21

DOUBLE GAS GA S BA RRIER (PRESS (PRESSURIZE URIZED D DUAL GAS): Very similar to cartridge double seals ... sealing involves an inert gas, like nitrogen, to act as a surface lubricant and coolant in place of a liquid barrier system or external flush required with conventional or cartridge double seals. This concept was developed because many barrier fluids commonly used with double seals can no longer be used due to new emission regulations. The gas barrier seal uses nitrogen or air as a harmless and inexpensive barrier fluid that helps prevent product emissions to the atmosphere and fully complies with emission regulations. The double gas barrier seal should be considered for use on toxic or hazardous liquids that are regulated or in situations where increased reliability is the required on an application.

Figure 7.22

TANDEM (DUAL UNPRESSURIZED): Due to health, safety, and environmental considerations, tandem seals have been used for products such as vinyl chloride, carbon monoxide, light hydrocarbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous liquids. Tandem seals eliminate icing and freezing of light hydrocarbons and other liquids, which could fall below the atmospheric freezing point of

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water in air (32° F or 0° C). {Typical buffer liquids in these applications are ethylene glycol, methanol, and propanol.) A tandem also increases online reliability. If the primary seal fails, the outboard seal can take over and function until maintenance of the equipment can be scheduled. Mechanic Mechanic al Seal Seal Selectio Selectio n: The proper selection of a mechanical seal can be made only if the full operating conditions are known: 1. 2. 3. 4. 5.

Liquid Pressure Temperature Characteristics of Liquid Reliability and Emission Concerns

1. Liquid: Liquid: Identification of the exact liquid to be handled is the first step in seal selection. The metal parts must be corrosion resistant, usually steel, bronze, stainless steel, or Hastelloy. The mating faces must also resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered. Stationary sealing members of Buna, EPR, Viton and Teflon are common. 2. Pressure: The Pressure: The proper type of seal, balanced or unbalanced, is based on the pressure on the seal and on the seal size. 3. Temperature: Temperature: In part, determines the use of the sealing members. Materials must be selected to handle liquid temperature. 4. Characteristics of Liquid: Liquid: Abrasive liquids create excessive wear and short seal life. Double seals or clear liquid flushing from an external source allow the use of mechanical seals on these difficult liquids. On light hydrocarbons balanced seals are often used for longer seal life even though pressures are low. 5. Reliability and Emission Concerns: Concerns: The seal type and arrangement selected must meet the desired reliability and emission standards for the pump application. Double seals and double gas barrier seals are becoming the seals of choice. Seal Seal Envi Envi ronm ent: The number one cause of pump downtime is failure of the shaft seal. These failures are normally the result of an unfavorable seal environment such as improper heat dissipation (cooling), poor lubrication of seal faces, or seals operating in liquids containing solids,

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air or vapors. To achieve maximum reliability of a seal application, proper choices of seal housings (standard bore stuffing box, large bore, or large tapered bore seal chamber) and seal environmental controls (CPI and API seal flush plans) must be made. STANDARD BORE STUFFING BOX COVER: COVER: Designed thirty years ago specifically for packing. Also accommodates mechanical seals (clamped seat outside seals and conventional double seals.)

Figure 7.23

CONVENTION CONVENTIONAL AL LARGE LA RGE BORE SEAL SEAL CHAMBER: Designed specifically for mechanical seals. Large bore provides Increased life of seals through improved lubrication and cooling of faces. Seal environment should be controlled through use of CPI or API flush plans. Often available with internal bypass to provide circulation of liquid to faces without using external flush. Ideal for conventional or cartridge single mechanical seals in conjunction with a flush and throat bushing in bottom of chamber. Also excellent for conventional or cartridge double or tandem seals.

Figure 7.24

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LARGE BORE SEAL CHAMBERS: Enlarged bore seal chambers with increased radial clearance between the mechanical seal and seal chamber wall, provide better circulation of liquid to and from seal faces. Improved lubrication and heat removal (cooling) of seal faces extend seal life and lower maintenance costs.

Figure 7.25: 7.25: Big Bor e

Figure 7.26: Taper Bore

API an d CPI Pl ans: ans : API and CPI mechanical seal flush plans are commonly used with API and CPI process pumps. The general arrangement arrangement of the plans are similar regardless of the designation whether API or CPI. The difference between the flush plans is the construction which provides applicable pressure-temperature capability for each type of pump. API plans have higher pressure and temperature capability than CPI plans. Each plan helps provide critical lubrication and cooling of seal faces to maximize seal reliability.

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Chapter 7

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