1. Introduction to Anti-Surge Valve:
The anti-surge valve piping is the recycle loop that connects the discharge of the compressor (generally downstream of the discharge cooler) back to the suction side(generally upstream of the scrubber). This allows the compressor to flow through bypass and keeps the compressor from surging (reverse flow thru compressor). The anti-surge (control) valve is installed in this line.
2. Process Description:
The performance of a process system depends a lot upon the quality of the layout engineering. The gas compressor surge control piping is critical for any gas compression plant and needs to be given due care during the design of piping.
The basic elements of a compressor surge control system are the flow measurement (on the suction side of compressor), pressure and temperature measurement on suction and discharge side of the compressor, discharge check valve, anti-surge valve and its actuation system and the surge control system controller.
Typically the surge control system has an Independent recycle loop for each compressor train and the surge controller for each compressor assures that adequate control is provided for every operation scenario which includes upsets in the compressor downstream gas system. Each compressor train also has an individual check valve for effective surge control.
A typical schematic flow diagram generally followed is attached in Appendix-I which depicts the surge control system of a gas compressor.
3. Layout Guidelines
Recommendations in the designing of the layout of a surge control piping system are as follows:
- The check valve on the downstream of the gas compressor discharge cooler outlet line shall be located at a minimum distance from the cooler outlet flange.
- The anti-surge line shall be connected immediately upstream of the check valve and the anti-surge valve shall be located at minimum distance from the discharge line. The performance of the surge control system is greatly dependent on the piping volume downstream of the compressor and upstream of the check valve, discharge shutdown valve and anti-surge valve being minimum.
- It may be noted that the requirement of the hot gas bypass may be avoided if the minimum pipe volume as described in the para above is ensured. Process shall check and confirm the requirement of hot gas bypass by performing a proper dynamic simulation.
- The branch connection for the pipe taking gas from discharge for the anti surge valve shall be located on the top of the discharge line.
- · The anti-surge valve shall be located at a high point in the piping such that the upstream and downstream piping is free draining into the suction and discharge lines and is without any pockets which may cause slug formation.
- The temperature transmitters (TITs) on the discharge cooler outlet shall beprovided on the common discharge line i.e. before the tapping for anti-surgeline. This is to ensure that the correct temperature is monitored during recycling of the gas through the anti-surge valve.
- Straight pipe lengths upstream and downstream of the anti-surge valve shallbe provided as per the recommendations of the Vendor to limit noise and pipevibrations. Normally, this should be about 10 times the pipe diameter on the upstream side and 5 times the pipe diameter on the downstream side.
- Any other specific requirements of the compressor Vendor shall be compliedwith.
An alternative typical schematic flow diagram is shown in Appendix-II. In this the tapping for anti-surge valve inlet is taken from the upstream of the compressor outlet cooler and the outlet of anti-surge valve is connected to upstream of compressor inlet cooler, instead of directly connecting to scrubber.
The actual piping arrangement shall be followed as indicated in the P&IDs.
Appendix III depicts a typical piping arrangement for the anti-surge valve piping.