R. J. AIMONE, W. E. FORSTHOFFER ANDR. M. SALZMANN

The first article in this series (p. 20,Jan./Feb. 2007) discussed best prac-tices for Dry Gas Seal (DGS) selectionand design. This article will cover:

Seal gas conditioning Seal gas control Primary vent systems

The source of seal gas may be the com-pressor discharge, an intermediate pressurepoint, or an external process or inert gas, or thecompressor discharge. For the majority ofapplications, the seal Gas Conditioning Unit(GCU) will consist only of filtration (5microns absolute) and moisture-removalequipment. The best practice is to use dualcoalescing filters and automatic drainers withhigh level alarms. However, two special situa-tions may require additional gas conditioning.

Seal gas conditioningCryogenic nitrogen (N2 that has beenliquified) can damage the carbon station-ary faces during slow-speed operation turning gear ratcheting or slow roll when the faces are in contact. CryogenicN2 is typically very dry, with a dew pointas low as -90C (-130F). But the self-lubricating quality of carbon is based onthe ability of its crystalline structure toadsorb and hold certain gases, includingwater vapor, which significantly reducerubbing friction [1].

In the absence of water vapor, carbonhas poor lubricating properties, and canwear rapidly. Therefore, dew point con-ditioning is required whenever carbonstationary elements are used in eitherface or circumferential seals, when rub-bing contact is anticipated for extendedperiods. For large steam or gas turbine-driven compressors that require slow rollfor extended periods below the DGS lift-off speed, the best practice is to condi-tion the N2 upstream of the coalescingfilter system, raising its dew point to -30C (-22F), or higher.

Methods to increase N2 dew pointinclude mixing saturated nitrogen from abubbler chamber with cryogenic nitro-gen in an appropriate ratio, or mixing moistair with cryogenic nitrogen, keeping theoxygen content below 5%. A dew-pointmonitor and low-dew-point alarm arerequired for safe operation.

Saturated seal gas, either in the system orentering downstream of the GCU, exposesthe DGS to liquid condensation and carry-over into the seal chamber and between theDGS faces. The risk of seal damage is highwhen liquid enters the area between thefaces. The best practice is to assure that thegas is superheated to approximately 15C(27F) above the gas condensing tempera-ture at the lowest operating pressure in theprimary vent. The addition of a heater to theGCU may be sufficient for this purpose.However, if the required temperature risecould cause polymerization, a cooler, separa-tion vessel and re-heater may be required [2].If the seal gas contains C6+ components,they must be identified and individually con-sidered in determining saturation conditions.

Regardless of the application, an alter-nate source of seal gas is required duringstart up or shut down, when the processcompressor is not delivering sufficient pres-sure, or to back up the independent seal gassource. This alternate seal gas must meet allof the requirements enumerated above forthe primary seal gas. N2 may be used if thesystem can tolerate it. If N2 is not accept-able, an amplifier unit (pressure booster)may be necessary when process gas is notavailable at sufficient pressure.

Reciprocating piston or diaphragm com-pressors are currently the most commonchoice, however small dry rotary screwshave been used successfully and are poten-tially more reliable. In mega-plant applica-tions, or where justified by potential rev-enue loss, dual amplifier units are recom-mended.

Seal gas controlThe DGS control system design depends onthe type of seal being used, and must con-sider all the anticipated operating condi-tions. This section will focus on double andtandem DGS applications (Figures 1, 2 arenot complete P&IDs). Regardless of thedesign of the seal gas control system, theoption to trip the machine based on mon-itored seal parameters and other parameters depends on the HazOp review, and eco-nomic evaluation of the consequences of theshutdown for each plant.

Double seals can help simplify the sealgas control system, minimize the quantity ofseal gas, and optimize system reliability. Asnoted in the previous article, double seals arenormally applied where an inert seal gas (usu-ally N2), which is compatible with theprocess, is available at a pressure exceedingthe maximum process pressure at the sealinterface (to prevent a seal pressure reversal).If N2 from a regulated system is used, the sealgas control valve can be eliminated (Figure 1).

If the process gas is sour, a sweetbuffer gas must be injected between theprocess labyrinth and DGS to prevent sourgas contact and potential DGS fouling.Differential pressure control is typicallyused. Flow control is also an acceptableoption, provided the flow is sufficient tomaintain a velocity of 15 m/sec (50 ft/sec)through the process labyrinth at twice themaximum design clearance.

Tandem seals are the most commonDGS application, and are required when aninert gas is either not available at sufficientpressure or not compatible with theprocess. The traditional control arrange-ment is to maintain a differential of 35 kPa- 70 kPa (5 psid -10 psid) over the balanceor equalization chamber pressure, usingone Pressure Differential Control Valve(PDCV). This arrangement is adequatewhen the sealing pressure is greater thanthe maximum pressure that can occur inthe primary seal vent cavity.

However, this control scheme can exposethe primary seal to possible pressure reversalsin low-pressure services, where the maxi-

OOIILL && GGAASS

Dry gas seal systems - part 2BEST PRACTICES FOR DESIGN AND SELECTION, WHICH CAN HELP PREVENT FAILURES

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Figure 1: Above is a best practice double DGSsystem. Differential pressure control of thebuffer gas is used. A seal gas control valve is notrequired because the system N2 headerpressure is regulated

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mum cavity pressure of the primary vent canexceed the sealing pressure. While the flareheader pressure is normally low (7 kPag - 21kPag or 1 psig -3 psig), the maximum designflare pressure that can exist during a majorupset, or during an Emergency Shut Down(ESD) can range from 140 kPag - 340 kPag(20 psig - 50 psig).

Therefore, the maximum pressure in theprimary seal vent cavity can be equal to themaximum flare pressure plus lossesthrough check valves, orifices, and pipingin the vent line. For services with low, orsub-atmospheric suction pressures, evennormal conditions in the primary ventcan cause a reverse differential on the pri-mary seal, unless the system design pre-cludes this possibility.

A significant number of DGS failures inrecent years have occurred in low-suction-pressure refrigeration, and other services.Figure 2 shows a system that has been usedsuccessfully in applications with low suc-tion pressure. A PDCV at each seal controlsthe seal gas pressure to the inlet cavity at anominal value of 35 kPa -70 kPa (5 psi - 10psi) above the higher of the following val-ues; reference gas pressure and primaryseal vent cavity pressure measuredupstream of the vent orifice. This is accom-plished through the use of a high signalselector device at each seal, which assuresthat the primary seal will always have apositive differential of at least 35 kPa - 70kPa (5 psid - 10 psid), even if the primaryvent pressure increases significantly duringan upset or ESD.

Another approach that has been usedsuccessfully for large machines, where the

seal gas flow represents a relatively smallrecirculation loss, is to use one PDCV forboth seals. In this scheme, the sensing pointfor the primary seal vent pressure is movedto a location in the primary vent that is com-mon to both seals. The differential pressurecontrolled by the PDCV is then set highenough (typically 200 kPa - 350 kPa or 30psid - 50 psid) to assure that it will alwaysbe greater than the maximum pressure dropacross the orifice, piping, and any othercomponent between the sensing point andthe primary seal-vent cavity. This pressurealso provides for the maximum allowableprimary seal gas leakage flow.

Regardless of whether one or twoPDCVs are used, the best practice is toassure that the control valve always oper-ates within the acceptable valve coefficient known as CV range. For large unitswhere seal gas flows can vary widely, anorifice can be installed in parallel with thePDCV for normal flow conditions. In thisarrangement, the PDCV will remain closedunless flow conditions dictate otherwise,and the smaller valve will always operate inthe acceptable CV range (10% - 90%). Thekey element in low-pressure service is tomonitor the primary seal differential, as

well as the process labyrinth differential,and control the seal gas pressure based onthe higher of the two values to assure that aprimary seal pressure reversal is not possi-ble under any anticipated circumstances.

Primary vent systemsThe term primary vent has traditionallybeen applied to tandem seal applications.For double seals, there is only one ventbetween the DGS and the separation seal(Figure 1). Since this vent will normally notcontain hydrocarbons, it is generally notconnected to the flare system, and may notalways be monitored. Therefore, for doubleseal applications, the best practice is to mon-itor and alarm, or trip the system if N2 inletflow to the seal assembly increases.Operators of large, high-revenue plants maydecide to trip only when outer seal failure isimminent. High N2 flow plus high outer sealflow may be used to trip these units.

For tandem seals, the primary vent,which normally contains hydrocarbons, isconnected to the flare system and is instru-mented to monitor the health of the prima-ry seal (Figure 2). This system must pro-vide the operating team with sufficientinformation to monitor the condition of theprimary seal, maintain safe operation of thecompressor for the longest possible time,

and safely shut down the compressor whena dangerous condition exists.

Below are best practices for the prima-ry vent:

Locate the primary seal chamber ventat the bottom (6 oclock) position on theseal cartridge, with a low point drain inthe vent piping adjacent to the machine.This will permit checking for liquids inthe primary vent chamber. The requiredpiping configuration, and when the drainwill be opened, should be determined dur-ing the HazOp review.

Intermediate labyrinths with N2 injec-tion are recommended for all tandem seals.To prevent masking the primary seal leak-age, flow control of intermediate N2 is thebest practice. (Details concerning the N2injection system for the intermediate seallabyrinth will be covered in the next article.)

A flow measuring device is recom-mended in the primary vent to monitorflow. The device may be an adjustable ori-fice but tamper-proof, with a set pointlock or cover or a fixed orifice. Orificesizing is to be based on the expected leak-age from the primary seal plus the interme-diate labyrinth flow. Measuring primaryvent flow to facilitate troubleshooting is

always recommended, regardless of whatparameters will be used for alarm and trip(flow or pressure).

Use low-pressure, loss-flow elementsthat measure loss of low pressure to moni-tor seal leakage and intermediate labyrinthflow rates. Piping should be generouslysized to minimize primary vent cavity pres-sure in the event of a primary seal failure.Seal cavity vent openings, internal to thecompressor, should be checked so that theyare not undersized.

If a complete primary seal failure cancause the pressure in the primary vent cavityto exceed the pressure that can be deliveredby the N2 system, the best practice is torelieve the vent pressure below the N2 sys-tem pressure. A weight-loaded, stem-guided,full-flow disc valve is recommended. A rup-ture disc can randomly fail and will require aunit shutdown for replacement.

A spring-loaded check valve is recom-mended in the piping at the flare header con-nection. This prevents the back flow of flaregas into the primary vent chamber, andmaintains a positive pressure differentialacross the secondary seal. The check valve isnormally designed to exert a minimum backpressure of 35 kPag (5 psig) in the primaryvent cavity, assuring a minimum 35 kPad (5psid) positive differential over the secondary

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Figure 2: Tandem seal systems can be designedto avoid failures due to sub-atmospheric suctionpressures

In low-pressure service the seal gas pressureshould be controlled to ensure that a primaryseal pressure reversal is not possible . . .

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seal. However, the required secondary seal pressure differentialshould be determined by the seal vendor, based on anticipated turn-ing gear operation and seal lift-off speed.

Primary vent systems have traditionally been provided withpressure or flow instrumentation usually Triple ModularRedundant (TMR) to trip the compressor unit on a significantincrease in pressure or primary vent flow rate. With the increaseof process unit size, and corresponding increase of daily revenueloss, many end-users are reconsidering this approach. Decisionsregarding machinery trips must ultimately be made by the operat-ing team for each plant as early as possible in the project (pre-Front End Engineering Design phase). The goal is to avoid spuri-ous trips and unscheduled shutdowns, while maintaining the safe-ty of the plant and the integrity of the machinery.

Specifying shutdownsDGS are precision components, and significant transient varia-tions can occur in monitored parameters. Therefore, the best prac-tices for shutdown are: Set trip levels as high as reasonably possible, but below the min-imum available N2 pressure Always use TMR (including wiring and logic) for trip functions Consider requiring more than one parameter to exceed allowablesettings to initiate an automatic trip Require provisions for monitoring the condition of the secondaryseal, as well as the primary seal, to permit action in the event of itsdeterioration or failure

References:[1] Paxton, R. Robert, "Manufactured Carbon: A Self-Lubricating Materialfor Mechanical Devices," CRC Press, Inc., 1979.[2] Forsthoffer, W.E., "How to prolong dry gas seal life", TurbomachineryInternational, Sept./Oct. 2005

Footnotes:The next article will focus on the best practices for intermediate and sepa-ration gas systems and discuss best practices forcondition monitoring of DGS systems.

No responsibility is assumed by the authors for anyinjury and/or damage to persons or property as a mat-ter of product liability, negligence or otherwise, orfrom any use or operation of any methods, products,instructions or ideas contained in these articles.

Authors:Robert Aimone has 47 years experience in themachinery field; in operations, maintenance,design, specification and troubleshooting. He spentover 30 years on the technical staff of Mobil OilCorporation and the last six years as president ofhis own consulting firm, REMO.

William Forsthoffer has over 36 years experiencein the turbomachinery industry as a designer(DeLaval), facilities engineer (Mobil), and fieldtroubleshooter and trainer. In 1990, he foundedForsthoffer Associates Inc., (FAI), a turbomachin-ery consulting firm.

Dick Salzmann has 44 years in the turbomachineryfield, including 36 years with Delaval Turbine Inc.and its successors. His experience covers machin-ery application, design, testing, training and trou-bleshooting. Salzmann has been consulting for thelast 6 years with FAI.

TI

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