The operating range of centrifugal or axial compressors is limited by an instability range commonly called 'Surge' which occurs during low flow or high head conditions and can result in extensive damage. In compressor surge, the energy of flow reversal is accompanied by a loud explosive noise and excessive vibration. Resulting flow reversal and pressure pulsation from surge can cause extensive damage to the compressor system through induced forces such as thrust reversal, radial and axial excitation of the rotor, etc.
Usually academic views such as ‘surge operation must be avoided in any case’ may not be practical in the industry particularly for centrifugal compressors. On the other hand, for axial compressors, surge should be really avoided at maximum extent possible. Design of many compressor components and systems is still mostly based on field operation feedback and experience. When the compressor operates closer to surge a higher discharge temperature occurs.
Surge control systems are usually not provided by the compressor manufacturer. Anti-surge systems are designed and supplied through third-party suppliers (suppliers specialized in anti-surge systems) based on the compressor manufacturer’s machinery performance maps. To prevent a compressor entering surge, generally a bypass is opened to maintain the flow above surge flow. This action should be taken before surge is reached.
Regardless of inlet pressures, surge for constant-speed machines occurs at a constant inlet volume flow rate. Variations in gas temperature and molecular weight cause the surge point of compressors to shift. Compressor speed change will shift surge point to greater extent. Temperature or mole weight variations normally change the safety margin which is the distance between surge and controlled flow point. Small-to-moderate gas variations can usually be accommodated by one set-point, which provides an adequate safety margin at the lowest gas temperature or the highest mole weight condition. To handle extreme variations, it may be necessary to change the controller set-point.
The inlet (suction) flow measurement system has some undesirable features, such as a relatively long inlet line to provide a sufficient straight run of pipe. Also, the pressure drop due to the flow element has considerable effect on operation. Many operators prefer to measure the flow at discharge. The discharge flow element is smaller, less expensive, and the pressure loss due to the flow element is less significant at the discharge. However from control point of view, suction flow measurement is preferred since it can detect flow reduction in upstream of compressor. Sometimes surge control schemes are based on measurements of power and discharge pressure.
A variable-speed compressor requires a complex surge control. Computer simulations of the complete system, i.e. compressor plant and control instruments, in full operating envelope, are required to design a satisfactory solution. The control action is usually based on measurement of compressor inlet flow, which is the input to the controller, and the set-point is based on the differential pressure between discharge and inlet.
Regarding flow element and the compressor surge line, consider the following:
• Flow is proportional to square root of the pressure drop produced by the flow element.
• Pressure rise produced by the compressor is proportional to rotating speed squared.
• The flow rate at which surge occurs is proportional to rotating speed.
For example, if a 100% pressure rise and 100% flow are used for the surge point at 100% speed, then at 90% speed the surge set point (surge pressure) is dropped to around 81%, and flow is controlled at around 90%. This can also be a pressure- and temperature-compensated system based on discharge volume. The flow measurement is usually temperature compensated. Therefore, the control line does not change with temperature variations .
In some designs, compressor's anti-surge recycle valve is taken from the after-cooler discharge to upstream the scrubber in the suction line. This design does not require recycle cooler. However, it may cause surge problems, hot-gas-bypass requirement and poor dynamic behavior due to large gas volume between machine and anti-surge valve.
Generally volume of piping and gas containing facilities between compressor discharge and anti-surge valve must be kept as small as possible. After-cooler and considerable associate piping are usually avoided between compressor and anti-surge loop. For many applications, optimum selection is compressor's anti-surge recycle taken out immediately from compressor discharge, upstream of discharge cooler (as close as possible to compressor) and routed back to upstream of cooler in suction (i.e. cooler of previous stage).
Modern surge control systems can be classified into two categories: Static and dynamic. A static surge control system operates on fixed parameters of flow and head. But a dynamic system corrects the base flow and head parameters, and is not influenced by errors in surge point prediction and other conditions, such as compressor fouling and gas variations (mole weight, temperature, pressure and machine speed). Presently, there are anti-surge systems available that correct themselves if surge occurs at or before the surge settings and then reset themselves. However these intelligent and self-learning systems should only be used in applications where really required. Sufficient studies are needed for each specific application. These intelligent systems are based on the theories and sensor technologies that measure and analyze parameters, such as changes in harmonic frequencies considering the number of blades or impellers, the speed of the compressor, changes in speed line slopes, dynamic pressure measurements and so on occur in the region near surge. They are new technologies which must be applied carefully. Reference check, proper evaluations and pre-assessments are always required.
The better the distance to surge can be measured; the operation closer to surge can be done without taking risk. The result is more efficient operation. The anti-surge controller must provide an optimum distance to surge calculation that is invariant of any change in inlet conditions or compressor operation parameters (such as speed). Results are bigger turndown range on the compressor and reduced energy consumption during part-load operating conditions.
Amin Almasi is a rotating machine consultant based in Brisbane, Australia
(More in the 2011 issues of the Turbomachinery International magazine)