Method and system for controlling net positive suction head of water feeding pump set of thermal power plant

Abstract

The invention discloses a method and a system for controlling the net positive suction head of a water feeding pump set of a thermal power plant, and belongs to the technical field of automatic control of thermal generator sets. The method comprises the following steps: determining an allowable pressure change lower limit of a deaerator by acquiring the actual available net positive suction head of an inlet of a water feeding pump; acquiring the time required for water in the deaerator to flow to the inlet of the water feeding pump; according to the lower pressure change limit and time of the deaerator, the condensed water flow increment for preventing insufficient net positive suction head of the water feeding pump is obtained; determining a condensed water flow limit value for preventing insufficient net positive suction head of the water feeding pump; when the actual condensed water flow is larger than the condensed water flow limiting value, the output of the deaerator liquid level automatic control module is limited to be increased; when the actual condensed water flow is smaller than the condensed water flow limit value or the condensed water flow increment is a negative number, the deaerator liquid level automatic control module works normally. According to the method, the insufficient net positive suction head of the water feeding pump set is prevented by limiting the flow of the condensed water in the accident state, and the safety and stability of thermal power plant operation are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of automatic control of thermal power generating units, and more particularly to a method and system for controlling the net positive suction head of a feed water pump group in a thermal power plant. Background Art

[0002] In the thermal system of a thermal power plant, the feed water pump group is one of the important equipment; the safe and reliable operation of the feed water pump group is directly related to the stability of the entire unit of the thermal power plant. During the actual operation of the unit, when the condensate system operates abnormally (such as the condensate pump trips and starts in series, and the low-pressure heaters with high liquid levels withdraw from operation) or the condensate throttling participates in the primary frequency regulation of the unit, a special condition of the deaerator liquid level drop is very likely to occur.

[0003] When the deaerator liquid level deviates from the normal liquid level (automatically set liquid level) by a sufficient amount, under the action of the existing common deaerator water level automatic control logic, an excessive amount of condensate will enter the deaerator in a short time in order to quickly replenish the deaerator liquid level to the normal liquid level (automatically set liquid level), which is likely to cause a drop in the deaerator pressure.

[0004] If the pressure in the deaerator drops too much in a short time, it will cause the available net positive suction head of the feed water at the inlet of the feed water pump group to be lower than the required net positive suction head of the feed water pump group, thereby causing cavitation of the feed water pump group, and ultimately causing the boiler to trip due to extremely low feed water flow. At the same time, this phenomenon of a large amount of condensate flow entering the deaerator is often accompanied by the over-flow operation of the condensate pump, resulting in the overload trip of the condensate pump frequency converter. The above two aspects are manifestations of the further expansion of accidents and are not conducive to the safe and stable operation of the thermal power plant. Summary of the Invention

[0005] Aiming at the problems existing in the above field, the present invention proposes a method and system for controlling the net positive suction head of a feed water pump group in a thermal power plant. By restricting the condensate flow rate in the accident state, it can prevent the insufficient net positive suction head of the feed water pump group, not only reduce the risk of cavitation of the feed water pump group, but also prevent the expansion of accidents, improve the anti-interference ability of the automatic control of the thermal system of the thermal power plant, and thus improve the safety and stability of the operation of the thermal power plant.

[0006] To solve the above technical problems, the present invention discloses a method for controlling the net positive suction head of a feed water pump group in a thermal power plant, including the following steps: Obtain the actual available net positive suction head at the inlet of the feed water pump, and determine the lower limit of the allowable deaerator pressure change according to the difference between the available net positive suction head and the known required net positive suction head of the feed water pump; According to the obtained length, radius of the deaerator downcomer and the feed water flow rate at the inlet of the feed water pump corresponding to the unit load, obtain the time required for the water in the deaerator to flow to the inlet of the feed water pump; Determine the mass - energy balance of the deaerator transient process according to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the inlet of the feed water pump, and obtain the condensate flow rate increment to prevent insufficient net positive suction head (NPSH) of the feed water pump; sum the condensate flow rate increment and the actual condensate flow rate corresponding to the obtained unit load to determine the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump. By adding a limiter to the automatic control module of the deaerator level, when the actual condensate flow rate is greater than the condensate flow rate limit value, limit the increase of the output of the automatic control module of the deaerator level; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is negative, the automatic control module of the deaerator level operates normally.

[0007] Preferably, the obtaining of the actual available NPSH at the inlet of the feed water pump specifically includes: Determine the pre - conditions for controlling to prevent insufficient NPSH of the feed water pump group, including: Set an automatic control module for the basic level of the deaerator, and automatically control the deaerator level through the condensate pump frequency converter or the deaerator water supply regulating valve. Set a condensate flow rate measuring point entering the deaerator and a feed water flow rate measuring point flowing out of the deaerator on the pipeline of the feed water pump system. The deviation amount between the value shown by the deaerator pressure measuring point and the absolute pressure A ; The deviation amount between the value shown by the deaerator level measuring point and the height difference from the deaerator water surface to the bottom of the deaerator B ; Collect system equipment data, including: the static differential pressure height from the bottom of the deaerator to the center line of the feed water pump and the flow resistance of the pipeline at the suction inlet of the feed water pump ; Calculate the actual available NPSH at the inlet of the feed water pump group according to the current operating parameters of the pump unit and the collected system equipment parameters : In the formula, is the deaerator pressure; is the density of the feed water in the deaerator; is the acceleration due to gravity, with a value of 9.807 m / s²; is the static differential pressure height from the bottom of the deaerator to the center line of the feed water pump; is the flow resistance of the pipeline at the suction inlet of the feed water pump; is the density of the feed water in the pump; is the saturation pressure of the feed water in the pump; is the deaerator level with the bottom of the deaerator as the zero position; In actual calculation, use to replace , it can not only obtain the conservative available net positive suction head (NPSH), but also simplify the logical configuration quantity, and simplify the available NPSH formula to: .

[0008] Preferably, the determination of the lower limit of the allowable deaerator pressure change includes the following steps: Obtain the required NPSH of the feed water pump ; Take the actual available NPSH at the inlet of the feed water pump group and the required NPSH of the feed water pump The difference is the lower limit of the allowable deaerator pressure change : Wherein, is The corresponding saturated water density.

[0009] Preferably, the obtaining of the time required for the water in the deaerator to flow to the inlet of the feed water pump includes the following steps: According to the actual stable operating conditions of the feed water pump unit, obtain the actual feed water flow rate when the unit load is stable ; Obtain the time required for the water in the deaerator to flow to the inlet of the feed water pump t as: In the formula, is the radius of the low-pressure feed water pipeline; is the length of the low-pressure feed water pipeline.

[0010] Preferably, the obtaining of the condensate flow rate increment to prevent insufficient NPSH of the feed water pump specifically includes: Obtain the length L , radius and the height of the elliptical head Y of the deaerator; Take as the parameters of the initial state in the transient process of the deaerator, take the lower limit of the deaerator pressure as the parameters of the end state in the transient process of the deaerator, take the time t as the time of the transient process of the deaerator, and calculate the condensate flow rate increment to prevent insufficient NPSH of the feed water pump by calculating the mass-energy balance of the transient process of the deaerator Among them, , are the enthalpy value and volume of the saturated water in the deaerator in the initial state during the transient process; , , are respectively the volume, density and enthalpy value of the saturated water in the deaerator in the end state during the transient process; H co is the enthalpy value of the condensate; The mass - energy balance of the deaerator transient process is obtained by simplifying the deaerator transient calculation model. The process of simplifying the deaerator transient calculation model is to ignore the mass and heat storage of the saturated steam in the deaerator, as well as the metal equivalent of the relevant pipelines and equipment.

[0011] Preferably, the determination of the condensate flow rate limit value to prevent insufficient net positive suction head (NPSH) of the feed water pump includes the following steps: According to the actual stable operating conditions of the feed water pump unit, obtain the condensate flow rate corresponding to the unit under different operating conditions ; Sum the condensate flow rate increment and the condensate flow rate corresponding to the unit load to obtain the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump which is: Among them, is the condensate flow rate increment to prevent insufficient NPSH of the feed water pump.

[0012] Preferably, the restriction on the increase of the output of the deaerator level automatic control module specifically includes: Obtain the actual condensate flow rate corresponding to the unit load under different operating conditions ; When the actual condensate flow rate is greater than the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump , restrict the increase of the output of the deaerator level automatic control module, and restrict the increase of the opening of the water regulating valve on the deaerator or the increase of the frequency of the condensate frequency converter.

[0013] Preferably, the normal operation of the deaerator level automatic control module specifically includes: When the actual condensate flow rate is less than the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump or when the condensate flow rate increment to prevent insufficient NPSH of the feed water pump is negative, the deaerator level automatic control module operates normally.

[0014] Preferably, it further includes a cavitation margin control system for a feed water pump group in a thermal power plant, including: The available net positive suction head calculation module is used to obtain the actual available net positive suction head at the inlet of the feed water pump; The deaerator pressure change lower limit calculation module is used to determine the allowable lower limit of the deaerator pressure change according to the difference between the available net positive suction head and the required net positive suction head of the known feed water pump; The transient time calculation module is used to obtain the time required for the water in the deaerator to flow to the inlet of the feed water pump according to the length, radius of the deaerator downcomer and the feed water pump inlet flow corresponding to the unit load; The condensate flow rate increment calculation module is used to determine the mass-energy balance of the deaerator transient process according to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the inlet of the feed water pump, and obtain the condensate flow rate increment to prevent the net positive suction head of the feed water pump from being insufficient; sum the condensate flow rate increment and the actual condensate flow rate corresponding to the obtained unit load to determine the condensate flow rate limit value to prevent the net positive suction head of the feed water pump from being insufficient; The deaerator water level automatic control module is used to add a limiter to the deaerator liquid level automatic control module. When the actual condensate flow rate is greater than the condensate flow rate limit value, limit the increase of the output of the deaerator liquid level automatic control module; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is negative, the deaerator liquid level automatic control module works normally.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The method for controlling the net positive suction head of the feed water pump group in a thermal power plant proposed by the present invention simplifies the condensate increment obtained from the deaerator transient calculation by ignoring various heat and mass inputs to the deaerator during the deaerator transient calculation process. It is a conservative value that meets the requirement of preventing the net positive suction head of the feed water pump from being insufficient, and provides data support for restricting the deaerator liquid level control. Summing the condensate flow rate increment and the condensate flow rate corresponding to the obtained unit load to determine the condensate flow rate limit value to prevent the net positive suction head of the feed water pump from being insufficient can match the data of the actual unit operation, and has strong applicability. When the actual condensate flow rate is greater than the condensate flow rate limit value, limit the increase of the output of the deaerator liquid level automatic control module; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is negative, the deaerator liquid level automatic control module works normally. This method can not only meet the deaerator liquid level control to prevent the net positive suction head of the feed water pump from being insufficient throughout the process, but also prevent the expansion of accidents caused by sudden changes in the deaerator liquid level or condensate flow rate, thereby improving the anti-interference ability of the automatic control of the thermal power plant system and enhancing the safety and stability of the operation of the thermal power plant. Description of the Drawings

[0016] Figure 1 It is a flow chart of the method for controlling the net positive suction head of the feed water pump group in a thermal power plant of the present invention; Figure 2For the control system architecture of the net positive suction head of the feed water pump set in the thermal power plant of the present invention; Figure 3 Schematic diagram of the calculation process of the available net positive suction head provided by the embodiment of the present invention; Figure 4 Schematic diagram of the calculation process of the lower limit of the allowable pressure of the deaerator provided by the embodiment of the present invention; Figure 5 Schematic diagram of the calculation process of the transient process time provided by the embodiment of the present invention; Figure 6 Schematic diagram of the calculation process of the allowable condensate flow rate increment provided by the embodiment of the present invention; Figure 7 Schematic diagram of the calculation process of the allowable condensate flow rate provided by the embodiment of the present invention; Figure 8 Schematic diagram of the automatic control process of the deaerator water level provided by the embodiment of the present invention. Detailed implementation manners

[0017] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. Figures 1-8 As shown in the figure, the present invention proposes a control method for the net positive suction head of the feed water pump set in a thermal power plant, including the following steps:

[0018] As Figure 1 shown, the present invention proposes a control method for the net positive suction head of the feed water pump set in a thermal power plant, including the following steps: S1: Obtain the actual available net positive suction head at the inlet of the feed water pump, and determine the lower limit of the allowable deaerator pressure change according to the difference between the available net positive suction head and the required net positive suction head of the known feed water pump; S2: According to the length, radius of the deaerator downcomer and the feed water pump inlet flow corresponding to the unit load obtained, obtain the time required for the water in the deaerator to flow to the feed water pump inlet; S3: According to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the feed water pump inlet, determine the mass-energy balance of the deaerator transient process, and obtain the condensate flow rate increment to prevent the net positive suction head of the feed water pump from being insufficient; Sum the condensate flow rate increment and the actual condensate flow rate corresponding to the unit load obtained to determine the condensate flow rate limit value to prevent the net positive suction head of the feed water pump from being insufficient; S4: Sum the condensate flow rate increment and the actual condensate flow rate corresponding to the unit load obtained to determine the condensate flow rate limit value to prevent the net positive suction head of the feed water pump from being insufficient, that is, the allowable condensate flow rate; S5: By adding a limiter to the automatic deaerator level control module, when the actual condensate flow rate is greater than the condensate flow rate limit value, the output increase of the automatic deaerator level control module is restricted; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is negative, the automatic deaerator level control module operates normally.

[0019] Specifically, in step S1, obtaining the actual available net positive suction head (NPSH) at the inlet of the feed pump specifically includes: Determining the preconditions for preventing insufficient NPSH of the feed pump group, including: Setting an automatic control module for the basic deaerator level, and automatically controlling the deaerator level through the condensate pump frequency converter or the deaerator water supply regulating valve; Setting a condensate flow rate measuring point entering the deaerator and a feed water flow rate measuring point flowing out of the deaerator on the feed pump system pipeline; Knowing the deviation between the value shown by the deaerator pressure measuring point and the absolute pressure A ; Knowing the deviation between the value shown by the deaerator level measuring point and the height difference from the deaerator water surface to the bottom of the deaerator B ; As shown in Table 1, obtaining the system equipment data.

[0020] Table 1 Relevant system and equipment data Calculating the actual available NPSH at the inlet of the feed pump group according to the current operating parameters of the pump unit and the obtained system equipment parameters : In the formula, is the deaerator pressure; is the density of the feed water in the deaerator; is the acceleration due to gravity, with a value of 9.807 m / s²; is the static differential pressure height from the bottom of the deaerator to the center line of the feed pump; is the flow resistance of the feed water suction pipeline of the feed pump; is the density of the feed water in the pump, obtained from the pump inlet pressure and temperature; is the saturation pressure of the feed water in the pump, obtained from the water temperature in the pump; is the deaerator level with the bottom of the deaerator as the zero position; A is the difference between the deaerator pressure measuring point and the absolute pressure, B is the difference between the zero position of the deaerator level measuring point and the bottom of the deaerator.

[0021] Determining the allowable lower limit of the deaerator pressure change, including the following steps: Taking the actual available NPSH at the inlet of the feed pump group The difference from the required net positive suction head (NPSH) of the feed water pump is the lower limit of the allowable deaerator pressure change : Wherein, is the corresponding saturated water density.

[0022] In step S2, obtaining the time required for the water in the deaerator to flow to the inlet of the feed water pump includes the following steps: According to the actual stable operating conditions of the feed water pump unit, obtaining the actual feed water flow rate when the unit load is stable ; Obtaining the time required for the water in the deaerator to flow to the inlet of the feed water pump t is: In the formula, is the radius of the low-pressure feed water pipeline; is the length of the low-pressure feed water pipeline.

[0023] In step S3, obtaining the condensate flow rate increment to prevent insufficient NPSH of the feed water pump specifically includes: Obtaining the radius of the deaerator R d , the length of the deaerator L and the height of the ellipsoidal head of the deaerator Y ; Taking as the parameters of the initial state in the transient process of the deaerator, taking the lower limit of the deaerator pressure as the parameters of the end state in the transient process of the deaerator, taking the time t as the time of the transient process of the deaerator, and calculating the conservative condensate flow rate increment to prevent insufficient NPSH of the feed water pump .

[0024] When calculating the transient process of the deaerator, the present invention assumes that the condensate water, extraction steam, drain water flowing into the deaerator and the feed water flowing out of the deaerator are in a mass-energy balance state during the normal and stable operation of the deaerator; ignoring the increase in the extraction steam volume after the pressure drop in the deaerator, ignoring the metal heat equivalent of the condensate pipeline, ignoring the metal heat equivalent of the deaerator itself, ignoring the mass and heat of the saturated steam in the deaerator, ignoring the latent heat of vaporization of the water in the deaerator; simplifying the problem of the deaerator pressure reduction caused by a large amount of cold water entering the deaerator into the problem of a certain amount of cold water entering a pressure vessel filled with saturated water, and calculating a conservative allowable cold water volume, which is called the condensate increment. This condensate increment needs to satisfy: Among them, 、 are the enthalpy value and volume of the saturated water in the deaerator at the initial state during the transient process; 、 、 are respectively the volume, density and enthalpy value of the saturated water in the deaerator at the end state during the transient process; is the enthalpy value of the condensate.

[0025] The mass - energy balance of the deaerator transient process is obtained by simplifying the deaerator transient calculation model. The process of simplifying the deaerator transient calculation model is to ignore the mass and heat storage of the saturated steam in the deaerator, specifically including: During the actual operation of the unit, except for the condensate system failure, when primary frequency modulation is carried out by the condensate throttling method, the deaerator water level will also decrease, resulting in a reduction in the available net positive suction head of the feed pump (booster pump).

[0026] After studying the influence of large load changes of the unit on the safe operation of the feed pump during the deaerator transient process, and the dynamic analysis of the heat, flow and pressure changes of the condensate, feed water and the water stored in the deaerator tank after the deaerator changes operating conditions, the designers finally ensure the net positive suction head of the feed pump under the maximum load rejection condition of the unit by setting a reasonable relative height difference between the deaerator and the inlet of the booster pump, so as to ensure that the net positive suction head of the feed pump meets the requirements under all load change conditions of the unit.

[0027] When the actual value of the deaerator operating water level deviates from the deaerator water level set value, more scholars focus on how to quickly adjust the deaerator water level to the normal value or reduce its dynamic disturbance amount, ignoring the risk of insufficient net positive suction head of the feed pump caused by a large amount of condensate surging into the deaerator under special operating conditions of the unit.

[0028] The present invention simplifies the deaerator transient calculation model, obtains the calculation formula for the conservative value of the condensate flow rate to prevent insufficient net positive suction head of the feed pump under special operating conditions, and takes this as a limit value into the conventional deaerator water level control logic in order to solve this problem.

[0029] Deaerator transient calculation During the deaerator transient calculation process, in order to ensure that the feed pump does not cavitate during operation, it is necessary to make the effective net positive suction head at the pump suction inlet, that is, the actual available net positive suction head at the inlet of the feed pump unit . This value is determined by the pipeline, system and device on the suction side of the feed pump and has nothing to do with the pump itself, but its value needs to be greater than the required net positive suction head of the feed pump 。

[0030] The available net positive suction head satisfies the following relationship: In the formula, is the deaerator pressure, A is the deviation between the value shown by the deaerator pressure measuring point and the absolute pressure; is the density of the feed water in the deaerator; is the acceleration due to gravity, with a value of 9.807 m / s²; is the static differential pressure height from the bottom of the deaerator to the center line of the feed water pump; is the flow resistance of the pipeline at the suction inlet of the feed water pump; is the density of the feed water in the pump; is the saturation pressure of the feed water in the pump; is the deaerator liquid level with the bottom of the deaerator as the zero position; B is the deviation between the value shown by the deaerator liquid level measuring point and the height difference between the water surface of the deaerator and the bottom of the deaerator.

[0031] Generally, when calculating the deaerator transient state, it is necessary to consider the metal equivalent mass of the last-stage low-pressure heater body and the pipelines from the low-pressure heater to the deaerator, and from the deaerator body to the inlet of the feed water pump. Calculate the enthalpy value of the saturated water in the deaerator and the enthalpy value of the feed water in the feed water pump through different processes in four stages, and then substitute them into the available net positive suction head formula for calculation. The calculation method is complex and not conducive to engineering control configuration application. Therefore, the present invention simplifies the deaerator transient model.

[0032] Model Simplification For the convenience of qualitative analysis, ignoring minor factors and retaining sufficient safety margins, the following assumptions are made for the deaerator transient model: 1) The working medium in the entire deaerator is in a saturated state, without considering the slight superheat of steam and the sub-cooling enthalpy of liquid working medium. Above the water surface in the deaerator is saturated gas, and below the water surface is saturated water.

[0033] 2) When calculating the available net positive suction head, substituting the water density in the feed water pump for the saturated water density in the deaerator can obtain a conservative value, and it is considered that the water density in the low-pressure feed water pipeline is the same as that in the feed water pump.

[0034] 3) Without considering the influence of the water volume and the metal equivalent mass of the pipeline from the first-stage low-pressure heater to the deaerator, directly using the final parameters of the condensate entering the deaerator as variables; by default, the condensate temperature does not change during the transient process.

[0035] 4) Without considering the influence of the metal equivalent mass of the deaerator body, ignoring the heat dissipation of the deaerator itself, and not considering the influence of non-condensable gases on heat transfer.

[0036] 5) Without considering the increase in extraction steam flow caused by the decrease in deaerator temperature during the transient process, it is considered that at a fixed load of the unit, the energy and mass of the feed water flowing out of the deaerator, the drain water from the last stage high-pressure heater entering the deaerator, the extraction steam entering the deaerator, and the condensate entering the deaerator are balanced and basically unchanged.

[0037] 6) In actual engineering, the flow velocity in the low-pressure feed water pipeline after the unit is in load operation often concentrates in the range of 1.0 - 2.5 m / s. The flow velocity is low and the change range is small, resulting in a small change in the frictional loss along the way. At the same time, it is defaulted that the local loss at the filter on the low-pressure feed water pipeline is fixed. Therefore, it is considered that the frictional loss of the low-pressure feed water pipeline is a fixed value.

[0038] 7) Excluding the part that has been balanced in assumption 5), the problem of insufficient net positive suction head (NPSH) of the feed water pump in the deaerator can be simplified to the problem of controlling the pressure change after a certain amount of condensate enters a pressure vessel filled with saturated water and saturated steam in layers. By reasonably controlling the mass of the condensate entering the pressure vessel, the pressure and liquid level change range in the pressure vessel can be controlled. That is, it is simplified to the problem of how to control the slow increase of the deaerator water level by controlling the condensate flow rate increment on the basis of the balance.

[0039] Due to the fact that the above assumptions ignore various heats input into the deaerator during the transient calculation process of the deaerator, and the conservative calculation method is used to calculate the available NPSH of the feed water pump, the condensate increment obtained through the mass-energy balance calculation in the deaerator is a conservative value that meets the requirement of preventing insufficient NPSH of the feed water pump.

[0040] Taking the mass-energy balance in the deaerator as the research object, when the unit is operating stably, the condensate, extraction steam, and drain water entering the deaerator reach a balance. When the extraction steam and drain water volumes are stable, only the excessive influx of condensate will disrupt this balance. Ignoring the balanced condensate, extraction steam, and drain water, the mass-energy balance model in the deaerator is simplified to a model of condensate flowing into a deaerator filled with saturated water and saturated steam, that is, by controlling the pressure drop amplitude in the deaerator, the size of the condensate increment is controlled. In the complete mass-energy balance of the deaerator transient calculation, it includes the condensate volume and the metal equivalent of the condensate pipeline, the metal equivalent of the deaerator body, the extraction steam volume, and the saturated steam volume in the deaerator. Among them, the influence of the extraction steam volume, the metal equivalent of the condensate pipeline, the metal equivalent of the deaerator itself, the extraction steam volume, and the saturated steam volume in the deaerator on the pressure of the deaerator (i.e., the available NPSH of the feed water pump) is positive. Therefore, the calculated available NPSH is smaller by ignoring these factors, and further a conservative condensate flow rate increment to prevent insufficient NPSH of the feed water pump can be obtained.

[0041] Mathematical calculation model of the deaerator transient model Calculation of water and steam parameters The densities of saturated water and steam in the deaerator are functions of the saturation pressure, i.e.: Wherein, is the temperature of the deaerator; , are the densities of saturated water and saturated gas in the deaerator respectively; , correspond to the enthalpy values of saturated water and saturated gas in the deaerator respectively.

[0042] Calculation of the volume of water and steam in the deaerator The volumes of saturated water and saturated steam in the deaerator are functions of the deaerator size and liquid level, and are specifically expressed as follows: Wherein, is the length of the deaerator, is the radius of the deaerator, is the height of the elliptical head of the deaerator, , are the volumes of saturated water in the initial state and in the deaerator, and the volume of saturated gas in the deaerator during the transient process respectively, is the total volume of the deaerator.

[0043] Calculation of transient mass - energy conservation in the deaerator In the deaerator with a pressure temperature of , and a liquid level of , within a fixed time , the parameters of the condensate inflow exceeding the reference condensate flow rate are , condensate increment , such that the parameters in the deaerator change to , , .

[0044] According to the law of conservation of mass, there is: According to the law of conservation of energy, there is: Wherein, , are the enthalpy values of saturated water and saturated gas in the deaerator respectively, is the enthalpy value of the condensate.

[0045] Relationship between the deaerator pressures before and after the transient process in the deaerator As shown in the effective net positive suction head formula, the available net positive suction head at the inlet of the feed pump is a function of the inlet pressure of the feed pump and the deaerator temperature. When the system parameters are determined, its available net positive suction head is determined. To allow a large amount of condensate to flow into the deaerator without cavitation occurring in the feed pump, the following relationship must hold:

[0046] where D is the static pressure value of the water column corresponding to the allowable change in net positive suction head, affected by the deaerator pressure and the deaerator level .

[0047] Determination of the transient process time in the deaerator Research shows that the minimum value of the available net positive suction head at the inlet of the feed pump during the transient process in the deaerator occurs during the period after the condensate enters the deaerator but before it reaches the inlet of the feed pump. Therefore, the time of the transient process can be determined and satisfies:

[0048] where is the time required for the water in the deaerator to flow to the inlet of the feed pump, is the radius of the low-pressure feed water pipe, is the length of the low-pressure feed water pipe, is the actual feed water flow rate when the unit load is stable.

[0049] Determination of the condensate flow rate increment Combining the formula in the mathematical calculation model of the deaerator transient model, it can be seen that the condensate increment is a function of the deaerator pressure , the deaerator level , and the inlet flow rate of the feed pump and satisfies: Since is known, then , , , , , , and It can be quickly obtained according to the saturated water and saturated steam parameter table.

[0050] Reduction and simplification of the calculation formula for condensate flow rate increment Since the mass ratio and heat storage ratio of saturated steam in the deaerator are low compared with saturated water, and according to the research of existing technologies, during the transient process of the two-phase mixing heat exchanger, the heat storage change caused by pressure and flow rate changes is mainly in the water part. Therefore, the mass and heat storage of saturated steam in the deaerator can be ignored, and the transient mass-energy conservation calculation formula in the deaerator and the condensate flow rate increment can be simplified as follows: In step S4, determining the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump includes the following steps: According to the actual stable operating conditions of the feed water pump unit, obtain the corresponding condensate flow rate of the unit under different operating conditions ; Add the condensate flow rate increment to the condensate flow rate corresponding to the unit load to obtain the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump which is: where is the condensate flow rate increment to prevent insufficient NPSH of the feed water pump.

[0051] The control process in step S5 specifically includes: When the actual condensate flow rate is greater than the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump , limit the output increase of the deaerator liquid level automatic control module, and prohibit the opening of the water regulating valve on the deaerator from increasing or the frequency of the condensate frequency converter from increasing.

[0052] When the actual condensate flow rate is less than the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump or when the condensate flow rate increment to prevent insufficient NPSH of the feed water pump is negative, the deaerator liquid level automatic control module operates normally.

[0053] The calculation result of the normal condensate increment will not be negative. However, considering that measurement points may malfunction during the actual operation of the unit, resulting in a negative calculated condensate flow increment, the logic of "when the condensate flow increment is negative, the deaerator level automatic control module operates normally" is added to avoid the influence of measurement point problems on the normal operation of the deaerator level automatic control module.

[0054] The present invention also provides a control system for the net positive suction head of a feed water pump group in a thermal power plant, including: An available net positive suction head calculation module for obtaining the actual available net positive suction head at the inlet of the feed water pump; A lower limit calculation module for the deaerator pressure change, which is used to determine the allowable lower limit of the deaerator pressure change according to the difference between the available net positive suction head and the required net positive suction head of the known feed water pump; A transient time calculation module for obtaining the time required for the water in the deaerator to flow to the inlet of the feed water pump according to the length, radius of the deaerator downcomer, and the feed water pump inlet flow corresponding to the unit load; A condensate flow increment calculation module for determining the mass - energy balance of the deaerator transient process according to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the inlet of the feed water pump, and obtaining the condensate flow increment to prevent the net positive suction head of the feed water pump from being insufficient; summing the condensate flow increment and the actual condensate flow corresponding to the obtained unit load to determine the condensate flow limit value to prevent the net positive suction head of the feed water pump from being insufficient; A deaerator level automatic control module, which is used to add a limiter to the deaerator level automatic control module. When the actual condensate flow is greater than the condensate flow limit value, the output increase of the deaerator level automatic control module is limited; when the actual condensate flow is less than the condensate flow limit value or the condensate flow increment is negative, the deaerator level automatic control module operates normally.

[0055] The method proposed by the present invention prevents the net positive suction head of the feed water pump group from being insufficient by restricting the condensate flow under accident conditions, improving the safety and stability of the operation of the thermal power plant.

[0056] Embodiment To verify the effectiveness of the method proposed by the present invention, this embodiment takes a 660MW unit as an example to give a detailed control process.

[0057] Determine the pre - conditions for the automatic control method to prevent the net positive suction head of the feed water pump group in a thermal power plant, including: a. The deaerator level has a basic automatic control module. In this embodiment, it is a typical three - impulse deaerator level control module.

[0058] b. There are relatively accurate condensate flow measurement points entering the deaerator and feed water flow measurement points flowing out of the deaerator on the system pipeline.

[0059] c. The deviation between the value shown by the deaerator pressure measuring point and the absolute pressure A = 0.06 MPa, that is, the pressure shown by the deaerator is 0.06 MPa lower than the absolute pressure.

[0060] d. The deviation between the value shown by the deaerator liquid level measuring point and the height difference from the deaerator water surface to the bottom of the deaerator B = 0.5 m, that is, the zero level of the deaerator liquid level is 0.5 m higher than the bottom of the deaerator.

[0061] e. As shown in Table 2, obtain the following relevant system and equipment data.

[0062] Table 2 Relevant system and equipment data obtained in the embodiment f. According to the actual stable operating conditions of the unit, obtain the corresponding condensate flow rate and feed pump inlet flow rate under different operating conditions, as shown in Table 3.

[0063] Table 3 Corresponding condensate flow rate and feed pump inlet flow rate under different operating conditions After all the above conditions are met, the automatic control steps of the automatic control method for preventing insufficient net positive suction head of the feed pump group in a thermal power plant specifically include: The first step is to calculate the actual available net positive suction head at the inlet of the feed pump group according to the current operating parameters and the collected system equipment parameters , as shown in the following formula, and the corresponding logic configuration is as Figure 3 shown.

[0064] The second step is to calculate the lower limit of the allowable deaerator pressure change within the transient time according to the difference between the available net positive suction head and the known required net positive suction head of the feed pump , as Figure 4 shown.

[0065] The third step is to calculate the time required for the water in the deaerator to flow to the inlet of the feed pump according to the length of the deaerator downcomer, the radius , and the feed pump inlet flow rate corresponding to the unit load , that is, the transient process time, as Figure 5 shown.

[0066] The fourth step is to As a parameter of the initial state in the transient process of the deaerator, the lower limit of the deaerator pressure As a parameter of the end state in the transient process of the deaerator, the time t As the time of the transient process of the deaerator, by calculating the mass - energy balance of the deaerator transient process, calculate the conservative condensate flow rate increment to prevent insufficient net positive suction head (NPSH) of the feed water pump , that is, the allowable condensate flow rate increment, as Figure 6 shown

[0067] Step 5, the condensate flow rate increment plus the condensate flow rate corresponding to the unit load at this time , to obtain the condensate flow rate limit value to prevent insufficient NPSH of the feed water pump , that is, the allowable condensate flow rate, as Figure 7 shown

[0068] The calculation results of the above Step 4 and Step 5 are as follows For a unit load of 495 MW, verification is carried out by two methods: varying the deaerator level at the same condensate temperature and varying the condensate temperature at the same deaerator level. The calculated results of the maximum allowable condensate flow rate to prevent feed water pump cavitation are shown in Table 3 and Table 4 below. In the tables, is the condensate temperature is the deaerator level with the bottom of the deaerator as zero

[0069] Table 3 Different deaerator levels, = 131 °C, 495 MW Table 4 Different condensate temperatures, = 2.45 m, 495 MW Step 6, add a blocking type limiter to the deaerator water level automatic control module

[0070] When the actual condensate flow rate is greater than , limit the increase of the output of the deaerator water level automatic control module, and prohibit the increase of the opening of the deaerator water supply regulating valve or the increase of the condensate frequency converter frequency; when the actual condensate flow rate is less than or is negative, the deaerator water level automatic control module works normally, as Figure 8 shown

[0071] Through this embodiment, it is shown that the invention clarifies the key points of the automatic control method for preventing the insufficient net positive suction head (NPSH) of the feed pump group, and reasonably sets them in the conventional deaerator water level automatic control module, enabling the automatic control logic of the deaerator water level in thermal power plants to have a certain ability to prevent the insufficient NPSH of the feed pump.

[0072] The method proposed by the present invention realizes the automatic control of preventing the insufficient NPSH of the feed pump group in thermal power plants, reduces manual operation, and also avoids risks such as misoperation, laying a foundation for the whole-process intelligentization (especially in accident conditions) of thermal power generating units.

[0073] The automatic control method proposed in the embodiment of the present invention is provided with a reasonable calculation method, which can not only prevent the insufficient NPSH of the feed pump, but also prevent the further expansion of accidents. This method realizes the continuous control of the deaerator water level during operation, and can reduce the test risk of condensate throttling participating in primary frequency modulation. This method has the ability to prevent misoperation and will not affect the normal operation of the normal deaerator water level automatic control module due to incorrect measurement point display.

[0074] As mentioned above, the above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

[0075] In addition, unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

Claims

1. A method for controlling the NPSH of a feedwater pump group in a thermal power plant, characterized in that: The following steps are involved: Obtain the actual available NPSH at the feedwater pump inlet, and determine the lower limit of the allowable deaerator pressure change based on the difference between the available NPSH and the known required NPSH of the feedwater pump; According to the length and radius of the deaerator downcomer and the feedwater pump inlet flow rate corresponding to the unit load, the time required for the water in the deaerator to flow to the feedwater pump inlet is obtained; According to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the feedwater pump, determine the mass-energy balance of the deaerator transient process, and obtain the condensate flow increment to prevent the feedwater pump from having insufficient NPSH; sum the condensate flow increment with the actual condensate flow corresponding to the obtained unit load, and determine the condensate flow limit value to prevent the feedwater pump from having insufficient NPSH; By adding a limiter to the deaerator liquid level automatic control module, when the actual condensate flow rate is greater than the condensate flow rate limit value, the output of the deaerator liquid level automatic control module is limited to increase; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is a negative number, the deaerator liquid level automatic control module works normally.

2. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 1, characterized in that: The obtaining of the actual available NPSH at the feedwater pump inlet specifically includes: Determine the prerequisites for preventing insufficient NPSH control of the feedwater pump group, including: Set up an automatic control module for the deaerator liquid level, and automatically control the deaerator water level through the condensate pump inverter or the deaerator water supply regulating valve; Set up condensate flow measurement points entering the deaerator and feed water flow measurement points out of the deaerator on the feed water pump system pipeline; The deviation between the displayed value and the absolute pressure at the known deaerator pressure measuring point A ; The deviation between the value displayed at the known deaerator liquid level measurement point and the height difference between the deaerator water surface and the deaerator bottom B ; Collect system equipment data, including: static differential pressure height from the bottom of the deaerator to the center line of the feed pump and the flow resistance of the water pump suction pipe ; Calculate the actual available NPSH at the inlet of the feedwater pump unit based on the current operating parameters of the pump unit and the collected system equipment parameters ; Theoretically, the formula for calculating the available NPSH is as follows: In the formula, is the deaerator pressure; is the feed water density in the deaerator; is the acceleration due to gravity, which is 9.807m / s²; It is the static differential pressure height from the bottom of the deaerator to the center line of the feed pump; is the flow resistance of the water pump suction pipe; is the density of water supply in the pump; is the saturated pressure of water in the pump; is the deaerator liquid level with the bottom of the deaerator as zero position; In actual calculation, use Substituting the formula , which can not only obtain the conservative available NPSH, but also simplify the logical configuration quantity, and simplify the available NPSH formula to: 。 3. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 2, characterized in that: Determining the lower limit of the allowable deaerator pressure variation comprises the following steps: Obtain the required NPSH of the feedwater pump ; Take the actual available NPSH at the inlet of the feed water pump group The feed water pump must have a NPSH The difference is the lower limit of the allowable deaerator pressure change : in, for The corresponding saturated water density.

4. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 3, characterized in that: The method of obtaining the time required for water in the deaerator to flow to the inlet of the water feed pump comprises the following steps: According to the actual stable operating conditions of the feedwater pump unit, obtain the actual feedwater flow rate when the unit load is stable ; Get the time required for water in the deaerator to flow to the inlet of the water pump t for: In the formula, is the radius of the low-pressure water supply pipe; is the length of the low-pressure water supply pipeline.

5. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 4, characterized in that: The step of obtaining the condensate flow increment for preventing the feedwater pump from having insufficient cavitation margin specifically includes: Get the length of the deaerator L ,radius and elliptical head height Y ; Will As the initial state parameter of the deaerator transient process, the deaerator pressure lower limit As the parameter of the end state of the deaerator transient process, the time t As the time of transient process of deaerator; Calculate the condensate flow increment to prevent insufficient NPSH of the feedwater pump through the deaerator transient process mass-energy balance for: in, , is the enthalpy and volume of saturated water in the deaerator in the initial state during the transient process; , , are the volume, density and enthalpy of saturated water in the deaerator at the end of the transient process; H co is the enthalpy of condensed water; The mass-energy balance of the deaerator transient process is obtained by simplifying the deaerator transient calculation model. The process of simplifying the deaerator transient calculation model is to ignore the mass of saturated steam in the deaerator and its heat storage capacity, and ignore the metal equivalent of related pipelines and equipment.

6. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 5, characterized in that: Determining the condensate flow rate limit value to prevent the feedwater pump from having insufficient cavitation margin comprises the following steps: According to the actual stable operating conditions of the feedwater pump unit, obtain the corresponding condensate flow rate of the unit under different working conditions ; Increase the condensate flow rate Condensate flow corresponding to unit load Sum the values ​​to obtain the condensate flow limit value to prevent the feedwater pump from having insufficient cavitation margin. for: in, To prevent the condensate flow rate from increasing due to insufficient cavitation margin of the feed water pump.

7. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 6, characterized in that: The method of limiting the increase in the output of the deaerator liquid level automatic control module specifically includes: Obtain the actual condensate flow corresponding to the unit load under different working conditions ; When the actual condensate flow Greater than the condensate flow limit value to prevent insufficient cavitation margin of the feedwater pump When the deaerator liquid level automatic control module output is limited to increase, the water regulating valve opening on the deaerator is limited to increase or the condensate inverter frequency increases.

8. The method for controlling the NPSH of a feedwater pump group in a thermal power plant according to claim 7, characterized in that: The deaerator liquid level automatic control module works normally, specifically including: When the actual condensate flow Less than the condensate flow limit value to prevent insufficient cavitation margin of the feedwater pump When the condensate flow increment to prevent the cavitation margin of the feed water pump from being insufficient is negative, the deaerator liquid level automatic control module works normally.

9. A NPSH control system for a feedwater pump group in a thermal power plant, characterized in that: include: Available NPSH calculation module, used to obtain the actual available NPSH at the feedwater pump inlet; A deaerator pressure change lower limit calculation module is used to determine the allowable deaerator pressure change lower limit according to the difference between the available NPSH and the known required NPSH of the feed water pump; The transient time calculation module is used to obtain the time required for the water in the deaerator to flow to the feedwater pump inlet according to the length and radius of the deaerator downcomer and the feedwater pump inlet flow corresponding to the unit load; The condensate flow rate increment calculation module is used to determine the mass-energy balance of the deaerator transient process according to the lower limit of the deaerator pressure change and the time required for the water in the deaerator to flow to the feed water pump inlet, and obtain the condensate flow rate increment to prevent the feed water pump from having insufficient cavitation margin; sum the condensate flow rate increment with the actual condensate flow rate corresponding to the obtained unit load, and determine the condensate flow rate limit value to prevent the feed water pump from having insufficient cavitation margin; The deaerator water level automatic control module is used to limit the increase of the output of the deaerator liquid level automatic control module by adding a limiter on the deaerator liquid level automatic control module when the actual condensate flow rate is greater than the condensate flow rate limit value; when the actual condensate flow rate is less than the condensate flow rate limit value or the condensate flow rate increment is a negative number, the deaerator liquid level automatic control module works normally.

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