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MAY 2004 www.pump-zone.com PUMPS & SYSTEMS32

Apump is a pump,right? Not exactly,given all the variations on the

theme. Take, for example those

pumps primarily used for trans-fer of liquefied natural gas (LNG)

and other liquefied gases. Theyrereally in a class unto themselves.

Over the years, various methods oftransferring LNG from ship to tankstorage for transfer later to a send-outsystem, or transfer directly from shipinto a regasification or send-out sys-tem have been studied, and some arealready in detailed design or underconstruction. This article focuses onthe use of submerged, electric motorpumps (SEMPs) for these types ofservices.

Unique pumps for unique duty. . .

Multistage LNG send-out pump

after removal from test stand

Submerged Motor

LNG Pumps In Send-OutSystem Service

Submerged Motor

LNG Pumps In Send-OutSystem Service

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Along with the rapid growthof the global LNG market hascome an escalating demand foradditional LNG receiving termi-nals and regasification systems

around the world. Such termi-nals, whether on- or offshore,contain gas send-out systems thatutilize SEMPs for LNG transferand pressurization. These pumpstypically feature an integral shaftwith the entire motor, bearingsand all other components com-pletely flooded with LNG.

SafetySubmerged motor pump

technology was first applied in

LNG applications in the early1960s. Since that time, SEMPShave been used in almost allLNG receiving terminals. Themain reason for their popularityis their inherently safe designcompared to that of externalmotor type pumps with dynamicshaft seals.

The motor and hydraulicsections of the SEMP are directlycoupled with a common shaft,submerged in the liquid, with nooxygen present. This means themotor is not located in the atmo-sphere in the hazardous area, andno rotating seals are required.The design almost completelyeliminates the possibility of leak-age of flammable gas into theatmosphere.

In addition to the safetyaspects, as no coupling isrequired between the motor andpump sections, there are noalignment problems normallyassociated with pumps that use

couplings. Furthermore, sincethe complete assembly is sub-merged in liquid that acts aseffective sound insulation, thesepumps operate very quietly.

Basic DesignIn a traditional, land-based

LNG receiving terminal, the sys-tem consists of a storage tank

that contains retractable (remov-able) or in-tank type pumps, anda send-out system, which con-tains vessel-mounted type high-pressure LNG pumps and vapor-

izers. This type of terminal alsoincludes a jetty where the LNGcarrier would dock and dischargeits LNG into the onshore storagetanks.

The pumps used in theonshore storage tanks are sub-merged motor, retractable types,as shown in Figure 1. For a typi-cal receiving terminal, this typepump, sometimes also referred toas a primary pump, will have aflow rate of approximately 200 to

over 400 m3

/h. This pump wouldnormally only require one or twoimpeller stages, as it only needsto transfer the LNG out of thetank and into the secondary sys-tem.

For vaporizer-feed duty, arelatively high pressure isrequired due to the high-pressuredrop across the vaporizer. Forthis application, a multistage ves-sel-mounted type pump (seeFigure 2) that can produce pres-sures up to approximately 140kg/cm2 is used.

For primary transfer pumps,the motor voltage is normally400 to 480 volts, three phase, butalso can be made at higher volt-ages, depending on the sitepower supply. The secondaryvaporizer-feed pumps are nor-mally higher power, whichrequire from 4160 to 6600 volts.Both types of pumps can bemanufactured for either 50 or 60Hz power, depending on site

requirements.The primary pump isinstalled into the storage tankthrough a discharge columnmounted inside the tank. At thebase of the column is a suctionvalve that is opened by the pumpitself. As the pump is loweredinto the tank, the valve opens,allowing the LNG to flow into

the inlet of the pump. The pumphas a seal located near its base,which allows the discharge liquidto be pumped out the top of thepump and out the top of the dis-

charge column.

Since the primary pump isinstalled in the storage tank,which is already provided with avent system, no other ventingconnection at the headplate orcolumn is required (althoughventing of the column to equal-

ize pressure to the main tank areais required prior to start-up). Theheat from the pump primarily istransferred to the pumped fluid,with only a small amount of heatbeing transferred back to the liq-uid in the storage tank.

The high-pressure secondarypump is installed in its own self-contained suction vessel, with

PUMPS & SYSTEMS www.pump-zone.com MAY 2004 33

By Steve Rush,Cryodynamics Division,Ebara International Corp.

Along with the

rapid growth

of the global

LNG market

has come an

escalatingdemand for

additional LNG

receiving

terminals and

regasification

systemsaround the

world.

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standard flanged inlet and dischargepipe connections. The vessel needsto be vented and is provided with avent nozzle at the top of the head-plate. Vessels can be made to the

ASME Section VIII code or to justabout any pressure vessel code, asrequired.

Apart from the piping connec-tions, both pump types requirepower cables to be connected attheir junction boxes located on ornear the headplate.

Primary Pump:Design Concerns

With a SEMP used for LNG, itis always important to keep in mind

that LNG is normally handled closeto its boiling point. Even just asmall increase in temperature or adrop in pressure can cause the LNGto vaporize.

As previously noted, the prima-ry pumps in the storage tank areused simply to transfer the LNG tothe send-out system. Normally, twoor three primary pumps are used,depending on total flow require-ments and the need for backup orredundancy. From a pump design

standpoint, there is no particularspacing requirement between thepump columns. Normally, the col-umn spacing will be dictated by thetank design and spacing of the pip-ing, valves, etc, at the tank top.

Care should be taken in thetank design for the location of theinlet piping to the tank. If the inletpipe is placed too close to the pumpcolumns, the warmer liquid enter-ing the tank can affect the NPSHRof the pumps when filling the tanksduring lower-level operation.

Another requirement is theneed for column venting prior tostart-up. Most columns will havedischarge piping that can be ventedto the tank top to equalize pressurebetween the tank and column; it iscrucial to ensure there are no lowspots that could trap liquid orprevent proper venting. This

Figure 1. Diagram of submerged motor, retractable (removable) type pumpused in onshore storage tanks.

Figure 2. Multistage vessel-mounted pump design for vaporizer-feed duty. PUMPS & SYSTEMS

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becomes very important at lowliquid levels, where two metersof LNG is equal to only0.09 kg/cm2 pressure. If the col-umn pressure is too high, the

lower section of the column,including the pump, can be com-pletely void of liquid, which cancreate problems during starting.

Secondary Pumps:Design Concerns

The secondary pumps arenormally installed in a bank orrow, with the suction nozzles allconnected to a common suctionheader. The discharge lines arealso normally connected to a

common line, but each pumpshould have its own dischargecontrol valve, as well as a by-passor kick-back line to allowproper starting and operation.Discharge control is extremelyimportant to allow proper start-up when operating severalpumps in parallel.

With a secondary pump, italso is critical to recognize properventing. Since the secondarypumps normally have muchhigher motor power, it is particu-larly important to take care ofany heated LNG or vapor. Ventlines should always be rising asthey leave the pump vessel andprovide good venting back to alow-pressure space. Many prob-lems with secondary pumps overthe years have been attributed topoor vent-system design.

In the secondary pump sys-tem, a phase separator, or recon-denser, is normally installed inthe suction area. This tank is typ-

ically used to allow the liquid tosettle long enough to allow anyvapor to be vented, and is alsoused to introduce LNG from theboil-off gas system to try andrecover as much of the LNG aspossible. The design of the sys-tem in this area should also betreated carefully to ensure thatthe LNG temperature is still well

into the liquid phase as it entersthe pump suction.

MotorsWith the recent interest in

offshore or remote send-out sys-tems, an important topic is theelectrical supply used to start thehigh-pressure send-out pumps.Since the motors in these pumpsare typically from 1000 to asmuch as 2300 kW, a large start-ing system is required. A cryo-genic motor is a unique design,and the starting current requiredis approximately 612 times the fullload current. It is difficult to

reduce this value because of theamount of torque required forstarting a cryogenic motor.

To reduce the starting cur-rent, soft starters, autotransform-ers and variable frequency drivescan be used with SEMPsandhave been used very successfullyin many applications. However,proper set-up of starting parame-

ters in any current reduction typestarting system is critical. To pre-vent problems, consult yourpump manufacturer.

In some offshore or remotelocations, using the LNG carrieras the primary system deliveringthe LNG directly to the sec-ondary pumps has been dis-cussed. It appears as though thebiggest obstacle in this type sys-tem is the venting and boil-offfrom the send-out system duringoperation. Using the boil-off gasto feed local gas turbine genera-tors for power, or pressurizingthe gas using compressors and

feeding into the downstream gassystem appear to be popularalternatives.

Monitor & ProtectBoth primary and secondary

SEMPs can have monitoring sys-tems installed to trend vibration.These systems typically consist ofa piezoelectric type accelerometer

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About EIC/CryodynamicsCryodynamics specializesin the design, manufac-ture and testing of sub-merged, electric motor-powered pumps andpower recovery turbinegenerators. Used in a vari-ety of land-based and

marine applications, Cryodynamics pumps are designed to handle liquefiedgasses with pumping temperatures ranging from ambient to cryogenic.

Through its affiliation with Ebara Corporation of Japan, one of thelargest pump companies in the world, Cryodynamics has developed a repu-tation for quality and flexibility, incorporating its own liquefied gas pumpengineering know-how with Ebara's centrifugal pump expertise dating backto 1912. The divisions large, modern headquarters and manufacturing plantin Sparks, NV (adjacent to Reno in Northern Nevada on the California bor-der), also is the site of one of the worlds largest and most sophisticated cryo-

genic liquefied gas pump test facilities.Cryodynamics supports its products with specialist service engineers

based in the U.S., Japan and the U.K. It is Quality System Certified to ISO9001: 2000 with a Current Continuous Registration.

For more information on Ebaras Cryodynamics Division, as well asEbaras Standard Pump Division or its Electric Motor Division, log onto:www.ebaracryo.com. Or, circle Reader Service Number 523.

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mounted to the outside of thepump casingyes, submerged inthe LNG. This is a very difficulttype of installation, and the relia-bility of these in-tank vibration

systems is not very good.For vessel-mounted secondary

pumps, the accelerometer can be

mounted outside of the suctionvessel, on the headplate. Whilenot as sensitive as a casingmount, it can be effective fortrend monitoring, and recently

has become the preferred methodfor mounting.

One of the best ways to mon-

itor SEMPs is to simply use thetrending capabilities of the dis-tributed control system (DCS). Ifmotor amperage, flow and pres-sure are checked regularly and

trended against one another, theperformance of the pump can beverified. Over time, any apparentdeterioration in performance isnormally an indication that theunit is in need of overhaul.

For protection, the pumpsshould be protected with at leastover- and under-current relays inthe electrical system out-of-spec-ification, connected to the motorpower input. A loss in amperageis an indication of loss of suction

pressure (cavitation), and anincrease in amperage is an indica-tion of a mechanical problem ora problem in the electrical wiringinto the motor. Furthermore, atypical system will contain pro-tection for low suction pressureor low liquid level, and low orout-of-specification flow and/orpressure. Newer DCS systemshave excellent capabilities for cre-ating acceptable operating para-meters for SEMPs.

ReliabilityThe reliability of a send-out

system is extremely important.Most systems have several pumpsoperating at all times, with theneed for pumps operating 24hours a day for long periods oftime.

With LNG systems, mainlybecause of the fact that the LNGis typically handled close to itsboiling point, there are severalfactors, including system design,

that are very important to consid-er when seeking to ensure a safe,reliable system. In a review ofLNG pump failures over theyears, the predominant root caus-es for failure have been as follows:

Cavitation due to poor sys-tem design or improperoperation.

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Operation at flow rates awayfrom the rated or best effi-ciency point for extendedperiods of time.

Debris or contamination inthe liquid.

Since LNG systems are nor-mally clean, the first two pointsseem to be the main causes offailure. To ensure reliable send-out systems, the designers andoperators of these systems needto be well trained and aware ofthe important issues surroundingdesign and operation.

Moreover, it is especially

important to consult with theSEMP manufacturer during ini-tial design or FEED stages, aswell as when writing specifica-

tions for equipment. As with anyapplication, proper systemdesign should result in a safe,simple, reliable and reasonablypriced installation. P&S

References

1. D. Cullen, J. Madison, HighPressure Technology, Hydro-carbon Asia, July/August2001.

2. G. Louis Weisser, ModernSubmersible Pumps forCryogenic Liquids, WorldPumps, January 1994.

3. D. Cullen, S. Rush, J. Madison,

Radial and Axial Diffusers forSubmerged Electric Motor-Driven Pumps, World Pumps,September 2000.

Steve Rush is the VicePresident of Sales and Service forthe Cryodynamics Division of Ebara International Corp (EIC).Headquartered in Sparks, NV, hesworked with this division for morethan 23 years in the design, devel-opment, testing, service and sales ofsubmerged motor cryogenic pumpsand liquid expanders. The authorof several papers and articles onvarious subjects related to liquefiedgas pump equipment manufac-tured by EIC/Cryodynamics, Rushbased this article on a presentationhe delivered for the 2004 AIChESpring National Meeting, Topical

Conference on Natural Gas Utili-zation, in New Orleans, LA, April27th. Contact him directly at:srush@ebaraintl.com

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