Problem of online reduction of set pressure in constant-pressure-controlled centrifugal compressors

Even for constant-pressure compressors, the outlet pressure generally falls within a certain range, and they also have their own characteristic curves; it is necessary to check whether they are operating within the allowed range. If it falls below the specified range, it has negative effects on both the equipment and energy consumption.

There is no problem with reducing the pressure, but the drop is too large; as a result, the opening of the anti-surge valve tends to happen too quickly, causing significant fluctuations in system pressure

Yes, the degree of pressure reduction needs to take anti-surge into account~~ How is the degree of pressure reduction determined? without causing backflow or the closure of the outlet check valve~

Different types of compressors and different drive units require different considerations. For those used in air separation, it is generally a constant-speed, fully gear-driven compressor equipped with inlet guide vanes and anti-surge valves (usually controlled using two parameters). Since the anti-surge valves have pressure-based opening controls, when the pressure drop is large, the speed of opening will increase due to the PI*D setting; it is possible for check valves to close as well. If the pressure drop is significant, the setting value can be adjusted slightly, for example by 20 Kpa. If the impact is minimal, the value can be increased a bit. The goal can be achieved by adjusting the setting value multiple times

Advice from the design institute: 1. The so-called constant pressure control of the compressor generally refers to maintaining the operating pressure in the container or tower at the compressor outlet. For example, some compressors are fed into the tower to participate in the reaction, and it is required that the reaction pressure not change too much in order to maintain a stable pressure level. This is called constant pressure control of the compressor. 2. There are various methods for constant pressure control. 1. The compressor outlet valve can be controlled; by closing this valve, the resistance in the pipeline increases, which leads to a decrease in the compressor’s flow rate and an increase in the outlet pressure. The increase in outlet pressure counteracts the throttling resistance caused by the valve, thereby stabilizing the pressure in the container behind it, but the flow rate decreases as a result. This is called adjusting the flow rate under constant pressure control. 2. Under constant pressure, controlling the flow rate can also be achieved by adjusting the rotation speed, among other methods. It must be illustrated with a diagram.

Also, you mentioned reducing the compressor outlet pressure, but you didn’t clarify why the outlet pressure needs to be reduced. If the speed is reduced and the outlet pressure drops, then it depends on the purpose of the compressor. If the compressor is used for circulation, there is no problem. It will not surge, and it is favorable for surge prevention. If the compressor is used to provide a stable pressure downstream, this is unfavorable for anti-surge protection; this is why surge can occur even when the anti-surge valve 1S is open during a sudden shutdown.

Is 20 kPa too risky? With a too large volume in the rear system, there is a risk that the check valve will close, and rotational stall may occur inside the compressor, leading to surging~

The purpose or method of maintaining a constant pressure is quite clear~ It mainly relates to the need to actively set a voltage reduction online; in such cases, it’s necessary to be able to determine the extent of this voltage reduction, based on factors such as the volume of the subsequent system and the PID gains for the guide vanes ~

Compressor manufacturers do not impose any limitations on the extent of voltage reduction in terms of design (such limitations are only set manually by adjusting the constant voltage value). In practice, voltage reduction does occur, but the extent of this reduction still needs to be specified during operation~

The relief valve is located upstream of the check valve; no matter how large the pressure drop is, it will not affect the compressor’s performance, only the system