【Q&A Question 071】March 12, 2017

To prevent surge occurrences, centrifugal compressors are equipped with active anti-surge relief valves that open automatically once the outlet pressure becomes too high, or when the compressor approaches the surge region or experiences surge. If it has not been flipped open, it should be manually flipped open promptly. The active relief valve should be checked and maintained regularly to ensure it remains sensitive and functional. There are two methods to prevent surge: 1) Pressure control. It falls under single-parameter control. Generally, a pressure regulator is installed, allowing the compressor to operate at a set pressure. When the pressure exceeds the set value, the anti-surge valve opens to release some pressure, thereby keeping the discharge pressure at the set level. The anti-surge valve is interlocked with the motor; when the motor trips and stops, the anti-surge valve opens automatically. 2) Two-parameter control. The two parameters refer to pressure and flow control. It is somewhat more advanced in terms of control methods.

①In accordance with the process requirements, ensure an adequate flow rate to keep the compressor in its stable operating range； ②Adjust the speed in a timely manner according to changes in process conditions, so as to keep the compressor’s operating point away from the surge point ； ③Measures such as export venting or adding a bypass return flow between the inlet and outlet (by opening the counter-rotation valve) should be taken to ensure an adequate gas flow rate at the inlet.

Anti-surge venting; anti-surge backflow. The PLC sends signals based on the decrease in pressure difference and flow rate, thereby controlling the frequency converter to reduce the power supply frequency. The pre-rotation of the airflow entering the impeller is adjusted. Flow and temperature monitoring instruments are installed at the inlet of the centrifugal compressor, while pressure monitoring instruments are installed at the outlet; this allows for timely alerts once the compressor approaches the surge condition zone

The operator should have the compressor performance curve with the surge line marked, so as to always know the position of the compressor’s operating point on the performance curve. To prioritize operational safety, an anti-surge line can be added at a flow rate that is 5% to 10% higher than the surge line, serving as a reminder to the operator. Reducing the operating speed can decrease the flow rate without causing surge, but the outlet pressure decreases as a result. Guide vane rotating mechanisms are installed at the primary stage or at various stages to adjust the angle of the guide vanes, so that the inlet shock angle does not become too large when the flow rate decreases, thereby preventing surge. A bypass pipe is installed at the compressor outlet; when it is necessary to reduce the flow rate delivered by the compressor during operation, the excess gas can either be vented or returned to the intake pipe after pressure reduction. It is better to incur additional flow and power consumption rather than allow the compressor to operate with an insufficient flow rate, in order to prevent it from entering a surge condition. Temperature and flow monitoring instruments are placed at the compressor inlet, while pressure monitoring instruments are installed at the outlet; these instruments provide early warnings in the event of any abnormalities or surges. Ideally, they should also be connected to surge prevention control systems as well as emergency shutdown mechanisms. The operating personnel should understand the working principle of the compressor, keep an eye on the operating conditions of the machine at all times, be familiar with the operation of various monitoring and control systems, and strive to prevent the machine from entering a surge damage state. Once surge occurs, the flow rate must be increased immediately to exit surge or the machine must be shut down right away. After shutting down the machine, it must be inspected to ensure there are no hidden hazards before restarting it

The operator should have the compressor performance curve with the surge line marked, so as to always know the position of the compressor’s operating point on the performance curve. To prioritize operational safety, an anti-surge line can be added at a flow rate that is 5% to 10% higher than the surge line, serving as a reminder to the operator. Reducing the operating speed can decrease the flow rate without causing surge, but the outlet pressure decreases as a result. Guide vane rotating mechanisms are installed at the primary stage or at various stages to adjust the angle of the guide vanes, so that the inlet shock angle does not become too large when the flow rate decreases, thereby preventing surge. A bypass pipe is installed at the compressor outlet; when it is necessary to reduce the flow rate delivered by the compressor during operation, the excess gas can either be vented or returned to the intake pipe after pressure reduction. It is better to incur additional flow and power consumption rather than allow the compressor to operate with an insufficient flow rate, in order to prevent it from entering a surge condition. Temperature and flow monitoring instruments are placed at the compressor inlet, while pressure monitoring instruments are installed at the outlet; these instruments provide early warnings in the event of any abnormalities or surges. Ideally, they should also be connected to surge prevention control systems as well as emergency shutdown mechanisms. The operating personnel should understand the working principle of the compressor, keep an eye on the operating conditions of the machine at all times, be familiar with the operation of various monitoring and control systems, and strive to prevent the machine from entering a surge damage state. Once surge occurs, the flow rate must be increased immediately to exit surge or the machine must be shut down right away. After shutting down, the machine should be inspected to ensure there are no hidden hazards before it can be restarted.

1. According to the process requirements, ensure an adequate flow rate so that the compressor operates in a stable operating range; 2. Adjust the speed in a timely manner as process conditions change, thereby keeping the compressor’s operating point away from the surge point; 3. Take measures such as implementing outlet venting or adding a bypass return line between the inlet and outlet (by opening the anti-surge valve) to ensure a sufficient gas flow rate at the inlet.

Surge is an inherent characteristic of centrifugal compressors. When the pressure or flow rate at the compressor’s suction inlet suddenly drops below the minimum allowable operating level, severe rotational separation of the gas inside the compressor occurs, leading to sudden stall (meaning a significant deviation in the flow direction of the gas at the inlet of the blades and in the blade inlet angles). In such situations, the impeller is unable to increase the gas pressure effectively, resulting in a decrease in the pressure at the compressor outlet. However, the pressure in the system piping did not drop instantly, which caused gas to flow back from the system piping toward the compressor; when the pressure in the system piping dropped below the compressor’s outlet pressure, gas flowed back into the system piping. Repeating this process leads to periodic axial, low-frequency, high-amplitude airflow oscillations within the unit and the piping network. When operating at low load, the centrifugal chiller reduces the opening of the compressor vanes, resulting in a decrease in the flow rate of refrigerant involved in the cycle. As the compressor displacement decreases, the impeller’s ability to generate pressure head also diminishes, and at this point surge occurs. The operator should have the compressor performance curve with the surge line marked, so as to always know the position of the compressor’s operating point on the performance curve. To prioritize operational safety, an anti-surge line can be added at a flow rate that is 5% to 10% higher than the surge line, serving as a reminder to the operator. Reducing the operating speed can decrease the flow rate without causing surge, but the outlet pressure decreases as a result. Guide vane rotating mechanisms are installed at the primary stage or at various stages to adjust the angle of the guide vanes, so that the inlet shock angle does not become too large when the flow rate decreases, thereby preventing surge. A bypass pipe is installed at the compressor outlet; when it is necessary to reduce the flow rate delivered by the compressor during operation, the excess gas can either be vented or returned to the intake pipe after pressure reduction. It is better to incur additional flow and power consumption rather than allow the compressor to operate with an insufficient flow rate, in order to prevent it from entering a surge condition. Temperature and flow monitoring instruments are placed at the compressor inlet, while pressure monitoring instruments are installed at the outlet; these instruments provide early warnings in the event of any abnormalities or surges. Ideally, they should also be connected to surge prevention control systems as well as emergency shutdown mechanisms. The operating personnel should understand the working principle of the compressor, keep an eye on the operating conditions of the machine at all times, be familiar with the operation of various monitoring and control systems, and strive to prevent the machine from entering a surge damage state. Once surge occurs, the flow rate must be increased immediately to exit surge or the machine must be shut down right away. After shutting down, the machine should be inspected to ensure there are no hidden hazards before it can be restarted.

1. Liquid on the raw material strip； 2. Poor rotor dynamic balance ； 3. Traffic is too low.

Ensure that the inlet flow rate is above the minimum flow rate

Ensure a stable inflow volume； Adjust the speed reasonably