As a general rule, the anti-surge control will rapidly prevent/interrupt surging of the compressor, owing to its highly dynamic features.
However, if the process upset happens again, multiple surges will occur with a requirement to interrupt a series of repeated surges.
The surge detector fulfills this purpose: it continuously monitors a compressor parameter that is significantly affected by surge, gives an alarm when surge is first detected, eventually allowing production personnel to intervene and stabilize the operation of the compressor, and ultimately stops the compressor when multiple surge occurrences are detected.
The surge detector exists in two versions.
If an ASC transmitter is no longer calibrated adequately due to faulty Transmitter then the ASC would no longer be in a position to bring the compressor away from surge because it is receiving wrong information.
In this case the SD is there to pick up this mal-operation and to trip the machine.