Surge Phenomenon and Anti-Surge Phenomenon

Surge Phenomenon  and Anti-Surge Phenomenon

Surge Phenomenon

In axial or centrifugal compressors, the phenomenon of momentary flow reversal is called surge.
During surging, the compressor discharge pressure drops off and then is re established on a fast cycle.
This cycling, or surging, can vary in intensity from an audible rattle to a violent shock.
Intense surges are capable of causing complete destruction of compressor parts, such as blades and seals.
The characteristic curves of compressors are such that at each speed they reach a maximum discharge pressure as the flow drops

Surge Phenomenon In Compressors

A line connecting these points (A to F) is the surge line. If flow is further reduced, the pressure generated by the compressor drops below that which is already existing in the pipe, and momentary flow reversals occur.
The frequency of these oscillations is between 0.5 and 10 Hz. The surge frequency of most compressor installations in the processing industries slightly less than 1 Hz.
Surge is usually preceded by a stall condition, which is caused by localized flow oscillations around the rotor at frequencies of 50–100 Hz.
At the beginning of surge, the total flow drops off within 0.05 seconds, and then it starts cycling rapidly at a period of less than 2 sec.1
This period is usually shorter than that of the flow control loop, which controls the capacity of the compressor.
If the flow cycles occur faster than the control loop can respond to them, this cycling will pass through undetected as uncontrollable noise.
Therefore, fast sensors and instruments are essential for this loop.

Anti-Surge Phenomenon

Anti-Surge Control System For Compressors

On a plot of ΔP vs. h these effects are most favourable. A decrease in suction pressure moves the surge line in the safe direction, temperature has no effect, and the effect of the other variables also less pronounced.
Therefore, although the ΔP vs. h plot is accurate only at low compression ratios, it does have the advantage of being independent from the effects of composition and temperature changes.
However, suction pressure should be included in the model in order to be exact; most ΔP vs. h plots disregard it.

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