Page 117 - Compression Machinery for Oil and Gas

104 SECTION II Types of Equipment

as surge, rotating stall, or choke. This is of special operational and safety con-
cern in mixed pipeline compressor stations where many centrifugal compres-
sors operate in series or parallel with reciprocating compressors. Over the
last 30 years, several authors have discussed the impact of piping flow pulsa-
tions on centrifugal compressor stability and specifically, on the impact on
surge margin and performance. For example, Sparks [23a], Kurz et al. [23b],
and Brun et al. [23c] provided analysis and numerical predictions on the impact
of discrete and periodic pressure pulsation on the behavior of a centrifugal com-
pressor. This interaction came to be known as the “Compressor Dynamic
Response (CDR) theory.” CDR theory explains how pulsations are amplified
or attenuated by a compression system’s acoustic response characteristic super-
imposed on the compressor headflow map. Although the CDR Theory describes
the impact of the nearby piping system on the compressor surge and pulsation
amplification, it provides only limited usefulness as a quantitative analysis tool,
primarily due to the lack of numerical prediction tools and test data for compar-
ison. In their work, Brun et al. utilized an efficient 1-D transient Navier-Stokes
flow solver to predict CDR in real life compression systems. Numerical results
showed that acoustic resonances in the piping system can have a profound
impact on a centrifugal compressor’s surge margin. However, although interest-
ing, the fundamental problem with both Spark’s and Brun’s approach was that
no experimental data was available to validate the analytical and numerical pre-
dictions. In 2014, laboratory testing of reciprocating and centrifugal compressor
mixed operation was performed in an air loop at Southwest Research Institute’s
(SwRI) compressor laboratory. Results provided clear evidence that suction
pulsations can significantly reduce the surge margin of a centrifugal compres-
sors and that the geometry of the piping system immediately upstream and
downstream of a centrifugal compressor will have an impact on the surge mar-
gin reduction. In severe cases, surge margin reductions of over 30% were
observed for high centrifugal compressor inlet suction pulsation. Pulsation
impact results are presented as both flow versus surge margin and operating
map ellipses. Some basic design rules were developed from the test results to
relate predicted flow pulsation amplitudes to corresponding reductions in surge
margin.

Erosion/Fouling/Plugging of Components and Mitigation Strategies

Erosion and fouling both cause degradation of compressor performance and can
result in reduced operating life.
Erosion describes the wearing of components due to abrasives in the gas
path. Typical culprits include dust, scale, catalyst fines, and other material,
which cause a type of wear known as solid-particle erosion. In addition, liquid
streams or droplets will cause a type wear known as liquid droplet erosion. In
essence, abrasives and liquids carried through the compressor by the high gas
velocity are quite capable of eroding impeller blades and stationary vanes.