DRY GAS SEAL SYSTEM DESIGN STANDARDS FOR CENTRIFUGALCOMPRESSOR APPLICATIONS

John StahleyDresser-Rand, Turbo Products, Olean, NY, USA

This paper will propose a set of gas seal support system design standards for process gas centrifugalcompressors on the basis of safety, reliability, and economics.

ABSTRACT

Dry gas seals have been applied in process gascentrifugal compressors for over 20 years. Over 80percent of centrifugal gas compressors manufacturedtoday are equipped with dry gas seals.

Despite the twenty-year trend of increasing dry gasseal applications, an industry accepted standard for gasseal support system design does not exist. The AmericanPetroleum Institute (API) has only recently addressed gasseal system design in its Standard 614 (1999). This paperwill propose a set of gas seal system design standards forprocess gas centrifugal compressors on the basis ofsafety, reliability, and economics.

This paper will present the philosophy of onecentrifugal compressor and dry gas seal originalequipment manufacturer (OEM) in regards to gas sealsystem design standards. These standards are based onover twenty years of experience in the area of gas sealsystem design, drawing from actual field experience ofthousands of compressors. The reader shall recognize,however, that numerous gas seal system designphilosophies can be applied to achieve the same systemobjectives.

INTRODUCTION

Dry Gas SealsDry gas seals are available in a variety of

configurations, but the "tandem" style seal (Fig. 1) istypically applied in process gas service and is the basisfor this paper. Other types of gas seals (such as doubleopposed) are not considered. Tandem seals consist of aprimary seal and a secondary seal, contained within asingle cartridge. During normal operation, the primaryseal absorbs the total pressure drop to the user's ventsystem, and the secondary seal serves as a backup shouldthe primary seal fail.

Dry gas seals are basically mechanical face seals,consisting of a mating (rotating) ring and a primary(stationary) ring (Fig. 2). During operation, grooves inthe mating ring (Fig. 3) generate a fluid-dynamic forcecausing the primary ring to separate from the mating ringcreating a "running gap" between the two rings. Inboardof the dry gas seal is the inner labyrinth seal, whichseparates the process gas from the gas seal. A sealing gasis injected between the inner labyrinth seal and the gasseal, providing the working fluid for the running gap and

the seal between the atmosphere or flare system and thecompressor internal process gas.

Barrier SealsOutboard of the dry gas seal is a barrier seal, which

separates the gas seal from the compressor shaft bearings(Fig. 1). A separation gas (typically nitrogen or air) isinjected into the barrier seals. The primary function ofthe barrier seal is the prevention of lube oil migration intothe gas seal. The barrier seal also serves as the lastdefense in the event of a catastrophic failure of theprimary and secondary gas seal. Traditional labyrinthseals or segmented carbon ring seals are used in mostbarrier seal applications today. Segmented carbon ringbarrier seals offer the advantage of substantially lowerseparation gas flow requirements due to the larger shaftclearance associated with labyrinth seals. The author haspreviously presented a more detailed comparison ofsegmented carbon ring versus labyrinth barrier seals(Stahley, 2001).

Gas Seal Support SystemsThe use of dry gas seals requires a support system,

which is normally supplied by the centrifugal compressorOEM mounted adjacent to the compressor. The purposeof the gas seal system is as follows:� To provide clean, dry sealing gas to the faces of the

dry gas seals.� To provide clean, dry separation gas to the barrier

seals.� To monitor the "health" of the dry gas seals and

barrier seals.

SEAL GAS SUPPLY

SourceThe end user must provide a source of seal gas

supply to the compressor OEM's gas seal system. Theseal gas source must be available at sufficient pressure tocover the entire operating range of the compressorincluding transient conditions such as startup, shutdown,or idle, and all static conditions. The seal gas should be atleast 50 psi above the required sealing pressure at thecustomer connection point on the gas seal system in orderto allow adequate regulation of the seal gas. If theprimary source of seal gas does not meet thisrequirement, an alternate gas source or gas pressureboosting equipment will be required. It is very commonin the industry to source the seal gas directly from thecompressor discharge. The author has previously

discussed various implications of this approach (Stahley,2001).

Another concern is the quality and composition of theseal gas. The seal gas should be free of solid particles 10microns and larger and 99.97 percent liquid free at thecustomer connection point on the gas seal system. It isalso critically important to assess the potential for liquidcondensation within the gas seal system or the gas sealsthemselves. To avoid such condensation, API 614 (1999)requires that the seal gas temperature into the gas seal beat least 20 �F above its dew point. This is a good rule ofthumb, but may not be sufficient in some cases.

Consider an example of a hydrocarbon gascompressor with a required sealing pressure of 1,000psia. Sealing gas is process (hydrocarbon) gas, suppliedto the customer connection point on the gas seal systemat 1,050 psia. As the sealing gas flows through the gasseal system, through the primary gas seal, and finally tothe primary vent, the pressure will drop to nearlyatmospheric. A corresponding decrease in gastemperature will result from the Joule-Thomson effect. Aphase diagram for the hydrocarbon gas (Fig. 4) indicatesthe dew point for the seal gas at 1,050 psia is about 100�F. Following the API 614 (1999) recommendation of 20�F superheat would require the sealing gas to be heated to120 �F at the customer connection point. However, acomputer simulation of the seal gas pressure andtemperature drops expected throughout the gas sealsystem reveals that the seal gas will pass through themixed (gas and liquid) phase even with 20 �F superheat(Fig. 4). Further computer simulation indicates that, inorder to maintain a 20 �F margin above the seal gas dewpoint throughout the entire gas seal system, the seal gaswould need to be heated to 200 �F (i.e. 100 �F superheat)at the customer connection point (Fig. 4).

To properly evaluate potential liquid condensation, acomputer simulation of the gas seal system, from thecustomer connection point to the primary seal vent, mustbe conducted during the system design phase. A 20 �Fmargin above the seal gas dew point should bemaintained throughout the entire gas seal system. Thecomputer simulation will determine the level of seal gassuperheating required to meet this criterion. Dependingon the quality of the seal gas and the result of the systemsimulation, special liquid separation and filtrationequipment, and possibly heating of the sealing gas, maybe required. Seal gas lines should be heat traced ifambient temperatures can fall below the dew point of theseal gas.

FiltrationSeal gas filters immediately follow the customer

connection point on the gas seal system. These filtersshould be used as "final" or "last chance" filtration andrequire compliance with the gas quality requirementsexplained in the previous section for maximum reliability.Duplex filter assemblies should be employed andprovided with a transfer valve allowing filter elementreplacement while in service. The filter housing shouldbe stainless steel, as required by API 614 (1999).

Since the running gap between the primary andmating rings of most gas seals is about three to fivemicrons, it is recommended that filter elements becapable of at least three micron (absolute) filtration. API614 (1999) requires the use of coalescing filter elementsunder certain conditions. It is recommended here that, inanticipation of a possible liquid presence, coalescingfilter elements be provided for all applications. API 614(1999) requires some type of automatic liquid drainage ofthe filter housing when coalescing filters are employed.An alternative, more economical approach is to equipeach filter housing with a manual liquid drain valve. Theuser's operational procedures should include, as part ofthe compressor operator's daily routine, the inspection ofthe filter elements and removal of any accumulatedliquids as required. If the seal gas quality conforms to therequirements explained previously, liquid accumulationat the filters should be minimal during normal operation.

The duplex seal gas filter assembly should beprovided with a differential pressure gage and a highdifferential pressure alarm to indicate when the filterelement has become fouled and needs to be replaced.The filter manufacturer normally advises a differentialpressure at which the filter element should be consideredno longer useful and therefore replaced with a newelement. The high differential pressure alarm should beset accordingly. A pressure gage should also be providedupstream of the filter assembly to indicate the seal gassupply pressure (Fig. 5).

ControlThere are two basic methods of controlling the

supply of sealing gas to the gas seals - differentialpressure (�P) control and flow control. �P systemscontrol the supply of seal gas to the seal by regulating theseal gas pressure to a predetermined value (typically 10psi) above a referenced sealing pressure. This isaccomplished through the use of a differential pressurecontrol valve (Fig. 6).

Flow control systems control the supply of seal gas tothe seal by regulating the seal gas flow through an orificeupstream of each seal. This can be accomplished withsimple needle valves or, when automatic control isdesired, through the use of a differential pressure controlvalve monitoring pressures on either side of the orifices(Fig. 7). Automatic control is recommended.

The primary objective of the seal gas control systemis to assure that sealing gas is injected between the innerlabyrinth seal and the gas seal at a rate sufficient toprevent reverse flow of unfiltered process gas across theinner labyrinth seal and into the gas seal. A flow rate of16 ft/s is an industry accepted standard for sealing withlabyrinth seals. This is considered the minimumacceptable seal gas velocity for gas seal applications.Therefore, in order to assure a positive flow of seal gasacross the inner labyrinth seal, gas seal systems should bedesigned to provide a minimum gas velocity of 16 ft/sacross the inner labyrinth seal at all times. The seal gasvelocity across the inner labyrinth seal will vary withlabyrinth clearance. In order to maintain the minimum 16ft/s velocity across the inner labyrinth seal at increasedlabyrinth clearance, the system should be designed toprovide twice the seal gas velocity (i.e. 32 ft/s) at designinner labyrinth clearance. This is a conservative approachto system design that allows for increasing labyrinthclearance that may result from normal operating wear ofthe labyrinth seal.

It is also desirable to minimize seal gas consumption.The majority of the injected seal gas flows across theinner labyrinth seal and back into the compressor, andvery little flow is actually required for the gas seal. This"recycled" flow into the compressor is inefficient anduses more energy at a cost to the user. Unnecessarily highseal gas flow can also result in increased initial gas sealsystem costs, since the high flow can result in largersized, and thus more expensive, gas seal systemcomponents such as filters, valves, piping, etc. Thisadded expense becomes even more significant if specialliquid separation and/or filtration equipment is requireddue to unacceptable seal gas quality.

In order to achieve the minimum seal gas velocity of32 ft/s across the inner labyrinth seal, and to minimize theamount of seal gas consumed, flow control isrecommended over �P control systems. Since flowcontrol systems are set to maintain the flow of seal gassupply through an orifice, the supply mass flow rate isconstant and will not vary with labyrinth clearance.

To demonstrate the advantages of flow control over�P control systems, consider the following example.Using a 25 mole weight hydrocarbon mixture, a chart wasconstructed depicting the sealing gas mass flow, velocity,and differential pressure across the inner labyrinth sealfor a range of sealing pressures using both flow controland �P control systems (Fig. 8). The data for the �Pcontrol system is based on a seal gas supply pressure of10 psi over the reference pressure. The data for the flowcontrol system is based on a constant seal gas velocity of32 ft/s across the inner labyrinth seal.

As can be seen from the chart (Fig. 8), the sealing gasmass flow and velocity across the inner labyrinth seal areequivalent for flow control and �P control systems at asealing pressure of about 2,900 psi. At sealing pressuresless than 2,900 psi, the sealing gas mass flow for �Pcontrol is much higher than that of flow control at thesame sealing pressure. For example, at a sealing pressureof 1,000 psi, �P control uses about 70 percent more sealgas (mass flow) than flow control. As explainedpreviously, the excess flow consumed by the �P controlsystem is inefficient and uneconomical, and the use offlow control is recommended.

At sealing pressures greater than 2,900 psi, theamount of sealing gas consumed by the flow controlsystem is actually greater than that of �P control at thesame sealing pressure. However, the velocity of thesealing gas across the inner labyrinth seal drops belowthe minimum recommended value of 32 ft/s when using�P control at these higher pressures. This increases thepossibility of gas seal contamination from unfilteredprocess gas and therefore is a threat to gas seal reliability.For this reason, the use of flow control is againrecommended.

The relationships between sealing gas mass flow andvelocity across the inner labyrinth seal for flow controland �P control systems demonstrated above hold true forall types of process gases, shaft sizes, and labyrinthclearances. For gases of different mole weights, thesealing pressure at which the two types of controlsystems have equivalent sealing gas mass flows andvelocities simply changes inversely proportional to thechange in mole weight. For example, for a 40 moleweight gas, constructing a similar chart (Fig. 9) of sealingpressures using the same assumptions as the previous 25mole weight example, it can be seen that the equivalentpressure is about 1,800 psi, as compared to 2,900 psi forthe 25 mole weight gas. For lower mole weight gases, theequivalent pressure increases. Constructing yet another

chart (Fig. 10) for a five mole weight gas indicates thatthe equivalent pressure is "off-the-chart" and beyond thesealing pressure capability of today's gas seal technology.

As can be seen from the three charts of various moleweights and sealing pressures, the differential pressureacross the inner labyrinth seal can become quite lowwhen using a flow control system at lower sealingpressures (relative to the equivalent pressure). Lowdifferential pressures across this labyrinth could besusceptible to process upsets, increasing the possibility ofgas seal contamination from unfiltered process gas andthreatening gas seal reliability. To compensate for thiscondition, it is recommended that the flow control systembe designed to maintain a minimum three psi differentialpressure across the inner labyrinth seal. This will increasethe seal gas consumption and velocity across the innerlabyrinth seal accordingly.

As demonstrated above, flow control systems havedefinite advantages over �P control systems, and flowcontrol is therefore recommended. The gas seal systemshould be designed to provide a minimum gas velocity of32 ft/s and a minimum differential pressure of three psiacross the inner labyrinth seal at design labyrinthclearance. The application of these criteria will assure apositive flow of sealing gas across the inner labyrinthseal, reduce the risk of gas seal contamination from theprocess gas, thereby adding to increased gas sealreliability. The use of flow control also has the addedadvantage of eliminating the need for measurement of thereference (sealing) pressure from a cavity internal to thecompressor, which is required when using �P controlsystems. Accurate reference pressure measurement canbe difficult in some instances as has been discussed indetail by the author (Stahley, 2001).

The flow control system should include a "high-select" feature for the reference pressure (low pressure,downstream) side of the orifices (Fig. 7). The high selectfeature includes reference lines on the downstream sideof the orifices in the seal gas supply piping to both gasseals. These lines include check valves to prevent crossflow of seal gas from each end of the compressor and aretied together into a single line before connecting to thedifferential pressure control valve. This allows the systemto seal against the "worst case" (highest referencepressure) condition in the event that the seal gas flowsrequired by each gas seal are slightly different. The checkvalves are drilled through to allow bleeding off of thebuilt up pressure. The system should also include a

pressure gage downstream of the control valve to indicatethe seal gas supply pressure and instrumentation toinitiate an alarm when the differential pressure across theorifices falls below a predetermined value.

PRIMARY GAS SEAL VENT

Sealing gas is injected between the inner labyrinthseal and the gas seal (Fig. 1). The vast majority of thisinjected gas flows across the inner labyrinth seal and intothe compressor, or "process" side of the gas seal. A verysmall amount of the sealing gas passes through theprimary seal and out the primary vent, which is normallyconnected to the user's flare system. The gas sealmanufacturer determines the gas seal leakage rate to theprimary vent based on the specified service conditionsand seal design. Leakage rates are typically between fiveand 15 scfm depending on seal size and serviceconditions.

The primary vent can be fabricated from carbon steelpiping. The vent should be equipped with a valved, lowpoint drain to allow removal of any built-up liquids in theprimary vent area that could cause damage to the primaryseal (Fig. 11). If the primary vent is connected to a flaresystem, a check valve must be included to prevent anypotential reverse flow from the flare system into theprimary vent area, which could cause damage to the gasseal.

Primary Gas Seal HealthThe suggested method of assessment of the condition

of the dry gas seal is by monitoring the gas seal leakagethrough the primary vent. This is accomplished bymeasuring the flow or pressure across a restriction orificein the primary vent piping. An increasing flow orpressure trend is indicative of increasing gas seal leakageand suggests deterioration of the primary seal. The flowrestriction orifice (FE in Fig. 11) should be provided witha differential pressure transmitter to monitor and recordseal leakage trends. An alarm should be included toinitiate upon increasing pressure or flow above apredetermined limit. The recommended alarm level variesdepending on the gas seal manufacturer, but twice thecalculated gas seal leakage rate is a conservativeapproach.

Safety IssuesThe primary vent system must be designed to cope

with a total failure of the primary seal. It is highlyrecommended that a shutdown and depressurization of

the compressor be initiated upon the failure of theprimary seal. The secondary seal is intended to act as abackup in case of primary seal failure, providing thenecessary shaft sealing until the compressor can be safelyshut down and depressurized. Due to increased safetyrisks, operation on the secondary seal for extendedperiods of time is strongly discouraged.

A pressure sensing device, installed upstream of theflow orifice, should be used to initiate a shutdown anddepressurization of the compressor upon increasingpressure above a predetermined limit. Again, therecommended shutdown level varies depending on thegas seal manufacturer, but three times the calculated gasseal leakage rate is a conservative approach.

In the event of a catastrophic failure of the primaryseal, the primary vent is subject to a much higher gasflow, causing a backpressure in the piping upstream ofthe flow restriction orifice. A rupture disc should beinstalled in the primary vent to relieve the backpressureand evacuate the gas. The rupture disc (PSE in Fig. 11),installed in parallel to the primary vent flow orifice,should be designed to burst at about 20 psi differential(depending on normal flare system design pressure). Itshould be noted that the high differential pressure or flowalarm and shutdown limits would be exceeded before therupture disc will burst.

Before the compressor can be restarted after the gasseal has been repaired or replaced, a new rupture discmust be installed. It must be recognized that it isphysically possible to restart the compressor with thedamaged rupture disc in place. If this is allowed to occur,the instrumentation installed in the primary vent toinitiate an alarm or shutdown will be renderedineffective. The primary seal gas leakage will flowunobstructed through the void created by the burstrupture disc and a high flow or pressure will not bedetected by the instrumentation. The user must be awareof these circumstances and maintenance procedures mustbe established accordingly.

To avoid this potential safety issue, the rupture discshould be fitted with an electronic continuity detector toindicate the disc has failed, thereby alerting the operatorto avoid further startup attempts. Or, the electronic devicecan be connected into the start control system to preventstartup if the rupture disc has not been replaced. Another,less economical alternative is to use a relief valve inplace of the rupture disc. However, the reader iscautioned to note the difficulties in sizing a relief valvefor high flow, low differential pressure applications.

SEPARATION GAS SUPPLY TO THE BARRIERSEAL

SourceThe end user must provide a source of separation gas

supply to the compressor OEM's gas seal system (Fig.12). The separation gas is required for the barrier seals,which are intended to prohibit lube oil migration into thegas seal. The separation gas is fed to the barrier sealsthrough stainless steel tubing. The separation gas must beavailable at sufficient pressure as defined by the barrierseal manufacturer with enough margin to account forbuildup of pressure drop through the gas seal systemcomponents. It is very common to use instrument air asthe separation gas medium. This requires carefulattention to safety considerations, which will bediscussed later. It is highly preferable to use nitrogen asthe separation gas medium.

Compared to the main seal gas supply, the quality andcomposition of the separation gas is of lesser concern.Barrier seal tolerances are not as small as gas sealtolerances, and therefore the gas quality requirements areless stringent. However, the typical sources of separationgas (nitrogen or instrument air) are generally very cleanin comparison to seal gas sources. Therefore, theseparation gas "requirement" at the customer connectionpoint on the gas seal system is that it be free of solidparticles 5 microns and larger and 99.97 percent liquidfree.

FiltrationA separation gas filter immediately follows the

customer connection point on the gas seal system. This isagain intended as "final" or "last chance" filtrationassuming compliance with the gas quality requirementsexplained in the previous section. API 614 (1999)requires a duplex filter arrangement, but a single filterelement with stainless steel housing has been proven tobe adequate for this service. The filter should include abypass line allowing filter element replacement while inservice.

Since, as mentioned previously, the typical sources ofseparation gas are generally very clean, it isrecommended that filter element be capable of fivemicron (absolute) filtration. Coalescing filter elementsand manual drain valves should be provided for allapplications.

The separation gas filter assembly should beprovided with a differential pressure gage and a highdifferential pressure alarm to indicate when the filter

element has become fouled and should be replaced. Apressure gage should also be provided upstream of thefilter assembly to indicate the separation gas supplypressure (Fig. 12).

ControlThe supply of separation gas to the barrier seals

should be controlled using a differential pressure (�P)control system. Approximately equal parts of theseparation gas will flow through the barrier seal into thecompressor bearing chamber (outboard side), and into thesecondary seal vent area (inboard side). The �P systemcontrols the supply of separation gas to the barrier sealsby regulating the separation gas pressure to apredetermined value above the secondary vent pressure.This is accomplished with a differential pressureregulator.

The barrier seal manufacturer determines therequired separation gas pressure to the barrier seal.Typical pressure requirements are three to five psidifferential for labyrinth barrier seals and five to 10 psidifferential above the secondary vent area pressure forcarbon ring barrier seals. The separation gas supplytubing should include a gage to indicate the differentialpressure between the gas supply and the secondary vent.It is important to note that the reliability and length ofservice of the barrier seal can be greatly influenced by theabsolute value of the separation gas pressure. The designof the system must take into consideration the maximumpressure that can be accepted by the barrier seal withoutcreating abnormal wear of the seal itself. The barrier sealmanufacturer must provide the maximum pressure vs.seal life characteristic.

It is vitally important to the reliability of the barrierseals and gas seals that lube oil is only supplied to thecompressor bearings when proper separation gas pressureexists. This can be assured with the following controls(Fig. 13):� An alarm is required if the differential pressure

between the separation gas supply and the secondaryvent falls below a predetermined level.

� If proper separation gas pressure is lost duringoperation (rotation) of the compressor, a delayedshutdown is recommended. If low separation gasdifferential pressure is detected, a shutdown shouldbe initiated after about 30 minutes. This will giveoperators time to attempt to reestablish properseparation gas supply and minimize the effects of oilmigration to the gas seals. When compressor shaft

rotation has come to a complete stop, lube oil flow tothe bearings must be halted. If this is not possible dueto turning gear requirements or for concern of heatsoak into the bearings, an emergency nitrogen supplymust be supplied to provide the required sealingduring these conditions.

� If proper separation gas pressure is lost while thecompressor is static (not rotating), lube oil flow tothe bearings must be immediately halted.

� Proper separation gas pressure must be confirmedbefore proceeding to provide lube oil flow to thebearings. A "permissive - start" the lube oil pumps isrequired.

SECONDARY GAS SEAL VENT

Reviewing overall seal operation, sealing gas isinjected between the inner labyrinth seal and the gas seal.A very small amount of the sealing gas passes through theprimary seal and out the primary vent. An even smalleramount (typically less than 0.1 scfm) of sealing gaspasses through the secondary seal and out the secondaryvent. The majority of the flow through the secondary ventis separation gas which has passed through the barrierseal (Fig. 1).

The secondary vent can be fabricated from carbonsteel piping. The vent system should be equipped with avalved drain at its lowest point to allow removal of anypotential lube oil carryover from the bearings. Thesecondary vent should be vented to atmosphere. If thesecondary vent is connected to a flare system, a checkvalve must be installed to prevent any potential backflowinto the vent, which could cause damage to the secondarygas seal and/or barrier seal. The system design must alsoconsider the possible increased flow to the compressorbearing housing and/or coupling guard area to avoidinterfering with the normal venting of these areas fromtoo high a pressure supplied to the barrier seal.

Secondary Gas Seal HealthIt is difficult to monitor the health of the secondary

seal. Unlike the primary seal vent, a flow or pressuremeasurement of the secondary vent is of little value forassessing the health of the secondary seal. Since the vastmajority of the gas flow through the secondary vent isinjected separation gas, measurement of this flow is notrepresentative of secondary seal performance.

As explained by the author (Stahley, 2001), thebiggest threat to the reliability of the secondary seal iscontamination from bearing lube oil. Therefore, an

evaluation of lube oil migration into the secondary sealcavity may provide the best means for monitoring thecondition of the secondary seal as well as theeffectiveness of the barrier seal. This can beaccomplished by routine inspection of the low point draininstalled in the secondary vent piping. The presence ofincreasing amounts of lube oil in the secondary vent drainover a period of time is indicative of deteriorating barrierseal performance. Progressive lube oil contamination willlead to degradation of the secondary seal. Conversely, ifprimary vent pressure or flow is normal, and thesecondary vent drains are dry, it is usually safe toconclude that the secondary seal and barrier seal are in anacceptable operating condition.

Safety IssuesIt is highly recommended that a nitrogen source be

employed for separation gas. If a nitrogen source is notreadily available, the user should consider the use ofspecial nitrogen generation equipment to avoid thecomplications arising from the use of air as the separationgas medium. A safety issue arises when air is used as theseparation gas for the barrier seal. Under this condition,it is possible to create an explosive mixture in the sealsystem secondary vent when air mixes with combustibleprocess gas. Combustion can occur within the secondaryvent if a certain gas to air mixture exists (i.e. within theexplosive levels) and a source of ignition is introduced.The worst case scenario is a catastrophic failure of theprimary seal, while the secondary seal remains intact.Under this condition, the secondary vent would beexposed to higher levels of sealing gas leakage, up to themaximum primary seal leakage rate advised by the gasseal manufacturer.

The explosive levels for a given gas will varydepending upon its components and the expected amountof gas seal leakage to the secondary vent (advised by thegas seal manufacturer), so the potential for explosivemixtures must be evaluated on a job-by-job basis.According to the Gas Processors Suppliers Association(1987), in general, a hydrocarbon gas and air mixture ispotentially explosive if between the range of about onepercent to 15 percent hydrocarbon gas by volume. Ifanalysis determines the existence of an explosivemixture, the issue must be addressed through the designof the separation air system. The system can be designedto create a "lean" or "rich" environment in the secondaryvent.

In a lean system, a quantity of air is injected into thesecondary vent to insure that the gas to air mixture is

below the lower explosive level (LEL) of the gas (i.e. theratio of gas to air is too low to allow combustion). Thisis accomplished by bypassing separation air from thesupply piping directly into the secondary vent piping(Fig. 14). It is also possible to include bypass ports withinthe secondary seal housing itself to bypass separation airdirectly from the barrier seal into the secondary ventcavity in the compressor. In a lean system, the secondaryvent can be routed to atmosphere. A conservative designapproach is to bypass enough separation air to create anenvironment of no more than 50 percent of the LELunder worst case (primary seal failure) conditions.

In a rich system, a quantity of process gas is injectedinto the secondary vent such that the gas to air mixture isabove the upper explosive level (UEL) of the gas (i.e. theratio of gas to air is too high to allow combustion). Thisis accomplished by bypassing process gas from the sealgas supply piping directly into the secondary vent piping(Fig. 14). A conservative design approach is to injectenough process gas to create an environment at least fivepercent more gas by volume above the UEL under worstcase conditions.

A rich system increases the amount of process gas tobe vented and environmental issues will probably requirethat the secondary vent be connected to the user's flaresystem. Connecting the secondary seal vent to a flaresystem when using an air separation system presentsfurther safety issues and is not recommended. A flaresystem upset could possibly create a reverse flow in thesecondary vent, forcing process gas into the compressorbearing cavity, and hence into the lube oil system. Thiscould create an explosive environment. It must also berecognized that an increase in separation air flow, such ascould be expected if the barrier seal were to malfunction,could alter the gas composition in the secondary vent,resulting in an explosive mixture (below the UEL).

It is ultimately the user's decision if the system isdesigned to run rich (above the UEL of the gas) or lean(below the LEL of the gas). A rich system will greatlyreduce the overall separation air consumption, but willincrease process gas consumption and increase theamount of hydrocarbon gas routed to the secondary vent,creating a potentially dangerous environment. A leansystem increases the overall separation air consumption,but will decrease process gas consumption and theamount of hydrocarbon gas routed to the secondary vent.These factors must be evaluated by the end user based onthe specific project conditions before the system designcan be finalized. The use of nitrogen for separation gas isagain highly recommended.

SUMMARY

The author has addressed the four main componentsof gas seal systems in detail and proposed designstandards for each:� Supply of sealing gas to the dry gas seals� Primary seal vent system� Separation gas supply to the barrier seals� Secondary seal vent systemThe gas seal system design standards proposed herein aresummarized in a single diagram for easy reference (Fig.15). Also provided is a tabulation of recommended gasseal system alarm, shutdown, and permissive-startconditions (Table 1).

CONCLUSION

The purpose of this paper was to put forth to thecommunity of process gas centrifugal compressor users adesign standard for dry gas seal support systems. Thedesign suggestions and recommendations presented arebased on the experience of one manufacturer of bothcentrifugal compressors and dry gas seals. Theserecommended design practices will provide the user withan effective, reliable, safe, and economical dry gas sealsupport system.

The gas seal system design recommendationsproposed in this paper are applicable to the industry'smost common arrangement of a beam-style compressorwith tandem dry gas seals. These system standards are"typical" and may need to be modified for different typesof compressor and/or gas seal arrangements, but the basicdesign philosophies are applicable to most applications.Each project should be evaluated on a case-by-case basisto assure an appropriate gas seal system is applied.

REFERENCES

API 614, 1999, "Lubrication, Shaft-Sealing, and ControlOil Systems and Auxiliaries for Petroleum, Chemical,and Gas Industry Services", Fourth Edition, AmericanPetroleum Institute, Washington, D.C.

Gas Processors Suppliers Association, 1987, EngineeringData Book, Tenth Edition, Gas Processors Association,Tulsa, Oklahoma.

Stahley, J. S., 2001, "Design, Operation, andMaintenance Considerations for Improved Dry Gas Seal

Reliability in Centrifugal Compressors", Proceedings ofthe Thirtieth Turbomachinery Symposium,Turbomachinery Laboratory, Texas A&M University,College Station, Texas, pp. 203-207.

ACKNOWLEDGMENTSThe author wishes to acknowledge the following

individuals for their assistance in reviewing this paper:David Shemeld, Norman Samurin, Dan Lowry, MichelRabuteau, George Talabisco, Ron Fox, Jr., and Jill Roulo.I also thank Dresser-Rand for allowing me to publish thisdocument.

LIST OF FIGURES:

Figure 1 - Tandem Dresser-Rand Gas Seal / Barrier Seal Configuration

Figure 2 - Dresser-Rand Dry Gas Seal Components

Figure 3 - Grooves in Dresser-Rand Gas Seal Mating Ring

Figure 4 - Typical Seal Gas Phase Diagram

Figure 5 - Duplex Gas Seal Filter Arrangement

Figure 6 - Differential Pressure Control

Figure 7 - Flow Control

Figure 8 - Inner Labyrinth Seal Gas Flow (25 Mole Weight Gas)

Figure 9 - Inner Labyrinth Seal Gas Flow (40 Mole Weight Gas)

Figure 10 - Inner Labyrinth Seal Gas Flow (5 Mole Weight Gas)

Figure 11 - Primary Gas Seal Vent Arrangement

Figure 12 - Barrier Seal Filter Arrangement

Figure 13 - Separation Gas Supply

Figure 14 - Secondary Gas Seal Vent Arrangement

Figure 15 - Gas Seal Support System

Table 1 - Alarm and Shutdown Conditions

Figure 1 - Tandem Gas Seal / Barrier Seal Configuration

INNERLABYRINTH

SEALBARRIER

SEAL

SEAL GASSUPPLY

PRIMARYVENT

SECONDARYVENT

SEPARATIONGAS SUPPLY

PRIMARYGAS SEAL

SECONDARYGAS SEAL

BEARINGSIDE

PROCESSSIDE

Figure 2 - Dry Gas Seal Components

Figure 3 - Grooves in Gas Seal Mating Ring

Figure 4 - Typical Seal Gas Phase Diagram

0

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80 100 120 140 160 180 200 220

Temperature (deg. F)

Pres

sure

(psi

a)

Seal Gas Dew Line Seal Gas with 20 deg. F Superheat Seal Gas with 100 deg. F Superheat

Mixed (Gas / Liquid) Phase Gas Phase

Figure 5 - Duplex Gas Seal Filter Arrangement

PDI

PDAH

PI

SEAL GAS SUPPLY� Clean & dry (free of particles 10 microns &

larger and 99.97% liquid free)� 50 PSI above sealing pressure� Superheated as required

To control system(Fig. 6 or 7)

Figure 6 - Differential Pressure Control

Seal gas reference fromcompressor ("process side"as shown in Fig. 1)

To compressor seal gassupply, high pressure ordischarge end

Seal gas from filters (Fig. 5)

To compressor seal gassupply, low pressure orintake end

PDAL PI

PDIC

Figure 7 - Flow Control

Seal gas reference

Seal gas from filters (Fig. 5)

To compressor seal gas supply, lowpressure or intake end

PI

PDIC

PDAL

FEFE

To compressor seal gas supply, highpressure or discharge end

Figure 8 - Inner Labyrinth Seal Gas Flow (25 Mole Weight Gas)

0

20

40

60

80

100

120

140

160

180

100 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Sealing Pressure (PSI)

Gas

Vel

ocity

(ft/s

)

0

5

10

15

20

25

30

35

Gas

Flo

w (l

b/m

in)

Diff

ernt

ial P

ress

ure

(psi

)

Velocity Using Differential Pressure ControlVelocity Using Flow Control Mass Flow Using Differential Pressure ControlMass Flow Using Flow ControlDifferential Presure Across Inner Laby Using Flow Control

32 ft/s

Figure 9 - Inner Labyrinth Seal Gas Flow (40 Mole Weight Gas)

0

20

40

60

80

100

120

140

100 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Sealing Pressure (PSI)

Gas

Vel

ocity

(ft/s

)

0

10

20

30

40

50

60

Gas

Flo

w (l

b/m

in)

Diff

ernt

ial P

ress

ure

(psi

)

Velocity Using Differential Pressure ControlVelocity Using Flow Control Mass Flow Using Differential Pressure ControlMass Flow Using Flow ControlDifferential Presure Across Inner Laby Using Flow Control

32 ft/s

Figure 10 - Inner Labyrinth Seal Gas Flow (5 Mole Weight Gas)

0

50

100

150

200

250

300

350

400

100 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Sealing Pressure (PSI)

Gas

Vel

ocity

(ft/s

)

0

2

4

6

8

10

12

Gas

Flo

w (l

b/m

in)

Diff

ernt

ial P

ress

ure

(psi

)

Velocity Using Differential Pressure ControlVelocity Using Flow Control Mass Flow Using Differential Pressure ControlMass Flow Using Flow ControlDifferential Presure Across Inner Laby Using Flow Control

32 ft/s

Figure 11 - Primary Gas Seal Vent Arrangement

PDI PDIT

PAHH

To flare

FEPSE

From compressorprimary seal vent area

Low point drain

Figure 12 - Barrier Seal Filter Arrangement

PDI

PDAH

PI

SEPARATION GAS SUPPLY� Clean & dry (free of particles 5 microns &

larger and 99.97% liquid free)

To control system(Fig. 13)

Figure 13 - Separation Gas Supply

Secondary vent referencefrom pipe

Separation gasfrom filters (Fig. 12)

To compressor barrierseal, high pressure ordischarge end

PDI

PDAL

Secondary ventfrom compressor

Low pressure alarm & permissive tostart lube oil pumps

PDALL Delayed shutdown

To compressor barrierseal, low pressure orintake end

Figure 14 - Secondary Gas Seal Vent Arrangement

To ventFrom compressor barrierseal, low pressure or intakeend

Low point drain

RO RO

From seal gas supply orseparation gas supply(if required)

From seal gas supply orseparation gas supply(if required)

From compressor barrierseal, high pressure ordischarge end

Figure 15 - Gas Seal Support System

Seal gas supply

Gas balance

Primary gas seal vents(Fig. 11, typical, both vents)

Secondary gas seal vents (Fig.12)

Seal gas filters(Fig. 5)

Flow control(Fig. 7)

Separation gas filters (Fig.12)

�P control (Fig.13)

Separation gas supply

Table 1 - Alarm and Shutdown ConditionsParameter Alarm

ConditionShutdownCondition

PermissiveStart

Seal gas filter differential pressure High NASeal gas supply flow Low NAPrimary vent pressure or flow High HighSeparation gas filter differential pressure High NASeparation gas supply differential pressure Low Low

(delayed)Required

Refer questions to:Contact D-R