A pump is a pump,retractable
right? No t exactly,
given all the variations on the
theme. Take, for example those
pumps primarily used for trans-
fer of liquefied natural gas (LNG)
and o ther liquefied gases. They’re
really in a class unto themselves.
Over the years, various methods of
transferring LNG fro m ship to tank
storage for transfer later to a send-out
system, or transfer directly from ship
into a regasificatio n o r send-o ut sys-
tem have been studied, and some are
already in detailed design o r under
co nstructio n. This article fo cuses o n
the use of submerged, electric motor
pumps (SEMPs) fo r these types o f
services.
Multistage LNG send-out pump
after removal from test stand Submerged Motor
32
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Along with the rapid growth of the global LNG market has c ome an esc alating demand for additional LNG receiving termi-nals and regasific ation systems around the world. Suc h termi-nals, whether on- or offshore,contain gas send-out systems that utilize SEMPs for LNG transfer and pressurization. These pumps typically feature an integral shaft with the entire motor, bearings and all other c omponents c om-pletely flooded with LNG. Safety
Submerged motor pump tec hnology was first applied in LNG applic ations in the early 1960s. Sinc e that time, SEMPS have been used in almost all LNG rec eiving terminals. The main reason for their popularity is their inherently safe design ompared to that of external motor type pumps with dynamic shaft seals.
The motor and hydraulic sections of the SEMP are directly c oupled with a c ommon shaft,submerged in the liquid, with no oxygen present. This means the motor is not located in the atmo-sphere in the hazardous area, and no rotating seals are required.The design almost c ompletely eliminates the possibility of leak-age of flammable gas into the atmosphere.
In addition to the safety aspe c ts, as no c
oupling is required between the motor and pump sec tions, there are no alignment problems normally assoc iated with pumps that use ouplings. Furthermore, sinc e the c omplete assembly is sub-merged in liquid that ac ts as effec tive sound insulation, these pumps operate very quietly.Basic Design
In a traditional, land-based LNG receiving terminal, the sys-tem c onsists of a storage tank
that contains retractable (remov-able) or in-tank type pumps, and a send-out system, whic h c on-tains vessel-mounted type high-pressure LNG pumps and vapor-izers. This type of terminal also inc ludes a jetty where the LNG carrier would dock and discharge its LNG into the onshore storage tanks.
The pumps used in the onshore storage tanks are sub-merged motor, retractable types,as shown in Figure 1. For a typi-c al rec eiving terminal, this type pump, sometimes also referred to as a “primary” pump, will have a flow rate of approximately 200 to over 400 m 3/h. This pump would normally only require one or two impeller stages, as it only needs to transfer the LNG out of the tank and into the secondary sys-tem.
For vaporizer-feed duty, a relatively high pressure is required due to the high-pressure drop ac ross t
he vaporizer. For this application, a multistage ves-sel-mounted type pump (see Figure 2) that can produce pres-sures up to approximately 140kg/cm 2is used.
For primary transfer pumps,the motor voltage is normally 400 to 480 volts, three phase, but also can be made at higher volt-ages, depending on the site power supply. The sec ondary vaporizer-feed pumps are nor-mally higher power, whi c h require from 4160 to 6600 volts.Both types of pumps c an be manufactured for either 50 or 60Hz power, depending on site requirements.
The primary pump is installed into the storage tank through a disc harge “c olumn”mounted inside the tank. At the base of the c olumn is a suc tion valve that is opened by the pump itself. As the pump is lowered into the tank, the valve opens,allowing the LNG to flow into
the inlet of the pump. The pump has a seal loc ated near its base,which allows the discharge liquid to be pumped out the top of the pump and out the top of the dis-charge column.
Sinc e the primary pump is installed in the storage tank,which is already provided with a vent system, no other venting c onnec tion at the headplate or olumn is required (although venting of the column to equal-ize pressure to the main tank area is required prior to start-up). The heat from the pump primarily is transferred to the pumped fluid,with only a small amount of heat being transferred back to
the liq-uid in the storage tank.
The high-pressure secondary pump is installed in its own self-ontained suc tion vessel, with
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Figure 1.Diagram of submerged motor,retractable (removable) ty pe pump
used simply to transfer the LNG to Array the send-out system. Normally, two
or three primary pumps are used,
depending on total flow require-
ments and the need for backup or
redundancy. From a pump design
standpoint, there is no particular
spacing requirement between the
pump columns. Normally, the col-
umn spacing will be dictated by the
tank design and spacing of the pip-
ing, valves, etc, at the tank top.
Care should be tak en in the
tank design for the location of the
inlet piping to the tank. If the inlet
pipe is placed too close to the pump
columns, the warmer liquid enter-
ing the tank can affect the NPSHR
of the pumps when filling the tanks
during lower-level operation.
Another requirement is the
need for column venting prior to
start-up. Most columns will have
discharge piping that can be vented
to the tank top to equalize pressure
between the tank and column; it is
crucial to ensure there are no low
spots that could trap liquid or
prevent proper venting. This
Figure 2.Multistage vessel-mounted pump design for vaporizer-feed duty.PUMPS & SYSTEMS
e
also is critical to recognize proper v nting. Sinc  th  s condary pumps normally hav  much higher motor power, it is particu-larly important to take  care  of any heated LNG or vapor. Vent line s should always be  rising as the y le ave  the  pump ve sse l and provide  good ve nting back to a low-pre ssure  space. Many prob-lems with secondary pumps over the years have been attributed to poor vent-system design.
In the  se condary pump sys-tem, a phase separator, or recon-de nse r, is normally installe d in the suction area. This tank is typ-ically used to allow the liquid to se ttle  long e nough to allow any vapor to be  ve nte d, and is also used to introduce LNG from the boil-off gas syste m to try and re cove r as much of the  LNG as possible. The  de sign of the  sys-te m in this are a should also be tre ate d care fully to e nsure  that the LNG temperature is still well
into the liquid phase as it enters
the pump suction.
Motors
With the  re ce nt inte re st in offshore or remote send-out sys-te ms, an important topic is the electrical supply used to start the high-pre ssure  se nd-out pumps.Since the motors in these pumps are  typically from 1000 to as much as 2300 kW, a large start-ing syste m is re quire d. A cryo-ge nic motor is a unique  de sign,and the starting current required is approximately 61⁄2times the full load curre nt. It is difficult to re duce  this value  be cause  of the amount of torque  re quire d for starting a cryogenic motor.
To re duce  the  starting cur-rent, soft starters, autotransform-ers and variable frequency drives can be  use d with SEMPs—and have been used very successfully in many applications. Howe ve r,proper set-up of starting parame-
ters in any current reduction type starting system is critical. T o pre-v nt probl ms, consult your pump manufacturer.
In some  offshore  or re mote locations, using the LNG carrier as the primary system delivering the  LNG dire ctly to the  se c-ondary pumps has be n dis-cussed. It appears as though the biggest obstacle in this type sys-te m is the  ve nting and boil-off from the send-out system during operation. Using the boil-off gas to feed local gas turbine genera-tors for powe r, or pre ssurizing the  gas using compre ssors and feeding into the downstream gas syste m appe ar to be  popular alternatives.
Monitor & Protect
Both primary and secondary SEMPs can have monitoring sys-tems installed to trend vibration.These systems typically consist of a piezoelectric type accelerometer
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C i r c l e  226o n  R e a d e r  S e r v i c e  C a r d
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