Page 458 - Compression Machinery for Oil and Gas

438 SECTION III Applications

evaluation of risk of the turbulent energy generated by fluid flow exciting the
MNFs of the piping. The amplitude of the turbulent energy is a function of the
2
fluid energy (ρν and dynamic viscosity, μ). The goal of this analysis is to quan-
tify the risk of piping vibration problems. It should be noted that this screening
method applies to continuous service operation.

System Dynamic Analyses
The compressor brings gas from a system of pipes and vessels at suction pres-
sure to a system of pipes and vessels at discharge pressure. Since gas is com-
pressible, the pipes and vessels have the capability to store gas. This means that
while the volume of the pipes and vessels is typically constant, the amount
(mass) of gas that can be stored depends on the pressure and temperature of
the gas. We therefore have a system where the piping and vessels determine
the suction and discharge pressure the compressor will see, and the compressor
responds to these conditions (and depending on the power available, or the com-
pressor speed, or some other compressor control setting) with a certain amount
of flow.
Operation of centrifugal gas compressors can be defined by three operating
parameters: speed, head, and flow. Centrifugal compressors have a maximum
head that can be achieved at a given speed. At that peak head there is a corre-
sponding flow. This is a stability limit. Operation of the compressor is stable
provided the head is lower (less resistance in series with the compressor) and
the flow is greater than these values, that is, the system is stable, as long as
reductions in head result in increases in flow. Surge occurs when the peak head
capability of a compressor is reached and flow is further reduced. Depending on
the dynamic behavior of the compression system, system surge can occur at
somewhat higher or, seldom, lower flows than the peak head capability. This
is a particular issue in systems with low-frequency pulsations. When the com-
pressor can no longer meet the head imposed by the suction and discharge con-
dition (which are imposed by the compression system), flow reverses.
Fig. 11.6 shows a compressor map including reverse flow, and the behavior
when the compressor transitions from normal operation to fully reversed flow.
When a compressor approaches its surge limit, some of its components (dif-
fusers, impeller) may start to operate in stall. Surge is the ultimate result of sys-
tem instability. Surge is what happens after the stability limit of the compression
system is passed. Not only is this detrimental to meeting the process objectives,
the resulting axial and radial movement of the rotor can cause damage, some-
times severe, to the compressor. Surge can be avoided by ensuring that the flow
through the compressor is not reduced below the flow at peak head.
The surge avoidance system prevents surge by modulating a surge control
(bypass) valve around the compressor. A typical system consists of pressure and
temperature transmitters on the compressor suction and discharge lines, a flow