Surge, compressor dynamic

The author has a patent for a dynamic surge-detection system, using a boundary-layer probe, presently undergoing field tests. This system consists of specially mounted probes in the compressor to detect boundary-layer flow... 

The use of dynamic pressure transducers gives early warning of problems in the compressor. The very high pressure in most of the advanced gas turbines cause these compressors to have a very narrow operating range between surge and choke. Thus, these units are very susceptible to dirt and... 

The dynamic compressors, axial and centrifugal, exhibit one other non-steady force, which is often questioned by foundation designers. This is the torque reaction force at the casing feet and, thus, the foundation due to surge. As a rule of thumb, a value of A the maximum steady-state torque reaction at the casing feet can be used. The frequency is the surge frequency, which is less than operating speed but not readily predictable, because it is heavily dependent on the connected system. 

All compressed air systems that use a positive displacement compressor must be fitted with a pressure relief or safety valve that will limit the discharge or inter-stage pressures to a safe maximum limit. Most dynamic compressors must have similar protection due to restrictions placed on casing pressure, power input and/or keeping out of surge range. 

All dynamic compressors have a limited range of capacity for a given selection of impellers at a fixed speed. Below the minimum value of 50 to 70% of the rated flow, the compressor will surge, that is, it will become unstable in operation. Excessive vibration and possibly sudden failure or shutdown may arise. 

Nowadays dynamic simulation is commonly used during process design, e.g. for compressor surge analysis. Simulation software tools like UniSim (Honeywell), DynSim (SimSci), Hysys, Aspen Dynamics (both AspenTech) and GPROMS (PSE Ltd) have become very powerful and are relatively easy to use. In this way the plant controllability and operability can be tested and its robustness against process upsets verified. 

It is necessary to install pressure-relief valves on most dynamic compressors to protect them due to restrictions placed on casing pressure, power input, and to keep out of the compressor s surge range. Always install a valve capable of bypassing the full-load capacity of the compressor between its discharge port and the first isolation valve. 

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