Page 406 - Compression Machinery for Oil and Gas

384 SECTION III Applications

safety of the public and personnel or to the equipment itself.” The standard
defines limits for H 2 S partial pressure in the gas, depending on the pH value
of the environment, beyond which special material considerations apply.

Liquid-Induced Failures
Typical upstream systems involve a large number of heaters, coolers, and sep-
arators to remove liquids from the compression stream. This equipment is
exposed to the (potentially corrosive) fluid mixtures directly from the well bore
and can experience failures that allow liquid introduction to the compressor.
Common system failure modes include separator level measurement failure
allowing liquid carryover to the compressor, and insufficient heating/cooling
allowing liquid dropout in the compressor inlet as the flow cools and is accel-
erated to high velocities. Liquid slugs, streams, or spray entering a compressor
can cause significant reliability challenges including large forces on many com-
pressor components. Although a more detailed discussion of wet gas compres-
sion is provided in another chapter, a brief overview of liquid-induced failures
in upstream applications is provided here.
Liquid slug ingestion in a reciprocating compressor can result in a very high
cylinder pressure spike if the trapped liquid volume exceeds the clearance vol-
ume in the cylinder. This catastrophic failure mode will damage rods, cross-
heads, cylinder heads, and valves. Liquid accumulation in valve pockets can
also reduce valve motion and cause valve plate failures. Finally, liquid contam-
ination of valve sealing faces can cause “stiction,” which delays valve opening/
closing events and increases impact velocities as described by Bauer [10] and
Allison and Brun [11]. Finally, ingestion of water or other non-lubricating fluids
can reduce the effectiveness of cylinder lubrication and increase wear.
Centrifugal compressors are also susceptible to increased wear and failures
due to liquid contamination. Liquid ingestion can cause large fluctuations in
compressor thrust and shaft torque. The surge margin is also affected by liquid
ingestion, potentially causing compressor surge and resulting rotor and thrust-
bearing damage [12]. Liquid in the process can also enter the compressor seals,
causing seal failures [13].

Changing Process Conditions
Changing reservoir pressure, flow, and gas composition can cause operational
challenges for both reciprocating and centrifugal compressors. As reservoir
pressure drops, varying pressures may require respringing of reciprocating com-
pressor valves to ensure appropriate dynamic operation, and flow reductions
may require the use of cylinder unloaders to efficiently reduce flow. Centrifugal
compressors may require restaging at some intervals to continue meeting
pressure and flow requirements [14].