Anti Surge Controller
Surge Phenomenon and the Needs of Anti Surge Controller
The surge phenomenon in the centrifugal compressor can be understood as a complete breakdown and reversal of the flow through the compressor. Every centrifugal compressor has its own operating characteristic map. This map will show us the surge line limit and the operating limit. See below example of the compressor operating map. The surge will happen if the compressor operates on the left side of the surge line.
The surge can’t be eliminated but it can be avoided or prevented. This surge phenomenon can be prevented by recycling or blow off a portion of the discharge flow to keep the compressor away from its surge limit. This prevention in particular is done by the anti surge controller.
The anti surge controller must be able to accurately determine how close the compressor is surging so it can maintain an adequate but not excessive recycle flow rate.
A compressor’s surge limit is not a fix value. It is a very complex function that depends on gas composition, suction temperature and pressure, rotational speed and guide vane angle. Surge protection is more complicated by the speed at which the surge develops. It can take only a fraction of a second for a compressor to move from a relatively healthy operating point to one in which the surge is inevitable. Therefore it is a logic reason to have a very fast final control element which we called an anti surge control valves.
In general application, the anti surge controller received a signal from flow transmitter, pressure transmitter, and temperature transmitter from downstream and upstream of the compressor. The anti surge controller is then analyzes the entire received signal. The surge margin (control line) usually determine as a 10% margin from the known compressor surge line (the surge line can be obtained from actual test or empirical data). Typically the PI (Proportional Integral) control algorithm will active and start to modulate the anti surge control valve if the compressor operates between the control line and the surge line. The set point of the PI controller is a flow rate that lies on the control line. Normally, a polynomial equation is used to approach the control line and determine the PI controller set point with the discharge and inlet pressure of the compressor as the variable. Then, the anti surge controller (using PI controller algorithm) will open and close the anti surge control valve as necessary to bring the compressor operates on the control line.
The Anti Surge Control Valves
As briefly explained above, to avoid the surge phenomenon, there are an anti surge control system that analyze the compressor operating point and modulate the anti surge control valve as necessary. Before designing the anti surge control valve, the compressor manufacturer usually has some anti surge system study with some set of process data required to design the anti surge control valve.
The process data it self has several cases for several surge possibility. The study also conclude the required anti surge control valve opening time during normal operation and during shutdown sequence operation.
In practice, there is no difference between anti surge control valve and ‘general’ control valves. The main difference is that the anti surge control valve has a certain required fast opening time to avoid the surge. This time constraint will make the anti surge control valve need some additional accessories such as volume booster and three way pilot operated valve. The valve accessories that will be used may differ from each anti surge control valve manufacturer; this is not a problem as long as the required opening time is achieved. But in general speaking, the anti surge control valve will has a general arrangement as below.
The anti surge controller is started up, the solenoid valve is energized and all pilot operated valve is actuated. The tubing line from I/P Positioner output A & output B are continuously connected to the actuator. When the compressor operating point drop in the area between surge line and control line, the anti surge controller start to operate and modulate the anti surge control valve through the I/P Positioner. The valve will fast open and fast close as necessary with the help of volume booster. The volume booster is used to increase the stroking speed by bypassing the instrument air supply or in other words, it amplify the pneumatic signal output of the I/P Positioner.
While there are an emergency cases (need to shutdown etc..), the anti surge controller will simply de-energized the solenoid valve and all pilot operated valve will un-actuated. The pressure of top side piston actuator is vented by the 3-way pilot operated valve no. 7 and 2-way pilot operated valve no. 6. The low side of piston actuator is pressurized by the regulated instrument air supply (or from volume bottle if emergency). This top side vented and low side pressurized will make the valve in the open position in fast manner. The anti surge control valve will open very fast in less than one second in this case.
The surge phenomenon in the compressor can’t be eliminated but it can be avoided or prevented. The anti surge control system is designed so that the compressor will not experience the surge by blow off some portion of the discharge flow. The anti surge controller together with the anti surge control valve will do this very important task in a very fast manner.