Adjustable guide vane control system of multi-working-condition centrifugal compressor

Abstract

The invention discloses an adjustable guide vane control system of a multi-working-condition centrifugal compressor, which comprises a cabin pressure PID (Proportion Integration Differentiation), a cabin pressure set value is input, and after PID operation is carried out, the cabin pressure set value is output to a switching module. And the flow PID is used for inputting a flow set value, carrying out PID operation and then outputting the flow set value to the switching module. And the switching module is used for outputting the calculation result of the cabin pressure PID or the flow PID to the split-ranging control module according to different working conditions and control objects of the centrifugal compressor. And the split-range control module comprises an anti-surge valve and an adjustable guide vane control sub-module, performs split-range according to the output of the switching module, and outputs an anti-surge valve control instruction or an adjustable guide vane control instruction to control the opening degree of the anti-surge valve or the adjustable guide vane. Appropriate control objects are selected for different working conditions, and the stability of process control is improved; and through the opening given value and speed control, the operation stability of the high-load compressor under severe fluctuation of the working condition is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of compressor control, and in particular relates to an adjustable guide vane control system for a multi-operating-mode centrifugal compressor. Background Art

[0002] The current liquefied natural gas (LNG) transportation market is developing rapidly, and large LNG carriers are an important tool for transporting LNG. During operation, a large amount of boil-off gas (BOG), nitrogen or inert gas will be generated in the cargo hold during the processes of LNG loading, warming up, gas replacement and initial cooling of the cargo hold. At this time, the BOG needs to be returned to the shore station for processing through a high-load low-temperature gas compressor (also known as a return compressor, hereinafter referred to as a high-load compressor) to maintain the cargo hold pressure within a safe range. Since high-load compressors have a large flow rate and a small pressure ratio, centrifugal compressors are generally used.

[0003] In different process steps and at different stages of the same process, the intake and exhaust parameters, gas flow rate, gas properties and compressor characteristic curve of high-load compressors are undergoing complex changes. Therefore, the control method of the compressor is the key to ensuring the safe, efficient and stable operation of the compressor under different working conditions.

[0004] The process flow of high load compressor is as follows Figure 1 As shown in FIG. 1 , it includes a cargo tank, a pressure transmitter, a flow transmitter, an adjustable guide vane, a high-load compressor, an anti-surge valve and a shore station. The control method is to use the cargo tank pressure or flow as the control parameter, and after passing through the PID controller, control the adjustable guide vane and the anti-surge valve in a split-range manner, such as 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 in the figure. Figure 3 shown.

[0005] In the above control mode, 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 actuated to adjust the pressure or gas volume in the cabin; the minimum opening A is obtained based on the compressor surge curve and characteristic curve, which can prevent the compressor surge caused by too small an opening during the closing process of the adjustable guide vane.

[0006] Since the minimum opening A is a fixed value, the above method is only applicable to applications with relatively stable working conditions. When the working parameters fluctuate greatly, it is easy to cause compressor surge. In addition, the control object of the above method is fixed, generally one of the cabin pressure and flow rate, and the requirements for the control object of different working conditions of high-load compressors are different; when the working parameters fluctuate greatly, PID adjustment alone cannot guarantee the stability of the working conditions, and may even cause the valve to operate frequently, resulting in a decrease in the life of the sealing packing. At the same time, when the controller output drops rapidly from more than 50% to less than 50%, if the adjustable guide vane adjustment speed is not enough at this time, the adjustable guide vane and the anti-surge valve will act at the same time, and the two may interfere with each other, and even increase the disturbance of the working conditions. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a centrifugal compressor adjustable guide vane control system for multiple working conditions, which realizes the switching of control modes and control objects under multiple working conditions by setting cabin pressure PID and flow PID and cooperating with a switching module.

[0008] The objective of the present invention is achieved through the following technical solutions: a multi-operating-mode centrifugal compressor adjustable guide vane control system, comprising:

[0009] Cabin pressure PID, input cabin pressure set value, perform PID calculation, and output to the switching module.

[0010] Flow PID, input the flow set value, perform PID calculation, and output it to the switching module.

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

[0012] The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule, which performs split-range according to the output of the switching module, and outputs an anti-surge valve control instruction or an adjustable guide vane control instruction to control the opening of the anti-surge valve or the adjustable guide vane.

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

[0014] Preferably, the centrifugal compressor operating conditions include: loading, initial stage of warm-up, final stage of warm-up, replacement and cold-up; wherein the replacement operating condition corresponds to flow control, and the remaining operating conditions select cabin pressure control.

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

[0016] Preferably, the adjustable guide vane range control instruction is first input into the first high selector before controlling the adjustable guide vane. The control system is also provided with a second rate controller. The second rate controller inputs an opening given value and outputs the opening given value to the first high selector. The first high selector selects the adjustable guide vane range control instruction or the opening given value according to preset rules to control the adjustable guide vane opening.

[0017] Preferably, the first high selector selects the larger value output between the adjustable guide vane range instruction and the opening given value.

[0018] Preferably, the control instruction output by the first high selector enters the second high selector before controlling the adjustable guide vane. The control system is also provided with a minimum opening calculation module. The minimum opening calculation module calculates the minimum opening of the adjustable guide vane according to 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 a larger output between the output of the first high selector and the minimum opening.

[0020] Preferably, the method for determining the minimum flow rate is as follows:

[0021] Assume that the surge line expression of the 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 line, and b is the intercept of the surge line; the surge line is shifted to the right to obtain the anti-surge control line, and its expression is p=k·(Q-Q1)+b. The minimum flow line is obtained by continuing to shift the anti-surge control line to the right, and its expression is p=k·(Q-Q1-Q2)+b. The pressure ratio is obtained according to the inlet and exhaust pressures of the high-load compressor, and is substituted into the minimum flow line expression to calculate the minimum flow. The minimum opening under the current operating conditions is determined according to the minimum flow.

[0022] Preferably, if the opening of the adjustable guide vane decreases during operation of the high-load compressor, when the flow rate is less than the minimum flow rate, the opening of the adjustable guide vane is no longer reduced, and the opening at this time is the minimum opening under the current operating conditions.

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

[0024] The present invention provides a multi-operating condition centrifugal compressor adjustable guide vane control system, which realizes the switching of control modes and control objects under multiple operating conditions by setting a cabin pressure PID and a flow PID, and cooperating with a switching module, and limits the PID fluctuation amplitude caused by large operating condition fluctuations by setting a first rate controller; by setting a second rate controller and a first high selector, it is ensured that the given value output will not cause operating condition disturbances due to too fast changes, and the theoretical opening is combined with the actual opening to ensure stable operation or switching of the operating condition; by setting a minimum opening calculation module and a second high selector, the minimum flow rate under the current operating condition is calculated to limit the minimum opening of the adjustable guide vane, and ensure that the flow value during normal operation of the high-load compressor is greater than the minimum flow rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the process flow of the existing high-load compressor;

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

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

[0028] Figure 4 Schematic diagram of the control principle of the adjustable guide vane in the embodiment of the present invention Figure 1 ;

[0029] Figure 5 Schematic diagram of the control principle of the adjustable guide vane in the embodiment of the present invention Figure 2 ;

[0030] Figure 6 Schematic diagram of the control principle of the adjustable guide vane in the embodiment of the present invention Figure 3 ;

[0031] Figure 7 Schematic diagram of the minimum flow line in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

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

[0034] The tank pressure PID inputs the tank pressure set value, performs PID calculation, and then outputs it to the switching module. The tank pressure PID performs PID calculation with the liquid cargo tank pressure as the set value and the actual cargo tank pressure as the process value.

[0035] Flow PID, input flow set value, perform PID calculation, and then output it to the switching module. Flow PID uses the high-load compressor flow as the set value and the actual compressor processed flow as the process value for PID calculation.

[0036] The switching module is used for switching between the cabin pressure control mode and the flow control mode. It outputs the calculation results of the cabin pressure PID or the flow PID to the split-range control module according to different centrifugal compressor working conditions and control objects. In this embodiment, the outputs of the cabin pressure PID and the flow PID are selected by the switching module. The selection method depends on the control mode selected under the current working condition, which are cabin pressure control and flow control, respectively. The purpose is to select the appropriate control mode according to the requirements of different working conditions.

[0037] The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule, which performs split-range according to the output of the switching module, and outputs an anti-surge valve control instruction or an adjustable guide vane control instruction to control the opening of the anti-surge valve or the adjustable guide vane.

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

[0039] Among them, each working condition is described as follows: Loading condition: The design target working condition of the high-load compressor. In this working 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 to ensure that the pressure in the cargo hold does not exceed the limit value. Warming-up condition: Before the cargo hold is inerted and ventilated, the cargo hold should be heated to ambient temperature to prevent the CO2 in the inert gas from condensing into dry ice during the inerting operation, damaging the insulation layer of the cargo hold or blocking the liquid cargo channel. The warming-up condition can be divided into two stages, namely the initial stage of warming-up and the end stage of warming-up. The initial stage of warming-up is mainly to heat and vaporize the LNG that cannot be pumped out of the cargo hold into BOG until all the residual LNG is vaporized and volatilized. When the residual LNG is completely vaporized, the warming-up operation enters the end stage of warming-up. In the initial stage of warming-up, the compressor cannot run at full load to prevent the remaining LNG in the hold from evaporating rapidly in a short period of time, and the cabin pressure rises rapidly, causing damage to the insulation layer of the cargo hold. In the end stage of warming-up, the compressor runs at full load, which can shorten the time of warming-up operation. Replacement process: Before the cargo hold cooling operation, the nitrogen in the hold should be replaced by natural gas. When replacing the gas, it is usually replaced in sequence in a single hold. Cold hold working condition: LNG is transferred from the liquid cargo manifold to the spray main pipe at a certain flow rate, and sprayed into the hold through the nozzle located in the gas dome. The BOG generated during the cooling process is extracted and pressurized by the high-load compressor and then transported to the shore station. The initial gas volume is the largest, and the gas volume gradually decreases as the cooling process proceeds.

[0040] Set a variable to distinguish the working conditions. The variable name is Flag, and the corresponding relationship is: Flag = 1, loading; Flag = 2, the initial stage of warm-up; Flag = 3, the end stage of warm-up; Flag = 4, replacement; Flag = 5, cold cabin. According to the previous working condition analysis, when Flag = 1 / 2 / 3 / 5, select the cabin pressure as the control object, and the cabin pressure PID is enabled; when Flag = 4, select the flow as the control object, and the flow PID is enabled. When the Flag is determined, the corresponding set value will be given to the corresponding PID. The set value is obtained according to the theoretical calculation of the working condition or input by the operator. The method for determining the opening set value and control rate under different working conditions is as follows:

[0041] When the Flag is determined, the corresponding opening given value will be given to the second rate controller, and the opening given value is obtained according to the theoretical calculation under the working condition. At the same time, the control rate setting value will be given to the second rate controller, and the setting value is determined according to the working condition. The determination method is as follows:

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

[0043] b. When Flag = 2, the load of the high-load compressor is 60% of the maximum load of the compressor, and the high-load compressor needs to control the loading speed not to be too fast, so the control rate setting value at this time is relatively small;

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

[0045] d. When Flag = 4, the flow rate is relatively stable during the entire replacement process, and the high-load compressor can reach the design load at a faster speed, so the control rate setting value is relatively large at this time;

[0046] e. When Flag = 5, the initial flow of the high-load compressor is large. As the cooling process progresses, the flow 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 end stage respectively, and a larger control rate setting value is given in the initial stage, and a smaller control rate setting value is given in the end stage, that is, it is first increased to the opening setting value of the initial stage at a larger rate, and then decreased to the opening setting value of the end stage at a smaller rate.

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

[0048] like Figure 6As shown, in one embodiment of the present invention, an adjustable guide vane control system provided with a minimum flow calculation module is provided. In this embodiment, the control instruction output by the first high selector enters the second high selector before controlling the adjustable guide vane. The control system is also provided with a minimum opening calculation module. The minimum opening calculation module calculates the minimum opening of the adjustable guide vane according to the intake and exhaust pressures and outputs it to the second high selector. The second high selector selects the larger value of the output of the first high selector and the minimum opening to control the adjustable guide vane opening. Among them, the minimum opening calculation module is used to calculate the minimum opening value under the current working condition, and the second high selector (high selector 2 in the figure) is used to select the larger value of the output of the first high selector and the minimum opening. In this embodiment, the given value of the opening is calculated based on the theoretical operating conditions. Different operating conditions have different given values, and the opening instruction after the split-range control belongs to the actual calculated value. The purpose of the first high selector is to combine the theoretical opening with the actual opening to ensure the stable operation or switching of the operating conditions, use the theoretical opening for coarse adjustment, and use the actual opening for fine adjustment; the high selector is used to take a large value because a larger adjustable guide vane opening will make the high-load compressor run safer. The purpose of the second high selector is to ensure that the flow value during the normal operation of the high-load compressor must be greater than the minimum flow. In this embodiment, the minimum opening calculation module calculates the minimum flow 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 the high-load compressor is shifted to the right to obtain the minimum opening line, and the current pressure ratio (exhaust pressure / intake pressure) value is substituted into the expression of the minimum opening line to obtain the minimum flow value, which is specifically as follows: suppose the surge line expression of the high-load compressor is p=k·Q+b, wherein p is the pressure ratio, Q is the flow, k is the slope of the surge line, and b is the intercept of the surge line; the surge line is shifted to the right to obtain an anti-surge control line, and its expression is p=k·(Q-Q1)+b, and the minimum flow line is obtained by continuing to shift the anti-surge control line to the right, and its expression is p=k·(Q-Q1-Q2)+b, and the pressure ratio is obtained according to the inlet and exhaust pressures of the high-load compressor, and substituted into the expression of the minimum flow line to calculate the minimum flow, and the minimum opening under the current operating conditions is determined according to the minimum flow. If the adjustable guide vane opening is reduced during the operation of a high-load compressor, when the flow rate is less than the minimum flow rate, the adjustable guide vane opening will no longer be reduced, and the opening at this time is the minimum opening under the current operating conditions.

[0050] The invention provides a centrifugal compressor adjustable guide vane control system with multiple operating conditions, which has the following advantages:

[0051] 1. The variable minimum opening value reduces the risk of high-load compressors entering the surge zone when operating conditions fluctuate, thereby improving safety;

[0052] 2. Appropriate control objects are selected for different working conditions to improve the stability of process control;

[0053] 3. The operating stability of high-load compressors under severe fluctuations in operating conditions is improved through opening setpoint and rate control;

[0054] 4. The original split-range control algorithm is retained, and energy-saving operation of high-load compressors can still be achieved.

[0055] 5. The principle of the method is simple and clear, and it is highly operational.

[0056] The above are preferred embodiments of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A multi-operating-mode centrifugal compressor adjustable guide vane control system, characterized in that: The control system comprises: Cabin pressure PID, input cabin pressure set value, perform PID calculation, and output to the switching module; Flow PID, input flow set value, perform PID calculation, and output to the switching module; The switching module outputs the calculation result of the cabin pressure PID or flow PID to the split-range control module according to different centrifugal compressor working conditions and control objects; The split-range control module includes an anti-surge valve and an adjustable guide vane control submodule, which performs split-range according to the output of the switching module, and outputs an anti-surge valve control instruction or an adjustable guide vane control instruction to control the opening of the anti-surge valve or the adjustable guide vane.

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

3. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 2, characterized in that: The centrifugal compressor working conditions include: loading, initial stage of warm-up, end stage of warm-up, replacement and cold-up; among them, the replacement working condition corresponds to flow control, and the other working conditions select cabin pressure control.

4. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 1, characterized in that: A first rate controller is arranged between the switching module and the step control module.

5. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 4, characterized in that: The adjustable guide vane range control instruction is first input into the first high selector before controlling the adjustable guide vane. The control system is also provided with a second rate controller. The second rate controller inputs an opening given value and outputs the opening given value to the first high selector. The first high selector selects the adjustable guide vane range control instruction or the opening given value according to preset rules to control the adjustable guide vane opening.

6. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 5, characterized in that: The first high selector selects the larger value of the adjustable guide vane range instruction and the opening given value for output.

7. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 6, characterized in that: The control instruction output by the first high selector enters the second high selector before controlling the adjustable guide vane. The control system is also provided with a minimum opening calculation module. The minimum opening calculation module calculates the minimum opening of the adjustable guide vane according to 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.

8. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 7, characterized in that: The second high selector selects a larger output between the output of the first high selector and the minimum opening.

9. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 8, characterized in that: The method for determining the minimum flow rate is as follows: Assume that the surge line expression of the 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 line, and b is the intercept of the surge line; the surge line is shifted to the right to obtain the anti-surge control line, and its expression is p=k·(Q-Q1)+b. The minimum flow line is obtained by continuing to shift the anti-surge control line to the right, and its expression is p=k·(Q-Q1-Q2)+b. The pressure ratio is obtained according to the inlet and exhaust pressures of the high-load compressor, and is substituted into the minimum flow line expression to calculate the minimum flow. The minimum opening under the current operating conditions is determined according to the minimum flow.

10. The adjustable guide vane control system for a multi-operating-mode centrifugal compressor according to claim 9, characterized in that: If the adjustable guide vane opening is reduced during the operation of a high-load compressor, when the flow rate is less than the minimum flow rate, the adjustable guide vane opening will no longer be reduced, and the opening at this time is the minimum opening under the current operating conditions.

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