Compressor is an equipment for increasing pressure of gas stream. One type of compressor is centrifugal compressor.
One thing that is interesting about this centrifugal compressor is the anti-surge control, where other types of compressor are not necessarily required. Why? Because centrifugal compressor is relatively a constant head-variable volume machine or we can say the flow entering centrifugal compressor fluctuates more than other type of compressor.
So let’s talk about this “surge”.
What is surge?
Surge is a condition involving volume and pressure fluctuation that happened when attempting to operate at higher pressure ratio than compressor design maximum. See the compressor chart below:
At chart above, there is a surge limit line that can be reached from stable operating point by either reducing flow or decreasing suction pressure. Surge usually occurs below 50-70% rated flow.
Anti-surge control system senses conditions approaching surge, and maintain the pressure ratio below the surge limit by recycling some flow to compressor suction. Recycle flow is taken from air cooler outlet after compressor to prevent overheat. Below is the simple diagram of anti-surge control:
1. Minimum volume
Flow element (FE) senses the flow and flow transmitter (FT) send it to surge control (SC) system. When suction flow to compressor is decreasing below minimum set point, surge control system order the surge control valve (SCV) to open and recycle discharge flow to suction flow until the surge condition is passed.
2. Pressure limiting
Pressure transmitter (PT) senses process pressure and send it to surge control (SC) system. When discharge pressure exceeds the set point pressure, surge control system order the surge control valve (SCV) to open and recycle discharge flow to suction flow until the surge condition is passed.
For example, I take the PHE-FOXTROT Compressor P&ID snapshot as seen below:
At P&ID above, there is two sensing that send information to surge control system, which are FT-100 (for sensing inlet flow) and PDT-100 (for sensing differential pressure between suction and discharge). The surge line is flowing through air cooler (HAL-100) outlet to gas scrubber (MBF-100) inlet via surge control valve (SCV-102).
If you want to read detail sharing knowledge about this anti-surge control, write on your email in comment and I will invite you to shared folder on Dropbox. Thank you 
References:
GPSA Section 13 Compressors and Expanders
credits: Nika Yunitri