Page 450 - Compression Machinery for Oil and Gas

430 SECTION III Applications

Analysis Methods

Piping Thermal Stress (Static Stress)
The purpose of a stress analysis (sometimes called a piping flexibility analysis) is
to calculate the static forces in a piping system created by thermal expansion,
static pressure loads, weight, etc. The resulting predicted forces are used in
the system design to ensure there is sufficient flexibility in the piping to absorb
the thermal expansion. The analysis is a balancing act between the movement
of the piping due to temperature or pressurization scenarios and the piping
restraints, soil, and rigid equipment nozzle connections that restrict pipe move-
ment. The requirements for thermal expansion of a piping system are often in
direct conflict with effective vibration control. The selection and accurate model-
ing of appropriate restraint types and supporting structures can be critical. The
thermal stress analysis typically includes calculations for the piping stresses,
compressor nozzle loads, cooler nozzle loads, and pipe support reaction loads.
The stress analysis should be performed according to the applicable piping
code to ensure compliance. ASME B31.8 and B31.3 are the most commonly
used codes for compressor station piping systems; these codes provide calcula-
tions for the maximum allowable stresses in the piping based on diameter, wall
thickness, and material properties. For equipment and reciprocating machinery
connections, the acceptability of the calculated loads on the nozzles must be
verified by the manufacturer. API 617 provides allowable loads for centrifugal
compressor nozzles; a multiplier of 3 for these allowable loads is considered
acceptable by many manufacturers. API 661 provides allowable loads for cooler
nozzles, with an additional allowable multiplier occasionally applied.
The piping system is evaluated for multiple cases. The typical range
includes the design and normal operating conditions, the sustained cases with
weight and pressure loads, as well as the combination of loads, such as the range
of temperatures from a cold to a hot condition. Optional cases include seismic
loads, wind loads, reaction forces from the opening of a relief valve, slug forces,
settlement, and other forces induced by dynamic events such as surge or pres-
sure equalizations.

Machinery-Induced Pulsations
Reciprocating Compressors
Reciprocating compressors receive and deliver gas in discrete slugs (or pulses)
as the pistons moves back and forth. For a double-acting compressor there will
be two pulses of gas for each revolution thus resulting in higher excitation on the
second running order. As these pulses of gas enter or exit the compressor cyl-
inder they will give rise to pressure pulsations that moves in both directions at
the speed of sound down the pipe. These pulses can be quite large and so atten-
uation devices generally called “pulsation bottles” or “dampers” are mounted