Anti Surge Control

Surge is part of the inherent operating characteristics of centrifugal and axial compressors. It is probably one of the most misunderstood of all the characteristics of these compressors. Regard for surge seems to vary from complacency to terror based sometimes on fact and experience and other times on rumor. While surge was described in Chapters 5 and 6 for the centrifugal and axial compressor, a quick review to introduce anti surge control is included here.

As flow is reduced in an axial or centrifugal compressor, there is a minimum limit when the geometric form of the internal blading can no longer move gas forward through the machine in a stable fashion. Unfortunately, the flow cannot remain static, so if it no longer moves forward, the residual volume in the machine moves in the reverse direction. The audible sound, normally associated with surge, comes from the reverse-flowing gas meeting forward-moving gas in the inlet. The noise is similar to thunder being caused by pressure waves of air colliding due to the local heating from lightning.

Is the surge phenomenon harmful? This can be compared to a wasp or bee sting to a human. If the human is healthy and not allergic, there is only temporary discomfort but no permanent damage. If the human is allergic, has other health problems, or is hit by a whole nest of wasps or bees at one time, we have another story. Similarly, a low-stressed, conservatively designed machine with no large amount of process temperature sensitivity can withstand surge occasionally with the only problem being the disturbance in the process from unsteady flow. If, however, the machine has highly stressed parts or exhibits other marginal design parameters, then we have a problem. Likewise, if the gas is sensitive to large temperature rises, where either process gas decomposition or temperature reactions can occur, we have an allergy. Finally, if any machine is surged long enough (wasp nest), problems are likely to occur. To understand the mechanics, the following occur in surge.

1. Temperature—during the back flow, gas at discharge temperature is introduced to the inlet. If more than one cycle occurs, the same gas is reheated and returned, getting hotter with each cycle.

2. Load—during the back flow the shaft torque is reduced, then restored with feed forward giving a torsional pulse with each cycle.

3. Component stress—during the back flow and forward reversals, all the components involved with the gas propulsion are loaded and unloaded, placing blading in a cyclic loading mode.

By evaluating the three physical parameters above as they apply to any compressor, one may anticipate the possibilities for problems.

Whenever normal operating limits or startup require the compressor flow to be less than the minimum flow for surge, an automatic anti-surge control should be considered. Process variables that affect the surge flow limit are flow, differential pressure, inlet temperature, molecular weight, other gas properties, and speed. To evaluate the possibility of surge, the variation of the above limits must be considered together with a com pressor performance curve, normally polytropic head plotted against inlet flow in actual operating conditions. This curve can also be plotted in terms of pressure and flow for different inlet temperatures and flows if the variations are not too great. The head capacity curve, while not as straight forward, is somewhat more useful if many variations must be considered. Figure 8-41 is a centrifugal compressor curve. Included is the unstable region normally not shown.

To control or prevent surge, gas must be bypassed around the compressor in order to increase inlet flow. On a gas turbine or in a multistage compressor, the bypass may have to come from more than one point in the compressor. In air service with atmospheric inlets, bypass means dumping back to atmosphere. In gas application or closed systems, bypassing must be done by returning discharge gas to the inlet with provision made to remove the heat of compression either with a shell and tube exchanger or by direct contact such as flashing liquid to vapor.

There seems to be one problem with anti-surge control, which is its name. Both surge control and anti-surge are used. Anti seems to be what the control is supposed to do, so it will be used here.

The best anti-surge control is the simplest and most basic that will do the job. The most obvious parameter is minimum-flow measurement, or if there is a relatively steep pressure-flow characteristic, the differential pressure may be used. The latter parameter allows for a much faster response system, as flow measurement response is generally slow; however, the speed of response need only be fast enough to accept expected transients. One major problem with the conventional methods of measurement and control is the need to move the set point for initiation of the control signal away from the exact surge point to allow some safety factor for control response time and other parameters not directly included

PRESSURE

RISE

Unstable

Stable

Unstable

Stable

Detail A

Figure 8-41. Unstable flow region of a centrifugal compressor curve.

in the surge control. A proper speed of response and other more exact variable compensations are ways of narrowing this margin, which represents wasted power and wasted capacity.

Variables such as speed, temperature, and molecular weight can be included as part of the compensation. These may be linearized and put into the system as analog weighting functions or, with computer control, be put into the equation in a more exact fashion. Inlet temperature and speed present little problem as signals proportional to these parameters are commonly measured. A well-designed, properly compensated antisurge control with a good response can probably function to a 5% flow margin on a conventional centrifugal compressor. On an axial compressor, differential pressure is normally used and a limit can only be defined for a given application. Needless to say, close control can be held. The computer controller also adds intelligence to permit the system to selectively narrow the margin based on plant conditions. With the high cost of power there has been a desire to operate without surge margin or to the level of incipient surge. Incipient surge is a narrow margin between full stability but before full complete flow reversal. This is represented at detail A of Figure 8-41. Various methods have been devised, such as measuring stage differential pressure and looking at the unsteady component of the signal. The unsteady component does increase with the onset of surge; however, on multi-impeller compressors, either all impellers must be instrumented or a guess must be made, based on probability, as to which impeller is the one most likely to go into surge first. Other methods have been proposed that require internal instrumentation which, however, have the same limits just presented, as well as the problem of simply existing in a hostile environment.

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