Methanol Segment Weekly Topics Causes of surge in centrifugal compressors 2016.08.08--08.15

It is usually caused by insufficient traffic. Increasing the backflow rate can reduce surge

The compressor inlet filter is clogged, the volume of gas entering the compressor is too low or the pressure is too low, the molecular weight of the gas entering the compressor is too low, or the inlet temperature is too high. The unit speed indication is low, and the anti-stall controller cannot function properly. Stall prevention valve failure. The compressor is clogged or its components are damaged. The unit's speed is unstable; the voltage rises too quickly when starting up, and it drops too quickly when reducing load during shutdown.

Causes of surge, Solutions: Low inlet pressure – Increase the inlet pressure; Clogged inlet filter – Shut down the system to clean the inlet filter; Closed outlet valve – Open the outlet valve; High outlet pressure – Reduce the outlet pressure; Broken inlet pipeline – Seal the leak urgently or shut down the system

The circulation volume is low; increase the circulation hydrogen bypass

1\ The centrifugal compressor experiences surge, and the root cause is a reduction in the air intake volume to the minimum level permitted by the compressor. Both theoretically and in practice, it can be proven that the following factors can all cause the compressor’s operating point to fall into the surge zone, and they are all causes of surge. a) The pressure in the system’s piping network increases, resulting in blockage at the outlet and surge due to poor exhaust ； b) The opening degree of the outlet valve or inlet valve is insufficient, acting as a throttle and resulting in a reduced flow rate ； c) A sudden drop in the speed of the centrifugal compressor leads to a reduction in flow rate and a sudden decrease in outlet pressure, increasing the risk of surge ； d) The intake pressure decreases, such as when the inlet filter is clogged or the suction negative pressure is high, which corresponds to an increase in the resistance in the piping system ； e) An increase in inlet temperature and a decrease in gas density cause the compressor’s surge line to shift downward to the right, thereby reducing the stable operating range ； f) The molecular weight of the gas decreases significantly ； g) The operating pressure at the outlet of the centrifugal compressor is set at the edge of the surge zone ； h) Design issue with the compressor itself; the expansion plate has an excessive chamfer. The main function of the diffuser is to convert the velocity of the gas exiting the impeller into the required static pressure energy. Changes in the diffuser design can result in an excessive flow demand on the compressor; if this demand cannot be adjusted accordingly, surge will occur. 2\ Measures to avoid and eliminate surge in centrifugal compressors: Due to the surge tendency of centrifugal compressors, manufacturers design anti-surge systems whose function is to activate anti-surge protection automatically when surge occurs in the machine. The specific setting involves comparing the outlet pressure value with the system pressure value; if the pressure change exceeds a certain set value within an extremely short period of time, the anti-surge automatic protection is activated. At that time, the pressure relief valve (backflow control valve) and the inlet regulating valve open simultaneously to eliminate surge, as shown in Figure 5. Then adjust the inlet and outlet valves, close the pressure relief valve (backflow control valve) and the inlet adjustment valve, and automatically adjust to restore the system to its normal pressure level.

What are the factors that cause surge in centrifugal compressors? How to eliminate surge in a centrifugal compressor? Answer: The reasons for surge occurring during actual operation are roughly as follows: 1) The gas flow rate is low, entering the surge region. 2) The gas outlet pressure is too high, entering the surge region. 3) Surge caused by improper operation. 4) Surge is caused by damage to mechanical components. 5) Additionally: a sudden drop in rotational speed, significant fluctuations in the temperature and specific weight of the compressed medium, as well as low intake pressure, can also cause surging. When surge occurs, the flow rate should be increased immediately (by opening the outlet vent valve or the bypass valve between the inlet and outlet – the anti-surge valve – to reduce the outlet pressure) in order to eliminate the danger. Afterwards, the cause of the surge must be identified and addressed, and the machine should be checked for any damage before normal operation can be resumed. If it is necessary to maintain pressure in the system, the speed should be increased appropriately after opening the anti-surge valve to allow gas to flow back, so that the outlet pressure returns to its original level.

Process issues: 1. The inlet pressure is too low, due to excessive fluctuations in the system conditions, especially regarding pressure. Since the surge point is directly affected by the pressure difference in the throat of the inlet flow meter, a sudden drop in inlet pressure occurs. Before the compressor system reaches a state of surge, the large pressure difference causes the surge point to cross the surge protection line and the surge line itself, thereby entering the surge zone and resulting in surge. The reasons include the shutdown of the previous system’s compressor, failure of the automatic control valve leading to its closure, liquid resistance in the separator, collapse of the packing filter, activation of the safety valve, or rupture of equipment pipelines. 2. Excessive pressure buildup or differential pressure in the system downstream of the compressor leads to high outlet pressure; faults in the vent valves of the synthesis system, liquid resistance in the methanol separator, and problems with proper functioning of the methanol cooling tower can also contribute to this issue. 3. The speed at which the manual control valves in the entire system are adjusted is too fast, especially those shut-off valves that have a significant impact on gas flow and pressure. 4. Too rapid changes in load. Control system issues: 1. The distance between the surge protection setting and the actual surge line is too small; for centrifugal compressors that are subject to system fluctuations, this distance should be increased. 2. The straight pipe sections required for the flow meter do not meet the standards; in particular, issues with the inlet filters and tees mean that even minor pressure fluctuations in the system can cause the displayed flow rate to be inaccurate. 3. The thermal resistors and pressure transmitters have large accuracy errors or are malfunctioning. 4. The control valves at the inlet are faulty. 5. The PID parameters set for various automatic control valves are inappropriate, resulting in too rapid, too slow, or overly drastic changes in the regulated parameters, such as those in the carbon supplementation flow control valve. Valve issues: 1. The pneumatic anti-surge valve may open automatically due to problems with the air supply, blockages, or leaks. 2. There are faults with the two-position five-way solenoid valves associated with the surge valve, or there are issues with the electrical control system. 3. The surge valve itself is malfunctioning. Equipment-related issues: The flow channels of the equipment’s impellers are too narrow, and the diffuser design is inadequate

The intake air temperature is too low, and the average molecular weight of the intake air is too high

The amount of hydrogen entering decreases, and the inlet pressure drops. Open the anti-surge valve wide.