Protecting A Centrifugal Compressor From Surge

The response time of a transmitter for the measurement of a process variable used by the surge control algorithm should be less than 100 milliseconds. Moreover, the emergency shutdown situation will require the fastest response time from the control system. The shutdown operation does not affect the surge control algorithm because the sole function of the controller under the shutdown operation is to fully open the control valve as quickly as possible. In such condition, the recycle control valves should be able to move from fully closed to fully open in less than 2.0 seconds.

Anti-Surge System layout
Figure 6 is an illustration of the Anti-Surge System layout. On centrifugal discharge, a check valve shall be installed to prevent reverse flow and minimize surge. The check valve shall be located as close as possible downstream of the compressor to minimize mass inventory. In other words, a check valve at the compressor outlet will limit the downstream volume and increase the required system response time of the surge control system. In parallel operations, compressor units should have check valves installed to assure effective surge control of each compressor.

In order to protect the compressor adequately from surge, the discharged flow must be restored very quickly. For this reason the anti-surge recycle connection and anti-surge control valve shall be located upstream of the check valve as close as practical to the compressor discharge connection. Moreover, a cooler in the recycle line may have to be considered, to suitably control the suction temperature and thereby prevent the compressor from going into surge. The line-up and tie-in point of the recycle line on the suction side of the compressor shall be at the upstream of the suction scrubber in order to prevent liquid from entering the compressor.

If the compressor bypass control line is taken downstream of the discharge cooler (called “cold bypass line”), an additional hot gas bypass line upstream of the discharge check valve is required to protect the compressor against surge during start-up, trip and normal operation (Figure 7).
In “hot bypass lines,” the suction cooler is installed after the recycle line, which is taken directly from the discharge and recycled to the upstream of the suction cooler.

Figure 7: Hot and Cold Recycle Valves.

Recycle lines and their components shall be sized to handle compressor flow rates under rated conditions and all other specified operating and start / stop conditions.

Anti-Surge Valve Specification
The anti-surge valve (ASV) is a fail open solenoid valve. This means that it needs a high signal of 20 mA to close the valve, and a low signal of 4 mA to open the valve. When a failure occurs, the valve will usually receive a low signal and it will open, which is the safe position.

In startup and normal stop, the compressor will initially operate with the valve at 100% open. The valve characteristic for 100% open should be sufficiently removed from the compressor operational region, during compressor startup. ASV should be sized for 1.8 to 2.2 times of maximum surge point flow rate. Oversized valves will not provide the sensitivity required for particular operational settings and will restrict the performance of the compressor; actually, the valve characteristic lines will become too far from the right of the operation.