Introduction to the control of anti-surge control in centrifugal compressors

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This post was last edited by MTO001 on 2021-7-21 at 17:14. 1. Speed control and steam extraction pressure control: During normal operation, the compressor’s first-stage suction pressure control circuit sets the required value for the decoupling module; once the performance controller is activated, the decoupling module then sets the speed setpoint. The speed controller outputs commands to adjust the steam inlet valve position in order to control the turbine speed. 2. Process/choke decoupling: The speed of the choke valve is used to control the suction pressure in the suction tank. Previously, while the anti-surge valve was already controlling surge, the suction pressure was controlled by the compressor speed. Unfortunately, this can lead to conflicts between the anti-surge controller and the suction pressure controller, ultimately resulting in unstable compressor operation. To avoid this situation, a method called \"process/surge decoupling\" will be used. This method separates the relationship between the suction pressure controller and the anti-surge controller by ensuring that different controllers do not adjust the anti-surge valve and the compressor speed simultaneously. The method by which the decoupling function prevents conflict between the two controllers is to try to ensure that the PV of the anti-stall controller never drops below its set value, thereby preventing the anti-stall controller from controlling the anti-stall valve. To achieve this goal, the suction pressure controller is allowed to open the compressor’s anti-surge valve. By opening the anti-surge valve, the process gas returns to the suction section of the compressor; as a result, the flow rate entering stage 1 in the forward direction decreases, while the flow rate through stage 1 of the compressor increases. Thus, by using an anti-surge valve, the pressure controller is able to control the suction pressure. It decouples functional control and determines when the suction pressure controller should control the speed or the anti-stall valve. This can be achieved through split control, which can regulate the suction pressure by controlling the compressor speed or by controlling the position of the anti-surge valve; however, it is not possible to control both at the same time. The uniqueness of this split-control system lies in the fact that the split point at which the suction pressure controller switches between using the speed control or the anti-surge valve is variable. We call this variable split point a dynamic breakpoint. For example, as the compressor’s inlet pressure decreases, the suction pressure controller reduces the compressor’s speed to maintain the same suction pressure. If reducing the compressor speed causes the anti-surge controller to open the anti-surge valve (as reducing the speed while maintaining a constant pressure difference will ultimately lead to surge in the compressor), then prior to that, the decoupling module will cause the suction pressure controller to open the anti-surge valve, thereby preventing the anti-surge controller from controlling the anti-surge valve and allowing pressure control to be maintained through that valve. Under normal operating conditions (i.e., when the operating point is to the right of the maximum area line), the characteristic of the dynamic cut-off point is that it is located at a specified distance below the output of the suction pressure controller. Through this operation, the compressor will operate based on the speed control of the decoupling algorithm, rather than based on the anti-surge valve. We call this distance the dynamic breakpoint suspension margin. The gradual changes of dynamic breakpoints are as follows. If the operating point is to the right of the maximum area line and to the left of the fastest breakpoint line (the preset distance from the anti-stall control line determines whether the dynamic breakpoint moves fast or slow), then the operating point will decrease gradually at a relatively slow preset slope rate. The slope of the dynamic breakpoint is relatively slow in this area, as the compressor is operating very close to the anti-surge control line; without the action of the anti-surge controller, the compressor speed cannot be reduced significantly. Therefore, it is advisable to reduce the compressor speed at a lower rate, because if the suction pressure continues to drop, the suction pressure controller is very likely to start acting on the anti-surge valve. If the operating point is to the right of the rapid break line, it will decrease at a relatively fast slope rate. This is because, before the compressor approaches the anti-surge control line, there are many speeds that can be sacrificed in this area. If the operating point is to the left of the minimum region line, it will rise at a pre-set deceleration slope. The purpose of increasing the compressor speed in this operating area of the surge diagram is to use the feedstock accumulated within the system to increase the suction flow rate, and to prevent the compressor from being affected by the anti-surge controller. Since increasing the speed also reduces the suction pressure, the suction pressure controller will open the anti-surge valve to increase the suction pressure. If these actions are carried out together, they should enable the compressor to move away from the anti-surge control line. If the operation point is located between the minimum region line and the maximum region line, the position of the dynamic breakpoint will be fixed at the location where it was when the operation point entered that region. It is crucial to keep in mind that, (in most cases), the position of the dynamic breakpoint is a small portion of the compressor’s operating point on the surge curve. The only exceptions are Examples 1 and 2 mentioned above. The distance at which the dynamic breakpoint operates to the left of the pressure controller’s output can be set, but it is usually around 1.5% below the controller’s output. This ensures that the suction pressure controller will act on the speed controller whenever possible. If the output of the compression force controller decreases at a faster rate than the rate permitted by the dynamic cut-off point (based on the operating range), then the pressure controller will use an anti-surge valve to control the suction pressure. The illustration in the figure below shows the different operating regions on the surge chart. It also includes examples for three different decoupling algorithm scenarios. Surge diagram Legend 1 Legend 1 shows the normal operating conditions of the compressor. In this case, the suction pressure of the compressor is controlled by the compressor speed, and the dynamic cut-off point operates right below the suction pressure controller. If the rate of decrease in the output of the pressure controller is faster than the speed at which the dynamic cut-off point can be moved (based on the position of the operating point relative to the surge curve as discussed above), then the suction pressure controller will stop decreasing in speed; instead, an anti-surge valve is used to control the suction pressure. Assuming the operating point remains above the maximum area line, the dynamic cut-off will continue to reduce its output (and thus affect the compressor’s operating speed), causing the initial cut-off margin to return below the pressure controller’s output. This speed reduction will cause the anti-stall valve to close. Legend 2: Legend 2 shows the operation of the compressor using an anti-surge valve in order to control the suction pressure. In this case, the compressor load has been reduced to the level corresponding to the speed at which it operates with minimum control speed. The compressor will continue to operate in that area until the load increases. Legend 3: Legend 3 shows the compressor operating at its minimum controlled speed, with the suction pressure being controlled by either closing or opening the anti-surge valve. If the load increases, the anti-surge valve will close, and the speed will start to rise to accommodate the increased load. As the speed increases, the dynamic breakpoint will move along the operating point into a condition similar to that in Figure 1. If the load decreases, the dynamic break will stop at the minimum controlled speed, and the primary anti-surge valve will open to control the suction pressure. Then, a condition similar to that in Figure 2 will occur. 3. Surge prevention control and surge map: The surge prevention controller is an independent controller. Its function is to ensure that one or more stages of the compressor it protects have sufficient flow under the current operating conditions in order to prevent surge. It is not used to control any pressure. It uses several equations and inputs to calculate the operating point. If compared with the surge line, the actual operating margin will be obtained. The anti-surge controller has several functions to help the compressor avoid surge. The figure below shows the surge diagram, as well as how the anti-surge controller measures the distance from the surge line. The head with suction pressure compensation is the setpoint tracking operating point of the anti-surge controller. Due to certain constraints, this setpoint will be kept at a certain distance from the operating point (a hover margin, usually around 5%, which can be adjusted as needed). It must not go to the left of a constant safety margin. It must also descend at a specified slope rate (hover slope). Since the setpoint hover line is the setpoint of the anti-choke controller, if the operating point moves rapidly toward choke, then, taking into account the slope of the setpoint hover line, the choke controller will respond before the operating point crosses the constant safety margin. This enables the anti-surge controller to respond before a process fault that leads to a surge event occurs. If the operating point remains on the right side of a constant safety margin, then as the setpoint returns along the slope to the left side of the operating point, the output of the anti-stall controller will decrease. In addition to the ordinary PID anti-surge controller, the anti-surge controller also has a surge relief function that ensures the circulation (anti-surge) valve opens when the compressor is approaching surge. This function is very useful because the conventional anti-surge control PID is likely to be tuned too slowly to prevent compressor surge caused by severe process failures. When the operating point moves into the proportional control region (see figure below), the surge relief function opens the anti-surge valve. The opening degree of the valve is proportional to the distance the operating point travels within that area. If the margin reaches 0, or if it passes the surge line, then the proportional time reaches its maximum value (the cycle anti-surge valve is fully open at 100%). The increase or decrease in the size of the proportional action zone depends on the movement of the setpoint hover line. Therefore, as the setpoint hover line moves to the right side of the surge diagram (as a result of the movement of the operating point), the proportional action region will \"extend,\" since this region is defined as a portion of the total distance between the surge line and the setpoint hover line. If a conventional surge PID controller is used, the surge overshoot output will be selectively amplified, so control is only possible when the PID response is too slow (as mentioned above). If the overshoot output is greater than the PID output, then the surge PID tracks the output of the overshoot controller. Surge delay, surge prevention control PID, and surge relief output are sent to the surge prevention valve selection module. For the low-pressure section of the compressor, this module uses these outputs in combination with the output from the decoupled anti-surge valve to make a high-level selection. If the decoupling module controls the anti-surge valve, then this module will use surge PID tracking. This prevents any time delay caused by the end of the surge PID. It also limits the opening and closing actions of the anti-surge valve. These limitations are known as conversion speed. The shutdown speed is usually very low to ensure stability when the anti-surge valve closes. We often observe that as the compressor load increases, the closing speed of the anti-surge valve is limited by the surge PID controller. In this case, the suction pressure controller (decoupling algorithm) will immediately increase the compressor speed (based on the increase in load), while it waits for the control from the anti-surge valve to return from the surge PID controller to the suction pressure controller. When control is restored, it will stop increasing the speed and close the anti-surge valve again. Manual valve control will remain engaged between the suction pressure controller and the surge PID controller until the valve is finally closed and the compressor is under pressure control (assuming the required compressor speed is higher than the minimum control speed). When the surge relief function is active, increasing the switching speed can prevent overshoot; otherwise, the rise rate of the surge relief function may be much faster than the speed at which the valve (and the corresponding process) can respond. 4. Process/choke decoupling module: This module controls the suction pressure at the first stage of the compressor, achieving the desired outcome by adjusting the compressor’s speed and/or opening the first-stage anti-choke valve. This will be achieved once the performance controller is put into use. If the anti-stall controller is only in full manual mode, the speed will be adjusted by the decoupling module. 5. Dynamic breakpoint: The point at which the capacity control switch switches from controlling the speed to opening the anti-surge valve is referred to as the dynamic breakpoint. This value is dynamic because it changes depending on the operating conditions of the compressor. If the performance controller’s output increases above the setpoint, the turbine speed will rise; if the controller’s output drops below the setpoint, the anti-surge valve will open and the speed will be maintained at the setpoint level. Logical explanation: Prevent the reduction caused by dynamic breakpoints from exceeding 0.5% (when the breakpoint is hovered over), and ensure it is less than the output of the suction pressure controller. Prevent rapid drops in dynamic breakpoints caused by a swift decrease in the output of the capacity controller. If the surge margin is more than 10% above the anti-surge control line, the breakpoint will move quickly to the left; if the surge margin is less than 10% above the anti-surge control line but greater than the maximum region line, the breakpoint will move slowly. If the capacity controller output drops below the trip point, the anti-surge valve will open. The cut-off point will continue to decrease until the surge margin is within 5% of the control line (maximum area line), or until it reaches 50% (minimum control speed), or until the capacity controller is satisfied, whichever occurs first. If the surge margin is within 2% of the control line, the dynamic breakpoint will rise. If the surge margin is between 2% and 5% of the control line, then the dynamic breakpoint remains unchanged. Limit the shutdown point to below 50% (minimum controlled speed). 6. Gain adjustment can be made to balance the operation between the speed setpoint and the anti-stall valve. Typically, the gain of 1 used means that a 1% change in the anti-surge valve is equivalent to a 1% change in the speed setting. If a gain greater than 1 is used, it indicates the use of a large anti-surge valve, and its effect is not limited to changing the speed setting. 7. Process relaxation: If decoupling is disabled, process relaxation is set to 0%. If decoupling is enabled, when the operating point approaches the control line or the output of the process controller is reduced rapidly, the overshoot will open the anti-surge valve. Logical explanation: As the output of the capacity controller increases from the dynamic cut-off value of -50 to a higher value, the anti-surge valve closes from 100% to 0%. Use an anti-surge controller to perform high-level selection on the process overshoot in order to open the anti-surge valve. If anti-surge control is not selected, the process override is set to be equal to the anti-surge valve output. If only anti-surge control is selected, a capacity controller will be used to directly set the process margin of 8 and the decoupling speed setting. If decoupling is disabled, the target speed on the human-machine interface will be set to be equal to the speed setting value. If the decoupling function is enabled, the target speed will be set to be equal to the decoupling speed setting. Logical explanation: The speed limit setting is reduced below the dynamic breakpoint. Adjust the speed setting value from 50%–100% of the suction pressure controller output to the minimum control speed–maximum control speed. Since dynamic breakpoints are restricted to below 50%, it is not possible to calculate a negative value for the speed setting. If performance control is not selected, the speed target value is set to be equal to the output of the speed setting generator (Process/Surge Decoupling Module). Only when performance control is selected can the speed target value be set directly by the process pressure controller. If all speed setpoint switches are not used for disturbance-free switching, then all speed setpoint switches will be tracked. Note: Due to space constraints, practical issues related to the anti-surge systems of centrifugal compressors will be discussed in detail in the next article: 1. How to adjust the compressor speed so as to move the surge point away from the surge line when the surge point is close to it? 2. How does the surge point change when the relative molecular mass of the pyrolysis gas changes? 3. For anti-surge systems with limit control, what are the effects when the limit control is activated? 4. What is the meaning of the formula for calculating the anti-surge flow rate? 5. What should be done if a measuring instrument in the anti-surge control circuit fails? 6. Does the anti-surge control circuit have a self-diagnosis function? How is this implemented in practical applications? 7. The pressure overshoot function (POC) in the CCC performance controller? 8. When does the anti-surge valve activate when starting the propylene refrigeration compressor? Due to my limited knowledge, if you have any good suggestions regarding the control of centrifugal compressors, please leave a message so that we can exchange ideas and improve together.