Page 66 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 57

compressor cannot produce the same head as at the stability limit. It is therefore
no longer able to overcome the pressure differential between suction and dis-
charge side. Because the gas volumes upstream (at discharge pressure) is
now at a higher pressure than the compressor can achieve, the gas will follow
its natural tendency to flow from the higher to the lower pressure. The flow
through the compressor is reversed. Due to the flow reversal, the system pres-
sure at the discharge side will be reduced overtime, and eventually the compres-
sor will be able to overcome the pressure on the discharge side again. If no
corrective action is taken, the compressor will again operate to the left of the
stability limit and the above described cycle is repeated: the compressor is in
surge. The observer will detect strong oscillations of pressure and flow in the
compression system.
It must be noted that the violence and the onset of surge are a function of the
interaction between the compressor and the piping system.

Stall
If the flow through a compressor at constant speed is reduced, the losses in all
aerodynamic components will increase. Eventually, the flow in one of the aero-
dynamic components, usually in the diffuser, but sometimes in the impeller
inlet, will separate (the last picture in Fig. 3.27 shows such a flow separation
for an airfoil). It should be noted that stall usually appears in one stage of a
compressor first.
Flow separation in a vaneless diffuser means, that all or parts of the flow will
not exit the diffuser on its discharge end, but will form areas where the flow
stagnates or reverses its direction back to the inlet of the diffuser (i.e., the impel-
ler exit; Fig. 3.27).
Stall in the impeller inlet or a vaned diffuser is due to the fact, that the direc-
tion of the incoming flow (relative to the rotating impeller changes with the flow
rate through the compressor). Usually, vanes in the diffuser reduce the operating
range of a stage compared to a vaneless diffuser. Therefore, a reduction in flow
will lead to an increased mismatch between the direction of the incoming flow
the impeller was designed for and the actual direction of the incoming flow. At
one point, this mismatch becomes so significant that the flow through the impel-
ler breaks down.
Flow separation can take on the characteristics of a rotating stall. When the
flow through the compressor stage is reduced, parts of the diffuser experience
flow separations. Rotating stall occurs if the regions of flow separation are not
stationary, but move in the direction of the rotating impeller (typically at 15%–
30% of the impeller speed). Rotating stall can often be detected from increasing
vibration signatures in the subsynchronous region. Onset of stall does not nec-
essarily constitute an operating limit of the compressor. In fact, in many cases,
the flow can be reduced further before the actual stability limit is reached.