Page 130 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 117

something in between. The compressor shown in Figs. 3.75 and 3.76 had a
rotordynamic stability test performed in addition to a full-load, full-pressure,
full-density, and full-speed test [28].

Refrigeration

Low-temperature service is described in API 617 eighth edition as a service
where the specified minimum design metal temperature is below 244K.
Low-temperature service typically requires stringent material certifications
including impact testing. One of the colder applications is LNG refrigeration.

LNG
All LNG processes run one or more compressed refrigerants through heat
exchangers to chill the treated gas stream down to a temperature of roughly
111K, which results in liquefaction and some subcooling. One such commonly
used process is the propane-precooled mixed-refrigerant (C3MR) cycle shown
in Fig. 3.79. This process consists of two closed-loop refrigeration streams: a
propane refrigeration cycle and a mixed refrigerant (MR) cycle. The propane
refrigeration cycle cools the gas to around 238K through a heat exchanger.
It also acts to precool the MR. The MR refrigeration cycle further cools the nat-
ural gas from 238 to 111K in the main cryogenic heat exchanger (MCHE).
The multiple streams across the heat exchanger are used to increase the ther-
mal efficiency of the process. These multiple streams of the propane refriger-
ation cycle can be handled by a single propane compressor arranged with
multiple sections with incoming sideloads. Per design, the sideload compressor
is well suited for the C3MR refrigeration application where the refrigerant is
introduced at progressively lower temperatures to the lower-pressure sections.

LNG

Propane
MR vapor
Feed Propane
precooling

MR liquid
Mixed refrigerant
FIG. 3.79 C3MR process for LNG refrigeration [29].