LNG Industry - April 2020

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catastrophic failures. Three case study examples
highlight this.
A closer look at liquid expanders
The liquid expander plays a major role in the propane
precooled mixed refrigerant process (C3MR) to make
LNG. For the mixed refrigerant (MR) expander (there
is also an LNG expander downstream), a pressurised
MR stream from the heat exchanger is isentropically
expanded with a pressure drop in the MR expander
down to -130°C, so that it can be put back into the heat
exchanger for further cooling. The liquid gas expansion
is performed by the turbine portion of the liquid
expander, where work is extracted from the gas/liquid
mixture to turn the rotor, which in turn drives the
generator. As the liquid expander is a cryogenic machine,
the thermal loading on the components, and especially
on rolling element bearings, can lead to component
failure if not properly brought to load and controlled during
startup.
Different plant, same fault
At one particular LNG plant, the liquid expanders had been
operating without mishap for several years, until a bearing
fault was suddenly detected in one of the expanders during
a routine startup (Figures 1 and 2). Shortly afterwards, the
monitoring system detected a bearing fault on the same
middle bearing of another liquid expander on a different
train. In fact, the same fault was then also detected by
the monitoring system within the same time period at an
entirely different LNG plant. A diagnostic specialist was
called in from the monitoring system provider to do a root
cause analysis with the customer. After looking at the data,
it was determined that the premature failure of the bearings
was caused by a recent change in the startup sequence of
the machines.
The importance of startup
Startup is critical for a liquid expander, since the bearings
have to be precooled prior to operation. At ambient
Figure 3. Vibration sensor configuration of the propane
compressor gearbox. The gear where high vibration was detected is
shown in red.
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