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8 Pump Mechanical Seal Flush Best Practices Experience shows that pump MTBFs (mean time between failures) are directly related to mechanical seal reliability. It is well known that pump mechanical seal MTBF is the lowest of all machinery components. This chapter will therefore present best practices for pump mechanical seals in the hope that these proven Best Practices will signicantly increase your plants mechanical seal and pump MTBFs. Copyright Ó 2011, W E Forsthoffer. Published by Elsevier Ltd. All rights reserved

Forsthoffer's Best Practice Handbook for Rotating Machinery || Pump Mechanical Seal Flush Best Practices

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Page 1: Forsthoffer's Best Practice Handbook for Rotating Machinery || Pump Mechanical Seal Flush Best Practices

8Pump Mechanical Seal FlushBest Practices

Experience shows that pump MTBFs (mean timebetween failures) are directly related to mechanicalseal reliability. It is well known that pumpmechanicalseal MTBF is the lowest of all machinery components.

Copyright � 2011, W E Forsthoffer. Published by Elsevier Ltd. All rights reserved

This chapter will therefore present best practices forpump mechanical seals in the hope that these provenBest Practices will significantly increase your plant’smechanical seal and pump MTBFs.

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F

B e s t P r a c t i c e 8 . 1 Pump Mechanical Seal Flush Best Practices

Best Practice 8.1Best Practice 8.1

Confirm the actual seal operating conditions on the datasheet (PT, vapor pressure, S.G., P1 and P2) to optimizemechanical seal MTBFs.

Of all machinery components, mechanical seals are the mostaffected by the process conditions.

Accurate definition of the following parameters on the pump andseal data sheet will go a long way towards assuring optimum pumpmechanical seal MTBFs:� Fluid vapor pressure� Fluid specific gravity and viscosity� Fluid pumping temperature (PT)� Suction pressure� Cooling medium temperatures (if a flush cooler will be used)� External flush fluid pressures, temperature, vapor pressure,

viscosity and specific gravity

Seal chamber temperatureSeal chamber pressureSeal fluid characteristics:

CleanlinessVapor pressureViscositySpecific heatSpecific gravity

•••••

ig 8.1.1 � Seal reliability and pump fluid conditions

470

Lessons LearnedFailure to properly specify the correct process conditionson the pump and/or mechanical seal data sheet will resultin lower than optimum seal MTBFs.

Most ‘bad actor’ seals (sealswithmore thanone failure per year) resultfrom improper specification of process conditions on the data sheets.

Always check all operating seal process conditions against the datasheet values when investigating a ‘bad actor’ seal.

If the proper instrumentation (pressure gauge, etc.) is not installed,have theappropriate instruments installedat the timeofseal replacement.

BenchmarksThis best practice has been used since 1990 to troubleshootmechanical seal problems and to ensure maximum mechanical sealMTBFs (greater than 100 months).

Temperature gun close to seal gland – be sure to monitor off ofa non-reflective surfaceSurface contact thermometer on flush line – add 5°C tomeasured value if pipe and use recorded value if SS tubingInstall a thermometer in the flush lineUse an infrared camera to record temperature and temperatureprofile

B.P. 8.1. Supporting MaterialConfirm process conditions are actually as stated in data sheets.If the ‘bad actor’ pump seal reliability, when operating in theEROE (see B.P. 2.7 for EROE details), does not significantlyimprove, a complete check of seal fluid conditions in the sealchamber is required. Figure 8.1.1 shows the process variablesthat influence seal reliability.

Fig 8.1.2 � Alternative methods to measure seal fluid temperature

Install a pressure gauge in the seal chamberModify the seal flush line for installation of a pressure gauge.

Fig 8.1.3 � Seal chamber pressure monitoring guidelines for ‘badactor’ seals

Temperature monitoring

Since seal chamber or seal flush line pressure gauges are notusually installed on new pumps, the first place to start iswith seal chamber temperature. Flush plans with coolers(21, 23, 41, etc.) have an option for a thermometer down-stream of the cooler which can be used to determine theseal chamber temperature. This value must be compared tothe PT (pumping temperature) value listed on the datasheet. If the measured value does not agree with the datasheet, consult with operations first to see if process changescan be made. If they cannot be made, discuss the measuredtemperature with the seal vendor representative. Alternativeoptions for measuring seal fluid temperature are noted inFigure 8.1.2.

Pressure monitoring

Unless the plant assigned machinery specialist is ‘worldclass’ there are usually no pressure gauges in the flush lineor seal chamber. Our recommendations concerning sealchamber pressure monitoring are presented in Figure 8.1.3.The measured seal chamber pressure must be compared tothe seal vendor’s assumed value, which should be on theseal layout drawing. If this value is not present, the sealvendor should be consulted. Note that the seal vendor as-sumed seal chamber pressure is the value that was used inthe calculation of the seal PV. If the measured value doesnot agree with the data sheet, consult with operations firstto see if process changes can be made. If process changescannot be made, discuss this fact with the seal vendorrepresentative.

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Orifice conditionSeal gland conditionThroat bushing clearanceCooler conditionStrainer conditionCyclone separator conditionBuffer fluid condition (tandem seal)

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 2

Seal fluid characteristics

If all of the above mentioned items are in accordance with theseal design (EROE, seal chamber temperature and seal chamberpressure), a check of the seal fluid characteristics is required.Figure 8.1.4 presents guidelines for checking the seal fluidcharacteristics.

Take a sample to determine if debris is presentIf seal fluid is not water or product fluid (which must meetproduct specifications) have sample analyzedSend sample results to seal vendor

Fig 8.1.4 � Seal fluid check guidelines

Buffer fluid level (tandem seal)Pumping ring conditionBuffer fluid pressure (tandem seal)

Fig 8.1.5 � Seal reliability as a function of flush system componentcondition

If the measured fluid sample parameters are not as stated onthe data sheet, consult with operations first to see if processchanges can be made. If process changes cannot be made, dis-cuss this with the seal vendor representative.

Variance in seal chamber pressures and temperatures, if thepump is operating in the EROE, will most likely be caused bymalfunction of components in the flush system. Figure 8.1.5defines those flush system components which can affect sealchamber pressures and temperatures.

Based on the seal flush plan used, the flush system compo-nents should be checked in the logical order e starting with thebeginning of the flush system and ending with the throat bush-ing. (The exception is flush plan 13 which begins with the throatbushing and ends at the pump suction pipe.) FAI ‘seal mainte-nance best practice’ requires that the flush system be com-pletely checked (including the throat bushing clearance) eachtime a seal is changed. Note that this recommendation applies toall seal configurations: single, tandem (dual un-pressurized) ordouble (dual pressurized).

Best Practice 8.2

Pre-select mechanical seal and flush system design duringthe pre-FEED project phase, based on plant, companyand/or industry experience, to optimize mechanical sealMTBF.

Use plant, company and industry lessons learned to properly selecta flush system that will result in optimum seal life e in your plant!

Frequently, the flush system is selected by the process licensor orthe EP&C (contractor) and does not reflect actual plant conditions.

Be proactive early in the project design (pre-FEED phase) to con-vince project management of the proper flush systems to apply for allpump services.

Lessons LearnedThe use of the following flush system components wherethey are not warranted has resulted in lowmechanical sealMTBFs and has exposed plants to safety issues:

� Flush line strainers� Cyclone separators� Flush line coolers

Flush line strainers can expose plants to seal failure and safetyissues, since they are not monitored in the control room and can resultin flush line blockage, which will fail the mechanical seal and canexpose the plant to significant safety issues in hydrocarbonapplications.

BenchmarksThis best practice has been used since the mid-1970s, when I becameinvolved with pump selection. Since that time, prohibiting the use offlush line strainers and cyclone separators has resulted in optimummechanical seal safety and reliability during field mechanical sealreliability optimization audits for all projects.

Mechanical seal flush must possess the following qualities foroptimal seal life:

CoolCleanApproximately 345 kPa (50 psi) above vapor pressure (psia)Most importantly it must be cost effective!

Fig 8.2.1 � Optimal flush plans for various applications e overview

B.P. 8.2. Supporting Material

Optimal seal plans for various applications

We know the design aspects and how to monitor different flushsystems, but when would certain flush systems be better thanothers? Figure 8.2.1 lists the parameters required for a reliableflush source.

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Hot oil service above 230°C (450°F [typically tower bottoms])A single seal with the use of a Plan 32 is the most reliable/costeffective if a reliable source is available nearby... note that if thesource is a product that has to be reprocessed it may not becost effective.A dual pressurized seal would be the best option (Plan 53 or 54)if a Plan 32 is deemed not feasible.

Fig 8.2.4 � Optimal flush plans for various applications: option 3

B e s t P r a c t i c e 8 . 3 Pump Mechanical Seal Flush Best Practices

Therefore, if you can say that the flush system for an appli-cation can provide all the qualities (most important is cost ef-fectiveness) listed in Figure 8.2.1, then you have selected theoptimal flush system.

The following points outline different seal flushing scenarios,listing considerations specific to each. Note that these are gen-eral points, and should not be taken as being definitive for everyapplication; however they can aid in flush plan selection.Ultimately, the seal vendor should be consulted when selectingthe seal and flush plan; the more input they have, the higher sealreliability your plant should see as a result.

Therefore, if you can say that the flush system for an appli-cation can provide all the qualities (most important is cost ef-fectiveness) listed in Figure 8.2.1, then you have selected theoptimal flush system.

The following points outline different seal flushing scenarios,listing considerations specific to each. Note that these are gen-eral points, and should not be taken as being definitive for everyapplication; however they can aid in flush plan selection.Ultimately, the seal vendor should be consulted when selectingthe seal and flush plan; the more input they have, the higher sealreliability your plant should see as a result.

Clean HC service (e.g. #2 FO) between 150 and 230°C (300 and450°F)

Plan 23 is most efficient in cooling, however proper installationand venting is required.Plan 21 will be sufficient in most cases and will be easier tooperate... note that the orifice sizing is critical here as thisdetermines the velocity of the fluid through the heat exchanger.

Fig 8.2.3 � Optimal flush plans for various applications: option 2

HC service with no known solid particles and temperature under150°C (300°F)

Assuming satisfactory vapor margin (approximately 345 kPa[50 psi] above vapor pressure) in seal chamber a Plan 11 wouldbe the optimal choice.A vertical pump application would require venting of the sealchamber back to suction if possible (Plan 13 or 14).A Plan 52, 53 or 54 (53 or 54 preferred) would be recommendedin low S.G. (< 0.7) services.

Fig 8.2.2 � Optimal flush plans for various applications: option 1

Acid service (e.g. H2SO4)Reliable Plan 32 can be used, however it is very critical for it tobe operating at times when pump is installed in field (standbyand startup situations).A dual pressurized seal, whether contacting (Plan 53/54) or non-contacting (Plan 74) will give the optimal seal life with ease ofoperation... note that process side seal components will needto be constructed of materials that are corrosion resistant to theparticular acid.

Fig 8.2.5 � Optimal flush plans for various applications: option 4

Dirty service (containing suspended solid particles)A cyclone separator (Plan 31) can be effective, however it mustbe sized correctly and particle size must not fluctuate greatly.A clean externalflush source (Plan 32 for single, Plan 54 for dual)will isolate the seal faces from solids that can cause prematurefailure due to abrasion.

Fig 8.2.6 � Optimal flush plans for various applications: option 5

Best Practice 8.3Best Practice 8.3

A cartridge seal design should be used whenever possibleto ensure proper seal assembly and optimum mechanicalseal MTBFs.

The use of cartridge mechanical seals significantly minimizes in-stallation errors.

While cartridge seals are more expensive than component seals,the additional cost can be justified in many cases based on the materialcosts and revenue losses of component seals.

Lessons LearnedModifying existing component mechanical seals for car-tridge seal assemblies, based on a life cycle cost analysis,has resulted in significantly higher seal MTBFs.

Many clients have standardized on cartridge seals for all pumpsthat can accommodate them, justified by a life cycle cost analysiswhich compares the material, maintenance and lost revenue costs ofcomponent seals to the additional costs of cartridge seals.

BenchmarksThis best practice has been used since 2000 for new projects, andrecommends modifications to cartridge seals for plant ‘bad actor’seals. This best practice has resulted in seal MTBFs of greater than48 months, compared with previous MTBFs below 12 months.

472

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Single coil spring – reduces potential for springs to hang up since one large diameter spring is used Filtration – can be high cost and needs to be maintained for high reliability External fl ush – if an external fl ush plan can be used, it will provide optimal seal life in this application

Fig 8.3.3 � Considerations for pusher type seal in a low S.G. dirtyservice

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 3

B.P. 8.3. Supporting Material

Pusher vs. non-pusher

Mechanical seals are typically categorized into two major types;pusher and non-pusher.

A pusher-type seal consists of a primary sealing ring assem-bled with an ‘O’ ring and springs (can be one or multiple). Thepurpose of this is to force the sealing fluid across the face andkeep it from leaking to the ID (atmospheric) side of the seal.The dynamic ‘O’ ring is designed to move axially (be pushed)along the shaft or sleeve (in a cartridge seal). The surface un-derneath the dynamic ‘O’ ring must therefore be very smooth(< 32 RMS) to allow for this axial movement. If solids areabundant in the sealing fluid, they can build up on the ‘O’ ringand prevent this axial movement (hang up).

A non-pusher type seal consists of a bellows assembly. Thebellows is a component that acts as both the load element (likea spring in a pusher type) and a secondary sealing element (likean ‘O’ ring in a pusher type). Because the bellows prevents anyleakage to the atmospheric side of the seal, and has a largeclearance between itself and the shaft or sleeve, it can movefreely in the axial direction (no dynamic ‘O’ ring), reducing thepotential for hang up.

Refer to Figure 8.3.1 for details on these two types of seals.Pusher type seals are used more commonly in low S.G.

(<0 .7) services.

laeSrehsuP-noNlaeSrehsuP

Closing force supplied byspring(s)Used in low temp. services‘O’ ring secondary sealsUsed in light end services(ethylene, propane, methane,butane, etc.)

Closing force supplied bybellows (no dynamic ‘O’ ring)Can be used in high tempservices (metal bellows)Metal bellows use ‘grafoil’secondary seals to handlehigh temperature

Fig 8.3.1 � Single seal (pusher vs. non-pusher)

Referring back to the previous discussion on balance ratio(Figure 8.3.2), which is balance of the ratio of closing area toopening area of the seal.

Our depiction (Figure 8.3.2) shows the concept of balanceratio in a pusher type seal. In a bellows seal, the secondarysealing element (bellows) generally has a larger diameter thana pusher seal, therefore the closing area is less. Since the closingarea is larger and the width of the primary ring face is limited(cannot be too large or it won’t fit in the bellows assembly), thebalance ratio cannot be varied as much as in a pusher seal. Withlight S.G. fluids, it is important to be able to have a range ofbalance ratios to control where the fluid will vaporize across thefaces. It is for this reason that a pusher type seal is desirable inlight S.G. services. Note that some applications can have a S.G.of less than 0.7 and contain solids. In these applications, it is stillrecommended to use a pusher-type seal, however provisionsneed to be made to ensure the seal will not hang up in operation.Take a look at Figure 8.3.3.

An advantage for using the bellows seal, apart from being lesslikely to hang up, is that they typically utilize ‘grafoil’ packingrings as their secondary seals. Grafoil packing rings can with-stand temperatures of approx. 425�C (800�F), allowing metalbellows seals to be used in refinery bottoms applications withgreat success.

Dual un-pressurized vs. dual pressurizedseals

Today, due to environmental restrictions, dual seals are beingselected for more and more applications. There are two ar-rangements in which dual seals can be used; namely dual un-pressurized (previously called tandem) and dual pressurized(previously called double). Refer to the diagram in Figure 8.3.4,which shows the dual un-pressurized arrangement.

Fig 8.3.2 � Balance ratio ¼ closing area/openingarea

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Fig 8.3.5 � Dual pressurized

Dual Un-pressurized Dual Pressurized

Primary leakage is process fl uid, therefore the secondary seal is essentially a backup seal Does not require the use ofnitrogen or other inert gases

Primary leakage is barrier fluidinto the process, therefore the process must be able to accept a small amount of barrier fl uid Requires use of nitrogen (or other compatible inert gas) to pressure seal reservoir approx .175 kPa (25 psig)above seal chamber pressure in a Plan 53A Plan 54 uses an externalfl uid (synthetic skid or another pump) to lubricate the seals at a pressure of 175 kPa (25 psi) above seal chamber pressure

Fig 8.3.6 � Dual un-pressurized vs. dual pressurized

Normally 316 SS for large components (sleeve, gland, retainer, etc.) Normally hastelloy C or other corrosion resistant alloy for springs Acid or chloride services may require hardware be constructed of hastelloy C, chrome alloys, monel or other corrosion resistant material

Fig 8.3.7 � Adaptive hardware materials

Fig 8.3.4 � Dual un-pressurized

Fluorocarbon (Viton) – most common (relatively cheap) and highly recommended for HC services under 175 ° °C (350 F).Per fl uoro-elastomer (Kalrez or AFLAS) – used in higher temperature services (175 to 260°C (350 to 500°F)) than Viton

B e s t P r a c t i c e 8 . 3 Pump Mechanical Seal Flush Best Practices

A dual un-pressurized seal uses a buffer fluid at, or near, at-mospheric pressure to lubricate the atmospheric side seal. Thebuffer fluid pressure is significantly less than the seal chamberpressure, so any leakage occurring across the process side sealwill leak into the buffer fluid. This arrangement is very commonin applications containing VOCs, as it can potentially reduce theleakage to the environment.

Refer to Figure 8.3.5, showing a dual pressurized seal ar-rangement. These seals use a barrier fluid (same fluid as dualun-pressurized) at a pressure that is 175 kPa (25 psi) above sealchamber pressure to lubricate the seals. Since the barrier fluidpressure is higher than the seal chamber pressure, any leakageoccurring at the process side seal will enter the pump. Thisarrangement (when working properly) ensures no leakage ofthe pumped fluid to atmosphere. Note that this arrangementrequires the barrier fluid to be compatible with the pumpedfluid, since the barrier fluid leaks into the pump. Refer toFigure 8.3.6.

It has been our experience that if an inert gas (or externalfluid) is available at the required pressure, and the pumped fluidcan accept the barrier fluid (compatible), a dual pressurized sealis potentially more reliable in VOC service.

and generally highly chemically resistant. EPDM – common in hot water (BFW) applications, as it is more resistant to thermal attack in hot water than the two listed above. Buna-N – not recommended over the above three materials for most (if not all) applications.

Fig 8.3.8 � ‘O’ ring material and usage guidelines

Material selection of faces and secondarycomponents

It is very important that all parts be resistant to corrosion by thesealing fluid, and allow for optimal sealing at the operatingconditions.

474

Metal parts (adaptive hardware)Adaptive hardware will typically be made of 316 SS, howevercertain applications can dictate different materials be selected.Refer to Figure 8.3.7.

Secondary sealing elements (‘O’ rings)‘O’ ring materials vary in their applicability, so caution must beused in their selection. Refer to Figure 8.3.8 for ‘O’ ring mate-rials and guidelines for use.

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HC service with no known solid particles and temperature under 150°C (300°F)

Assuming satisfactory vapor margin (approximately 345 kPa

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 3

Teflon (PTFE), PEEK and Grafoil packing have the highesttemperature resistance, but are stiff, and poor secondary sealingelements compared to ‘O’ rings (‘O’ rings should be used wherethe application allows).

[50 psi] above vapor pressure) in seal chamber a Plan 11 would be the optimal choice. A vertical pump application would require venting of the seal chamber back to suction if possible (Plan 13 or 14). A Plan 52, 53 or 54 (53 or 54 preferred) would be recommendedin low S.G. ( < 0.7) services.

Fig 8.3.11 � Optimal flush plans for various applications: option 1

Seal facesSeal face materials also need to be selected to be compatiblewith the sealing fluid, however there is another reason. If yourecall our discussion on face generated heat, you will rememberthe term ‘f’ (coefficient of friction). This term describes theamount of friction between the two face (primary ring andmating ring) materials. Refer to Figure 8.3.9 on material selec-tion for faces.

Two dissimilar face materials are typically used (one softer than the other)

Carbon vs. silicon carbide – f = 0.1 Carbon vs. tungsten carbide – f = 0.12

Abrasive services may require the use of two hard faces (tungsten carbide vs. silicon carbide – f = 0.15 or more)

Note that if fluid has a potential toflash, two hard faces shouldnever be used.

Fig 8.3.9 � Seal face materials

Clean HC service (e.g. #2 FO) between 150 and 230°C (300 and 450°F)

Plan 23 is most ef fi cient in cooling, however proper installation and venting is required. Plan 21 will be suf fi cient in most cases and will be easier to operate ... note that the ori fi ce sizing is critical here as this determines the velocity of the fl uid through the heat exchanger.

Fig 8.3.12 � Optimal flush plans for various applications: option 2

Hot oil service above 230°C (450°F [typically tower bottoms])A single seal with the use of a Plan 32 is the most reliable/costeffective if a reliable source is available nearby... note that if thesource is a product that has to be reprocessed it may not becost effective.A dual pressurized seal would be the best option (Plan 53 or 54)if a Plan 32 is deemed not feasible.

Note the last point on Figure 8.3.9. If the sealing fluid is closeto its vapor margin (potential to flash), the last thing you want isto create more heat by having more face friction. Hard facecombinations (silicon carbide vs. tungsten carbide) should neverbe used in a sealing fluid with a high vapor pressure. Carbon vs.silicon carbide has the lowest coefficient of friction value and isrecommended for these applications.

Fig 8.3.13 � Optimal flush plans for various applications: option 3

Acid service (e.g. H2SO4)Reliable Plan 32 can be used, however it is very critical for it tobe operating at times when pump is installed in field (standbyand startup situations).

Optimal seal plans for various applications

We know the design aspects and how to monitor different flushsystems, but when would certain flush systems be preferableover others? Figure 8.3.10 lists the parameters required fora reliable flush source.

Mechanical seal fl ush must possess the following qualities for optimal seal life:

Cool Clean Approximately 345 kPa (50 psi) above vapor pressure (psia) Most importantly it must be cost effective!

Fig 8.3.10 � Optimal flush plans for various applications e overview

A dual pressurized seal, whether contacting (Plan 53/54) or non-contacting (Plan 74) will give the optimal seal life with ease ofoperation... note that process side seal components will needto be constructed of materials that are corrosion resistant to theparticular acid.

Fig 8.3.14 � Optimal flush plans for various applications: option 4

Dirty service (containing suspended solid particles)A cyclone separator (Plan 31) can be effective, however it mustbe sized correctly and particle size must not fluctuate greatly.A clean externalflush source (Plan 32 for single, Plan 54 for dual)will isolate the seal faces from solids that can cause prematurefailure due to abrasion.

Fig 8.3.15 � Optimal flush plans for various applications: option 5

Therefore, if you can say that the flush system for an appli-cation can provide all the qualities (most important is cost ef-fectiveness) listed in Figure 8.3.10, then you have selected theoptimal flush system.

The following points outline different seal flushing sce-narios, listing considerations specific to each. Note that

475

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B e s t P r a c t i c e 8 . 4B e s t P r a c t i c e 8 . 5 Pump Mechanical Seal Flush Best Practices

these are general, and should not be taken as beingdefinitive for every application; however they can aid inflush plan selection. Ultimately, the seal vendor should be

476

consulted when selecting the seal and flush plan; the moreinput they have, the higher seal reliability your plant shouldsee as a result.

Best Practice 8.4Best Practice 8.4

All cartridge seals should be statically pressure testedprior to installation to ensure that the seal has not beendamaged in transit.

To ensure maximum cartridge seal MTBF, the plant should havea static seal test facility to ensure that the seals are in ‘as manufacturedcondition’ at the time of installation.

This best practice is analogous to the low speed balance of a rotorprior to installation to ensure proper operation.

Lessons LearnedCartridge mechanical seals can be damaged in transitand/or at customs! Static testing of all cartridge seals prior

to installation will more than justify the cost of a static sealtest facility in your plant.

BenchmarksThis best practice has been recommended since 2000 to ensuremaximum cartridge seal MTBFs for new projects, as well as in existingfacilities that have been increasing their cartridge seal population.

B.P. 8.4. Supporting MaterialSee B.P: 8.3 for supporting material.

Best Practice 8.5

Use pressurized dual seals whenever possible in applica-tions that can potentially be hazardous to personnel safetyand/or the environment.

The use of pressurized dual seals, which require a safe, non-toxicbarrier fluid at a higher pressure than the process fluid at the seal face,will positively ensure that the process fluid will be contained, sincea positive barrier fluid to process fluid differential pressure will bemaintained by the flush system.

Confirmation of compatibility of the barrier and process fluids isrequired.

The barrier fluid can be pressurized by any of the following means:� Plant nitrogen header pressure e if the lowest value of header

pressure is sufficient� Nitrogen bottles and a regulator� A dedicated barrier fluid seal system

It is recommended that a differential pressure gauge be installed torecord the differential pressure between the reference fluid pressure

(pressure that is being sealed) and the pressure of the barrier fluid. Thiswill ensure that the proper barrier fluid to seal fluid pressure is main-tained, and that the pressure of the barrier fluid is not high enough toforce the inner seal open.

Lessons LearnedThe use of unpressurized seals in hydrocarbon and/ortoxic applications has resulted in flammable, toxic and/orsour fluid releases to the plant environment, which hasexposed the plant and personnel to hazards.

BenchmarksThis best practice has been used since the mid-1990s when multiple,unpressurized (tandem) seal failures and process fluid releases wereexperienced, due to improper operational and monitoring procedures.Since that time this best practice has resulted in optimum plant safetyand seal MTBFs (greater than 100 months in some cases).

B.P. 8.5. Supporting Material

Dual unpressurized vs. dual pressurizedseals

Today (2010), dual seals are being selected for more and moreapplications, due to environmental restrictions. There are two

arrangements in which dual seals can be used, namely dual un-pressurized (previously called tandem) and dual pressurized(previously called double).

Refer to the diagram in Figure 8.5.1, which shows the dualunpressurized arrangement. This type of seal uses a bufferfluid at or near atmospheric pressure to lubricate theatmospheric side seal. The buffer fluid pressure is signifi-cantly less than the seal chamber pressure, so any leakage

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Fig 8.5.1 � Dual un-pressurized

Fig 8.5.2 � Dual pressurized

Dual Un-pressurized Dual Pressurized

Primary leakage is processfluid, therefore the secondaryseal is essentially a backupsealDoes not require the use ofnitrogen or other inert gases

Primary leakage is barrier fluidinto the process, therefore theprocess must be able toaccept a small amount ofbarrier fluidRequires use of nitrogen (orother compatible inert gas) topressure seal reservoirapprox .175 kPa (25 psig)above seal chamber pressurein a Plan 53A Plan 54 uses an externalfluid (synthetic skid or anotherpump) to lubricate the seals ata pressure of 175 kPa (25 psi)above seal chamber pressure

Fig 8.5.3 � Dual un-pressurized vs. dual pressurized

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 6

occurring across the process side seal will leak into the bufferfluid.

This arrangement is very common in applications containingVOCs as it can potentially reduce the leakage to the environment.

Refer to Figure 8.5.2, showing a dual pressurized seal ar-rangement. These seals use a barrier fluid (same fluid as dualun-pressurized) at a pressure that is 175 kPa (25 psi) above seal

chamber pressure to lubricate the seals. Since the barrier fluidpressure is higher than the seal chamber pressure, any leakageoccurring at the process side seal will enter the pump. Thisarrangement (when working properly) ensures no leakage ofthe pumped fluid to atmosphere. Note that this arrangementrequires the barrier fluid to be compatible with the pumpedfluid, since the barrier fluid leaks into the pump. Refer toFigure 8.5.3.

It has been our experience that if an inert gas (or externalfluid) is available at the required pressure and the pumped fluidcan accept the barrier fluid (compatible), a dual pressurized sealis potentially more reliable in VOC service.

Best Practice 8.6Best Practice 8.6

Require that a pressure gauge is installed in the sealchamber or the flush line immediately before the sealchamber to ensure optimum mechanical seal MTBF.

Maintaining the proper seal chamber pressure above the vaporpressure is a key factor in obtainingmaximummechanical seal MTBFs.

Most seal applications do not use a pressure gauge to continuouslymonitor seal chamber pressure.

It is recommended that this best practice be specified for all hy-drocarbon seal applications and in other applications where ‘bad actorseals’ (more than one seal failure per year) have been experienced.

Lessons LearnedFailure to identify seal chamber pressure caused by thefollowing issues has resulted in mechanical seal MTBFs farbelow expected values (lower than 12 months):

� Blocked flush line orifice� Eroded or missing flush line orifice� Plugged cyclone separator� Plugged flush line strainer� Worn throat bushing

BenchmarksThe installation of seal chamber pressure gauges has been recom-mended for all projects since 1990, especially for all ‘bad actor’ seals(more than one seal failure per year).

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B e s t P r a c t i c e 8 . 7 Pump Mechanical Seal Flush Best Practices

B.P. 8.6. Supporting Material

Install a pressure gauge in the seal chamberModify the seal flush line for installation of a pressure gauge

Fig 8.6.1 � Seal chamber pressure monitoring guidelines for ‘badactor’ seals

Pressure monitoring

Unless the plant assigned machinery specialist is ‘world class’there are usually no pressure gauges in the flush line or sealchamber. Our recommendations concerning seal chamberpressure monitoring are presented in Figure 8.6.1. The mea-sured seal chamber pressure must be compared to the sealvendor’s assumed value, which should be on the seal layoutdrawing. If this value is not present, the seal vendor should beconsulted. Note that the seal vendor assumed seal chamber

Fig 8.7.1 � Importance of seal face lubrication

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pressure is the value that was used in the calculation of the sealPV. If the measured value does not agree with the data sheet,consult with operations first to see if process changes can bemade. If process changes cannot be made, discuss this fact withthe seal vendor representative.

Best Practice 8.7Best Practice 8.7Best Practice 8.7

Perform checks of all associated seal systems every timethat themechanical seal is replaced, to optimize safety andMTBF.

Like bearings, the reliability of mechanical seals is a direct functionof the reliability of all of its support system components.

To ensure that the root cause of a mechanical seal failure was notcaused by a flush system component, all of the components in the flushsystem should be checked each time a mechanical seal is replaced.

As a minimum, the following items should be checked:� The flush line orifice for proper dimensions and cleanliness� The flush line does not contain any restrictions (especially where

flush line tubing is used and may be crimped)� All flush line components (cooler, strainer, cyclone separator, etc.)� The gland plate to ensure all ports are fully open and do not contain

any debris� Throat bushing clearances

Lessons LearnedFailure to completely check the flush line, gland plate portsand all flush line components during a mechanical sealreplacement has resulted in low seal MTBFs and has ex-posed plant personnel and assets to damage.

The root cause of many repeated mechanical seal failures has oftenbeen eventually found in the flush system.

BenchmarksThis best practice has been used since 1990. Since that time, thisrecommended maintenance procedure has resulted in optimum me-chanical seal safety and reliability for all projects and field mechanicalseal reliability optimization audits.

B.P. 8.7. Supporting Material

The importance of seal face lubrication

In order to understand the functions and design parameters ofmechanical seal flush systems one must first understand theneed for these flush systems. Refer to Figure 8.7.1, outlining theimportance for seal face lubrication.

As is the case with hydrodynamic bearings, a mechanical sealconsists of a rotating part (shaft or thrust disc) and a stationarypart (shoes or brg housing). Contact between these two, just likerubbing your hands together, creates heat generated by friction.Therefore, some type of lubricant (whether it be the pumpedfluid or an external fluid) is required to keep a gap between thetwo surfaces to keep them from contacting. This gap (created bylubrication) results in a reduction of friction (or heat) generatedbetween the two faces.

The fluid used to lubricate and take the heat away from theseal faces must have certain characteristics to ensure optimalseal face life. Refer to Figure 8.7.2.

Vapor pressure – the fluid should be at least 345 kPa (50 psi)above its vapor pressureViscosity – typical limitations are no more than 100 Cst atminimum pump temperature and no less than 1 Cst at highestpump temperatureCleanliness – fluid shall contain little to no suspended solids

Fig 8.7.2 � Sealing fluid characteristics

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Where M (LPM or GPM) is the seal fluid massflow requiredM = Q/(500 CP SG T)

Where Q (BTU/hr) is the total heat load in the seal chamberWhere S.G. is the specific gravity of the seal fluidWhere T is the desired temperature rise of the sealingfluid(this varies depending on the fluid characteristics)500 is a conversion factor to convert the flow to GPM (125 usedfor LPM)

Fig 8.7.4 � Equation for seal flow

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 7

The most critical of these characteristics is the fluid’s vaporpressure. The sealing fluid needs to be 345 kPa (50 psi) above itsvapor pressure (at the pump’s operating temperature) in the sealchamber to ensure it does not vaporize too early across the sealfaces. Seal vendors refer to this value as the ‘vapor margin’. It isa great concern for fluids that have a specific gravity of less than0.7 at the pump’s operating temperature, as they will havea higher vapor pressure.

If the pressure and/or temperature within the seal chamberchanges during operation (this could be the result of operating ina region of the centrifugal pump curve where vaporization canoccur), the vapor margin will become smaller. Therefore it isessential to know the conditions within the seal chamber duringprocess changes. Figure 8.7.3 represents estimated seal chamberpressures for different pump configurations.

Single stage overhung withbalance holes – PSC = P1(0.15 (P2 P1))Single stage overhungwithout balance holes –PSC = .8 P2Double suction – PSC = P1Multistage with balancedrum and new bushing –PSC1 = P1PSC2 = P1 525 kPa(75 PSI)Multistage with balancedrum and old bushing –PSC1 = P1PSC2 = P2

Multistage opposed impellerswith new bushing – PSC1 = P1PSC2 = P1 525 kPa (75 PSI)Multistage opposed impellerswith old bushing – PSC1 = P1PSC2 = P1 (0.5 (P2 P1))Multistage vertical with bleedoff –PSC = P1 525 kPa (75 PSI)Multistage vertical no bleedoff – PSC = P2

Fig 8.7.3 � Seal chamber pressure estimations

In Figure 8.7.3 PSC is seal chamber pressure, P1 is pumpsuction pressure, and P2 is pump discharge pressure.

The calculations shown are estimates, and should be verifiedaccurately before consulting the vendor about bad actor seals.The best way to confirm the seal chamber pressure, as will bediscussed later, is simply by installing a pressure gauge in the sealchamber. If ports are not available to do so, the pump OEMshould be contacted as they can give an accurate estimate basedon the pump being in good condition.

In addition to the fluid characteristics listed in Figure 8.7.2,the specific heat (CP) of the sealing fluid also has an effect onseal life. The specific heat describes howmuch heat is needed toincrease the temperature of a fluid by one degree. Therefore,a fluid with a higher specific heat would be affected less (tem-perature won’t increase as much) than a fluid with a lowerspecific heat in the same conditions.

All of the fluid characteristics, along with heat generation,influence the amount of seal fluid flow to the seal faces that isnecessary. Take a look at Figure 8.7.4, which shows the equationused to determine the minimum required flow to the seal faces.

The amount of flow required (M) to the seal faces is directlyproportional to the heat load (Q) and inversely proportional tothe fluid’s specific heat, specific gravity, and desired tempera-ture rise in the seal chamber. Note that the heat load used indetermining the flush flow required is the total heat load. This

not only accounts for the seal face generated heat but also addsany heat generated via heat, or conduction through the pumpcasing. An example where heat soak needs to be taken intoaccount is a BFW service. In this service the heat from the pumpfluid is transmitted through the shaft to the seal. In addition,heat will also be transferred through the casing to the seal fluid.The heat soak value is an estimate, and varies betweenseal vendors. It typically is negligible in the calculation inFigure 8.7.4 except in applications above 177�C (350�F).

Considerations for process flush systems

Now that we understand why it is so important to lubricatemechanical seal faces and what fluid characteristics need to beconsidered for optimal seal life, we will discuss the design con-siderations for all the major mechanical seal flush plans. First, wewill take a look at the flush plans categorized as process flushplans, which utilize the pumped fluid to lubricate the seal faces.

API Plan 11The most commonly used flush plan, an API Plan 11 flush,utilizes the pumped fluid to lubricate the seal faces. Thepumped fluid is taken from discharge and sent to the sealchamber through an orifice. Refer to Figure 8.7.5 for a flush planschematic.

The orifice is used to control the flow of the pumped fluidabove the minimum required flow rate. Seal vendors require anorifice to ensure the flow is not too great either, as high flowrates can cause erosion of the seal faces. Equally important, isthe fact that an orifice limits the amount of recirculation throughthe seal chamber back to the pump (the pump pumps money).A 3mm (1/8") orifice is the most commonly used size, as it is thesmallest practical size and Plan 11 flushes are normally used inservices that are easy to seal (good lubricating qualities). Refer toFigure 8.7.6, outlining general guidelines for orifice sizes.

Remember that Figure 8.7.6 just shows guidelines for orificesizes that are not always followed, but if followed they shouldnot harm the seal if the pump is operating in a region of thecentrifugal pump curve where vaporization can occur. Note thatorifice sizing does not give an exact flow, due to the systemfriction (piping, coolers, etc.), therefore the more informationabout the system the seal vendor has, the more accurately theycan size the orifice.

A vendor may require a close clearance throat bushing beinstalled in certain instances to increase the pressure in the sealchamber above the vapor pressure of the pumped fluid. As the

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Fig 8.7.5 � Plan 11 e recirculation fromdischarge through orifice and tomechanical seal

Single stage pumps – 1/8" Multistage pumps – gang ori fi ces (series of orifices back toback, sized by seal vendor)Low S.G. fl uids may require higher fl ow rates to prevent fl ashing (early vaporization) where a 3/16" orifice may be necessary

Fig 8.7.6 � Orifice size guidelines e where used

Fig 8.7.8 � Typical API Plan 11

B e s t P r a c t i c e 8 . 7 Pump Mechanical Seal Flush Best Practices

throat bushing wears over time, the seal chamber pressurewill drop.

In a Plan 11 flush system, the main thing to monitor is thetemperature across the orifice. If this drops by more than 10%,the orifice is most likely plugging up. If this is the first occur-rence and the fluid does not normally contain solids, the bestoption would be to clean the orifice out quickly and continue tocheck the temperature drop after the pump is restarted. If theorifice has plugged up more than once, another flush plan optionshould be considered. Refer to Figure 8.7.7.

Monitor temperature across the ori fi ce If temperature decreases by more than 10% it is beginning to plug If fi rst occurrence, clean and continue to monitor If problem reoccurs, consider using a different fl ush plan

Fig 8.7.7 � Orifice monitoring

Look at Figure 8.7.8, which shows a typical Plan 11. Thisis a between bearing double suction pump with an orifice toeach seal.

API Plan 13An API Plan 13 is widely used in vertical pump applications, orwhen seal chamber pressure is at or near the discharge pressureof the pump. This flush plan basically vents the seal chamber at

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a high point back to the suction of the pump (ideally a high pointin the suction piping). Refer to Figure 8.7.9 for a schematic ofa Plan 13 flush.

As with a Plan 11, this flush plan also utilizes an orifice,however it is more to create a back pressure on the seal chamberthan to control flow. The orifice in a Plan 13 flush is typically6 mm (¼"), which is usually large enough to vent vapors accu-mulated in a vertical pump, while keeping the vapor margin at345 kPa (50 psi).

Since a Plan 13 uses a larger orifice, it is not typically moni-tored with the frequency of a Plan 11. From time to time,however, it is a good idea during rounds to check the temper-ature across the orifice as in a Plan 11, to ensure flow out of theseal chamber and no plugging of the orifice.

Very commonly, a Plan 13 will be used in conjunction witha Plan 11; this is defined by API as a Plan 14 flush. The samemonitoring rules apply to a Plan 14 as to flush Plans 11 and 13.

API Plan 21A flush Plan 21 is a Plan 11 with the addition of a cooler to lowerthe temperature of the pumped fluid. This plan is generally used

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Fig 8.7.9 � Plan 13 e recirculation from seal chamber through orifice and back suction

Fig 8.7.10 � Plan 21 e recirculation from pump discharge through an orifice and heat exchanger to the mechanical seal

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 7

when the pumped fluid is naturally a good lubricant, but thevapor margin is low at the current pump operating temperature,and it therefore requires the addition of a cooler to increase thevapor margin. Refer to the schematic of a Plan 21 inFigure 8.7.10.

Just like a Plan 11, a Plan 21 utilizes an orifice to control theflow of the pumped fluid. In this plan, orifice sizing is morecritical, since it is preferred to have a flow that is close to theminimum required for maximum cooling of the pumped fluid. Ithas also been found to be beneficial to place this orifice down-stream of the cooler (especially in low S.G. fluids) to preventvaporization before the cooler.

With the addition of the cooler, it is essential to check thetemperature differential across it. This should be done at initialinstallation (or after cleaning of the cooler) and trended ona time basis. Typical seal flush coolers should provide a tem-perature drop greater than 38�C (100�F), however this dependson the cooling media temperature, showing why trending isimportant. If a water-cooled heat exchanger is being used,

cooling water (CW) temperatures should also be checked reg-ularly, as a decrease in cooling efficiency could be the result ofCW temperature or flow changes. It is ideal to have a ther-mometer installed after the cooler to allow easy check of theseal fluid temperature. A thermometer is recommended by APIas an option, and if possible, FAI recommends it be installed ona Plan 21. The pump operating conditions need to be consideredas they could alter the cooler inlet temperature.

Refer to Figure 8.7.11. This pump is utilizing a process flushwith a cooler, before making its way to the seal. Note that thisinstallation also includes a component called a cyclone sepa-rator. This is used in services that contain suspended solids,and it works by sending the heavier solids back to the suctionof the pump while the ‘clean’ liquid goes to the seal chamber.Our experience with cyclone separators has been one offrustration at times. It is essential that it be sized correctly,and that the solids are significantly heavier (and don’tchange size) than the pumped fluid. If not, the solids will carryover to the seal and may even potentially plug the cyclone

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Fig 8.7.11 � (Cyclone Separator) API Plan 41

B e s t P r a c t i c e 8 . 7 Pump Mechanical Seal Flush Best Practices

separator, preventing seal face lubrication. In addition, thesolids that cause heavy seal face wear are smaller than 2 mi-crons (typical gap between seal faces), which a cyclone sepa-rator cannot separate from the pumped fluid. With that said,when a cyclone separator is properly sized it can reduce thepotential of seal hang-up due to solids building up on thesprings or dynamic ‘O’ ring.

The picture shown above is an example of an excellent flushplan installation. As you can see, a pressure gauge and ther-mometer are installed in the piping just before entering the seal.These instruments give you an idea of the flush system perfor-mance (and aid in troubleshooting failures) by allowing the sealchamber pressure and temperature to be monitored easily(vapor margin!).

Considerations for external flush plans

When the pumped fluid contains significant amounts of solids, isvery corrosive, or is at a very high or low temperature, an ex-ternal flush should be considered. An external flush plan usesa fluid from another pump, or it may be a separate console witha process compatible liquid to lubricate the seal faces. We willnow discuss two types of external flush plans; a Plan 32 anda Plan 54.

API Flush Plan 32A flush Plan 32 utilizes a fluid from another pump with goodlubricating qualities (cool, clean and at an acceptable vapormargin) at a higher pressure than the seal chamber (by at least66 kPa (10 psi), to keep the pumped fluid out of the sealchamber. Refer to Figure 8.7.12.

Since the Plan 32 is at a higher pressure than the sealchamber, the Plan 32 fluid will leak into the pump, therefore itmust be compatible with the pumped fluid and at an acceptablevapor margin. The leakage into the pump is controlled bya bushing located either in the seal itself or the pump casingknown as the throat bushing.

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For optimal seal life with a Plan 32 flush (very common toexceed 3 years), the flow is very critical. A typical value used byseal vendors is 3 LPM (0.75 GPM) per inch of seal size, howeverit is essential to ensure the minimum required flow is met, whileminimizing the flow into the pump. Using the equation fromFigure 8.7.4 and the Plan 32 fluid qualities, the seal vendor willcalculate the minimum required flow for the seal. This flow canbe controlled using a variety of methods (usually manual). Referto Figure 8.7.13, which describes the methods for controllingthe flow to the seal in a Plan 32.

Although they can be somewhat expensive, a flow meterinstalled in conjunction with a throttle valve is a very accurateway of controlling the flow to the seal. No matter which methodis used, operator training on the method is important in assuringa reliable system.

A flush Plan 32 can provide very long seal life, however italways needs to be justified. A compatible fluid (with theprocess and with vapor margin) is required and it must beprovided at all times from start-up through to shutdown ofthe pump. Also, if the selected fluid is a product of the plant,long term costs (reprocessing a product because the product isnot compatible with the pumped fluid) may need to beconsidered.

API Plan 54A flush Plan 54 is an external flush used on dual pressurized(double) seals. This flush plan is typically used in applicationswith very corrosive fluids, or when a Plan 32 is not feasible.Refer to Figure 8.7.14 for a schematic of a Plan 54 flush.

A Plan 54 can use a fluid from another pump in the plant(process Plan 54), like a Plan 32, or it can be a separate consolewith process compatible liquid with sufficient vapor margin(synthetic Plan 54). The advantage of a separate console is thata potential product of the plant is not used, therefore long-termcosts for reprocessing need not be considered. The syntheticPlan 54 can potentially produce the longest seal life, but theup-front cost (capital expenditure) of this flush plan needs to bejustified.

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Fig 8.7.12 � Plan 32 e clean seal flush from external source

Ori fi ce – not very accurate, since all system losses need to be considered Throttle valve and pressure gauge – can be effective if seal chamber pressure does not fl uctuate much Throttle valve and fl ow meter – most effective manual fl ow control

Fig 8.7.13 � Manual flow control options for API Plan 32

Fig 8.7.14 � Plan 54 e dual pressurized seal using external source to lubis preferred)

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 7

As with a Plan 32 flush, the seal vendor will calculate theminimum flow required for the seal (using the heat generatedfrom two sets of faces). This flow is used in sizing the seal pump(value can be from 24 to 40 LPM [6 to 10 GPM]). Since the flowis not going directly to the seal chamber (through the bushing intothe pump) but is a ‘through’ flow back to the seal reservoir, thepressure is controlled at 175 kPa (25 psi) above the seal chamberpressure. This ensures the Plan 54 fluid is pushed across the faces,providing adequate lubrication. A process Plan 54 will most likelyuse a throttle valve and pressure gauge to set the pressure, while

ricate both seals (source can be a process fluid, or synthetic oil, which

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B e s t P r a c t i c e 8 . 7 Pump Mechanical Seal Flush Best Practices

a synthetic Plan 54 is usually provided with a pressure controlvalve to automatically control the pressure. Note that pressurecontrol is adequate in most applications, however if the sealchamber pressure fluctuates during operation, a differentialpressure control should be considered.

Another consideration when making a decision on a syntheticPlan 54 vs. a Process 54 or a Plan 32, is the potential addition ofextra components. A synthetic Plan 54 will contain pump(s),coolers, filters, and instrumentation. As everybody knows, themost reliable equipment has a balance of quality components andis as simple as possible (fewest number of components).

Fig 8.7.16 � Plan 23 flush

Considerations for closed loop system

API Flush Plan 23A flush Plan 23 is used on single seals, and consists of piping(usually tubing) connected to the seal and a cooler. The fluid(seal chamber is filled with the pumped fluid) is circulated tothe cooler and back to the seal (see Figure 8.7.15).

This flush plan offers more efficient cooling than a flushPlan 21, since it does not have to continually cool down thepumped fluid. This can be related to your car air conditioning;most cars today have a button for recirculation of air. This closesthe valve to the atmosphere, so that the A/C just has to cool theair in the car, not the air pulled in from outside. A Plan 23 worksin the same fashion. This, however, creates some concerns,because it relies heavily on the pumping ring to circulate thefluid to the cooler. High system friction or vapor pockets (notvented properly) will result in limited to no circulation, due tothe limited pumping capability of the pumping ring.

Therefore, the cooler needs to be placed close to the seal(3 meters of total tubing and 7e10 cm above the seal [10 feet oftotal tubing and 18e24" above the seal]) to ensure the pumpingring will be sufficient. The cooler needs to be above the seal toallow for venting and flow back to the seal (through gravity and

Fig 8.7.15 � Plan 23 e closed loop circulation of process fluid through

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thermosyphon). It is also essential in a Plan 23 to have a highpoint vent and block valve installed, to allow for proper ventingbefore pump start-up. Operators should be trained to un-derstand the necessity of venting these systems.

In monitoring this system, the temperature drop needs to bechecked across the cooler. Note that this temperature drop willbe less than in a Plan 21, due to the cooler not needing to providethe same amount of cooling. If the temperature drop across thecooler is high (like a Plan 21 should be), it indicates that thethroat bushing clearance has increased and mixing of the pumpedfluid and seal fluid is occurring. Typical temperature drop ina Plan 23 can be anywhere from 7�C to 10�C (20�F to 50�F).

This flush plan is most common in BFW applications, sincewater has poor lubricating qualities and needs to be as cool aspossible. Refer to Figure 8.7.16, which shows a flush Plan 23.

The installation shown in Figure 8.7.16 is once again excel-lent as the location of the cooler is very close to the seal, giving

a heat exchanger via a pumping ring

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Fig 8.7.17 � Plan 52 e dual un-pressurized seal using synthetic buffer fluid to lubricate the atmospheric side seal. A pumping ring in the sealcirculates the buffer fluid (pressure less than seal chamber) to the reservoir.

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 7

the pumping ring as little system resistance as possible. Also, thevent is in its proper location and a thermometer is installed inthe return line to the seal for easy cooler and throat bushingcondition monitoring.

API Flush Plan 52Dual un-pressurized seals (tandem) rely on a buffer fluid at ornear atmospheric pressure to lubricate the atmospheric sideseal. This buffer fluid is circulated via a pumping ring from theseal to the seal reservoir and back to the seal (in a closed loop).Take a look at Figure 8.7.17 for a schematic.

This flush plan is very common in applications with VOCs(Volatile Organic Compounds), however if not set up similarlyto the schematic in Figure 8.7.16 it may not be effective.

The reservoir is at atmospheric pressure (less than seal chamberpressure), so the leakage across the process side seal facesmigratesinto the seal reservoir, and will either increase pressure, level, orboth in the reservoir. Since every seal does leak a certain amount, itis essential to have the reservoir vented to a flare or vapor collec-tion system. If the reservoir is allowed to reach the seal chamberpressure, the atmospheric side seal will most likely fail (if it hasn’talready) as it is not typically designed to handle seal chamberpressure. If this is a concern in the plant, youmaywant to considerrequesting the seal vendor to redesign the atmospheric seal to

handle maximum seal chamber pressure. In addition, as the pro-cess side seal leaks in this flush plan, the atmospheric side seal willessentially be sealing the pumped fluid, exposing the plant to therelease of flammable and/or toxic vapors.

Monitoring of seal leaks can be done by checking the level andpressure of the reservoir, as one or both may increase in theevent of excessive leakage. The seal vendor (or support systemvendor) may supply high level and/or pressure switches whichwould alert the operators to a seal leak. It is highly recom-mended to specify this instrumentation in new projects, as it willcut down on the already high workload of operators (if the alarmsounds then the level or pressure can be personally verified inthe field).

In addition to checking for excessive leakage (pressure orlevel increase), temperature in and out of the seal can help verifyproper circulation. In a reservoir that is not cooled, there shouldbe a temperature drop of approx. 1 to 2�C (5�F to 10�F) fromthe seal outlet to the seal inlet. If there is no temperature drop(or if the temperature increases), this indicates zero or possiblyreverse circulation.

It has been our experience that Plan 52 flushes may not bevented properly (blocked in) and level may be low or at zero inthe reservoir. For these reasons, it is very important for theoperators to be trained to understand the necessity of moni-toring this system.

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Fig 8.7.18 � Plan 53 e dual pressurized seal using a pressurized barrier fluid (usually pressurized by a nitrogen blanket) to lubricate the seals

B e s t P r a c t i c e 8 . 7 Pump Mechanical Seal Flush Best Practices

API Flush Plan 53A Plan 53 is basically a combination of a 52 and 54. It uses thesame reservoir as a Plan 52, however it is pressurized at 175 kPa(25 psi) above the seal chamber pressure, just like a Plan 54.Refer to Figure 8.7.18.

The reservoir is pressurized, however the system holdsa constant pressure of 175 kPa (25 psi) above seal chamberpressure, hence requiring a means of circulation (pumping ring).The pumping ring circulation can be monitored the same way asin a Plan 52.

Fig 8.7.19 � Plan 62 e a medium (usually steam or water) is introducedof solids

486

Also, as in a Plan 52, pressure and level switches arerecommended to set off an alarm on excessive leakage. Sincethis system is at a higher pressure than the seal chamber, leakagewill migrate into the pump. A low pressure or level in the res-ervoir will indicate this excessive leakage.

Considerations for quench

A quench (known as an auxiliary flush plan) is a flush plan thatuses a medium (steam, nitrogen, or water) on the atmospheric

on the atmospheric side of the seal to prevent crystallization or buildup

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Using information in this section (or other source), discuss parameters to monitor on different fl ush plans Walk to the equipment and monitor these parameters Discuss action plans to be executed to increase reliability of these systems Remember, operators see this equipment every day; the morethey understand about fl ush system, the more reliable the seals will be

Fig 8.7.20 � Flush system training for operators

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 8

side of the seal to wash away any solid buildup from the faces.The buildup is drained to a collection system. Refer toFigure 8.7.19 for a schematic.

A quench is most commonly used in single seals with steam asthe medium, if the seal fluid is hot and can form coke particlesand/or if the seal fluid is flammable or toxic. Note: manycountries today require dual (tandem or double) seals if the sealfluid is flammable or toxic, in order to meet environmental re-quirements. The steam should be regulated to a pressure ofapprox. 20 to 33 kPa (3 to 5 psi), which is just enough to washthe accumulated solids off the atmospheric side of the faces. It isessential for the steam to be superheated (dry), to preventflashing of water at the faces, causing premature failures and toensure that moisture does not enter the bearing chamber. Wehave experienced plant fires that resulted from water contami-nation in the bearing housing, leading to a hot bearing, whichserved as an ignition source for a single seal leaking a flammable

vapor. We recommend that an ‘oil condition bottle’, to monitorwater in the oil, always be installed in the bearing housings whena steam or water quench is used. Finally, refer to Figure 8.7.20,highlighting the importance of operator training on flushsystems.

Best Practice 8.8Best Practice 8.8

Use medium pressure steam in seal jackets for hot pump(above 300�C) services to cool the seal fluid during oper-ation and keep standby pump seal fluid warm.

Most hot pump services (bottoms and gas oil refinery services) usebellows seals (to eliminate the dynamic secondary seal) and deadended (no flush) configurations to minimize ingress of fluid particlesinto the seal chamber.

As a result, seal chamber jacket cooling is required duringoperation.

Medium pressure steam in the seal chamber jacket has proven tobe the best solution in this case, since it will provide adequate coolingduring operation and keep the seal fluid viscosity in the standby pump(frequently on auto-start in these applications) at an acceptable level toprevent excessive seal wear during start-up and operation.

Lessons LearnedLow seal MTBFs have been experienced in hot services(bottoms and gas oil) caused by incompatible externalflushes (too high a vapor pressure) or failure of the standbypump seal during start-up.

This best practice solves both issues by eliminating the need for anexternal flush as well as ensuring the seal fluid in the standby pumpchamber will be at an acceptable viscosity under start-up conditions.

BenchmarksThis best practice has been used since the mid-1970s to optimizerefinery bottoms and gas oil pump mechanical seal MTBFs (greaterthan 36 months).

B.P. 8.8. Supporting MaterialThe part of the pump that is exposed to the atmosphere and thatthe rotating shaft or reciprocating rod passes through is called

the stuffing box. A properly sealed stuffing box prevents theescape of pumped liquid. Mechanical seals are commonlyspecified for centrifugal pump applications (refer toFigure 8.8.1).

Fig 8.8.1 � Typical single mechanical seal

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B e s t P r a c t i c e 8 . 8 Pump Mechanical Seal Flush Best Practices

Function of mechanical seals

The mechanical seal is comprised of two basic components(refer to Figure 8.8.2).

Stationary member fastened to the casingRotating member fastened to shaft, either direct or with shaft

sleeve

Fig 8.8.2 � Basic seal components

The mating faces of each member perform the sealing. Themating surface of each component is highly polished, and theyare held in contact with a spring or bellows which results in a netface loading closure force (refer to Figure 8.8.1).

In order to prevent fluid escaping to the atmosphere, addi-tional seals are required. These seals are either ‘O’ rings, gas-kets or packing (refer to Figure 8.8.1). For high temperatureapplications (above 200�C [400�F]) the secondary seal isusually ‘Graphoil’ or ‘Kalrez’ material in a ‘U’ or chevronconfiguration. An attractive alternative is to eliminate thesecondary seal entirely by using a bellows seal, since the

488

bellows replaces the springs and forms a leak-tight elementthus eliminating the requirement for a secondary seal (refer toFigure 8.8.3).

To achieve satisfactory seal performance over extended pe-riods of time, proper lubrication and cooling is required. Thelubricant, usually the pumped product, is injected into the sealchamber and a small amount passes through the interface of themating surfaces. Therefore, it can be stated that all seals leak,and the amount of leakage depends on the pressure drop acrossthe faces. This performance can be considered to be flowthrough an equivalent orifice (refer to Figure 8.8.4).

The amount of heat generated at the seal face is a function ofthe face loading and the friction coefficient, which is related tothe materials and lubrication of the faces. Figure 8.8.5 shows theequation for calculating the amount of heat that needs to beremoved by the flush liquid.

As the lubricant flows across the interface, it is prone tovaporization. The initiation point of this vaporization is de-pendent upon the flush liquid pressure, and its relationshipto the margin of liquid vapor pressure at the liquid temperature.The closer the liquid flush pressure is to the vapor pressure ofthe liquid at the temperature of the liquid, the sooner vapor-ization will occur (refer to Figure 8.8.6).

Fig 8.8.3 � Metal bellows seal

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Fig 8.8.4 � Equivalent orifice flow across seal faces

Q = 500 S.G. Q,NJ Cp Twhere: Q = heat load (BTU/HR)S.G. = specific gravity of injection liquid

Cp = specific heat of injection liquid

T = temperature rise of injection liquid (°F)°Flb

QINJ = injection liquid flow rate (G.P.M.). If flow is in LPM,constant = 125.

Fig 8.8.5 � Heat generated by a mechanical seal

Fig 8.8.6 � Typical seal face pressure temperature relationship tovaporization

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 8

The seal system

To ensure reliable, trouble-free operation for extended periodsof time, the seal must operate in a properly controlled envi-ronment. This requires that the seal be installed correctly, sothat the seal faces maintain perfect contact and alignment, andthat proper lubrication and cooling be provided. A typical sealsystem for a simple, single, mechanical seal is comprised of theseal, stuffing box throat bushing, liquid flush system, auxiliaryseal and auxiliary flush or barrier fluid (when required) (refer toFigure 8.8.7).

The purpose of the seal is to prevent leakage of pumpedproduct from escaping to the atmosphere. The liquid flush(normally pumped product from the discharge) is injected intothe seal chamber to provide lubrication and cooling. An auxiliaryseal is sometimes fitted to the gland plate on the atmosphericside of the seal chamber. Its purpose is to create a secondarycontainment chamber, when handling flammable or toxic fluidsthat would be considered a safety hazard to personnel if theywere to leak to atmosphere. A liquid (non-toxic) flush or barrierfluid, complete with a liquid reservoir and appropriate alarmdevices can be used to ensure toxic fluid does not escape to theatmosphere.

Controlling flush flow to the seal

The simple seal system shown in Figure 8.8.8 incorporates anorifice in the flush line from the pump discharge to the me-chanical seal. Its purpose is to limit the injection flow rate to theseal and to control pressure in the seal chamber. A minimumbore diameter or 3 mm (1/8") is normally specified (to minimizepotential of blockage) and the orifice can either be installedbetween flanges or in an orifice nipple.

Examining some causes of seal failures

An indication of some causes of seal failures can be obtainedwhile the seal is operating. When you consider the seal as anequivalent orifice, an examination of ‘tell-tale’ symptoms cancauses of indicate potential failure for which corrective actioncan be implemented or at least can provide direction of

Fig 8.8.7 � Simple seal system

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Fig 8.8.8 � Seal flow control orifice

Comments Possible causes Comments/recommendations

B e s t P r a c t i c e 8 . 8 Pump Mechanical Seal Flush Best Practices

subsequent failure analysis (refer to Figure 8.8.9). It should benoted that improper application, installation, and/ormanufacturing errors can also result in mechanical seal failures.

Seal squealduring operation

Insufficientamount of liquidto lubricate sealfaces

Flush line may need to beenlarged and/or orificesize may need to beincreased

Carbon dustaccumulating onoutside of sealarea

Insufficientamount of liquidto lubricate sealfaces

See above

Liquid filmvaporizing/flashing betweenseal faces

Pressure in seal chambermay be too low for sealtype

Seal spitsand sputters inoperation(popping)

Productvaporizing/flashing acrossseal faces

Corrective action is toprovide proper liquidenvironment of theproduct at all times1. Increase seal

chamber pressure if itcan be achieved withinoperating parameters(maintain ata minimum of 175 kPa(25 psig) abovesuction pressure)

2. Check for proper sealbalance withmanufacturer

3. Change seal design toone not requiring asmuch producttemperature margin ( T)

4. Seal flush line and/ororifice may have to beenlarged

5. Increase cooling ofseal faces

Note: A review of sealbalance requires accuratemeasurementof seal chamber pressure,temperature and productsample for vapor pressuredetermination

Fig 8.8.9 � Possible causes of seal failure

Seal configurations

Mechanical seals are the predominant type of seals currently inuse in centrifugal pumps. They are available in a variety ofconfigurations, depending upon the application service condi-tions and/or the user’s preference (refer to Figures 8.8.10 to8.8.13 for the most common arrangements used in refinery andpetrochemical applications).

Single mechanical seal applicationsSingle mechanical seals (refer to Figure 8.8.10) are the mostwidely used seal configuration, and should be used in any ap-plication where the liquid is non-toxic and non-flammable.

As mentioned earlier in this section, many single mechanicalseals are used with flammable and even toxic liquids, and relysolely on the auxiliary seal throttle bushing to prevent leakage toatmosphere. Since the throttle bushing does not positively con-tain leakage, state and federal environmental regulations nowrequire the use of a tandem or double seal for these applications.In some plants, a dynamic type throttle bushing (‘Impro’ orequal) is used to virtually eliminate leakage of the pumped fluidto atmosphere in the event of a mechanical seal failure.

Tandem mechanical seal applicationsTandem mechanical seals (refer to Figure 8.8.11) are used inapplications where the pumped fluid is toxic and/or flammable.

They consist of two (2) mechanical seals (primary and back-up). The primary seal is flushed by any selected seal flush plan.The back-up seal is provided with a flush system incorporatinga safe, low flash point liquid. There is a pressure alarm whichactuates on an increase in stuffing box pressure between theprimary and back-up seal thus indicating a primary seal failure.Since the pumped product now occupies the volume betweenthe seals, failure of the back-up seal will result in leakage of thepumped fluid to atmosphere. In essence any time a tandem sealin alarm, it is actually a single seal and should be shut downimmediately to ensure that the toxic and/or flammable liquiddoes not leak to atmosphere.

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Fig 8.8.10 � Single mechanical seal

Fig 8.8.11 � Tandem seal arrangement

Fig 8.8.12 � Double mechanical seal

Fig 8.8.13 � Liquid/gas tandem seal combination

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 8

Double mechanical seal applicationsDouble mechanical seals (refer to Figure 8.8.12) are used inapplications where the pumped fluid is flammable or toxic, andleakage to atmosphere cannot be tolerated under any circum-stances. Typical process applications for double seals are H2Sservice, hydrofluoric acid alkylation services or sulfuric acidservices.

Leakage of the pumped fluid to the atmosphere is positivelyprevented by providing a seal system, whose liquid is compatiblewith the pumped liquid, which continuously provides a safebarrier liquid at a pressure higher than the pumped fluid. Theseals are usually identical in design with the exception that oneseal incorporates a pumping ring to provide a continuous flow ofliquid to cool the seals. Typical double seal system componentsare: reservoir, cooler, pressure switch and control valve.

Liquid/gas tandem mechanical seal applicationsIn this configuration (refer to Figure 8.8.13) a conventionalsingle liquid mechanical seal is used as the primary and a gas seal(non-contacting faces) that can temporarily act as a liquid seal inthe event of primary seal failure serves as the back-up seal. This

seal configuration is used in low specific gravity applicationswhere the pumped fluid is easily vaporized. Using a gas seal asthe back-up has the advantage of eliminating the vessel, coolerand pumping ring necessary for conventional tandem liquidseals.

This application is well proven, and has been used success-fully for natural gas liquids, propane, ethylene, ethane andbutane pump applications.

Double gas seal applicationsBefore leaving this subject, a relatively new application utilizestwo (2) gas seals in a double seal configuration, and uses N2 orair as a buffer, maintained at a higher pressure than thepumped fluid to positively prevent the leakage of pumpedfluid to atmosphere. This configuration, like the tandemliquid/gas seal mentioned above, eliminates the seal systemrequired in a conventional liquid double seal arrangement.Note however, that the pumped product must be compatiblewith the small amount of gas introduced into the pumpedfluid. This configuration cannot be used in recycle (closedloop) services.

An excellent resource for additional information coveringdesign, selection and testing criteria is API Standard 682.

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B e s t P r a c t i c e 8 . 9B e s t P r a c t i c e 8 . 1 0 Pump Mechanical Seal Flush Best Practices

Best Practice 8.9Best Practice 8.9

Avoid the use of flush line strainers and cyclone separatorsin dirty services, and use an external flush or a dual pres-surized seal to optimize mechanical seal MTBFs:

Mechanical seal reliability is significantly affected by the operationalcharacteristics of the flush system and its components.

In services where the pumped fluid can contain solids, API 610 andAPI 682 both offer the option of using flush line strainers or cycloneseparators.

Flush line strainers can become blocked resulting in immediate sealfailure.

Cyclone separator effectiveness is dependent on the relative densitydifference between the fluid and solid particles and the flush line piping(adequate slope for the debris drain line back to the pump suction).

Using an external clean flush, if available, or a dual pressurized sealarrangement will eliminate the need for flush line strainers and cycloneseparators and ensure optimum mechanical seal MTBFs.

Lessons LearnedFlush line strainers and cyclone separators have been thecause of low seal MTBFs (less than 12 months) in many

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applications where the seal fluid contains solid particles.Eventual modification to a clean external flush or a dualpressurized seal has significantly increased seal MTBFs(greater than 48 months).

BenchmarksThis best practice has been used since the 1990s to thoroughly in-vestigate, with the process engineers, the possibility of using a cleanexternal flush source in services where solid particles were containedin the seal fluid. A contingency recommendation where a clean externalflush was not available was to use a dual pressurized seal.

In many cases, the additional cost of an external flush or dualpressurized seal was justified on the basis of past plant mechanicalseal maintenance history and the loss of revenue when the standbypump was under maintenance and the operating pump failed.

B.P. 8.9. Supporting MaterialSee B.P: 8.2 for supporting material.

Best Practice 8.10Best Practice 8.10

Properly design and monitor quench systems for optimumseal reliability and to ensure that moisture does not enterthe bearing housing.

The proper function of seal quench systems is to remove solidparticles from the lower seal face to prevent premature seal wear.

The term ‘quench’ originally came from the use of steam to bufferthe outer seal chamber, between the seal and throttle bushing, to dilutethe hydrocarbon fluid leaking from the seal to a safe non-flammablelevel. Today (2010) this practice is not acceptable and dual seals arerequired to be used for all hydrocarbon services.

When a quench is used today (2010) either steam (where solidhydrocarbon particles can form) or water (where water soluble particlescan form) are used. It is solely for the purpose of removing solid par-ticles from the lower seal face. Both of these alternatives expose thebearing housing to entrance of water vapor which will impact bearingreliability and MTBFs.

Proper system design to control the amount and condition of thequench fluid and the mandated use of a bearing bracket oil condition

monitoring bottle is essential to the reliability of mechanical sealsemploying quench systems.

Lessons LearnedFailure to regulate quench fluid conditions and to monitorthe operation of seal quench systems has resulted in lowmechanical seal MTBFs (lower than 12 months). In onecase, it resulted in a refinery fire when the steam quenchbecame saturated and displaced all of the oil in a pumpbearing bracket. An excessive hydrocarbon seal leak wasignited by the ‘red hot’ bearing bracket.

BenchmarksThis best practice has been used since the mid-1980s when I wasfaced with contamination of pump bearing brackets with saturatedsteam from malfunctioning steam quench systems in refinery service.Since that time, this best practice has optimized mechanical sealquench system safety and reliability.

B.P. 8.10. Supporting Material

Considerations for quench

A quench (known as an auxiliary flush plan) is a flush plan thatuses a medium (steam, nitrogen, or water) on the atmospheric

side of the seal to wash away any solids buildup from the faces.The buildup is drained to a collection system. Refer to Figure8.10.1 for a schematic.

A quench is most commonly used in single seals with steam asthe medium, if the seal fluid is hot and can form coke particles,and/or if the seal fluid is flammable or toxic. Note: many

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Fig 8.10.1 � Plan 62 e a medium (usually steam or water) is introduced on the atmospheric side of the seal to prevent crystallization or buildupof solids

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 1 1

countries today require dual (tandem or double) seals to meetenvironmental requirements if the seal fluid is flammable ortoxic. The steam should be regulated to a pressure of approx.20 to 33 kPa (3 to 5 psi), which is just enough to wash the solidsaccumulation off the atmospheric side of the faces. It is essentialfor the steam to be superheated (dry), to prevent flashing ofwater at the faces, causing premature failures and to ensure that

moisture does not enter the bearing chamber. We have expe-rienced plant fires as a result of water contamination in thebearing housing resulting in a hot bearing which served as anignition source for a single seal leaking a flammable vapor. Werecommend that an ‘oil condition bottle’, to monitor water inthe oil, always be installed in the bearing housings when a steamor water quench is used.

Best Practice 8.11Best Practice 8.11Best Practice 8.11

Execute the following mechanical seal monitoring bestpractices for optimum mechanical seal MTBFs:� Confirm that the pump is operating in its EROE (see B.P 2.7 for

EROE details)� Check for a plugged flush line orifice by taking temperature reading

across it� Confirm seal chamber pressure is at least 345 kPa (50 psi) above the

fluid’s vapor pressure� If the flush system contains a cyclone separator or strainer, check

for plugged components by taking temperature readings� If the flush fluid is cooled, confirm the proper function of the cooler

by checking the temperature drop across the cooler (should beapprox. 10 to 38�C [50 to 100�F] normally) and the temperature riseof the cooling medium

� Check the temperature difference on seal reservoirs (pots) betweenbuffer/barrier in and out lines to ensure circulation to and from theoutboard seal. If there is no temperature difference, the circulationthrough the buffer/barrier circuit has halted

� Always vent seal pot systems at the highest point to ensurepumping ring circulation

� For dual pressurized seal applications install a permanentdifferential pressure gauge to ensure that the seal pot fillingpressure is not excessive, which will force open the inner seal

We recommend that the above predictive maintenance (PDM)guidelines be followed for all centrifugal pumps in your facility at thefollowing times:

� At pump commissioning� After the pump operating conditions are normalized� Whenever the plant condition monitoring results indicate a change

(in vibration, temperature, seal leakage, noise, etc.)� Immediately after seal replacement

Lessons LearnedFailure to monitor centrifugal pump performance and theeffect of the process conditions on pump flow rate are themajor contributors to centrifugal pump mechanical com-ponent failure (seals, bearings, wear rings and impeller).

Most plant condition monitoring programs do not integrate cen-trifugal pump performance (operating point and produced head) withmechanical condition (vibration and temperature). Neglecting pumpperformance, in FAI experience, neglects consideration of approxi-mately 80% of the potential root causes for mechanical seal failure.

BenchmarksThis best practice has been used since the mid-1990s to recommendplant PDM practices for pumps with mechanical seals to optimize plantcentrifugal pump safety and mechanical seal reliability (MTBFs above48 and as high as 80 months).

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Increased P2Decreased P1Decreased S.G.

Increased Pump Flow:

Decreased Pump Flow:

Decreased P2

B e s t P r a c t i c e 8 . 1 1 Pump Mechanical Seal Flush Best Practices

B.P. 8.11. Supporting MaterialSeal vendors design the seal balance ratio and select face ma-terials based on a parameter known as the PV to be able tochange the fluid to a vapor approximately 3/4 of the way downthe seal faces (see Figure 8.11.1).

Fig 8.11.1 � Mechanical seal primary face vaporization point

Increased P1Increased S.G.

Fig 8.11.3 � Process effects on centrifugal pump flow

As can be seen in Figure 8.11.2, head required is a function ofthe pressure differential across the pump flanges and inverselyproportional to the specific gravity of the pumped fluid.Therefore, process changes of P2, P1 and/or S.G. will changethe pump flow in any centrifugal pump if a process controlsystem is not present or operational (failed or in manual mode).These facts are presented in Figure 8.11.3.

In Figure 8.11.1, three distinct operating modes are shownfor the primary ring (on the left) and the mating ring (on theright).

- The top mode shows a condition of early vaporization whichcan occur on light fluids, or hot fluids or where seal chamberpressure is lower than designed (note: seal chamber pressure

494

is designed to be at least 345 kpa (50 psi) or above the sealfluid vapor pressure).

- The middle figure shows the desired design condition ofchanging the fluid to a vapor approximately 3/4 of the waydown the faces. This is the design basis since it is known thatfluid characteristics, temperature and/or pressure willchange during operation a certain amount. Excessive changesin any or all of these parameters will lead to seal failure.

- The last mode shows the case of no vaporization andapparent seal failure. In reality, this is not a failure but onlya seal fluid condition change that does not allow the seal fluidto reach the fluid vapor pressure between the seal facesbefore it exits the seal.

As can be seen from Figure 8.11.1, the ability to achieve theobjective of vaporization 3/4 or 75% down the seal faces (seecenter case in Figure 8.11.1) depends on the following:

- Seal fluid characteristics e cleanliness, specific heat, vaporpressure and viscosity

- Pressure of the seal fluid in the seal chamber- Temperature of the seal fluid entering the seal chamber

What determines the condition of items listed above? Theprocess. Therefore, effective mechanical seal condition moni-toring requires that all seal process conditions, as noted aboveare considered.

Fig 8.11.2 � Head required by lower density fluid

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Fig 8.11.4 � Typical centrifugal pump head vs. flow curve

1. The EROE flow range is + 10% and – 50% of the pump best ef fi ciency point (BEP) fl ow

2. All ‘ bad actor ’ pumps (more than one component failure peryear) should be checked for EROE

3. To determine that the pump is operating in EROE: • Calculate the pump head required • Measure the fl ow • Plot the intersection of head and fl ow on the pump shop test

curve

Fig 8.11.5 � EROE facts

1. Measure motor amps and calculate power 2. Record control valve position, valve differential pressure, fl uid

S.G. and calculate valve flow (pump flow)3. Measure pump pipe differential temperature and calculate

pump ef fi ciency 4. Obtain an ultrasonic fl owmeter to measure fl ow 5. For items 1 and 3, locate the calculated value (power or

ef fi ciency) on the pump test curve to determine pump fl ow

Fig 8.11.6 � Available pump flow determination options

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 1 1

EROE (equipment reliability operatingenvelope) determination

Figure 8.11.2 shows the effect of the head required by theprocess on centrifugal pump flow rate. As noted inFigure 8.11.3, process changes will vary the flow of any cen-trifugal pump.

If the centrifugal pump flow is too high or too low, hydraulicdisturbances will be present that can change the pumped fluidpressure and/or temperature. Since the majority of mechanicalseal applications use the pumped fluid in the seal chamber, theseal chamber pressure and/or temperature will be affected. Ascan be seen in Figure 8.11.1, these changes will directly impactmechanical seal life and reliability.

Figure 8.11.4 shows a typical centrifugal pump head vs. flowcurve with the following items noted:

- The ‘desirable region’ of operation e heart of the curve orEROE

- Regions of hydraulic disturbances e on the upper portion ofthe curve

- The pump components affected e on the lower portion ofthe curve

The ‘heart of the curve’ is the flow region for any centrifugalpump that will be free of hydraulic disturbances and where theseal fluid should be free of vapor if the seal fluid conditionsstated on the pump and seal data sheets are present duringpump field operation.

This flow region is also called the EROE e the equipmentreliability operating envelope

Figure 8.11.5 presents facts concerning the EROE.In many pump installations, neither a flow meter nor a suction

pressure gauge is installed. A calibrated suction pressure gaugecan be installed in the suction pipe drain connection (alwayspresent). Be sure to obtain a MOC (management of change) and

work permit, and any other plant required permission prior toinstalling a suction pressure gauge as the pumped fluid could besour (H2S), flammable and/or carcinogenic.

If a flow meter is not installed, Figure 8.11.6 defines theoptions available to determine the pump flow so the EROE canbe obtained.

The flow values in Figure 8.11.6 can be determined by handcalculations using the equations available in any pump text(power equation and pump temperature rise equation). It can beseen that the EROE will provide a reasonable guide that usuallywill eliminate the hydraulic disturbances that could cause seal

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B e s t P r a c t i c e 8 . 1 1 Pump Mechanical Seal Flush Best Practices

chamber pressures and temperatures to change and lead topremature seal wear and/or failure. Note that the stated EROElow flow range can be reduced if the pump or fluid have any ofthe characteristics noted in Figure 8.11.7.

Temperature gun close to seal gland – be sure to monitor off ofa non-reflective surfaceSurface contact thermometer on flush line – add 5 C tomeasured value if pipe and use recorded value if SS tubingInstall a thermometer in the flush lineUse an infrared camera to record temperature and temperature

°

Pumps with suction specific speeds > 8,000 (customary units)Double suction pumps Water pumps with low NPSH marginFluids with S.G.< 0.7

Fig 8.11.7 � Factors that can reduce low flow EROE range

Therefore, we always recommend that the first step in sealcondition monitoring is determination of pump operation withinits EROE. If the ‘bad actor’ pump is operating outside its EROE,we recommend the action shown in Figure 8.11.8.

profile

Fig 8.11.10 � Alternative methods to measure seal fluid temperatureConsult operations to determine if process changes can be made to operate in EROE De fi ne target EROE parameters for operations (flow, amps,control valve position, delta T)

Fig 8.11.8 � If a centrifugal pump is outside its EROE:

If seal reliability does not improve when operating within theEROE, further investigation is required concerning the processconditions in the seal chamber and/or flush system as noted inthe remaining sections.

Confirm process conditions are as stated indata sheets

If the ‘bad actor’ pump seal reliability, when operating in theEROE, does not significantly improve, a complete check of sealfluid conditions in the seal chamber is required. Figure 8.11.9shows the process variables that influence seal reliability.

Install a pressure gauge in the seal chamberModify the seal flush line for installation of a pressure gauge

Fig 8.11.11 � Seal chamber pressure monitoring guidelines for ‘badactor’ seals

1. Seal Chamber Temperature2. Seal Chamber Pressure3. Seal Fluid Characteristics:

CleanlinessVapor PressureViscositySpecific HeatSpecific Gravity

Fig 8.11.9 � Seal reliability and pump fluid conditions are based on:

Temperature monitoring

Since seal chamber or seal flush line pressure gauges are notusually installed on new pumps, the first place to start is with

496

seal chamber temperature. Flush plans with coolers (21, 23, 41etc.) have an option for a thermometer downstream of thecooler which can be used to determine the seal chamber tem-perature. This value must be compared to the PT (pumpingtemperature) value listed on the data sheet. If the measuredvalue does not agree with the data sheet, consult with operationsfirst to see if process changes can be made. If they cannot bemade, discuss the measured temperature with the seal vendorrepresentative. Alternative options for measuring seal fluidtemperature are noted in Figure 8.11.10.

Pressure monitoring

Unless the plant assigned machinery specialist is ‘world class’there are usually no pressure gauges in the flush line or sealchamber. Our recommendations concerning seal chamberpressure monitoring are presented in Figure 8.11.11. Themeasured seal chamber pressure must be compared to the sealvendor’s assumed value, which should be on the seal layoutdrawing. If this value is not present, the seal vendor should beconsulted. Note that the seal vendor assumed seal chamberpressure is the value that was used in the calculation of the sealPV. If the measured value does not agree with the data sheet,consult with operations first to see if process changes can bemade. If process changes cannot be made, discuss this fact withthe seal vendor representative.

Seal fluid characteristics

If all of the above mentioned items are in accordance with theseal design (EROE, seal chamber temperature and seal chamberpressure), a check of the seal fluid characteristics is required.Figure 8.11.12 presents guidelines for checking the seal fluidcharacteristics.

If the measured fluid sample parameters are not as stated onthe data sheet, consult with operations first to see if processchanges can be made. If process changes cannot be made, dis-cuss this fact with the seal vendor representative.

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Take a sample to determine if debris is presentIf seal fluid is not water or product fluid (which must meetproduct specifications) have sample analyzedSend sample results to seal vendor

Fig 8.11.12 � Seal fluid check guidelines

Orifice conditionSeal 4Throat bushing clearanceCooler conditionStrainer conditionCyclone separator conditionBuffer fluid condition (tandem seal)Buffer fluid level (tandem seal)Pumping ring conditionBuffer fluid pressure (tandem seal)

Fig 8.11.13 � Seal reliability as a function of flush system componentcondition is based upon:

The following field seal information is suggested to be exchangedwith the seal vendor for seal reliability issues:- If the pump is operating in the EROE (provide curve)- Seal chamber temperature- Seal chamber pressure- Seal fluid sample results- Seal flush system component operation confirmation- Throat bushing clearance confirmation- Seal jacket temperature change (if applicable)- Seal quench fluid pressures, temperatures and condition

(if applicable)

Fig 8.11.14 � Seal vendor field information

Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 1 2

Variance in seal chamber pressures and temperatures, if thepump is operating in the EROE, are most likely to be caused bymalfunction of components in the flush system. Figure 8.11.13defines the flush system components which can affect sealchamber pressures and temperatures.

Based on the seal flush plan used, the flush system compo-nents should be checked in the logical order e starting with thebeginning of the flush system and ending with the throat bush-ing. (The exception is flush Plan 13 which begins with the throatbushing and ends at the pump suction pipe.) FAI ‘seal mainte-

nance best practice’ requires that the flush system be com-pletely checked (including the throat bushing clearance) eachtime a seal is changed. Note that this recommendation applies toall seal configurations: single, tandem (dual un-pressurized) ordouble (dual pressurized).

Coordination with the seal vendorrepresentative

After all mechanical seal and flush system condition parametershave been measured, coordination with the seal vendor repre-sentative and/or in-house seal vendor engineer is necessary. A‘team’ approach is very effective and is being used by all sealvendors today in large gas plants, refineries and chemical plants.This approach provides immediate seal condition monitoringand troubleshooting assistance as well as on site manufacturingcapability. Suggested seal vendor field information is presentedin Figure 8.11.14.

Best Practice 8.12Best Practice 8.12

All flush Plan 52s (un-pressurized tandem seals) must becontinuously vented to flare or safe point to ensure that theseal pot is not contaminated with the inner seal processfluid which will expose the plant to a process fluid releasethrough the outer seal. A laminated warning sign isrecommended at each application of this kind.

Un-pressurized dual seals (formerly called ‘tandem’) require anopen path to flare or high point safe location to remove all seal fluidvapors (usually flammable and/or toxic) from the seal pot and directthem to a safe location.

A valve closure and/or restriction in the seal pot vent line willprevent removal of the process fluid, contaminate the seal pot safebarrier fluid and expose the plant and its personnel to a flammable ortoxic fluid release in the event of an outer (atmospheric side) sealfailure.

It is recommended that a local, laminated, warning sign be posi-tioned at the pump.

In addition, a short ‘mini information’ presentation by site rotatingmachinery specialists has been effective in making operators aware ofthis potential safety hazard.

Lessons LearnedMost plants have experienced a release of process fluidfrom dual un-pressurized (tandem) seal systems when thevent line had been improperly closed off.

Lack of awareness of dual un-pressurized (tandem) seal systemoperational principles has resulted in flammable and toxic fluid re-leases in many plants.

BenchmarksThis best practice has recommended since the mid-1980s. Since thattime, this best practice, when implemented by plant operations, hasresulted in optimum dual un-pressurized (tandem) seal safety and re-liability (MTBFs > 48 and as high as 80 months).

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B e s t P r a c t i c e 8 . 1 2 Pump Mechanical Seal Flush Best Practices

B.P. 8.12. Supporting Material

API Flush Plan 52

Dual un-pressurized seals (tandem) rely on a buffer fluid at ornear atmospheric pressure to lubricate the atmospheric sideseal. This buffer fluid is circulated via a pumping ring from theseal to the seal reservoir and back to the seal (in a closed loop).Take a look at Figure 8.12.1 for a schematic.

Plan 52 e dual unpressurized seal using synthetic buffer fluidto lubricate the atmospheric side seal. A pumping ring in the sealcirculates the buffer fluid (pressure less than seal chamber) tothe reservoir.

The reservoir is at atmospheric pressure (less than sealchamber pressure), so the leakage across the process side sealfaces migrates into the seal reservoir, and will either increasepressure, level, or both in the reservoir. Since every seal doesleak a certain amount, it is essential to have the reservoir ventedto a flare or vapor collection system. If the reservoir is allowed toreach the seal chamber pressure, the atmospheric side seal willmost likely fail (if it hasn’t already) as it is not typically designedto handle seal chamber pressure. If this is a concern in the plant,you may want to consider requesting the seal vendor to redesignthe atmospheric seal to handle maximum seal chamber pres-

Fig 8.12.1 � Plan 52

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sure. In addition, as the process side seal leaks in this flush plan,the atmospheric side seal will essentially be sealing the pumpedfluid, exposing the plant to the release of flammable and/or toxicvapors.

Monitoring of seal leaks can be done by checking the level andpressure of the reservoir, as one or both may increase in theevent of excessive leakage. The seal vendor (or support systemvendor) may supply high level and/or pressure switches whichwould alert the operators to a seal leak. It is highly recom-mended to specify this instrumentation in new projects, as it willcut down on the already high workload of operators (if the alarmsounds then the level or pressure can be personally verified inthe field).

In addition to checking for excessive leakage (pressure orlevel increase), temperature in and out of the seal can help verifyproper circulation. In a reservoir that is not cooled, there shouldbe a temperature drop of approx. 1 to 2�C (5�F to 10�F) fromthe seal outlet to the seal inlet. If there is no temperature drop(or if the temperature increases), this indicates zero or possiblyreverse circulation.

It has been our experience that Plan 52 flushes may not bevented properly (blocked in) and level may be low or at zero inthe reservoir. For these reasons, it is very important for theoperators to be trained to understand the necessity of moni-toring this system.

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Pump Mechanical Seal Flush Best Practices B e s t P r a c t i c e 8 . 1 3

Best Practice 8.13Best Practice 8.13

Always execute the following seal installation best practiceguidelines for component and cartridge mechanical sealsto achieve maximum pump mechanical seal MTBFs:� Component seal faces must be cleaned with clean cloth and

rubbing alcohol (or alcohol wipe) just before installation. Proper caremust be taken to not touch the faces after cleaning.

� Limited grease should be used on seal gland ‘O’ ring as it canmigrate to the seal faces and cause damage. Proper practice is tosparingly apply grease to ‘O’ ring groove then install it in the gland.This will ensure ‘O’ ring will stay in the groove when installing theseal.

� Use liquid soap sparingly on sleeve ‘O’ ring to assist in installation.

It is recommended that this best practice be incorporated into theplant maintenance procedure manual for mechanical seal installation.

Lessons LearnedFailure to have and implement a required mechanical sealinstallation procedure has been a principle cause of lowmechanical seal MTBFs (lower than 12 months).

BenchmarksFAI has used this best practice since the 1990s to ensure that plantmechanical seal installation procedures are in complete accordancewith mechanical seal vendors’ recommendations and to ensure opti-mum installed seal safety and reliability.

B.P. 8.13. Supporting Material

Component and cartridge mechanical sealinstallation guidelines

Component mechanical seal installation

- Refer to the vendor instruction book and obtain all requiredmechanical seal components.

- Have a clean area available for the components and ensurethat there are no finger prints, oil or grease on the primaryseal faces. Wipe with alcohol using a lint free cloth ifnecessary.

- Primary seal face flatness will have been confirmed by theseal manufacturer and should be within 1e2 helium lightbands. For this reason, the primary seal faces must be free offingerprints, oil and/or grease.

- Inspect the rotating head assembly as follows:- Secondary ‘O’ ring in pusher seal e inspect for

discontinuities, confirm proper material and durometer.Note: do not lubricate ‘O’ ring.

- Ensure that all springs (if a pusher seal) are sitting upright andhave not been dislodged from their counter-bores.

- Confirm that shaft or shaft sleeve surface finish, wheresecondary ‘O’ ring or Teflon wedge will ride, is maximum of32 rms, using a comparator.

- Clean and de-burr the shaft area and seal chamber area asrequired using a solvent that will be compatible with theprocess fluid. Note: assure all piping/tubing is isolated fromflush/quench ports. Plastic plugs need to be installed in allflush/quench after isolation of piping/tubing up to the pointof the piping/tubing reinstallation.

- Install the seal head carefully being sure not to damage thesecondary ‘O’ ring, or the ID of the primary ring on the shaft/sleeve.

- Proper spring compression check (‘working height’) e obtaindimension from seal drawing which shows the distancebetween the back of the seal retainer and the seal chamberface. Then fit the bearing bracket and shaft up against the

casing and mark (with bluing pen) the point on shaft or sleevedirectly below the seal chamber face. Remove shaft andbearing bracket from casing and mark the dimension wherethe back of the retainer sits when assembled.

- Attach seal head to shaft/sleeve and ensure that set screwsare installed on clean and undamaged shaft areas. If this is notpossible, carefully de-burr with a file and smooth out withemery cloth.

- Note: If sleeve ‘O’ ring is installed, confirm proper material,size and durometer and apply a small amount of ‘Krytox’ orequal lubrication and install. Assure shaft surface is clean andfree of any burrs and carefully install sleeve being sure ‘O’

ring is not damaged.- Install the stationary seal face into gland evenly to ensure face

is not cocked. Assure that any finger prints are removed withalcohol and lint free towel.

- Stationary ‘O’ ring checks e confirm proper material,durometer and inspect for discontinuities.

- Install gland to stuffing box using opposite and eventightening technique. Prior to doing so, a sweep of the sealchamber face is required to ensure proper sealing betweengasket/‘O’ ring on seal gland and seal chamber. A dialindicator should be attached to the shaft and the shaft will berotated to cover a complete sweep of the seal chamber face.TIR should be no more than 0.005’.

- After seal is completely installed, conduct a static pressurecheck. Consult the seal vendor for the proper pressure ifstatic limit is not indicated on seal drawing. If any leaks areobserved, consult seal vendor immediately.

- Typical installation errors:- Contaminated faces e Can affect face flatness, resulting in

excessive leakage. In addition, certain oils can set-up likeadhesives and pull material out of the carbon primary ringfaces after start-up, resulting in premature seal failures.

- Applying lubricant to secondary seals which can enterprimary seal face areas e the same results apply as above.

- Not setting proper spring compression: excessivecompression e more seal wear. Low compression e will notprovide sufficient face contact during start-up and stand byconditions allowing excessive leakage.

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B e s t P r a c t i c e 8 . 1 3 Pump Mechanical Seal Flush Best Practices

- Improper installation of mating ring into the gland resulting inexcessive leakage due to improper face contact. Note that ifbackofmating ring is visibly ‘notflat’, the same result can apply.

- Stuffing box face TIR should be checked with dial indicatoron shaft. Seal chamber face flatness should be no more than0.005’ TIR.

- Installation of gland to stuffing box not using equal andopposite tightening technique. Failure to tighten the nuts/capscrews properly can cock the mating ring, which will notallow for proper contact between the faces and result inexcessive leakage between the seal faces.

- Sleeve ‘O’ ring cut during installation, resulting in excessiveleakage between shaft and sleeve.

Cartridge mechanical seal installation

- Refer to pump vendor manual and maintenance records toensure proper cartridge seal will be used.

- Assure the seal chamber is cleaned and shaft is de-burred toremove any sharp material that could damage seal sleeve ‘O’

ring.- Assure seal chamber is isolated from all flush and quench

connections systems. Plastic plugs need to be installed in all

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flush/quench after isolation of piping/tubing up to the pointof the piping/tubing reinstallation.

- Assure that the proper durometer, material and size ‘O’ ringis supplied and lubricate seal sleeve ‘O’ ring with ‘Krytox’ orequivalent lubricant.

- Slide sleeve over shaft carefully.- Install gland to seal chamber using equal and opposite

tightening technique. Prior to doing so, a sweep of the sealchamber face is required to ensure proper sealing betweengasket/‘O’ ring on seal gland and seal chamber. A dialindicator should be attached to the shaft and the shaft will berotated to cover a complete sweep of the seal chamber face.TIR should be no more than 0.005’.

- Tighten set screws to engage shaft/sleeve by equal andopposite technique. Assure that a clean shaft surface isavailable for all set screw locations. If not, use a file andemery cloth to remove any damaged spots. Be sure tocompletely clean shaft to remove all debris.

- Completely remove setting clips. If setting clips are slid upon the gland, they can potentially fall back down to thesleeve or drive collar resulting in an ignition source (thewriter has experienced fires that have started due to thisissue).