Explain Anti-surge control?

Anti-surge control is a system that is used to prevent compressor surge in gas compression applications. Compressor surge is a phenomenon that occurs when there is a sudden pressure reversal in the compressor, which can cause damage to the compressor and the piping system.

To understand how anti-surge control works, it’s important to first understand the basics of compressor surge. Compressor surge is caused by a disturbance in the gas flow, which can occur for various reasons, such as a sudden change in the inlet flow rate, changes in the process conditions, or a malfunction in the compressor itself. When this happens, the pressure in the compressor drops, causing the gas to flow backward, which can lead to vibration, noise, and damage to the compressor blades and other components.

Anti-surge control systems are designed to detect these disturbances and take corrective action to prevent compressor surge from occurring. The system consists of a controller, sensors, and an actuator.

The controller continuously monitors the process conditions, such as the inlet and outlet pressures, flow rate, and temperature, and calculates the surge margin, which is the difference between the operating point and the surge limit. The surge limit is determined by the compressor design and is the point at which the compressor is most likely to surge.

If the surge margin decreases below a certain threshold, the controller takes corrective action to prevent surge from occurring. The corrective action may involve reducing the compressor speed, adjusting the recycle valve, or opening the bypass valve to maintain the required flow rate. The controller may also adjust the discharge pressure or the suction pressure to maintain a safe operating range.

One of the primary methods used to prevent surge is through speed control. The controller may adjust the speed of the compressor to maintain a safe operating range. If the surge margin decreases, the controller may reduce the compressor speed to increase the surge margin and prevent surge from occurring.

Another method used to prevent surge is through recycle control. Recycle control involves diverting a portion of the gas back to the inlet of the compressor, which can help stabilize the flow and prevent surge from occurring. The controller may adjust the recycle valve to maintain the required flow rate and prevent surge.

In addition to speed and recycle control, the anti-surge control system may also use bypass control to prevent surge. Bypass control involves opening a valve to bypass some of the gas around the compressor, which can help reduce the load on the compressor and prevent surge. The controller may adjust the bypass valve to maintain a safe operating range and prevent surge from occurring.

The ratio of compressor pressure rise to intake flow rate is used in this surge control technique to determine the flow in the by-bass loop. The compressor surges as the suction pressure declines and the discharge pressure increases. When the PDT detects this, it signals the FRC to open the by-pass valve.

In conclusion, anti-surge control is an essential system in gas compression applications to prevent compressor surge, which can cause severe damage to the compressor and the piping system. The system uses a combination of speed, recycle, and bypass control to maintain a safe operating range and prevent surge from occurring. By continuously monitoring the process conditions and taking corrective action when necessary, anti-surge control systems help ensure the safe and reliable operation of gas compression systems.