Compressor Seals for Hydrogen Recycle Service

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FEATURE

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IntroductionHydrogen recycle compressors are used in a vari-ety of refinery processes including catalyticreforming (platforming), hydrocracking, andhydrotreating. While these different processeshave unique operating pressures, temperaturesand catalysts, many basic elements are commonto them all. Since hydrotreating to remove sul-phur is currently an important topic in refining,a simplified process schematic of this process isshown in Figure 1 as an example.

Hydrotreating A liquid feedstock is sent to a heater by a chargepump. The liquid is heated by a combination of heat exchangers and a fired heater, but noexchangers are shown to simplify the diagram.The heated charge is combined with hydrogenrich recycle gas and fed to a reactor. Within thereactor, in the presence of a catalyst, sulphurcompounds are decomposed to form a hydrocar-bon and hydrogen sulphide (H2S). Other reac-tions can take place as well. Oxygen compounds

are converted to hydrocarbons and water.Nitrogen compounds are converted into hydro-carbons and ammonia (NH3). Olefins and otherunsaturated hydrocarbons are saturated by theaddition of hydrogen, producing stable hydro-carbons. Metallic impurities are absorbed physi-cally onto the catalyst surface.

The reactor effluent is then cooled and flowsthrough a series of gas/liquid separators. A water wash of the eff luent may be included. Inthis example, a steam stripper removes gas andnaphtha leaving the low-sulphur product. Thegas from the final separator flows to a H2Sscrubber where liquid amines are used toremove H2S from the recycle gas. Gas from thescrubber enters the inlet of the recycle compres-sor. Since hydrogen is consumed by the chemi-cal reactions, a makeup compressor supplies

additional hydrogen to the loop. This compres-sor is typically a reciprocating unit.

Seal design challengesSeal design for hydrogen recycle compressors has

always been a challenge to the compressor origi-

nal equipment manufacturer (OEM), and a

potential source of downtime for the user. Some

of the seal design challenges are:

• High pressure, up to 2500 psi.• Rotordynamic considerations – the low

molecular weight of hydrogen recycle gas

demands many stages (a long rotor) and

high rotational speeds for just a moderate

pressure rise. Seals can contribute to vibra-

tion problems on an already sensitive rotor.

• Venting of flammable process gas.• Contamination from process gas.

While all these considerations are important,it is the contamination issue that causes mostseal-related shutdowns.

Types of seals used inhydrogen recycle service

Dry gas seals A dry gas seal uses gas, typically the process gas,as a sealing medium. The gas leakage to atmos-phere is minimized and controlled by a small,self-regulating gap between a rotating seal ring and a stationary seal ring. A balance of spring forces, hydrostatic forces, and hydrodynamicforces acting on the stationary seal ring controlsthe width of this gap. The hydrodynamic forces

are generated by a pattern of shallow grooves inthe sealing surface of the rotating seal ring. A secondary seal , typical ly an O-ring, is

located behind the stationary seal ring to pre-vent gas from bypassing the gap. Gas pressure isbroken down across one or two sets of seal ringsand the leakage is vented to the flare system. A

Sealing Technology May 2003

Compressor seals for

hydrogen recycle serviceBy Stephen L. Ross and M. Theodore Gresh, Elliott Turbomachinery Company Inc, Jeannette, Pennsylvania, USA and Robert M. Kranz, Phillips Petroleum Company, Belle Chase, Louisiana, USA

Hydrogen recycle service has presented a special challenge for centrifugal com-pressors and particularly the casing seals, mainly because of contamination fromprocess gas. This feature, reproduced with permission from the Turbomachinery Laboratory, reviews a simplified process to identify the sources of contaminantsaffecting both dry and wet seals. It lists precautions for the application of dry seals in this service, and discusses improvements to materials used for a wet seal

design. Finally, it highlights two case histories where retrofits with the improved liquid film seal have significantly reduced sour seal oil leakage rates.

Figure 1. Simplified hydrodesulphurization process.



6Sealing Technology May 2003

second or third set of seal rings serves as a back-up in the event of damage to the primary set(Figure 2).

Gas requirementsTo function properly, dry gas seals require a steady flow of gas that is both clean and dry.This is provided by a buffer gas system that con-sists of filters, a regulating device, and instru-mentation. The amount of gas supplied to theseals is determined by supplying it at a pressure5–15 psi above the pressure to be sealed, or sup-plying a certain minimum flow. In either case,the excess flow of filtered gas returns to theprocess across a labyrinth seal inboard of the gasseal cartridge.

FailureDry gas seals have proven to be very reliable inmost services, but have a much higher incidenceof failure in high-pressure hydrogen recycle ser-vice. In some applications, mean time betweenfailures is as low as 3–18 months. The exactcause of failure is difficult to determine because a

damaged seal contaminates itself, ranging fromcarbon dust to catastrophic break up of rotating rings. External contaminants are almost alwayspresent and likely failure scenarios can be theo-rized. These include:

• Oil and dirt from the process gas filling upthe shallow lift groves in the rotating ring until lift is lost and the rotating and station-ary faces contact.

• Oil and dirt from the process gas fouling the secondary O-ring seal so it will notallow the stationary ring to axially track therotating ring. Eventually, the rotating and

stationary faces contact.• Liquid breaking up the stable gas film

between the rotating and stationary facesuntil they contact.

Precautions Any of these scenarios can happen as the result of inadequate filters, insufficient flow of cleanbuffer to the seals, or a complete lack of cleanbuffer during startup/shutdown of the compres-sor. The contaminants can come from cylinderlubricating oil of the make-up gas compressor,chlorides in the process, liquid carryover fromupstream vessels, or condensation of the buffergas.

The following precautions should be consid-ered when applying dry gas seals in hydrogenrecycle service:

• Prefilters or a mechanical separator toremove larger quantities of liquid upstreamof coalescing filters.

• Differential pressure control of buffer gas with buffer gas supplied at least 15 psiabove the pressure to be sealed. This is toensure an adequate supply of clean gas tothe seal area.

• Use of an air-operated booster compressorto supply a flow of filtered buffer gas to the

seals at times when the recycle compressor isnot running

• Heat tracing of buffer gas supply lines toprevent condensation from occurring

Oil sleeve seals An oil sleeve seal functions on the principle of maintaining a film of oil in a floating bushing ata slightly higher pressure than the process gasthat is being sealed. Oil is injected at a very pre-cisely controlled pressure between a gas side sealand breakdown bushing(s). By maintaining anoil pressure of 5–10 psi above the gas pressure,the process gas is prevented from leaking to

atmosphere. Close clearances minimize theamount of oil that leaks inward to the contami-nated oil trap. The volume of oil flowing through the seal cavity both lubricates and cools

the seal. The seal rings are typically lined with a bearing-type tin- or lead-based babbitt to avertshaft wear.

LeakageH2S and/or chlorides in the process gas attack the babbitt, and the resulting buildup of corro-sion products reduces clearances. The reduced

clearance results in localized overheating of thetin-based babbitt and “washing out” of the bab-bitt. The buildup of corrosion products and lack of babbitt also results in scoring of the shaftsleeves. The end result is increased clearance andextremely high leakage rates for the sweet oil as well as contaminated oil.

Oil supply flow rates will increase until eitherpump capacity is exceeded or pressure losses inthe pipes become equal to the design oil to gasdifferential pressure. In either case, an oil pres-sure higher than the process gas pressure cannotbe maintained and gas escapes to atmosphere.Life of the seals can be as short as six months to a

year, depending on the particular situation.

Resolution of oil sealproblemsIt is obvious that protecting the seal parts with a clean buffer gas, although desirable, is not realis-tically feasible as the H2 loop is the highest pres-sure in the plant. Resolution of the oil seal prob-lem involves upgraded seal parts with animproved babbitt material to resist the chemicalattack of the H2S and chlorides in the processgas and hardened shaft sleeves to resist scoring.

Improving the babbitt material A decision was made to use gold for the babbittmaterial. Gold is chemically resistant to bothchlorides and H2S, while it has a very soft andmalleable nature like tin- or lead-based babbitt.The gold babbitt also has a significantly highermelting temperature, preventing the babbittfrom “washing out” during a rub. In general,

FEATURE

Figure 2. Typical dry gas seal.

Driver Steamturbine

Speed (RPM) 12 300

Inlet pressure (PSIA) 1723

Inlet temperature (°F) 150

Discharge pressure (PSIA) 2185

Discharge temperature (°F) 203

Power (HP) 4039

Molecular weight 5.7

Capacity (ICFM) 1625

Table 1. Operating conditions forCase Study A.



metals tend to get very soft and putty-like beforethey reach their melting temperature, like butterthat sits out in room temperature for a long peri-od of time. Typically this occurs at about 70% of the melting temperature in degrees Fahrenheit.For tin or lead babbitt, this “soft” region begins

at around 350°F. For gold, this soft region isestimated to be around 1200°F, as the melting temperature is 1950°F.

Hardening the shaft sleeves A high velocity oxygen fuel (HVOF) spray coat-ing of tungsten carbide with a nickel binder was

recommended for the shaft sleeves. The coating is applied to the assembled rotor, and then the

sleeve outside diameters are finish ground tomaintain size and concentricity with the journal

bearing surfaces.

CleaningFor a retrofit application of upgraded seals, it isstrongly suggested to thoroughly clean the entireseal oil system to remove any accumulation of dirt and corrosion products. This cleaning

should include the reservoir, any other vesselssuch as rundown tanks, and all piping. Flushing a clean charge of oil through the system is notsufficient. Cleaning with a chemical agent ispreferred.

Case study A

Background A small hydrogen recycle compressor operating at an oil refinery in Oklahoma reported having seal problems. Contaminated seal leakage rates were three barrels a day and higher. The timebetween seal parts changeouts was between six to 12 months. Seal parts that were removedlooked overheated and a gritty black substance was found on the surface of all the parts. Bothgas side seal and breakdown bushings werefound to be wiped and/or scored. The compres-sor inner barrel was covered with a black oily residue and salt deposits. The journal bearings

were found to have some of the same black deposits. Operating conditions are given inTable 1.

Problem resolutionIt was obvious from the seal condition thatimproved materials were necessary, as cleanbuffer gas, although desirable, was not realisti-cally feasible. Resolution of the seal problem in July 1998 involved a thorough chemical clean-ing of the oil system, including the overheadtank that controlled the seal oil to gas differen-tial. New seal bushing parts with gold babbitt

were installed. The inboard labyrinth seal, locat-ed between the process and the oil seal assembly, was made from carbon-filled PEEK (polyether-etherketone) thermoplastic instead of alumini-um. The spare rotor was reworked with tung-sten-carbide-coated shaft sleeves and installed.The seal configuration is similar to Figure 3.

These changes resulted in leakage rates on theorder of 1 to 2 gal/day/seal. Four years later the

original upgraded seals are still in service. According to the user, leakage rates are stil l“very low.”

Case study B

Background

Four hydrogen recycle compressors operating atan oil refinery in Louisiana had been having sealproblems for many years. Contaminated sealleakage rates were several barrels a day and high-er. The time between seal parts changeouts was18–24 months. Chemical attack of the babbitthad forced a change to cast-iron bushing parts with no lining at all in the bores. Wear and scor-ing of the shaft sleeves required completeremoval of the compressor bundle and replace-ment with a refurbished spare rotor wheneverseal leakage rates became too high. At times,raised metal of the sleeve required the use of a cold chisel to allow removal of the bushings. The

basic seal design dated from the 1970s and wasdifficult to assemble. Missing or pinched O-rings often led to startup delays. In operationthe compressor internals would foul with chlo-rides and require frequent water washing tomaintain performance. Operating conditionsare given in Table 2.

Problem resolution A cartridge design seal assembly was suggestedby the user and agreed to by the compressormanufacturer. This would allow the seals to bebench assembled and checked prior to installa-

tion, thus eliminating concerns about damagedor missing O-rings. All the material upgradesused in Case Study A were applied (refer toFigures 3–6).

Tungsten carbide spray coating Additional ly, tungsten carbide spray coating was applied to the vertical surfaces of the hous-ings where they are contacted by the floating


FEATURE

Compressor string 1 LP 1 HP 2 LP 2 HP

Driver Steam turbine Same Steam turbine Same

Speed (RPM) 8640 8640 10 370 10 370

Inlet pressure (PSIA) 233 439 550 965

Inlet temperature (°F) 100 100 100 130

Discharge pressure (PSIA) 449 768 975 1565

Discharge temperature (°F) 228 217 246 273

Power (HP) 2467 2315 2142 2143

Molecular weight 7.7 7.0 5.5 5.5

Capacity (ICFM) 2420 1265 817 495

Table 2. Operating conditions for Case Study B.

Figure 3. Upgraded two bushing oil sleeveseal for Case Study B. (a) PEEK labyrinth seal;(b) gas side seal bushing; (c) atmospheric sidebushings; (d) hardened shaft sleeve; (e) oilbypass orifice; (f) contaminated seal oil drain;(g) seal oil supply; (h) Sweet oil drain.

Figure 4. Upgraded single bushing oil sleeveseal for Case Study B. (a) PEEK labyrinth seal;(b) gas side seal bushing; (c) atmospheric sidebushing; (d) hardened shaft sleeve; (e) oilbypass orifice; (f) contaminated seal oil drain;(g) seal oil supply; (h) sweet oil drain.



bushings. This coating provides two related ben-efits. The hardness of the coating minimizes thechance of the bushing brinelling the surface of the housing. This has occasionally happened onhigh-pressure seals. If the damage is severe, itcan prevent the bushing from floating with the

shaft and cause it to act like a bearing, creating subsynchronous rotor vibration. The other ben-efit is the smooth surface finish of the coating will reduce the coefficient of friction betweenthe bushing and the housing; again helping thebushing to float and remain centered on theshaft.

Enlargement of oil passagesOil passages were enlarged on the new design toreduce internal pressure drops of the supply oiland to promote better drainage of the contami-nated or sour oil. The contaminated drain was a particular concern to the user, as the traps

would no longer be vented. The series of eightradial slots in the drain area was replaced by twolarge bottom slots aligned with the casing drainand two slots in the top that connect to an

equalizing line with the overhead seal oil tank (Figure 5).

Bypass passage Another new feature was the addition of an

internal seal oil bypass passage. In the past, all

the sweet oil flowed across the atmospheric side

bushings. The amount of oil required to keep the

seals cool was controlled by the clearancebetween the bushing and the shaft sleeve. The

bushing bore was a standard diameter, and the

outside diameter of the shaft sleeve was ground

to various dimensions to achieve the necessary

clearance. For these seals all bushings are the

same and all shaft sleeves are ground to the samediameter. If this clearance provides enough flow

for cooling, the bypass passage is plugged. If

more oil is required, an orifice (minimum 1/16

inch diameter) is fit into the bypass passage. This

allows for simple adjustments if process condi-

tions are changed (Figure 6).

Installing the new cartridgesDuring a scheduled turnaround at the end of October 2001, all the affected compressors andturbines from the refinery were sent to two of the manufacturer’s service shops. The compres-sors receiving upgraded seals were disassembled,cleaned, and inspected. Any worn or damagedparts were replaced. The casings were machinedto add larger annuluses in the seal oil feed areas. Any restrictions in internal oil passages wereremoved by grinding. Vents were drilled into thesweet oil drain cavities, as these were not includ-ed in the original design. Other modifications to

bearings and couplings were made as well. Thespare rotors had been reworked previously andat-speed balanced by the manufacturer.

The only problem encountered during instal-lation of the new seal cartridges was difficulty aligning slots on the inboard side of the seal car-tridge with existing antirotation pins in the com-pressor cases. This alignment was done “blind”and was further complicated because some of thepins had been relocated from top-dead-center tonew holes 5 degrees to the left or right.

Performance

After the compressors were completely assem-bled, the gas side seals were fed oil at the samedifferential pressure they would operate at in thefield. The leakage rate appeared reasonable. Actual leakage was expected to be better as thebushing would be more concentric to the sleeve,and the windback groove would be effective inreducing oil flow due to shaft rotation. The windback groove feature had been in use since1984.

Before startup, the oil system was cleaned by circulating heated oil with a high flow pump. Inthe future, hydroblasting or chemical cleaning would be used.

The first two compressors were started 16November 2001. The new seals continue tooperate with very low leakage. One seal wasrecently measured at 3.9 gal/day. This is better

than expected for seals of this diameter with anoil to gas differential pressure of 8 or 9 psid.

Conclusions• Hydrogen recycle gas can contain a number

of contaminants harmful to seal operation.Many of these contaminants are not identi-

fied to equipment designers.• Dry gas seals in hydrogen recycle service

require more than the standard buffer sys-tems that are acceptable for most other ser-vices.

• Oil film seal reliability can be improved by the proper selection of materials suitable forhydrogen recycle gas. Gold babbitt resistscorrosive attack by H2S and has a higheroperating temperature limit. Tungsten car-bide coated sleeves resist wear and maintaindesign clearances. Carbon-filled PEEK labyrinths resist corrosive elements that canattack aluminium.

• Clean oil systems are necessary for properseal operation. A simple oil flush is inade-quate, chemical cleaning is preferred.

• Cartridge seal design makes for easierassembly.

Further reading API Standard 617 (1995) Centrifugal Compressors for Petroleum, Chemical, and Gas Service Industries , Sixth Edition, AmericanPetroleum Institute, Washington, DC.Carter, D. R. (1988) Application of dry seals on a

high pressure hydrogen recycle compressor. InProceedings of the Seventeenth Turbomachinery Symposium , Turbomachinery Laboratory, Texas A&M University, College Station, Texas, pp 3–8.Staff Report (2000) Refining Processes 2000,Hydrocarbon Processing 79(11) 85–143.

AcknowledgementsThe authors would like to recognize ElliottCompany and Phillips Petroleum Company forpermission to publish this paper, Joe Brittain forhis contributions to the project for Case Study B, the many individuals at Elliott and PhillipsPetroleum who assisted in implementing theproject, and Raymond Long for his assistance with the illustrations.

Contact:

Stephen Ross, Senior Service Engineer, Elliott

Turbomachinery Co Inc, 901 North Fourth Street,

Jeannette, PA 15644-1473, USA.

FEATURE


Figure 5. Inboard view of upgraded oil sleeveseal cartridge showing thermoplasticlabyrinth seal (front) and contaminated drainand vent slots.

Figure 6. Outboard view of upgraded oilsleeve seal cartridge showing windbackgroove (bore) and oil bypass passage (slot attop).

Reproduced with permission from theTurbomachinery Laboratory (http://turbolab.tamu.edu). From Proceedings of the 31st Turbo-

machinery Symposium , Turbomachinery Labora-tory, Texas A&M University, College Station,TX 77843-3254, USA, pp 85–89. © 2002.

Compressor Seals for Hydrogen Recycle Service

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