Page 439 - Compression Machinery for Oil and Gas

418 SECTION III Applications

dry gas seal buffer systems, torquemeters, and vibration monitoring systems.
Any fit up or assembly problems can be resolved during the string test which
is preferred over correcting these issues in remote field locations. Auxiliary
and control systems can be tuned, thus minimizing these activities in the field.
For extremely long compressor trains, with several compressor bodies, a gas
turbine driver and large VFD helper/starter motor, it is also possible to measure
torsional critical speeds during the string test if a torsiograph connection has
been provided.

Auxiliary Compressors
Besides the main refrigerant compressors there are other smaller compressors
for auxiliary duties in an LNG Plant. The number and size of these compressors
depend on the LNG process and in some cases, on feed gas conditions. Some of
these compressor services might include feed gas booster compressors, CO 2
compressors, end flash compressors, and boil of gas (BOG) compressors.
The BOG compressors are usually installed close to the LNG tank, in order
to recover gas vapors that boil off from the LNG tank. The BOG is usually
driven by an electric motor, and the compressor may be a horizontally split
machine or an integral compressor design. The most critical aspects of this com-
pressor are the very low operating temperature ( 120°C/ 161°C) and the
design aspects associated with it in terms of materials, dry gas seals, inlet guide
vanes and in the case of some integral-type compressors, diffuser guide vanes.
In some LNG processes, the BOG compressor may have to compress the BOG
from close to atmospheric pressure to fuel gas pressure which in the case of
some aeroderivatives can range from 40 to 60bar. In these cases, centrifugal
multistage machines may be used and variable IGVs are often used for flow
control.
End flash service can be accommodated with an electric motor-driven hor-
izontally split compressor with a gear box and barrel compressor as needed.
CO 2 in an LNG plant is not used as part of the process, but may be removed
from the natural gas, compressed and transported to be stored in an appropriate
geological formation. The gas handled by the compressors is not pure and may
contain other hydrocarbons, water and H 2 S which would constitute an acid gas.
Material choice should therefore be NACE MR0175/ISO15156 compliant. In
acid gas environments, dangerous phenomena such as stress corrosion cracking
(SSC) and simple corrosion can be present jeopardizing the life of the compo-
nents. The material selection is of critical importance and in some cases the final
choice is to use all stainless steel for compressor components (casing, dia-
phragm, and rotor). CO 2 compressors can be beam type or integrally gear type.
Often the pressure ratios are high of the order of 100:1, as the initial CO 2 pres-
sure is a little above atmospheric pressure, and the final discharge pressure has
to meet the reinjection requirements. Recent studies have combined compressor
with pumps to increase final discharge pressure.