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The compressor operating range: surge, stall and choke.

The operating range of any compressor is limited on e high flow side by the choke region and on the low flow side by surge. Sometimes a phenomenon called rotating stall (or diffusor stall) further limits the operating range of the compressor. Due to this, there are often numerous opinions on what constitutes a compressor's operating range. This is further confused by the fact that the surge line of a compressor can shift with changes in fuel composition, system impedance, and compressor degradation.

When a compressor is operated away from its design point, the gas flow into the aerodynamic components (impellers, blades, diffusors, etc.) deviates from its design direction. If the angle of deviation (or incidence angle) is large, flow separation occurs. At higher incidence angles, the flow fully separates at the impeller leading edge or diffusor inlet, and the flow is said to be stalled. The rotating stall is a special form of stall, where one or multiple flow regions in the diffusor (or impeller) are stalled but where other regions of the same impeller or diffusor are not stalled yet. The stall regions usually travel in the direction of the rotation at a speed that is fractionally lower than the rotating speed of the compressor. Stall and flow separation may be precursors to surge, but not necessarily so. Many compressors operate with local flow separations over parts of their operating range and function perfectly fine.

However, compressors will surge when forward flow through the compressor can no longer be maintained due to an increase in pressure across the compressor, and a momentary flow reversal occurs. This can be caused by the above described stall, impeller fouling/degradation, the imposed impedance of the piping system connected to the compressor, flow obstructions, combinations of these factors, or anything else that limits the amount of head a compressor generates. Once a surge occurs, the reversal of flow reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume again until the pressure rise again reaches the surge point. This surge cycle continues at a low frequency until some change is made in the process or compressor conditions. Surge is a global instability in a compressor's flow that results in a complete breakdown and reversal of flow through the compressor. Full surge is a source of violent axial (and radial) dynamic forces on the compressor's elements and must be completely avoided.

Choke, on the other hand, is simply a very high flow point on the compressor's map where the total amount of energy available to the impeller is utilized for pumping gas, but at a very low head or pressure ratio. ("Choke" is somewhat of a misnomer, as the flow in most centrifugal compressors never reaches the speed of sound.) But the losses due to increasingly unfavorable incidence angles, combined with the performance characteristic of impellers to produce less head at higher flows, effectively limits the maximum flow through the compressor at a given operating speed. At high speeds and with heavy gases, true inducer choke is possible, and the inducer flow path is blocked due to reaching the speed of sound. True choke is very noticeable because the compressor speedline will drop vertically from some head level to very low head without change in flow.

For many compressors on the market, operating in choke does not cause problems. However, as the compressor is far away from its design point with large flow incidence angles, continued operation in choke can result in blade flutter and high cycle blade metal fatigue failure for some compressor models. The compressor manufacturer's recommendations as to a particular model's ability to continuously operate in choke should be followed. As a minimum, operating in choke should be avoided because it is very inefficient.

Flow separation, stall, and rotating stall may be detectable by monitoring shaft vibrations, especially in high pressure machines. The turbulence caused by off design incidence angles in some components as well as local flow separation may cause broad spectrum noise in the shaft vibration signature; that is, the overall compressor vibration levels will increase, but there is no distinct frequency. Rotating stall events also increase the measured vibration levels, but at a distinct frequency that is lower than the shaft rotating frequency (typically between 10-50%). Both types of events, however, are not surge events. Nonetheless, in some instances they are used to define the lowest flow for the compressor before recycling is necessary. On the other side of the map, choke is easily detected from differential pressure measurements across the compressor. Choke can be easily avoided.

Surge, mild surge, violent surge, stability limit, and stall are often used interchangeably. One of the key problems is that the surge phenomenon is not just simply a compressor characteristic but is a systems issue; for example, it is the result of the interaction of the centrifugal compressor with the piping system it is connected to. The net effect is that the compressor may enter into surge even though the maximum head at a given speed has not been reached. On the typical head-flow compressor map, following a constant speed line from the left to the right, there will be at a local maximum. This is usually considered to be the extreme stability (or surge) limit for the compressor. At this peak on the speed line, any small flow disturbance entering the compressor will lead to surge (as the maximum head capability of the compressor is exceeded). However, in real life, the compressor will surge well before this point on the map because of the influence of the connected piping system's impedance. This follows from simple stability considerations: At flows higher than maximum head, any disturbance reducing the flow (for example the closing of a downstream valve) will cause the compressor to increase the pressure ratio, thus counteracting the change. At flows lower than maximum head, where the slope of the head-flow relationship is positive, the same disturbance would cause the pressure ratio to fall. The net effect is an amplification of the flow disturbance which rapidly leads to full surge.

It is also important to distinguish between full flow reversal where gas actually flows backwards through the compressor, which is called violent surge, and any other condition where the compressor loses the capability to make the necessary head, but is able to catch the flow before it can totally reverse, which is usually called mild surge. Violent surge has to be completely avoided, but mild surge can be tolerated for short times as long as the shaft vibrations do not exceed allowable limits. Mild surge and violent surge are very much functions of the piping geometry rather than the performance of the compressor and thus are difficult to determine from performance predictions or factory tests. The piping geometry, placement of check valves, and the surge avoidance system characteristics are critical and should be considered early in the design process of any new compression installation.

In general, systems to avoid operation in surge, and where necessary, in choke, are well established, and reliably prevent compressors from damage.

By Dr. Klaus Brun, Southwest Research Institute and Dr. Rainer Kurz, Solar Turbines Inc.
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Title Annotation:GLOBAL Gas Turbine News
Comment:The compressor operating range: surge, stall and choke.(GLOBAL Gas Turbine News)
Author:Brun, Klaus; Kurz, Rainer
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:May 1, 2009
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