CN108591041B - Feedwater pump minimum flow recirculation valve control system and method - Google Patents

Abstract

A minimum flow rate recirculation valve control system and method for a feedwater pump, used to drive an adjustment device for the feedwater pump to adjust the power of the drive equipment of the feedwater pump, and drive the variable speed feedwater pump through a rotating shaft to control the feedwater flow by changing the speed; The water pump sucks the working fluid from the upstream, and pumps the working fluid to the downstream system through the variable speed feed water pump. The inlet pressure signal measured by the inlet pressure sensor is sent to the saturation temperature module, which is used to calculate the saturation temperature corresponding to the inlet pressure according to the physical properties of water and water vapor. , the saturation temperature output by the saturation temperature module and the outlet temperature signal output by the outlet temperature sensor are sent to the minimum flow protection recirculation controller at the same time, and the outlet temperature output by the outlet temperature sensor is controlled to be lower than the corresponding pump inlet pressure output by the saturation temperature module Saturation temperature, when the difference between the two is lower than the preset value, the minimum flow recirculation valve of the feed water pump is opened to send a part of the hydraulic fluid from the outlet pipe of the variable speed feed water pump back to the upstream pipe of the variable speed feed water pump.

Description

Feedwater pump minimum flow recirculation valve control system and method

technical field

The invention relates to the field of automatic control of a thermal power plant, in particular to a control system and method for a minimum flow recirculation valve of a feed water pump.

Background technique

The minimum flow valve can ensure the minimum flow rate of the feedwater pump. When the outlet flow rate of the feedwater pump is lower than the minimum flow rate, the minimum flow valve will automatically open to return a part of the feedwater to the deaerator to prevent the feedwater pump from working below the minimum flow rate. The control of the minimum flow valve is related to the safe operation of the feed water pump, but the conventional control logic design only considers the start-stop conditions of short-term operation. With the in-depth transformation of my country's industrial structure and the rapid development of new energy power generation, thermal power units are faced with a strong demand for improved operational flexibility and deep peak regulation. During deep peak regulation, the operating condition of the unit deviates from the rated operating condition more, the flow rate of the feed water pump is low, and the minimum flow protection action needs to be frequent. Under the new demand for deep peak-shaving operation, there are often a series of problems such as severe fluctuations in low-load water supply and serious wear of the minimum flow valve:

(1) The flow measurement uncertainty and fluctuation of the minimum flow protection of the feed water pump are high

The outlet flow measurement point of the feed water pump itself works in a high-pressure area, so the measurement accuracy is not high. With the decrease of load and feed water flow, the measurement deviation is significantly enlarged, and the fluctuation of signal data is also more obvious. However, the signal measurement of the minimum flow protection control loop is currently Without correction of low load under deep peak shaving, and without necessary filtering, it is very easy to cause large vibration of the adjustment valve and then cause oscillation of water supply control, and may even threaten the safety of the pump due to the amplification of measurement error at low load.

(2) The commonly used minimum flow valve control strategy is not suitable for operation in deep peak shaving conditions

The minimum flow protection logic of the common feed water pump adopts the most conventional PID control method, and the control target is set at 230t/h, which means that when the outlet flow of the feed water pump is less than 230, the valve is opened to increase the flow to 230t/h and when it is higher than 230t/h When the h flow rate is turned off, it is necessary to keep the pump working at the minimum flow rate under low load. Since the control target of the pump is the water level of the steam drum, the switch of the minimum flow valve will affect the control of the pump, which will cause the fluctuation of the pump outlet flow. Due to the coupling oscillation of the PID control, and because there is a certain control overshoot in the PID control, it is very easy to cause the pump to work at a condition of less than the minimum flow rate during the control fluctuation process, which has an adverse effect on the safety of the pump.

(3) The operation law of the minimum flow protection fixed value no-load variable speed pump

The minimum flow protection of the feed water pump is to prevent the pump from cavitation due to small flow and threaten the safety of the pump. Therefore, the minimum flow protection setting is related to the speed of the variable speed pump. As the pump speed decreases, the protection setting also changes lowered. In the prior art, the minimum flow protection setting value of the feed water pump is constant, which is generally set according to the characteristics of the pump at the rated speed. During the deep peak-shaving operation, the speed of the feed pump decreases a lot, and the protection setting value also needs to decrease with the speed in theory. , if it is controlled according to the protection fixed value, it will inevitably cause the recirculation to run in the open state for a long time, which will not only cause the valve flushing loss, but also cause long-term economic loss in operation.

Contents of the invention

In view of the above existing problems in the prior art, the present invention proposes a minimum flow recirculation valve control system and method for feed water pumps, which can avoid the switching action of the minimum flow valve in most working conditions during low-load operation, with better stability and energy saving The effect is obvious.

In order to achieve the above object, the present invention adopts following technical scheme:

A control system for the minimum flow recirculation valve of a feed water pump, comprising an adjustment device 1 for driving the feed water pump, a drive device 2 of the feed water pump connected to the adjustment device 1 , and a variable speed variable speed valve connected to the drive device 2 of the feed water pump through a rotating shaft 3 Feed water pump 4, inlet pressure sensor 5 installed at the inlet of variable speed feed water pump 4, outlet temperature sensor 6 installed at the outlet of variable speed feed water pump 4, connected with inlet pressure sensor 5 for calculation according to the physical properties of water and water vapor The saturation temperature module 7 of the saturation temperature corresponding to the inlet pressure, the minimum flow protection recirculation controller 8 connected with the outlet temperature sensor 6, the saturation temperature module 7 is connected with the minimum flow protection recirculation controller 8, and is used to send the saturation temperature to the minimum The flow protection recirculation controller 8, the minimum flow protection recirculation controller 8 is used to compare the outlet temperature output by the outlet temperature sensor 6 and the saturation temperature corresponding to the pump inlet pressure output by the saturation temperature module 7; it also includes the minimum flow protection recirculation The controller 8 and the feedwater pump minimum flow recirculation valve 9 connected to the outlet pipeline of the variable speed feedwater pump 4, when the saturation temperature deviation corresponding to the outlet temperature output by the outlet temperature sensor 6 and the pump inlet pressure output by the saturation temperature module 7 is lower than the predetermined When setting the value, the minimum flow protection recirculation controller 8 opens the minimum flow recirculation valve 9 of the feedwater pump, and sends a part of the hydraulic fluid from the outlet pipeline of the variable speed feedwater pump 4 back to the upstream pipeline of the variable speed feedwater pump 4 .

The minimum flow rate recirculation valve control system of the feedwater pump is based on the measurement of the pressure at the inlet of the variable speed feedwater pump 4 and the temperature signal at the outlet, starting from the minimum flow protection principle of the variable speed feedwater pump 4, by comparing the variable speed feedwater pump 4 The difference between the outlet temperature of the feed water pump 4 and the saturation temperature corresponding to the inlet pressure is used to control the opening of the minimum flow valve; the minimum flow protection recirculation controller 8 can adopt a mature conventional PID controller, and use the proportional, integral and differential functions to always It is ensured that the deviation between the outlet temperature of the variable speed feed water pump 4 and the saturation temperature corresponding to the inlet pressure is smaller than the preset protection margin and control error.

The minimum flow rate recirculation valve control system of the feedwater pump does not set an absolute limit on the minimum flow rate, uses the working principle of the variable speed feedwater pump 4, and compares the deviation between the outlet temperature of the variable speed feedwater pump 4 and the saturation temperature corresponding to the inlet pressure , under the condition of variable speed, it can avoid the risk of cavitation caused by the variable speed feed water pump 4 working or mixing high temperature sealing water, and automatically adapt to the speed change of the variable speed feed water pump 4, without opening the minimum flow recirculation valve prematurely when reducing the load 9, thereby greatly reducing the damage to the stability of the water supply system and the life of the minimum flow recirculation valve 9 when the unit is in deep peak regulation.

The control method for the minimum flow rate recirculation valve control system of the feedwater pump is used to drive the adjustment device 1 of the feedwater pump to adjust the power of the drive equipment 2 of the feedwater pump, and drive the variable speed feedwater pump 4 through the rotating shaft 3. By changing the speed Realize the control of the feed water flow rate; the variable speed feed water pump 4 sucks the working fluid from the upstream, and sends the working fluid to the downstream system through the variable speed feed water pump 4, and the inlet pressure signal measured by the inlet pressure sensor 5 is sent to the saturation temperature module 7 for Calculate the saturation temperature corresponding to the inlet pressure according to the physical properties of water and water vapor. The saturation temperature output by the saturation temperature module 7 and the outlet temperature signal output by the outlet temperature sensor 6 are simultaneously sent to the minimum flow protection recirculation controller 8 to control the outlet temperature sensor. The outlet temperature output by 6 is lower than the saturation temperature corresponding to the pump inlet pressure output by the saturation temperature module 7. When the deviation between the two is lower than the preset value, the minimum flow protection recirculation controller 8 opens the minimum flow recirculation valve 9 of the feed water pump, Send part of the hydraulic medium from the outlet pipe of the variable speed feed water pump 4 back to the upstream pipe of the variable speed feed water pump 4, the feed water flow rate decreases, and the unit will automatically open the large adjustment device 1 to increase the speed of the variable speed feed water pump 4 and the pumping rate. In this way, the working flow rate of the variable-speed feedwater pump 4 is increased, the efficiency is increased, and the working point is further away from the risk of cavitation, so that the adaptive speed change realizes the protection of the pump and avoids economic losses caused by unnecessary bypass flow.

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

1. The protection of the minimum flow rate of the feed water pump starts from the basic principle of pump cavitation, does not use the minimum flow rate setting value of the rated working condition for protection control, directly uses the parameters of steam and water, and ensures that the working flow rate of the pump is at a safe level from the source. Compared with the current For the mainstream fixed-value flow protection, it is more suitable for the new situation of thermal power units with deep peak-shaving operation. It can avoid the switching action of the minimum flow valve in most working conditions during low-load operation, with better stability and obvious energy-saving effect.

2. The calculation of NPSH required in the design stage of the pump, and the cavitation analysis and calculation of related mainstream methods are only related to the pressure and temperature parameters of the pump inlet, but the position where cavitation occurs first is generally in the working area of the pump impeller , during the rotation of the impeller of the pump, due to the mechanical movement, a local low pressure will be formed on the suction surface of the impeller, and at the same time, it will cause the temperature of the fluid working medium to rise, which will easily make the local temperature lower than the saturation temperature corresponding to the local low pressure. It is easy to make the local temperature higher than the saturation temperature corresponding to the local pressure, resulting in flash evaporation and cavitation damage. According to the above analysis, it can be seen that there is a certain deviation between the local low pressure and high temperature and the pump inlet parameters, and a larger safety margin should be adopted according to the inlet parameters. The present invention adopts the inlet pressure measuring point and the outlet temperature measuring point as the protection benchmark, because the outlet temperature increases due to the enthalpy rise of the pump, and it is safer for protection than the inlet parameters; on the other hand, when the flow rate and rotating speed decrease, the protection conditions are correspondingly Adaptive changes occur. In comparison, the existing technology uses inlet parameters to judge. Generally, only the NPSH corresponding to the rated working condition is used for protection, but in the low-load and low-flow working conditions, there is often a lack of data on variable working conditions. As a result, the minimum flow recirculation valve opens prematurely during the load reduction process, which not only causes unnecessary economic losses in recirculation, but also seriously interferes with the water supply control of the unit during the switching process, bringing a series of hidden dangers to safety and valve life.

3. For feedwater pumps with liquid seals, low flow and low speed conditions can easily cause sealing water to mix into the feedwater, further aggravating the risk of cavitation, and this type of temperature rise can be further increased by the increase in the outlet temperature of the feedwater pump. Reflect, thereby compensating for the resulting minimum flow protection risk. Therefore, the system of the present invention can still effectively function for the liquid-sealed water pump, but the conventional structure and method for judging cavitation cannot be applied.

Description of drawings

Fig. 1 is a diagram of the minimum flow recirculation control system of the feed water pump in the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Technical principle of the present invention is as follows:

The minimum flow rate of the centrifugal pump is generally measured and provided by the pump factory. Generally, the allowable NPSH represented by the hydraulic height difference is used as the design standard to avoid the pump working under the cavitation condition as much as possible during the design, and the design has The minimum flow recirculation system is used to increase the flow through the pump when the flow rate of the process system is below a certain level, so that the pump can avoid the occurrence of cavitation damage.

According to the principle of the centrifugal pump, when the pump is running at a small flow rate, the working condition deviates from the most economical working condition, and the efficiency drops significantly. Due to the movement of the impeller of the pump and the action on the working medium to generate temperature rise, and cause the local temperature to be higher than the saturation temperature corresponding to the local pressure, flashing and cavitation of the working medium will occur, causing damage to the pump. When the liquid temperature reaches the effective NPSH of the system equal to the necessary NPSH of the pump, this temperature is the protection temperature for no cavitation, and the pump needs to maintain a flow rate higher than this temperature, which is to determine the pump General method for minimum continuous heat flow.

However, due to the remarkable energy-saving effect, current high-power thermal power units use variable-speed feed pumps driven by steam turbines or variable-frequency motors. Since the effective NPSH and the minimum protection flow rate of the variable speed pump decrease with the decrease of the speed, they are far less than the protection value corresponding to the rated speed. However, most of the large-power thermal power units generally adopt the above-mentioned method to determine according to the rated parameters. The minimum flow value is protected and controlled, which directly leads to the premature opening of the minimum flow recirculation valve under low load, which not only causes unnecessary economic losses, but also the opening of the minimum flow recirculation valve will directly affect the stability of the water supply control, which is likely to cause frequent switching and oscillation. In the context of deep peak shaving, the above problems may cause stability problems during deep peak shaving and thus cause non-stop risks.

The composition of the minimum flow recirculation valve control system of the feed water pump proposed by the present invention is shown in Fig. 1 . The adjusting device 1 for driving the feedwater pump adjusts the power of the driving device 2 of the feedwater pump, and drives the variable speed feedwater pump 4 through the rotating shaft 3 to realize the control of the feedwater flow by changing the speed. The variable speed feed water pump 4 sucks working fluid from the upstream, and an inlet pressure sensor 5 is installed near the pump inlet, and the variable speed feed water pump 4 sends the working fluid to the downstream system for use, and an outlet temperature sensor 6 is installed near the pump outlet. The inlet pressure signal measured by the inlet pressure sensor 5 is sent to the saturation temperature module 7 for calculating the saturation temperature corresponding to the inlet pressure according to the physical properties of water and water vapor. The temperature signal is sent to the minimum flow protection recirculation controller 8 at the same time to control the outlet temperature output by the outlet temperature sensor 6 to be lower than the saturation temperature corresponding to the pump inlet pressure output by the saturation temperature module 7. When the deviation between the two is lower than a certain value (such as 0), the minimum flow protection recirculation controller 8 opens the minimum flow recirculation valve 9 of the feedwater pump, and sends a part of the hydraulic fluid from the outlet pipeline of the variable speed feedwater pump 4 back to the upstream pipeline of the variable speed feedwater pump 4, and the feedwater flow rate decreases. The unit will automatically open the adjustment device 1 to increase the speed and pumping flow of the variable speed feed water pump 4, so that the working flow of the variable speed feed water pump 4 increases, the efficiency increases, and the working point is further away from the risk of cavitation, so as to adapt to the speed The change realizes the protection of the pump and avoids the economic loss caused by unnecessary bypass flow.

Example:

As shown in Figure 1, when the unit is in deep peak-shaving operation, the feedwater flow required by the unit is greatly reduced, and the adjustment device 1 automatically adjusts and reduces the power of the drive equipment 2 of the feedwater pump, and then reduces the speed of the variable-speed feedwater pump 4 through the rotating shaft 3 , so that the water flow can be greatly reduced. At this time, since the working point of the variable speed feed water pump 4 deviates significantly from the optimal rated working condition, the efficiency is greatly reduced, and more mechanical power is directly converted into the internal energy of the feed water, so that the feed water temperature at the outlet of the variable speed feed water pump 4 At the same time, the sealing water of the high-temperature feed water pump is mixed in, which further increases the feed temperature, so that the pump faces the risk of potential cavitation. At this time, the temperature signal measured by the outlet temperature sensor 6 installed at the outlet of the variable speed feed water pump 4 is sent to the minimum flow protection recirculation controller 8 for deviation control with the saturation temperature corresponding to the inlet pressure calculated by the saturation temperature module 7, When the difference between the two is less than a given fixed value (such as 0), the minimum flow recirculation valve 9 of the feed water pump is opened, so that part of the flow at the outlet of the variable speed feed water pump 4 is sent back to the upstream of the pump inlet, the feed water flow decreases, and the unit will automatically start. The large adjustment device 1 further increases the speed and pumping flow of the variable speed feed water pump 4 to maintain the required final feed water flow, so that the working flow of the pump increases, the efficiency increases, and the working point is further away from the risk of cavitation, so as to adapt to the speed change to achieve pump protection.

Claims (4)

1. A feed pump minimum flow recirculation valve control system, characterized by: the device comprises an adjusting device (1) for driving a water feed pump, driving equipment (2) of the water feed pump, which is connected with the adjusting device (1), a variable-rotation-speed water feed pump (4) which is connected with the driving equipment (2) of the water feed pump through a rotating shaft (3), an inlet pressure sensor (5) which is arranged at the inlet of the variable-rotation-speed water feed pump (4), an outlet temperature sensor (6) which is arranged at the outlet of the variable-rotation-speed water feed pump (4), a saturation temperature module (7) which is connected with the inlet pressure sensor (5) and is used for calculating the saturation temperature corresponding to the inlet pressure according to the physical properties of water and steam, a minimum flow protection recirculation controller (8) which is connected with the outlet temperature sensor (6), wherein the saturation temperature module (7) is connected with the minimum flow protection recirculation controller (8) and is used for feeding the saturation temperature into the minimum flow protection recirculation controller (8), and the minimum flow protection recirculation controller (8) is used for comparing the outlet temperature output by the outlet temperature sensor (6) with the saturation temperature corresponding to the pump inlet pressure output by the saturation temperature module (7); the water supply system further comprises a water supply pump minimum flow recirculation valve (9) connected with the minimum flow protection recirculation controller (8) and an outlet pipeline of the variable-rotation-speed water supply pump (4), and when the saturation temperature deviation of the outlet temperature output by the outlet temperature sensor (6) and the pump inlet pressure output by the saturation temperature module (7) is lower than a preset value, the minimum flow protection recirculation controller (8) opens the water supply pump minimum flow recirculation valve (9) to send a part of water working medium back to an upstream pipeline of the variable-rotation-speed water supply pump (4) from an outlet pipeline of the variable-rotation-speed water supply pump (4).

2. The feedwater pump minimum flow recirculation valve control system of claim 1, wherein: the control of the opening of the minimum flow valve is realized by comparing the deviation of the saturation temperature corresponding to the outlet temperature and the inlet pressure of the variable-rotation-speed water feed pump (4) based on the measurement of the pressure at the inlet and the outlet temperature signals of the variable-rotation-speed water feed pump (4) and based on the minimum flow protection principle of the variable-rotation-speed water feed pump (4); the minimum flow protection recirculation controller (8) utilizes the proportional, integral and differential actions to always ensure that the deviation of the saturation temperature corresponding to the outlet temperature and the inlet pressure of the variable-rotation-speed water supply pump (4) is smaller than the preset protection margin and control error.

3. The feedwater pump minimum flow recirculation valve control system of claim 1, wherein: the minimum flow absolute value limit is not set, the working principle of the variable rotation speed water feed pump (4) is utilized, the deviation of the saturation temperature corresponding to the outlet temperature and the inlet pressure of the variable rotation speed water feed pump (4) is compared, the cavitation risk caused by acting or mixing high-temperature sealing water of the variable rotation speed water feed pump (4) is avoided under the variable rotation speed condition, the rotation speed change of the variable rotation speed water feed pump (4) is automatically adapted, the minimum flow recirculation valve (9) is not required to be opened prematurely when the load is reduced, and therefore the damage to the stability of a water feed system and the service life of the minimum flow recirculation valve (9) during the deep peak regulation of a unit is relieved.

4. A control method of a feed pump minimum flow recirculation valve control system according to any one of claims 1 to 3, characterized by: the adjusting device (1) for driving the water feed pump is used for adjusting the power of the driving equipment (2) of the water feed pump, the rotating shaft (3) is used for driving the variable-rotating-speed water feed pump (4), and the control of the water feed flow is realized by changing the rotating speed; the variable-rotation-speed water feed pump (4) sucks working media from the upstream, the working media are fed to a downstream system for use through the variable-rotation-speed water feed pump (4), an inlet pressure signal measured by an inlet pressure sensor (5) is fed into a saturation temperature module (7) for calculating the saturation temperature corresponding to the inlet pressure according to physical properties of water and vapor, the saturation temperature output by the saturation temperature module (7) and an outlet temperature signal output by an outlet temperature sensor (6) are simultaneously fed into a minimum flow protection recirculation controller (8), the outlet temperature output by the outlet temperature sensor (6) is controlled to be lower than the saturation temperature corresponding to the pump inlet pressure output by the saturation temperature module (7), when the deviation of the minimum flow protection recirculation controller and the inlet pressure signal is lower than a preset value, the minimum flow protection recirculation valve (9) of the water feed pump is started, a part of the water working media is fed back to the upstream pipeline of the variable-rotation-speed water feed pump (4) from an outlet pipeline of the variable-speed water feed pump (4), the water feed flow is reduced, a unit can automatically open a regulating device (1) to further improve the rotation speed of the variable-rotation-speed water feed pump (4) and the flow of the pump, the variable-rotation-speed water feed pump is further increased, the working speed is further increased, the necessary flow protection is prevented from being increased, and the risk of the variable-rotation pump is further is not increased, and the self-speed is prevented from being changed, and the working pump is further is protected.

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