The transfer of gas along a pipeline is a common process in the oil, chemical and petro-chemical industries. For cost-effectiveness, gas is usually transported at high pressure via a compressor before entering the pipeline
Compressor efficiency is maximised when the flow rate through it is kept low and the pressure high, with the minimum possible flow rate being restricted by the risk of compressor entering surge condition.
Surge is an unstable and undesirable operating condition of the compressor, occurring when the flow through it is reduced to the point where the compressor discharge pressure is less than the line pressure. This causes a momentary flow reversal, reducing line pressure and causing erratic flow output.
With the reduced line pressure, flow through the compressor is re-established, causing line pressure to increase and the cycle to begin again. If the factors leading to the surge condition are not correctly and quickly rectified, the output will continue to oscillate resulting in damage to the compressor.
Although all anti-surge control techniques are based on a similar concept - to maintain a minimal flow at extreme conditions which can be achieved with a robust and efficient control module offering configuration and operator interface flexibility.
The optimum flow rate may be calculated from a simple graph of pressure difference against flow, as shown on Figure
The position of the lines is unique to a particular compressor. The operating setpoint is at the minimum flow rate and pressure difference which avoids surge conditions. The application module for anti-surge control provides a fine and reliable control mechanism maintaining the process close to this setpoint.
The control system explains:
The application module processes the pressure difference to calculate the correct flow setpoint. Two different sets of time constants are used in the PID controller. For a positive deviation, where the flow rate is too high, the output signal to the flow control element produces a gradual correction to allow time for the control system to react, thus avoiding over-correction and risk of surge. If there is a negative deviation from the setpoint, a different set of time constants is used to open the by-pass valve quickly. This allows more gas to recirculate, increasing the flow rapidly enough to avoid surge conditions. The application module offers bumpless switching between the two sets of constants. Figure 2 gives a schematic representation of the compression process.
In order to avoid turbulence, the application module also monitors the rate of change of the flow through the compressor. If the flow rate begins to change rapidly, the output to the flow control element by-passes the control loop and opens the valve fully. This allows the maximum amount of gas to recirculate and increase the flow through the compressor. In general, the control system should be tuned so that the valve is shut most of the time, since recirculating the gas consumes energy.
Reduced maintenance and downtime