A multi-condition centrifugal compressor adjustable guide vane control system

Abstract

The application discloses a kind of multi-working condition centrifugal compressor adjustable guide vane control systems, comprising: cabin pressure PID, input cabin pressure set value, after PID operation, output to switching module.Flow PID, input flow set value, after PID operation, output to switching module.Switching module, according to different centrifugal compressor working conditions and control object, the calculation result of cabin pressure PID or flow PID is output to range control module.Range control module, including anti-surge valve and adjustable guide vane control submodule, according to the output of switching module, range is carried out, and anti-surge valve control instruction or adjustable guide vane control instruction controls the opening of anti-surge valve or adjustable guide vane.The application selects suitable control object for different working conditions, improves the stability of process control;Through opening degree given value and rate control, the running stability of high load compressor under working condition severe fluctuation is improved.

Description

Technical Field

[0001] This invention belongs to the field of compressor control technology, specifically relating to an adjustable guide vane control system for a multi-condition centrifugal compressor. Background Technology

[0002] The liquefied natural gas (LNG) transportation market is currently experiencing rapid growth, and large LNG carriers are crucial for transporting LNG. During operation, processes such as LNG loading, warming, gas purging, and initial cooling of the cargo tanks generate significant amounts of boil-off gas (BOG), nitrogen, or inert gases. These BOGs need to be returned to the shore terminal for processing using a high-load cryogenic gas compressor (also known as a return-to-shore compressor, hereinafter referred to as a high-load compressor) to maintain the cargo tank pressure within a safe range. Due to the high flow rate and low pressure ratio of high-load compressors, centrifugal compressors are generally selected.

[0003] High-load compressors undergo complex changes in their intake and exhaust parameters, gas flow rate, gas properties, and compressor characteristic curves during different processes and at different stages of the same process. Therefore, compressor control methods are crucial to ensuring the safe, efficient, and stable operation of compressors under various operating conditions.

[0004] The process flow of the high-load compressor is as follows: Figure 1 As shown, the system includes a cargo tank, pressure transmitter, flow transmitter, adjustable guide vanes, high-load compressor, anti-surge valve, and shore station. The control method involves using the cargo tank pressure or flow rate as the control parameter, which, after passing through a PID controller, controls the adjustable guide vanes and anti-surge valve in a split-range manner. Figure 2 As shown. The initial value of the split point is 50%, which can be adjusted according to actual needs. When the controller output changes from 0 to 50%, the anti-surge valve opening changes from 100% to 0%; when the controller output changes from 50% to 100%, the adjustable guide vane opening changes from the minimum opening (A in the figure) to 100%, as shown. Figure 3 As shown.

[0005] In the above control method, when the controller output increases from 0 to 100%, the anti-surge valve is first closed until it is fully closed, and then the adjustable guide vane opening is increased. Under normal operating conditions, the anti-surge valve remains fully closed, and only the adjustable guide vane is activated to regulate the cabin pressure or gas volume; the minimum opening A is obtained based on the compressor surge curve and characteristic curve, which can prevent compressor surge caused by excessively small opening during the closing process of the adjustable guide vane.

[0006] Because the minimum opening A is a fixed value, the above method is only applicable to applications with relatively stable operating conditions. When operating parameters fluctuate greatly, it can easily cause compressor surge. Furthermore, the controlled object in this method is fixed, generally one of the chamber pressure or flow rate, while the requirements for the controlled object vary depending on the operating conditions of a high-load compressor. When operating parameters fluctuate greatly, PID control alone cannot guarantee stable operating conditions and may even cause frequent valve actuation, leading to a decrease in the lifespan of the sealing packing. Simultaneously, when the controller output drops rapidly from greater than 50% to less than 50%, if the adjustable guide vane adjustment speed is insufficient, the adjustable guide vane and anti-surge valve will act simultaneously, potentially interfering with each other and even increasing operational disturbances. Summary of the Invention

[0007] To solve the above-mentioned technical problems, the present invention provides an adjustable guide vane control system for a multi-condition centrifugal compressor. By setting chamber pressure PID and flow PID, and in conjunction with a switching module, the control mode and the controlled object can be switched under multiple conditions.

[0008] The objective of this invention is achieved through the following technical solution: an adjustable guide vane control system for a multi-condition centrifugal compressor, comprising:

[0009] The chamber pressure PID controller takes the chamber pressure setpoint as input, performs PID calculations, and outputs the result to the switching module.

[0010] The flow rate PID is calculated by taking the flow rate setpoint as input and then outputting the result to the switching module.

[0011] The switching module outputs the calculation results of chamber pressure PID or flow PID to the split-range control module according to different centrifugal compressor operating conditions and controlled objects.

[0012] The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule. It splits the range according to the output of the switching module and outputs anti-surge valve control commands or adjustable guide vane control commands to control the opening degree of the anti-surge valve or adjustable guide vane.

[0013] Preferably, the initial value of the split point of the split control module is 50%, which can be adjusted according to actual needs.

[0014] Preferably, the centrifugal compressor operating conditions include: loading, initial stage of warming the compartment, final stage of warming the compartment, replacement, and cooling the compartment; wherein, the replacement operating condition corresponds to flow control, and the other operating conditions select compartment pressure control.

[0015] Preferably, a first rate controller is provided between the switching module and the split-range control module.

[0016] Preferably, the adjustable guide vane split-range control command is first input to the first high selector before controlling the adjustable guide vane. The control system is also equipped with a second speed controller. The second speed controller inputs the opening setpoint and outputs the opening setpoint to the first high selector. The first high selector selects the adjustable guide vane split-range control command or the opening setpoint to control the opening of the adjustable guide vane according to a preset rule.

[0017] Preferably, the first high selector selects the larger value between the adjustable guide vane split-stroke command and the opening setpoint for output.

[0018] Preferably, the control command output by the first high selector enters the second high selector before controlling the adjustable guide vane. The control system is also equipped with a minimum opening calculation module. The minimum opening calculation module calculates the minimum opening of the adjustable guide vane based on the intake and exhaust pressures and outputs it to the second high selector. The second high selector selects the output of the first high selector or the minimum opening to control the opening of the adjustable guide vane according to preset rules.

[0019] Preferably, the second high selector selects the larger of the outputs of the first high selector and the minimum opening.

[0020] The preferred method for determining the minimum flow rate is as follows:

[0021] Let the surge line expression for a high-load compressor be p = k·Q + b, where p is the pressure ratio, Q is the flow rate, k is the slope of the surge line, and b is the intercept of the surge line. Shifting the surge line to the right, we obtain the anti-surge control line, which is expressed as p = k·(Q-Q1) + b. The minimum flow line is obtained by shifting the anti-surge control line to the right again, and its expression is p = k·(Q-Q1-Q2) + b. Based on the inlet and outlet pressures of the high-load compressor, we obtain the pressure ratio, substitute it into the minimum flow line expression, calculate the minimum flow rate, and determine the minimum opening under the current operating conditions based on the minimum flow rate.

[0022] Preferably, if the adjustable guide vane opening decreases during the operation of a high-load compressor, and the flow rate is less than the minimum flow rate, the adjustable guide vane opening will not be further decreased. The opening at this point is the minimum opening under the current operating conditions.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] This invention provides a multi-condition centrifugal compressor adjustable guide vane control system. By setting chamber pressure PID and flow rate PID, and cooperating with a switching module, it realizes the switching of control modes and controlled objects under multiple conditions. By setting a first rate controller, the amplitude of PID fluctuation caused by large fluctuations in operating conditions is limited. By setting a second rate controller and a first high selector, it is ensured that the output value of the setpoint will not cause operating condition disturbance due to rapid changes. The theoretical opening degree is combined with the actual opening degree to ensure stable operation or switching of operating conditions. By setting a minimum opening degree calculation module and a second high selector, the minimum flow rate under the current operating condition is calculated to limit the minimum opening degree of the adjustable guide vane and ensure that the flow rate value during normal operation of the high-load compressor is greater than the minimum flow rate. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the existing high-load compressor process.

[0026] Figure 2 Here is a block diagram of the control principle of an existing high-load compressor;

[0027] Figure 3 This is a schematic diagram of the split-range control of an existing high-load compressor;

[0028] Figure 4 This is a schematic diagram of the adjustable guide vane control principle in an embodiment of the present invention. Figure 1 ;

[0029] Figure 5 This is a schematic diagram of the adjustable guide vane control principle in an embodiment of the present invention. Figure 2 ;

[0030] Figure 6 This is a schematic diagram of the adjustable guide vane control principle in an embodiment of the present invention. Figure 3 ;

[0031] Figure 7 This is a schematic diagram of the minimum flow rate line in an embodiment of the present invention. Detailed Implementation

[0032] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0033] like Figure 4 As shown, the technical solution of the present invention provides an adjustable guide vane control system for a multi-condition centrifugal compressor, comprising:

[0034] The tank pressure PID controller takes the tank pressure setpoint as input, performs PID calculation, and outputs the result to the switching module. The tank pressure PID controller uses the liquid cargo tank pressure as the setpoint and the actual cargo tank pressure as the process value for PID calculation.

[0035] The flow PID is calculated by inputting the flow setpoint, performing PID calculation, and then outputting it to the switching module. The flow PID is calculated using the high-load compressor flow rate as the setpoint and the actual flow rate processed by the compressor as the process value.

[0036] The switching module is used to switch between chamber pressure control mode and flow control mode. Based on different centrifugal compressor operating conditions and the controlled object, it outputs the calculation results of chamber pressure PID or flow PID to the split-range control module. In this embodiment, the outputs of chamber pressure PID and flow PID are selected by the switching module. The selection method depends on the control mode selected under the current operating condition, which is chamber pressure control or flow control. The purpose is to select the appropriate control mode according to the requirements of different operating conditions.

[0037] The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule. It splits the range according to the output of the switching module and outputs anti-surge valve control commands or adjustable guide vane control commands to control the opening degree of the anti-surge valve or adjustable guide vane.

[0038] In one embodiment of the present invention, the initial value of the split point of the split-range control module is 50%, which can be adjusted according to actual needs. When the controller output changes from 0 to 50%, the anti-surge valve opening changes from 100% to 0; when the controller output changes from 50% to 100%, the adjustable guide vane opening changes from the minimum opening to 100%. In this embodiment, the centrifugal compressor operating conditions include: loading, initial warm-up stage, final warm-up stage, displacement, and cooling; wherein, the displacement condition corresponds to flow control, and the other operating conditions select chamber pressure control.

[0039] The operating conditions are described as follows: Loading Condition: This is the target operating condition for the high-load compressor. In this condition, the high-load compressor is responsible for transporting the excess boil-off gas (BOG) generated in the cargo hold during loading to the shore station, ensuring that the pressure in the cargo hold does not exceed the limit. Warming Condition: Before inerting and ventilating the cargo hold, the cargo hold should be heated to ambient temperature to prevent CO2 in the inerting gas from condensing into dry ice and damaging the cargo hold insulation or blocking the liquid cargo passage during the inerting process. The warming condition can be further divided into two stages: the initial warming stage and the final warming stage. The initial warming stage mainly involves heating and vaporizing the LNG that cannot be pumped out of the cargo hold into BOG until all the remaining LNG has vaporized. After all the remaining LNG has vaporized, the warming operation enters the final warming stage. During the initial warming stage, the compressor cannot operate at full load to prevent the remaining LNG in the hold from rapidly evaporating in a short time, causing a rapid increase in tank pressure and damaging the cargo hold insulation. During the final warming stage, the compressor operates at full load, which can shorten the warming operation time. Replacement Process: Before the cargo hold cooling operation, nitrogen in the hold is replaced with natural gas to purge it. Gas replacement is generally performed one hold at a time, sequentially. Cooling Tank Operation: LNG is transferred from the cargo manifold to the spray header at a certain flow rate and injected into the hold through nozzles located in the gas dome. BOG generated during the cooling process is extracted and pressurized by a high-load compressor and transported to the shore terminal. The initial gas volume is the largest, gradually decreasing as the cooling process progresses.

[0040] One variable, named "Flag," is set to distinguish operating conditions, with the following correspondences: Flag = 1, loading; Flag = 2, initial warm-up stage; Flag = 3, end of warm-up stage; Flag = 4, replacement; Flag = 5, cold compartment. Based on the previous operating condition analysis, when Flag = 1 / 2 / 3 / 5, compartment pressure is selected as the controlled object, and the compartment pressure PID controller is enabled; when Flag = 4, flow rate is selected as the controlled object, and the flow rate PID controller is enabled. When the Flag is determined, the corresponding setpoint will be assigned to the corresponding PID controller. The setpoint is obtained from theoretical calculations for that operating condition or input by the operator. The determination methods for the opening setpoint and control rate under different operating conditions are as follows:

[0041] Once the Flag is determined, the corresponding opening setpoint is sent to the second speed controller. This opening setpoint is calculated theoretically for this operating condition. Simultaneously, the control speed setpoint is also sent to the second speed controller. This setpoint is determined based on the operating conditions, using the following method:

[0042] a. When Flag = 1, the flow rate does not change much during the entire loading process, and the high-load compressor will reach the design load at a relatively fast speed. Therefore, the control rate setting value is relatively large at this time.

[0043] b. When Flag = 2, the load of the high-load compressor is 60% of the compressor's maximum load, and the loading speed of the high-load compressor needs to be controlled to avoid being too fast. Therefore, the control rate setting value is relatively small at this time.

[0044] c. When Flag = 3, the load of the high-load compressor is 100% of the compressor's maximum load, and the high-load compressor needs to accelerate the load to shorten the warm-up time. Therefore, the control rate setting value is relatively large at this time.

[0045] d. When Flag = 4, the flow rate is relatively stable throughout the replacement process, and the high-load compressor can reach the design load at a relatively fast speed. Therefore, the control rate setpoint is relatively large at this time.

[0046] e. When Flag=5, the initial flow rate of the high-load compressor is large. As the cooling process progresses, the flow rate gradually decreases. The high-load compressor needs to accelerate to increase the load first and then gradually reduce the load. Therefore, the opening setting value is given in the initial stage and the final stage respectively. A larger control rate setting value is given in the initial stage and a smaller control rate setting value is given in the final stage. That is, the opening setting value is first increased to the initial stage opening setting value at a larger rate and then decreased to the final stage opening setting value at a smaller rate.

[0047] like Figure 5 As shown, in one embodiment of the present invention, a multi-condition centrifugal compressor capable of controlling the output of a switching module is provided. A first rate controller is provided between the adjustable guide vane control system switching module and the split-range control module. The adjustable guide vane split-range control command is first input to a first high-speed selector before controlling the adjustable guide vane. The control system also includes a second rate controller, which inputs an opening setpoint and outputs the opening setpoint to the first high-speed selector. The first high-speed selector selects the larger value between the adjustable guide vane split-range command and the opening setpoint to control the opening of the adjustable guide vane. In this embodiment, the first rate controller (rate controller 1 in the figure) is used to control the rate of change of the PID output, the second rate controller (rate controller 2 in the figure) is used to control the rate of change of the opening setpoint, and the first high-speed selector (high-speed selector 1 in the figure) is used to select the larger value between the adjustable guide vane split-range command and the opening setpoint. In this embodiment, the output of the switching module is limited in rate of change after passing through the first rate controller in order to limit the amplitude of PID fluctuations caused by large fluctuations in operating conditions. Then, split-range control is performed, where the opening command corresponding to 50%-100% enters the first high selector. The opening setpoint is set according to different operating conditions. The second rate controller gradually changes the output from the current value to the setpoint according to the preset time or rate of change, in order to ensure that the output of the setpoint does not cause operating condition disturbances due to excessively rapid changes.

[0048] like Figure 6As shown, in one embodiment of the present invention, an adjustable guide vane control system equipped with a minimum flow calculation module is provided. In this embodiment, the control command output by the first high-pressure selector enters the second high-pressure selector before controlling the adjustable guide vane. The control system is also equipped with a minimum opening calculation module, which calculates the minimum opening of the adjustable guide vane based on the intake and exhaust pressures and outputs it to the second high-pressure selector. The second high-pressure selector selects the larger value between the output of the first high-pressure selector and the minimum opening to control the opening of the adjustable guide vane. Specifically, the minimum opening calculation module is used to calculate the minimum opening value under the current operating conditions, and the second high-pressure selector (high-pressure selector 2 in the figure) is used to select the larger value between the output of the first high-pressure selector and the minimum opening. In this embodiment, the opening setpoint is calculated based on theoretical operating conditions. Different operating conditions have different setpoints, while the opening command after split-range control is the actual calculated value. The purpose of the first high-selector is to combine the theoretical opening with the actual opening to ensure stable operation or switching of operating conditions. The theoretical opening is used for coarse adjustment, and the actual opening is used for fine adjustment. The high-selector uses a larger value because a larger adjustable guide vane opening makes the operation of the high-load compressor safer. The purpose of the second high-selector is to ensure that the flow rate of the high-load compressor during normal operation must be greater than the minimum flow rate. In this embodiment, the minimum opening calculation module calculates the minimum flow rate under the current operating conditions to limit the minimum opening of the adjustable guide vanes.

[0049] like Figure 7 As shown, in one embodiment of the present invention, a method for determining the minimum opening of an adjustable guide vane is provided. When calculating the minimum opening, the surge curve of a high-load compressor is shifted to the right to obtain the minimum opening line. The current pressure ratio (exhaust pressure / intake pressure) is substituted into the expression of the minimum opening line to obtain the minimum flow rate. Specifically, the expression for the surge curve of a high-load compressor is p = k·Q + b, where p is the pressure ratio, Q is the flow rate, k is the slope of the surge curve, and b is the intercept of the surge curve. The surge curve is shifted to the right to obtain the anti-surge control line, whose expression is p = k·(Q-Q1) + b. The minimum flow rate line is obtained by further shifting the anti-surge control line to the right, and its expression is p = k·(Q-Q1-Q2) + b. The pressure ratio is obtained based on the intake and exhaust pressures of the high-load compressor and substituted into the expression of the minimum flow rate line to calculate the minimum flow rate. The minimum opening under the current operating condition is determined based on the minimum flow rate. If the adjustable guide vane opening decreases during high-load compressor operation, and the flow rate is less than the minimum flow rate, the adjustable guide vane opening will no longer decrease. The opening at this point is the minimum opening under the current operating conditions.

[0050] The adjustable guide vane control system for a multi-condition centrifugal compressor provided by this invention has the following advantages:

[0051] 1. By using a variable minimum opening value, the risk of high-load compressors entering the surge zone during operating condition fluctuations is reduced, thus improving safety;

[0052] 2. Appropriate control objects were selected for different operating conditions, which improved the stability of process control;

[0053] 3. By controlling the opening setpoint and rate, the operational stability of the high-load compressor under drastic fluctuations in operating conditions is improved;

[0054] 4. The original split-range control algorithm is retained, which can still achieve energy-saving operation of the compressor under high load.

[0055] 5. The method is simple and clear in principle, and highly operable.

[0056] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A multi-condition centrifugal compressor adjustable guide vane control system, characterized in that: The control system includes: The pressure PID controller takes the pressure setpoint as input, performs PID calculations, and outputs the result to the switching module. The flow rate PID is calculated by taking the flow rate setpoint as input and then outputting the result to the switching module. The switching module outputs the calculation results of chamber pressure PID or flow PID to the split-range control module according to different centrifugal compressor operating conditions and controlled objects; The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule. It splits the range according to the output of the switching module and outputs anti-surge valve control commands or adjustable guide vane control commands to control the opening degree of the anti-surge valve or adjustable guide vane. A first rate controller is provided between the switching module and the split-range control module; Before controlling the adjustable guide vane, the adjustable guide vane split-range control command is first input to the first high selector. The control system is also equipped with a second speed controller. The second speed controller inputs the opening setpoint and outputs the opening setpoint to the first high selector. The first high selector selects the adjustable guide vane split-range control command or the opening setpoint to control the opening of the adjustable guide vane according to a preset rule.

2. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 1, characterized in that: The initial value of the split point of the split control module is 50%, which can be adjusted according to actual needs.

3. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 2, characterized in that: The centrifugal compressor operating conditions include: loading, initial warming stage, end warming stage, replacement, and cooling; among them, the replacement operating condition corresponds to flow control, and the other operating conditions select chamber pressure control.

4. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 3, characterized in that: The first high selector selects the larger value between the adjustable guide vane split-stroke command and the opening setpoint and outputs it.

5. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 4, characterized in that: Before controlling the adjustable guide vane, the control command output by the first high selector enters the second high selector. The control system is also equipped with a minimum opening calculation module. The minimum opening calculation module calculates the minimum opening of the adjustable guide vane based on the intake and exhaust pressures and outputs it to the second high selector. The second high selector selects the output of the first high selector or the minimum opening to control the opening of the adjustable guide vane according to a preset rule.

6. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 5, characterized in that: The second high selector selects the larger of the outputs of the first high selector and the minimum opening.

7. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 6, characterized in that: The method for determining the minimum flow rate is as follows: Let the surge line expression for a high-load compressor be p = k·Q + b, where p is the pressure ratio, Q is the flow rate, k is the slope of the surge line, and b is the intercept of the surge line. Shifting the surge line to the right, we obtain the anti-surge control line, which is expressed as p = k·(Q-Q1) + b. The minimum flow line is obtained by shifting the anti-surge control line to the right again, and its expression is p = k·(Q-Q1-Q2) + b. Based on the inlet and outlet pressures of the high-load compressor, we obtain the pressure ratio, substitute it into the minimum flow line expression, calculate the minimum flow rate, and determine the minimum opening under the current operating conditions based on the minimum flow rate.

8. The adjustable guide vane control system for a multi-condition centrifugal compressor as described in claim 7, characterized in that: If the adjustable guide vane opening decreases during high-load compressor operation, and the flow rate is less than the minimum flow rate, the adjustable guide vane opening will no longer decrease. The opening at this point is the minimum opening under the current operating conditions.

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