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R. J. AIMONE, W. E. FORSTHOFFER AND

R. M. SALZMANN

So far in this series we have dis-cussed seal selection, seal gas con-ditioning, seal gas control and pri-mary vent systems. We encourage

readers to visit www.turbomachinery-mag.com to obtain the previous two arti-cles in this series. This final article willdiscuss best practices for: Intermediate labyrinths for tandem DryGas Seal (DGS) systems Vent systems between the seal cartridgeand separation seal Separation seal systems DGS system monitoring and protectionoptions

Specifying nitrogen useIn most cases, nitrogen is used as theintermediate and separation seal gas. If asite N2 source is not available, anotheralternative is to manufacture a low-grade,non-cryogenic N2 gas (minimum N2 con-tent of 95%), using instrument or plantair. Process designs and commercial unitsfor N2production are available and have

been used successfully.Regardless of the gas used, the best

practice is to filter all the gas for both sys-tems using dual coalescing filters (5micron absolute), to assure that there isno free moisture. If cryogenic nitrogen(N2 that has been liquefied) is used, car-

bon face damage on the DGS can occurduring slow-speed operation when thefaces are in contact, or damage to radial-contact carbon seals can result (part 2, p.24, March/April 2007).

The best practice is to condition theN2 upstream of the coalescing filter sys-tem and raise its dew point to -30C (-22F) or higher. Specific details of the

plants nitrogen system should be dis-cussed with seal suppliers in the pre-feed

project phase if contact-type separationseals are used, or if extended periods ofturning gear operation are anticipated.

Intermediate labyrinths for tandemDGS systems: Since double seal applica-tions use N2 as the seal gas (Figure 1),only N2 will flow through the atmospher-ic side seal. Therefore, an intermediateseal is not required.

As stated in part 1 of this series (p. 20,

Jan./Feb. 2007), an intermediatelabyrinth is always recommended for tan-dem DGS applications. The intermediatelabyrinth assures that N2 is always pre-sent between the seals, and limits theflow of process gas to the secondary sealin the event of a primary seal failure.

The best practice is to use flow con-trol, complete with local flow indicator,to each seal. The goal is to achieve avelocity of 50 ft/sec through the interme-diate labyrinth, without masking the pri-mary seal leakage (Figure 2).

Vent systems between the DGS car-tridge and separation seal: Usuallyreferred to as the secondary vent, its pur-

pose is to direct the gas present betweenthe DGS (tandem or double arrangement)and the separation seal to a safe location.Most importantly, conditions in this ventcan provide information on the health ofthe outer seal (secondary seal for tandemarrangements and atmospheric seal fordouble seals) and the separation seal.The majority of installations do notspecifically monitor the condition of thesecondary or outer seal. Undetected fail-ure of the secondary or the outer seal

exposes the plant to a process gas releasein the event of primary or inner seal fail-ure. If process gas can blow through theseparation seal and into the bearing hous-ing, a catastrophic equipment failurecould occur. Therefore, the best practiceis to monitor the condition of the sec-ondary or outer seal by measuring one ofthe following (Figures 1, 2): High pressure in the secondary vent -suggested setting: 1 kpag - 2 kpag (5inches - 10 inches water column) Low pressure differential between theseparation seal inlet and secondary vent

pressure (if the separation gas is con-trolled at a fixed pressure) Low pressure in the primary seal vent

this assumes that a pressure of 30 kpag- 40 kpag (5 psig) is normally maintained

between the primary and secondary seals(see part 1)

The decision to alarm or trip willdepend on the application and the potentialdaily revenue loss of the plant. Since thisvent can also contain oil or oil mist in theevent of a separation seal system malfunc-tion, the best practice is to monitor theeffectiveness of the separation seal by locat-ing the vent in the seal chamber at the low

point (6 oclock position), and installing adevice to indicate oil contamination (levelglass as a minimum), with a drain valve toa safe location in the vent line.

Separation systems: Regardless ofthe type of seal configuration (double ortandem), the function of the separationsystem is to prevent process gas fromentering the bearing housing in the eventof a seal failure, and oil from entering theseal cartridge (See Figures 1, 2 for sys-tem details). Entrance of process gas intothe bearing housing exposes the plant tocatastrophic consequences and extendeddowntime.

There are several types of separationseals. The choice depends on the avail-ability of the separation gas (usually N2).The alternatives, arranged in order ofhighest usage of separation gas, are: Labyrinth seals Abradeable labyrinth seals Non-contact carbon seals Segmented carbon contact seals

The best practice is to use labyrinth orabradeable labyrinth separation seals, ifsufficient N2 is available. This recommen-dation is based on the reliability of

OIL & GAS

Dry gas seal systems -part 3BEST PRACTICES FOR INTERMEDIATE AND SEPARATION GAS SYSTEMS, DGS

MONITORING AND PROTECTION OPTIONS

24 Turbomachinery International May/June 2007 www.turbomachinerymag.com

Figure 1: An intermediate seal is not required

for double seal applications as they use N2 as

the seal gas and only N2 will flow through the

atmospheric side seal

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labyrinth-type seals compared to carbonseals, and the fact that the differential pres-sure across labyrinth seals is not limited,as is the case for most carbon ring seals.

If carbon ring seals are used, the con-trol system must limit the differential

pressure to the design maximum. In addi-tion, if carbon contact seals use cryogenic

N2, the best practice is to condition the N2(described in part 2).

Experience shows that in the case of acatastrophic seal failure, there is a possi-

bility that process gas could enter thebearing housing through the separationseal. For this reason, the best practice isto individually vent each of the bearinghousings to a safe location.

The method of separation gas con-trol depends on the type of seal select-ed. For labyrinth and abradeablelabyrinth seals, the best practice is touse differential pressure control sealsupply pressure minus secondary vent

pressure to each seal. For carbon ringseals, pressure control could limit themaximum differential pressure acrossthe carbon rings.

The condition of each separation sealcan be determined by monitoring andalarming on low differential pressure forlabyrinth and abradeable labyrinth seals.For carbon ring seals, monitoring andalarming on low pressure is recommend-ed. These parameters should be used as

permissive signals to prevent starting theoil system if N2 gas is not being suppliedto the separation seals.

Monitoring and protectionThe majority of current DGS installa-

tions trip on primary seal failure by mea-suring primary vent pressure or flow. Inthe case of a high-value product, whereavailability of the plant is critical, oper-ations may choose to continue runningfor short periods after the primary sealfails, while preparing for shutdown. Inthis case, the recommendation is to tripon indicated primary AND secondaryseal failure (atmospheric side for doubleseals). Trip options are based on the typeof seal configuration and secondary seal.They are: Tandem seals trip on high-high

primary seal flow or pressure ANDhigh secondary vent pressure Tandem seals with carbon contact sepa-ration seal trip on high-high prima-ry seal flow or pressure AND low N2separation gas supply minus secondaryvent pressure Double seals trip on high-high sealgas supply flow AND vent high-high

pressureFor any of the above options, instru-

ments associated with the shutdown cir-cuit should be triple modular redundant.The final decision regarding unit trips is

of critical importance to plant safety andreliability, and will require a HazOpreview for each application.

Defining the seal systemIn concluding this series, the followingguidelines are recommended to promoteDGS system safety and reliability.

Always use a proactive approach asearly as possible in the project. Completely

define the entire system during the pre-feedphase of the project (using data sheets or aP&ID) to all quoting suppliers.

Work as a team with the compressorand seal suppliers. Listen to their recom-mendations and consider action based onlessons learned.

We sincerely hope that the DGS discus-sion and best practices presented in thisseries will be helpful in anticipating poten-tial problems, and assuring optimum safetyand reliability for all DGS installations.

Footnotes:No responsibility is assumed by the authors for any

injury and/or damage to persons or property as a

matter of product liability, negligence or otherwise,

or from any use or operation of any methods,prod-

ucts, instructions or ideas contained in these articles.

Authors:Robert Aimone has 47

years experience in the

machinery field; in opera-

tions,maintenance, design,

specification and trou-

bleshooting. He spent over

30 years on the technical

staff of Mobil Oil

Corporation and the lastsix years as president of his

own consulting firm

REMO.

William Forsthoffer has

over 36 years experience

in the turbomachinery

industry as a designer

(DeLaval), facilities engi-

neer (Mobil), and field

troubleshooter and trainer.

In 1990, he founded

Forsthoffer Associates

Inc., (FAI), a turbomachin-

ery consulting firm.

Dick Salzmann has 44

years in the turbomachin-

ery field, 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 the last

6 years with FAI.

TI

May/June 2007 Turbomachinery International 25www.turbomachinerymag.com

Figure 2: Flow control should be used to achieve a velocity of 50 ft/sec through the intermediate

labyrinth, without masking the primary seal leakage