Compressor Anti Surge Control Systems
Surge is a potentially dangerous phenomenon that can occur in centrifugal and axial compressors. Compressor surge can cause mechanical damage to the internals of a compressor. Surging can also cause the compressor to overheat to the point where the maximum allowable temperature of the unit is exceeded causing damage to seals. Further, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side due to the high forces induced by flow reversal. For axial compressors, the damage may be measureable after only a few surge cycles.
Compressor surge can occur in a fraction of a second so a fast acting antisurge control system is required to avoid surge and expensive compressor damage.
What is surge?
In simple terms, surge can be described as a situation where the flow through a centrifugal compressor is reversed intermittently, in other words gas is flowing from the discharge nozzle to the suction nozzle.
What is a surge cycle?
As described above, surge is a reversal of flow. Due to this flow reversal, the system pressure at the discharge side will be reduced over time, and eventually the compressor will again be able to overcome the pressure on the discharge side. If no corrective action is taken then the discharge pressure will increase and the compressor will again surge. This is known as a surge cycle.
What causes compressor surge?
The causes of surge are generally a function of the interaction between the compressor and the piping system. There are three main causes of surge;
Suction Throttling - if inlet flow is reduced such that the head developed by the compressor is insufficient to overcome the pressure at the discharge of the compressor.
Discharge Throttling - if the discharge side system resistance increases due to discharge throttling then the pressure developed by the compressor will increase and flow rate will start to decrease to such an extent it will reverse.
Change in compressor speed - an increase in the operating speed of a compressor can also cause compressor surge.
How to avoid compressor surge?
Compressor manufacturers produce an operating characteristic map, sometimes called a compressor performance map, for each compressor they produce. This map shows the surge limit line (SSL) where surge limit defines the flow at which, for a given speed, the operation of the compressor becomes unstable. The compressor needs to be operated to the right of the surge limit line, so a suggested operating line is also produced which is a safe distance from the surge zone. How large the surge offset (the margin) needs to be depends on the speed of instruments, the speed of actuators and the speed of the fastest disturbances that should not trip the anti-surge open-loop backup system. The operating line is usually in the region of 3 to 10% distant from the surge limit line. Operating too close to the surge line increases the chances of encountering a surge, and operating too far from the surge line is inefficient, therefore an anti-surge control system is used to maintain the compressor as close to the operating limit line as possible. A typical compressor map is shown below:
Anti Surge Control System
Surge happens quickly, and conventional instruments, control loops, and actuated control valves usually fail to recognise the onset of surge and react to it quick enough. Therefore axial and centrifugal compressors are usually fitted with a dedicated antisurge control system. Typical components in an anti-surge control system include inlet flow transmitter, inlet an outlet pressure and temperature transmitters, antisurge controller, and anti-surge control valve as depicted below.
Antisurge control systems detect when a compression stage is approaching surge and rapidly take action to reverse the movement of the operating point towards the surge line. This is normally achieved by opening a fast acting control valve (an antisurge control valve) in a recycle line, thereby returning the discharge gas to the inlet of the compressor via a suction cooler. The resulting increase in compressor inlet volume flow moves the operating point away from surge. The specialist compnents in the antisurge control system are the antisurge controller, and the antisurge control valve.
Anti Surge Controller
As discussed above, an anti-surge controller modulates the anti-surge valve to prevent the compressor’s operating point from reaching the surge limit while maintaining other process variables within safe or acceptable limits.
The antisurge controller is a specialised controller which is fast acting (faster than a typical DCS) and contains special surge detection algorithms.
The controller will have an inbuilt closed and open loop control responses; this helps minimise the control margin and gas recycling without unnecessary upsets to the process.
The more advanced antisurge controllers will automatically compensates for changes in gas molecular weight, temperature, compressibility, pressure, and compressor rotor speed.
It is not unusual for anti-surge controllers to be rack mounted, with a graphical user interface or HMI (human machine interface) on the front. This display provides access to all user settings and system configuration. The controller will "hand off" alarms, and process readings to the DCS for display and logging.
Specialist anti-surge controllers are available from Compressor Controls Corporation (CCC), and Yokogawa.
Anti Surge Control Valves
The antisurge valve is located in a recycle line connecting the compressor discharge to the inlet. For multisection compressors, it is good practice to install a separate recycle line with an antisurge valve for each of the compressor sections.
Antisurge control valves are designed to be fast-opening and relatively slow-closing. For relatively small valves (say below 6 inches), full stroking time is less than 1 second, for larger anti-surge valves the stroke time could be 1 to 1.5 seconds and for very large valves slightly less than 2 seconds. A typical design for a 12 inch antisurge valve could have an opening time of 1 to 2 seconds and a closing time of 3 to 6 seconds.
Antisurge valves should be configured to "fail open", that is on loss of pneumatic signal (either unintentional loss, or intentional loss due to process trip or emergency shutdown) the valve will move to the fully open position.
To achieve high opening speeds it is common for anti-surge valves to be fitted with a volume booster. A volume booster is a pneumatic relay that receives a small control signal and allows a large regulated flow to the actuator, in other words an amplifier. The volume booster should be mounted to minimise interconnecting pipework as excessive pipework will introduce delays into the system.
Although in practice there is no real difference between an antisurge control valve and a general control valve, (the only real difference being the accessories required by the antisurge control valve to achieve its fast opening time) many valve manufacturers, e.g. Mokveld, Kent Introl, Samson Controls market anti-surge valves as special valves.