A method and system for controlling net positive suction head (NPSH) of feedwater pump sets in thermal power plants

Abstract

This invention discloses a method and system for controlling the net positive suction head (NPSH) of feedwater pump sets in thermal power plants, belonging to the field of automatic control technology for thermal power generating units. The method involves obtaining the actual available NPSH at the feedwater pump inlet to determine the lower limit of allowable deaerator pressure variation; obtaining the time required for water to flow from the deaerator to the feedwater pump inlet; obtaining the condensate flow increment to prevent insufficient NPSH based on the lower limit of deaerator pressure variation and the time; determining the condensate flow limit value to prevent insufficient NPSH; limiting the output of the automatic deaerator level control module when the actual condensate flow is greater than the condensate flow limit value; and ensuring the automatic deaerator level control module operates normally when the actual condensate flow is less than the condensate flow limit value or when the condensate flow increment is negative. This method prevents insufficient NPSH in feedwater pump sets by limiting condensate flow under emergency conditions, thereby improving the safety and stability of thermal power plant operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of automatic control technology of thermal power generating units, in particular to a method and system for controlling the NPSH of a feedwater pump set in a thermal power plant. BACKGROUND

[0002] In the thermal system of a thermal power plant, the feedwater pump set is one of the important equipment; the safe and reliable operation of the feedwater pump set is directly related to the stability of the entire unit of the thermal power plant. In the actual operation process of the unit, when the condensate water system is abnormally operated (such as the condensate pump tripping and starting, the low-pressure heater liquid level three high exiting) or the condensate water throttling participates in the primary frequency modulation of the unit, the special working condition of the deaerator liquid level lowering is prone to occur.

[0003] When the deaerator liquid level deviates from the normal liquid level (the automatically set liquid level) enough, under the action of the existing common deaerator water level automatic control logic, excessive condensate water will enter the deaerator in a short time in order to rapidly supplement the deaerator liquid level to the normal liquid level (the automatically set liquid level), which is easy to cause the drop of the deaerator pressure in the deaerator.

[0004] If the pressure in the deaerator drops too much in a short time, the available NPSH of the feedwater at the inlet of the feedwater pump set will be lower than the necessary NPSH of the feedwater pump set, which further causes the cavitation of the feedwater pump set, and finally causes the boiler to trip due to the low feedwater flow. At the same time, the phenomenon of a large amount of condensate water flow entering the deaerator is often accompanied by the over-flow operation of the condensate pump, which causes the frequency converter of the condensate pump to overload and trip. The above two aspects are the manifestations of the further expansion of the accident, which is not conducive to the safe and stable operation of the thermal power plant. SUMMARY

[0005] In view of the problems existing in the above field, the present application provides a method and system for controlling the NPSH of a feedwater pump set in a thermal power plant, which prevents the insufficient NPSH of the feedwater pump set by limiting the condensate flow in an accident state, not only can reduce the risk of cavitation of the feedwater pump set, but also can prevent the expansion of the accident, 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 application discloses a method for controlling the NPSH of a feedwater pump set in a thermal power plant, comprising the following steps:

[0007] acquiring the actual available NPSH at the inlet of the feedwater pump, and determining the lower limit of the allowed deaerator pressure change according to the difference between the available NPSH and the known necessary NPSH of the feedwater pump;

[0008] acquiring the time required for the water flow in the deaerator to flow to the inlet of the feedwater pump according to the length, radius of the deaerator downcomer and the feedwater pump inlet flow corresponding to the unit load;

[0009] According to the deaerator pressure change lower limit and the time required for water flow in the deaerator to the feed water pump inlet, the deaerator transient process mass-energy balance is determined to obtain the condensate flow increment to prevent the insufficient NPSH of the feed water pump; the condensate flow increment and the actual condensate flow corresponding to the obtained unit load are summed to determine the condensate flow limit value to prevent the insufficient NPSH of the feed water pump;

[0010] By adding a limiter to the deaerator liquid level automatic control module, when the actual condensate flow is greater than the condensate flow limit value, the deaerator liquid level automatic control module output is limited to increase; when the actual condensate flow is less than the condensate flow limit value or the condensate flow increment is negative, the deaerator liquid level automatic control module works normally.

[0011] Preferably, the actual available NPSH of the feed water pump inlet is obtained, specifically including:

[0012] The preconditions for preventing insufficient NPSH control of the feed water pump group are determined, including:

[0013] The deaerator liquid level automatic control module is set to automatically control the deaerator water level through the condensate pump frequency converter or the deaerator water regulating valve;

[0014] The condensate flow measurement point into the deaerator and the feed water flow measurement point out of the deaerator are set on the feed water pump system pipeline;

[0015] The deviation amount of the deaerator pressure measurement point display value from the absolute pressure is known A ;

[0016] The deviation amount of the deaerator liquid level measurement point display value from the height difference of the deaerator water surface to the deaerator bottom is known B ;

[0017] The system equipment data is collected, including: the static differential pressure height of the deaerator bottom to the center line of the feed water pump And the flow resistance of the feed water pump suction inlet pipeline ;

[0018] According to the current operating parameters of the water pump unit and the collected system equipment parameters, the actual available NPSH of the feed water pump group inlet is calculated :

[0019]

[0020] In the formula, The deaerator pressure; The deaerator feed water density; The gravitational acceleration, the value is 9.807 m / s²; The static differential pressure height of the deaerator bottom to the center line of the feed water pump; flow resistance of the pipeline for the suction inlet of the feed water pump; feed water density in the pump; saturated pressure of the feed water in the pump; de-aerator liquid level with the bottom of the de-aerator as zero level;

[0021] In actual calculation, the is used instead of the in the formula, so that the conservative available NPSH can be obtained and the logic configuration quantity is simplified, and the available NPSH formula is simplified as

[0022] .

[0023] Preferably, the determining the lower limit of the allowable de-aerator pressure variation comprises the following steps:

[0024] obtaining the required NPSH of the feed water pump ;

[0025] taking the difference between the actual available NPSH of the feed water pump group inlet and the required NPSH of the feed water pump as the lower limit of the allowable de-aerator pressure variation :

[0026]

[0027] wherein, is the corresponding saturated water density.

[0028] Preferably, the obtaining the time required for the water flow in the de-aerator to the inlet of the feed water pump comprises the following steps:

[0029] obtaining the actual feed water flow when the unit load is stable according to the actual stable operation condition of the feed water pump unit ;

[0030] obtaining the time required for the water flow in the de-aerator to the inlet of the feed water pump t is:

[0031]

[0032] wherein, is the radius of the low-pressure feed water pipeline; is the length of the low-pressure feed water pipeline.

[0033] Preferably, the obtaining the condensate flow increment for preventing insufficient NPSH of the feed water pump comprises the following steps:

[0034] obtaining the length L , the radius and the height of the elliptical head of the de-aerator​Y ;

[0035] Will As a parameter for the initial state during the transient process of the deaerator, the lower limit of the deaerator pressure is used. As a parameter for the final state during the transient process of the deaerator, time is used. t As the time of the deaerator transient process, the condensate flow increment to prevent insufficient feedwater pump NPSH is calculated by calculating the mass-energy balance of the deaerator transient process. for:

[0036]

[0037]

[0038]

[0039]

[0040] in, , The enthalpy and volume of saturated water in the deaerator in the initial state during the transient process; , , These represent the volume, density, and enthalpy of the saturated water in the deaerator at the end of the transient process, respectively. H co This is the enthalpy of condensate;

[0041] 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, as well as the metal equivalent of related pipelines and equipment.

[0042] Preferably, determining the condensate flow rate limit to prevent insufficient cavitation margin of the feedwater pump includes the following steps:

[0043] Based on the actual stable operating conditions of the feedwater pump unit, obtain the corresponding condensate flow rate of the unit under different operating conditions. ;

[0044] Increase the condensate flow rate Condensate flow rate corresponding to unit load Summing yields the condensate flow limit to prevent insufficient net positive suction head (NPSH) of the feedwater pump. for:

[0045]

[0046] in, To prevent the increase in condensate flow rate due to insufficient cavitation margin of the feedwater pump.

[0047] Preferably, the increased output of the automatic control module for limiting the deaerator liquid level specifically includes:

[0048] Obtain the actual condensate flow rate corresponding to the unit load under different operating conditions. ;

[0049] When the actual condensate flow rate The condensate flow rate exceeds the limit value for preventing insufficient net positive suction head (NPSH) of the feedwater pump. At this time, the output of the automatic control module for limiting the deaerator liquid level increases, limiting the opening of the water regulating valve on the deaerator to increase or the frequency of the condensate frequency converter to increase.

[0050] Preferably, the automatic liquid level control module of the deaerator operates normally, specifically including:

[0051] When the actual condensate flow rate Less than the condensate flow rate limit to prevent insufficient cavitation margin of the feedwater pump When the condensate flow rate increment is negative to prevent insufficient cavitation margin of the feedwater pump, the deaerator level automatic control module will operate normally.

[0052] Preferably, it further includes a net positive suction head (NPSH) control system for a feedwater pump set in a thermal power plant, comprising:

[0053] Available NPSH calculation module is used to obtain the actual available NPSH at the inlet of the feedwater pump;

[0054] The deaerator pressure variation lower limit calculation module is used to determine the allowable lower limit of deaerator pressure variation based on the difference between the available net positive suction head (NPSH) and the known required NPSH of the feedwater pump.

[0055] The transient time calculation module is used to obtain the time required for water in the deaerator to flow to the feedwater pump inlet based on the obtained length and radius of the deaerator downcomer and the feedwater pump inlet flow corresponding to the unit load.

[0056] The condensate flow increment calculation module is used to determine the mass-energy balance of the deaerator transient process based on the lower limit of deaerator pressure change and the time required for water in the deaerator to flow to the feedwater pump inlet, and obtain the condensate flow increment to prevent insufficient feedwater pump NPSH; the condensate flow increment is summed with the actual condensate flow corresponding to the obtained unit load to determine the condensate flow limit value to prevent insufficient feedwater pump NPSH.

[0057] The deaerator water level automatic control module is used to limit the increase of the deaerator water level automatic control module output when the actual condensate flow rate is greater than the condensate flow rate limit value, and 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 water level automatic control module works normally.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] The method for controlling the net positive suction head (NPSH) of feedwater pump sets in thermal power plants proposed in this invention simplifies the condensate flow increment obtained from the deaerator transient calculation by ignoring various heat and mass inputs to the deaerator during the transient calculation process. This simplified condensate flow increment provides a conservative value that meets the requirements for preventing insufficient NPSH of the feedwater pumps, offering data support for limiting deaerator level control. By summing the condensate flow increment with the condensate flow corresponding to the unit load, a condensate flow limit value for preventing insufficient NPSH of the feedwater pumps is determined. This value can be matched with actual unit operating data, demonstrating strong applicability. When the actual condensate flow rate is greater than the condensate flow rate limit, the output of the deaerator level automatic control module is increased; when the actual condensate flow rate is less than the condensate flow rate limit or the condensate flow rate increment is negative, the deaerator level automatic control module operates normally. This method can both meet the requirements of deaerator level control to prevent insufficient feedwater pump cavitation margin throughout the entire process and prevent the expansion of accidents caused by sudden changes in deaerator level or condensate flow rate, thereby improving the anti-interference capability of the automatic control of the thermal power plant's thermal system and thus improving the safety and stability of the thermal power plant's operation. Attached Figure Description

[0060] Figure 1 This is a flowchart of the method for controlling the net positive suction head (NPSH) of a feedwater pump set in a thermal power plant according to the present invention.

[0061] Figure 2 This invention relates to the architecture of the net positive suction head (NPSH) control system for feedwater pump units in thermal power plants.

[0062] Figure 3 This is a schematic diagram illustrating the calculation process of available net positive suction head (NPSH) provided in an embodiment of the present invention.

[0063] Figure 4 This is a schematic diagram illustrating the calculation process of the lower limit of allowable pressure for a deaerator provided in an embodiment of the present invention.

[0064] Figure 5 This is a schematic diagram of the transient process time calculation process provided in an embodiment of the present invention;

[0065] Figure 6 This is a schematic diagram illustrating the calculation process for the allowable condensate flow rate increment provided in an embodiment of the present invention.

[0066] Figure 7This is a schematic diagram illustrating the calculation process of permissible condensate flow rate provided in an embodiment of the present invention;

[0067] Figure 8 This is a schematic diagram of the automatic water level control process of the deaerator provided in an embodiment of the present invention. Detailed Implementation

[0068] The following will refer to the appendices in the embodiments of the present invention. Figures 1-8 The technical solutions in the embodiments of the present invention will be clearly and completely described. It should be understood that the terminology used in the present invention is only for describing particular implementation methods and is not intended to limit the present invention.

[0069] like Figure 1 As shown, this invention proposes a method for controlling the net positive suction head (NPSH) of feedwater pump sets in thermal power plants, comprising the following steps:

[0070] S1: Obtain the actual available net positive suction head (NPSH) at the feedwater pump inlet. Based on the difference between the available NPSH and the known required NPSH for the feedwater pump, determine the lower limit of the allowable deaerator pressure variation.

[0071] S2: Based on the obtained length and radius of the deaerator downcomer and the feedwater pump inlet flow rate corresponding to the unit load, obtain the time required for water in the deaerator to flow to the feedwater pump inlet;

[0072] S3: Based on the lower limit of deaerator pressure change and the time required for water in the deaerator to flow to the feedwater pump inlet, determine the mass-energy balance of the deaerator transient process and obtain the condensate flow increment to prevent insufficient feedwater pump NPSH; sum the condensate flow increment with the actual condensate flow corresponding to the obtained unit load to determine the condensate flow limit value to prevent insufficient feedwater pump NPSH.

[0073] S4: Sum the condensate flow increment with the actual condensate flow corresponding to the obtained unit load to determine the condensate flow limit value to prevent insufficient feedwater pump NPSH, i.e., the allowable condensate flow.

[0074] S5: By adding a limiter to the deaerator level automatic control module, when the actual condensate flow rate is greater than the condensate flow rate limit value, the output of the deaerator 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 negative, the deaerator level automatic control module works normally.

[0075] Specifically, in step S1, the actual available net positive suction head (NPSH) at the feedwater pump inlet is obtained, which includes:

[0076] The prerequisites for preventing insufficient net positive suction head (NPSH) control in feedwater pump units include:

[0077] An automatic control module for setting the deaerator liquid level is set up to automatically control the deaerator water level through the condensate pump frequency converter or the deaerator water inlet regulating valve.

[0078] Install condensate flow measurement points entering the deaerator and feedwater flow measurement points exiting the deaerator on the feedwater pump system pipeline;

[0079] The deviation between the pressure readings at the deaerator pressure measuring points and the absolute pressure is known. A ;

[0080] The deviation between the reading displayed at the deaerator level measuring point and the height difference between the water surface and the bottom of the deaerator is known. B ;

[0081] As shown in Table 1, obtain system device data.

[0082] Table 1. Data on relevant systems and equipment

[0083]

[0084] Based on the current operating parameters of the pump unit and the obtained system equipment parameters, calculate the actual available net positive suction head (NPSH) at the inlet of the feedwater pump unit. :

[0085]

[0086] In the formula, This refers to the deaerator pressure. This refers to the feedwater density inside the deaerator; This is the acceleration due to gravity, with a value of 9.807 m / s². The static differential pressure height from the bottom of the deaerator to the center line of the feedwater pump; The flow resistance of the feedwater pump suction inlet pipe; The density of the water supplied to the pump is obtained from the pump inlet pressure and temperature. The saturation pressure of the feed water inside the pump is obtained from the water temperature inside the pump. The deaerator liquid level is set with the bottom of the deaerator as the zero point. A It is the difference between the pressure measurement point on the deaerator and the absolute pressure. B This is the difference between the zero point of the deaerator level measuring point and the bottom of the deaerator.

[0087] Determining the lower limit of permissible deaerator pressure variation includes the following steps:

[0088] Take the actual available net positive suction head (NPSH) at the inlet of the feedwater pump set. The required net positive suction head (NPSH) for the feedwater pump The difference is the lower limit of the allowable deaerator pressure variation. :

[0089]

[0090] in, for The corresponding saturated water density.

[0091] In step S2, the time required for water to flow from the deaerator to the feedwater pump inlet is obtained, including the following steps:

[0092] Based on the actual stable operating conditions of the feedwater pump unit, the actual feedwater flow rate when the unit load is stable is obtained. ;

[0093] The time required for water to flow from the deaerator to the feedwater pump inlet is obtained. t for:

[0094]

[0095] In the formula, The radius of the low-pressure water supply pipeline; This refers to the length of the low-pressure water supply pipeline.

[0096] In step S3, the condensate flow rate increment to prevent insufficient cavitation margin of the feedwater pump is obtained, specifically including:

[0097] Obtain the radius of the deaerator R d Length of deaerator L and the height of the deaerator elliptical head Y ;

[0098] Will As a parameter for the initial state during the transient process of the deaerator, the lower limit of the deaerator pressure is used. As a parameter for the final state during the transient process of the deaerator, time is used. t As the time of the deaerator transient process, the conservative condensate flow increment to prevent insufficient feedwater pump NPSH is calculated by calculating the mass-energy balance of the deaerator transient process. .

[0099] When calculating the transient process of the deaerator, this invention assumes that the condensate flowing into the deaerator, extraction steam, condensate, and feedwater flowing out of the deaerator are in a mass-energy balance state during normal and stable operation. It neglects the increase in extraction steam after the pressure drop within the deaerator, the metallic heat equivalent of the condensate pipes, the metallic heat equivalent of the deaerator itself, the mass and heat of the saturated gas within the deaerator, and the latent heat of vaporization of the water within the deaerator. The invention simplifies the pressure drop in the deaerator caused by a large influx of cold water into the deaerator as a problem of a pressure vessel filled with saturated water receiving a certain amount of cold water. A conservative allowable amount of cold water is calculated, referred to as the condensate increment. Must meet:

[0100]

[0101]

[0102]

[0103]

[0104] in, , The enthalpy and volume of saturated water in the deaerator in the initial state during the transient process; , , These represent the volume, density, and enthalpy of the saturated water in the deaerator at the end of the transient process, respectively. This is the enthalpy of condensate.

[0105] The transient process mass-energy balance of the deaerator is obtained by simplifying the transient calculation model of the deaerator. The process of simplifying the transient calculation model of the deaerator is to ignore the mass of saturated steam and its heat storage in the deaerator, specifically including:

[0106] In actual operation of the unit, in addition to condensate system failures, the deaerator water level may also drop when frequency regulation is performed by condensate throttling, which will lead to a decrease in the available net positive suction head (NPSH) of the feedwater pump (pre-pump).

[0107] After studying the impact of significant load changes in the unit during the deaerator transient process on the safe operation of the feedwater pump, and conducting dynamic analysis of the changes in heat, flow rate, and pressure of condensate, feedwater, and water stored in the deaerator tank after the deaerator changes its operating conditions, the designers ultimately ensured the net positive suction head (NPSH) of the feedwater pump under the unit's maximum load shedding condition by setting a reasonable relative height difference between the deaerator and the inlet of the pre-pump. This ensured that the NPSH of the feedwater pump met the requirements under all load changes of the unit.

[0108] When the actual operating water level of the deaerator deviates from the set water level, most scholars focus on how to quickly adjust the water level back to the normal value or reduce its dynamic disturbance, neglecting the risk of insufficient net positive suction head (NPSH) of the feedwater pump caused by a large influx of condensate into the deaerator under special operating conditions of the unit.

[0109] This invention simplifies the transient calculation model of the deaerator to obtain a conservative calculation formula for the condensate flow rate to prevent insufficient cavitation margin of the feedwater pump under special operating conditions. This formula is then used as a limit value in the conventional deaerator water level control logic to solve this problem.

[0110] Deaerator transient calculation

[0111] During the transient calculation of the deaerator, in order to ensure that the feedwater pump does not experience cavitation during operation, the effective net positive suction head (NPSH) at the pump inlet must be set, i.e., the actual usable NPSH at the feedwater pump set inlet. This value is determined by the feedwater pump's suction side piping, system, and equipment, and is independent of the pump itself, but it must be greater than the feedwater pump's required net positive suction head (NPSH). .

[0112] Effective net positive suction head (NPSH) satisfies the following relationship:

[0113]

[0114] In the formula, For deaerator pressure, A The deviation between the pressure reading displayed at the deaerator pressure measuring point and the absolute pressure; This refers to the feedwater density inside the deaerator; This is the acceleration due to gravity, with a value of 9.807 m / s². The static differential pressure height from the bottom of the deaerator to the center line of the feedwater pump; The flow resistance of the feedwater pump suction inlet pipe; This refers to the density of the water supplied to the pump. This is the saturation pressure of the water supply inside the pump; The deaerator liquid level is set with the bottom of the deaerator as the zero point. B This refers to the deviation between the value displayed at the deaerator level measuring point and the height difference between the water surface and the bottom of the deaerator.

[0115] Traditional deaerator transient calculations typically require considering the equivalent metal mass of the final stage low-pressure heater body and its piping to the deaerator, as well as the deaerator body and its piping to the feedwater pump inlet. This involves calculating the saturated water enthalpy within the deaerator and the feedwater enthalpy within the feedwater pump through four different processes, and then substituting these values ​​into the effective net positive suction head (NPSH) formula to calculate the NPSH. This calculation method is complex and not conducive to engineering control configuration applications. Therefore, this invention simplifies the deaerator transient model.

[0116] Model simplification

[0117] To facilitate qualitative analysis, secondary factors are ignored, and sufficient safety margin is retained. The following assumptions are made for the deaerator transient model:

[0118] 1) The working fluid inside the entire deaerator is in a saturated state, without considering slight superheating of steam or underenthalpy of liquid working fluid. The area above the water surface inside the deaerator is saturated gas, and the area below the water surface is saturated water.

[0119] 2) When calculating the net positive suction head (NPSH), a conservative value can be obtained by replacing the saturated water density in the deaerator with the water density in the feedwater pump, assuming that the feedwater density in the low-pressure feedwater pipeline is consistent with the density in the feedwater pump.

[0120] 3) The influence of the water flow rate and the equivalent metal mass of the pipeline from the first stage low pressure heater to the deaerator is not considered. The final parameters of the condensate entering the deaerator are directly used as variables; it is assumed that the condensate temperature does not change during the transient process.

[0121] 4) The influence of the metal equivalent mass of the deaerator body is not considered, the heat dissipation of the deaerator itself is ignored, and the influence of non-condensable gas on heat transfer is not considered.

[0122] 5) The increase in steam extraction caused by the drop in deaerator temperature during the transient process is not considered. It is assumed that under a fixed load, the energy and mass of the feedwater flowing out of the deaerator, the condensate entering the deaerator from the last high-pressure heater, the steam entering the deaerator, and the condensate entering the deaerator are in balance and remain basically unchanged.

[0123] 6) In actual engineering projects, after the unit is running under load, the flow velocity in the low-pressure water supply pipeline is often concentrated between 1.0 and 2.5 m / s. The flow velocity is low and the variation range is small, resulting in small changes in friction loss along the pipeline. At the same time, it is assumed that the local loss at the filter screen on the low-pressure water supply pipeline is fixed. Therefore, it is assumed that the friction loss along the low-pressure water supply pipeline is a fixed value.

[0124] 7) Excluding the balanced portion in assumption 5), the problem of preventing insufficient NPSH (Net Positive Suspension) of the feedwater pump in the deaerator can be simplified to controlling the pressure change after a certain amount of condensate enters a pressure vessel filled with stratified saturated water. By reasonably controlling the quality of the condensate entering the pressure vessel, the pressure and liquid level changes within the pressure vessel can be controlled. In other words, it simplifies to the problem of how to control the slow increase of the deaerator water level by controlling the increase in condensate flow rate, based on the already balanced condition.

[0125] Since the above assumptions ignore the various heat inputs to the deaerator during the transient calculation process, and the calculation of the available net positive suction head (NPSH) of the feedwater pump uses a conservative calculation method, the condensate increment obtained by the internal mass-energy balance calculation of the deaerator is a conservative value that meets the requirement of preventing insufficient NPSH of the feedwater pump.

[0126] Taking the mass-energy balance within the deaerator as the research object, when the unit is running stably, the condensate, extraction steam, and condensate entering the deaerator reach a balance. When the extraction steam and condensate volumes are stable, only an additional influx of excessive condensate will disrupt this balance. Ignoring the already balanced condensate, extraction steam, and condensate, the mass-energy balance model within 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 within the deaerator, the magnitude of the condensate increment is controlled.

[0127] The complete mass-energy balance calculation for the deaerator transient process includes the condensate flow rate and condensate pipeline metal equivalent, deaerator body metal equivalent, extraction steam flow rate, and saturated steam flow rate within the deaerator. Among these, the extraction steam flow rate, condensate pipeline metal equivalent, deaerator body metal equivalent, extraction steam flow rate, and saturated steam flow rate within the deaerator have a positive impact on the deaerator pressure (i.e., feedwater pump net positive suction head). Therefore, ignoring these factors results in a smaller usable net positive suction head, which further leads to a conservative condensate flow rate increment to prevent insufficient feedwater pump net positive suction head.

[0128] Mathematical calculation model of deaerator transient model

[0129] Calculation of water and steam parameters

[0130] The densities of saturated water and steam inside the deaerator are functions of the saturation pressure, i.e.:

[0131]

[0132]

[0133]

[0134]

[0135] In the formula, This refers to the deaerator temperature. , These are the densities of saturated water and saturated gas in the deaerator, respectively. , This corresponds to the enthalpy values ​​of saturated water and saturated gas inside the deaerator.

[0136] Calculation of water and steam volume in deaerator

[0137] The volume of saturated water and saturated steam within the deaerator is a function of the deaerator dimensions and the liquid level, as shown below:

[0138]

[0139]

[0140] In the formula, The length of the deaerator, The radius of the deaerator. This refers to the height of the elliptical head of the deaerator. , These represent the initial volume of saturated water and the volume of saturated gas in the deaerator during the transient process, respectively. This refers to the full volume of the deaerator.

[0141] Transient mass-energy conservation calculation within the deaerator

[0142] At pressure and temperature , The liquid level is Inside the deaerator, at a fixed time The parameters for internal inflow exceeding the baseline condensate flow rate are: , condensate increment This causes the parameters inside the deaerator to change as follows: , , .

[0143] According to the law of conservation of mass, we have:

[0144]

[0145] According to the law of conservation of energy, we have:

[0146]

[0147] In the formula, , These are the enthalpy values ​​of saturated water and saturated gas in the deaerator, respectively. This is the enthalpy of condensate.

[0148] Pressure relationship of deaerator before and after transient process inside deaerator

[0149] As shown in the formula for effective net positive suction head (NPSH), the available NPSH at the feedwater pump inlet is a function of the feedwater pump inlet pressure and the deaerator temperature. When the system parameters are fixed, the available NPSH is definite. For a large amount of condensate to flow into the deaerator without cavitation in the feedwater pump, the following relationship must hold:

[0150]

[0151] In the formula, D The static pressure of the water column corresponding to the allowable variation of the net positive suction head (NPSH), subject to the deaerator pressure. Deaerator liquid level The impact.

[0152] Determination of the transient process time within the deaerator

[0153] Studies have shown that the minimum net positive suction head (NPSH) at the feedwater pump inlet during the deaerator transient process occurs during the period between when condensate enters the deaerator and before it reaches the feedwater pump inlet. Therefore, the duration of the transient process is determinable and satisfies the following condition:

[0154]

[0155] In the formula, This is the time required for water to flow from the deaerator to the feedwater pump inlet. The radius of the low-pressure water supply pipeline. The length of the low-pressure water supply pipeline. This represents the actual water supply flow rate when the unit load is stable.

[0156] Determination of condensate flow increment

[0157] Based on the formulas in the mathematical calculation model of the deaerator transient model, the condensate increment can be determined. It is the deaerator pressure. Deaerator liquid level Flow rate of water pump inlet A function that satisfies:

[0158]

[0159]

[0160]

[0161]

[0162]

[0163] because Given, then , , , , , , and It can be quickly obtained from the saturated water and saturated water vapor parameter tables.

[0164] Further simplification of the formula for calculating condensate flow increment

[0165] Since the saturated steam in the deaerator has a lower mass ratio and lower heat storage ratio compared to saturated water, and according to existing research, the heat storage change caused by pressure and flow rate changes during the transient process of a two-phase mixing heat exchanger is mainly due to water, the mass and heat storage of the saturated steam in the deaerator can be ignored. Therefore, the transient mass-energy conservation calculation formula and the condensate flow rate increment in the deaerator can be simplified as follows:

[0166]

[0167]

[0168]

[0169] In step S4, determining the condensate flow limit to prevent insufficient net positive suction head (NPSH) of the feedwater pump includes the following steps:

[0170] Based on the actual stable operating conditions of the feedwater pump unit, obtain the corresponding condensate flow rate of the unit under different operating conditions. ;

[0171] Increase the condensate flow rate Condensate flow rate corresponding to unit load Summing yields the condensate flow limit to prevent insufficient net positive suction head (NPSH) of the feedwater pump. for:

[0172]

[0173] in, To prevent the increase in condensate flow rate due to insufficient cavitation margin of the feedwater pump.

[0174] The control process in step S5 specifically includes:

[0175] When the actual condensate flow rate The condensate flow rate exceeds the limit value for preventing insufficient net positive suction head (NPSH) of the feedwater pump. At this time, the output of the automatic control module for deaerator liquid level is limited, and the opening degree of the water regulating valve on the deaerator or the frequency of the condensate frequency converter is prohibited from increasing.

[0176] When the actual condensate flow rate Less than the condensate flow rate limit to prevent insufficient cavitation margin of the feedwater pump When the condensate flow rate increment is negative to prevent insufficient cavitation margin of the feedwater pump, the deaerator level automatic control module will operate normally.

[0177] The calculated result of normal condensate flow increment will not be negative. However, considering that the measuring points may malfunction during actual operation of the unit, which may lead to a negative calculated condensate flow increment, the logic of "when the condensate flow increment is negative, the deaerator level automatic control module works normally" is added to avoid the measuring point problem affecting the normal operation of the deaerator level automatic control module.

[0178] This invention also proposes a net positive suction head (NPSH) control system for feedwater pump sets in thermal power plants, comprising:

[0179] Available NPSH calculation module is used to obtain the actual available NPSH at the inlet of the feedwater pump;

[0180] The deaerator pressure variation lower limit calculation module is used to determine the allowable lower limit of deaerator pressure variation based on the difference between the available net positive suction head (NPSH) and the known required NPSH of the feedwater pump.

[0181] The transient time calculation module is used to obtain the time required for water in the deaerator to flow to the feedwater pump inlet based on the obtained length and radius of the deaerator downcomer and the feedwater pump inlet flow corresponding to the unit load.

[0182] The condensate flow increment calculation module is used to determine the mass-energy balance of the deaerator transient process based on the lower limit of deaerator pressure change and the time required for water in the deaerator to flow to the feedwater pump inlet, and obtain the condensate flow increment to prevent insufficient feedwater pump NPSH; the condensate flow increment is summed with the actual condensate flow corresponding to the obtained unit load to determine the condensate flow limit value to prevent insufficient feedwater pump NPSH.

[0183] The deaerator water level automatic control module is used to limit the increase of the deaerator water level automatic control module output when the actual condensate flow rate is greater than the condensate flow rate limit value, and 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 water level automatic control module works normally.

[0184] The method proposed in this invention prevents insufficient net positive suction head (NPSH) of feedwater pump sets by limiting the condensate flow rate under accident conditions, thereby improving the safety and stability of thermal power plant operation.

[0185] Example

[0186] To verify the effectiveness of the method proposed in this invention, this embodiment takes a 660MW unit as an example and provides a detailed control process.

[0187] The prerequisites for determining automatic control methods to prevent insufficient net positive suction head (NPSH) in feedwater pump units of thermal power plants include:

[0188] a. The deaerator level has a basic automatic control module. In this embodiment, it is a typical three-impulse deaerator level control module.

[0189] b. The system pipeline has relatively accurate condensate flow measurement points entering the deaerator and feedwater flow measurement points exiting the deaerator.

[0190] c. Deviation between the pressure reading displayed at the deaerator pressure measuring point and the absolute pressure A =0.06MPa, meaning the pressure displayed by the deaerator is 0.06MPa lower than the absolute pressure.

[0191] d. The deviation between the value displayed at the deaerator level measuring point and the height difference between the water surface and the bottom of the deaerator. B =0.5m, meaning the zero level of the deaerator liquid level is 0.5m higher than the bottom of the deaerator.

[0192] e. As shown in Table 2, obtain the following relevant system and equipment data.

[0193] Table 2. Relevant system and device data obtained from the embodiments.

[0194]

[0195] f. Based on the actual stable operating conditions of the unit, obtain the corresponding condensate flow rate and feedwater pump inlet flow rate under different operating conditions, as shown in Table 3.

[0196] Table 3. Condensate flow rate and feedwater pump inlet flow rate under different operating conditions

[0197]

[0198] Once all the above conditions are met, the automatic control steps of the automatic control method for preventing insufficient net positive suction head (NPSH) of feedwater pump units in thermal power plants specifically include:

[0199] The first step is to calculate the actual available net positive suction head (NPSH) at the inlet of the feedwater pump set based on the current operating parameters and the collected system equipment parameters. As shown in the following formula, the corresponding logic configuration is as follows: Figure 3 As shown.

[0200]

[0201] The second step is to determine the available net positive suction head (NPSH). With known feedwater pumps, the required net positive suction head (NPSH) The difference is used to calculate the lower limit of the allowable deaerator pressure change during the transient time. ,like Figure 4 As shown.

[0202]

[0203] The third step is to determine the length of the deaerator downcomer. ,radius Feedwater pump inlet flow rate corresponding to unit load Calculate the time required for water to flow from the deaerator to the feedwater pump inlet. That is, the time of the transient process, such as Figure 5 As shown.

[0204]

[0205] The fourth step is to As a parameter for the initial state during the transient process of the deaerator, the lower limit of the deaerator pressure is used. As a parameter for the final state during the transient process of the deaerator, time is used. t As the time of the deaerator transient process, the conservative condensate flow increment to prevent insufficient feedwater pump NPSH is calculated by calculating the mass-energy balance of the deaerator transient process. This means that the allowable increase in condensate flow rate is, for example... Figure 6 As shown.

[0206] Step 5: Increase condensate flow rate Add the condensate flow rate corresponding to the unit load at this time To obtain the condensate flow limit value to prevent insufficient cavitation margin of the feedwater pump. That is, the allowable condensate flow rate, such as Figure 7 As shown.

[0207] The calculation results of steps four and five above are as follows:

[0208] The unit load is 495MW. Verification was conducted using two methods: varying the deaerator level under the same condensate temperature and varying the condensate temperature under the same deaerator level. The calculated maximum condensate flow rate to prevent feedwater pump cavitation is shown in Tables 3 and 4 below. In the tables... This refers to the condensate temperature. The deaerator liquid level is defined with the bottom of the deaerator as the zero point.

[0209] Table 3. Liquid levels in different deaerators =131 ℃, 495 MW

[0210]

[0211] Table 4. Different condensate temperatures, =2.45 m, 495 MW

[0212]

[0213] The sixth step is to add a lockout limiter to the automatic water level control module of the deaerator.

[0214] When the actual condensate flow rate Greater than When the deaerator level automatic control module output is increased, it prevents the deaerator water inlet regulating valve from increasing its opening or the condensate frequency converter from increasing its frequency; when the actual condensate flow rate... Less than Sometimes When the value is negative, the deaerator level automatic control module works normally, such as... Figure 8 As shown.

[0215] This embodiment demonstrates that the invention clarifies the key points of the automatic control method for preventing insufficient net positive suction head (NPSH) of feedwater pump units, and rationally sets it in the conventional automatic deaerator water level control module, enabling the automatic control logic of the deaerator water level in thermal power plants to have a certain ability to prevent insufficient NPSH of feedwater pumps.

[0216] The method proposed in this invention realizes automatic control to prevent insufficient NPSH of feedwater pump sets in thermal power plants, reduces manual operation, and avoids risks such as misoperation, laying the foundation for intelligent operation of thermal power generating units throughout the entire process (especially in accident conditions).

[0217] The automatic control method proposed in this invention incorporates a reasonable calculation method, which not only prevents insufficient cavitation margin of the feedwater pump but also prevents the further escalation of accidents. This method achieves continuous control of the deaerator water level during operation, reducing the experimental risks associated with condensate throttling participating in primary frequency regulation. The method also has anti-maloperation capabilities, ensuring that errors in the measurement point display will not affect the normal operation of the automatic deaerator water level control module.

[0218] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0219] Furthermore, unless otherwise stated, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. All references to this specification are incorporated by way of citation to disclose and describe methods relating to those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

Claims

1. A method for controlling the NPSH of a boiler feed water pump set of a thermal power plant, characterized in that, The method comprises the following steps: acquiring actual available net positive suction head of the feed water pump inlet, determining lower limit of allowed deaerator pressure variation according to difference between the available net positive suction head and known necessary net positive suction head of the feed water pump; acquiring time required for water flow in the deaerator to flow to the feed water pump inlet according to length and radius of the deaerator downcomer and feed water pump inlet flow corresponding to the unit load; determining deaerator transient process mass-energy balance according to the lower limit of the deaerator pressure variation and the time required for the water flow in the deaerator to flow to the feed water pump, acquiring condensate flow increment for preventing insufficient net positive suction head of the feed water pump, and summing the condensate flow increment and actual condensate flow corresponding to the unit load to determine condensate flow limit value for preventing insufficient net positive suction head of the feed water pump; increasing a limiter on the deaerator liquid level automatic control module, when the actual condensate flow is greater than the condensate flow limit value, limiting the deaerator liquid level automatic control module output from increasing, and when the actual condensate flow is less than the condensate flow limit value or the condensate flow increment is negative, normally working the deaerator liquid level automatic control module.

2. The method of claim 1, wherein, The acquiring actual available net positive suction head of the feed water pump inlet specifically comprises: determining preconditions for preventing insufficient net positive suction head control of the feed water pump group, including: setting a deaerator liquid level basic automatic control module to automatically control the deaerator water level through a condensate pump frequency converter or a deaerator water feeding regulating valve; setting a condensate flow measuring point into the deaerator and a feed water flow measuring point out of the deaerator on a feed water pump system pipeline; Known is a deviation amount of a deaerator pressure measuring point display value from an absolute pressure A ; Known is a deviation amount of a display value of a deaerator level measuring point from a height difference of a water surface of a deaerator to a bottom of the deaerator B ; Collecting system equipment data, including: static differential pressure height from deaerator bottom to feedwater pump centerline and flow resistance of feedwater pump suction inlet piping ; According to the current operating parameters of the water pump unit and the collected system equipment parameters, the actual available net positive suction head of the water pump unit inlet is calculated ; theoretically calculating an available net positive suction head formula as follows: wherein Pd is the deaerator pressure; Pd is the deaerator pressure; g is the acceleration of gravity, having a value of 9.807 m / s2; h is the static head of the deaerator to the feedwater pump centerline; R is the flow resistance of the feedwater pump suction inlet piping; Pd is the deaerator pressure; Pd is the deaerator pressure; H is the deaerator level with the deaerator bottom as zero. In actual calculation, use instead of the formula in , both the conservative available NPSH and the simplified logic configuration quantity can be obtained, and the available NPSH formula is simplified to: 。 3. The method for controlling the NPSHa of a boiler feed water pump set of a thermal power plant according to claim 2, characterized by, The determining lower limit of allowed deaerator pressure variation comprises the following steps: Necessary net positive suction head for water supply pump ; The difference between the actual available net positive suction head at the inlet of the feed water pump set and the required net positive suction head for the feed water pump : the lower limit of the allowable deaerator pressure variation :​ wherein is corresponding saturated water density.

4. The method for controlling NPSH of a boiler feed water pump set in a thermal power plant according to claim 3, characterized by, The acquiring time required for water flow in the deaerator to flow to the feed water pump inlet comprises the following steps: According to the actual stable operation condition of the feed water pump unit, the actual feed water flow rate when the unit load is stable is obtained ; To obtain the time required for the flow of water in the deaerator to the feedwater pump inlet t Is: wherein R is the radius of the low-pressure feedwater pipe; L is the length of the low-pressure feedwater pipe.

5. The method for controlling NPSH of a boiler feed water pump set in a thermal power plant according to claim 4, characterized by, The acquiring condensate flow increment for preventing insufficient net positive suction head of the feed water pump specifically comprises: Obtaining the length of the deaerator L , the radius , and the elliptical head height Y ; Will As a parameter for the initial state during the transient process of the deaerator, the lower limit of the deaerator pressure is used. As a parameter for the final state during the transient process of the deaerator, time is used. t The time of the deaerator transient process; The increment of condensate flow to prevent the shortage of NPSH of feed water pump is calculated by the mass-energy balance of transient process of deaerator To: wherein, , is the enthalpy of the saturated water in the deaerator at the initial state in the transient process, the volume; , , are the volume, the density and the enthalpy of the saturated water in the deaerator at the end state in the transient process, respectively; H co is the enthalpy of the condensed water; The deaerator transient process mass-energy balance is obtained by simplifying a deaerator transient calculation model, and the process of simplifying the deaerator transient calculation model is ignoring mass and heat storage of saturated steam in the deaerator and ignoring metal equivalent of related pipelines and equipment.

6. The method for controlling NPSH of a boiler feed water pump set in a thermal power plant according to claim 5, characterized by, The determining condensate flow limit value for preventing insufficient net positive suction head of the feed water pump comprises the following steps: According to the actual stable operation condition of the feed water pump unit, the corresponding condensate water flow of the unit under different conditions is obtained ; The condensate flow rate increment is calculated The condensate flow rate corresponding to the unit load The sum is obtained to prevent the condensate flow rate limit value of the feed water pump cavitation margin is: wherein, To prevent the condensate flow from increasing to the extent that the NPSHA of the feedwater pump is insufficient.

7. The method for controlling NPSH of a boiler feed water pump set in a thermal power plant according to claim 6, characterized by, The limiting the deaerator liquid level automatic control module output from increasing specifically comprises: Obtaining actual condensate flow corresponding to unit load under different working conditions ; When the actual condensate flow is greater than the condensate flow limit value to prevent the feed water pump from cavitation , the deaerator level automatic control module output is limited to increase, the water regulating valve opening on the deaerator is increased or the condensate frequency converter frequency is increased.

8. The method for controlling NPSH of a boiler feed water pump set in a thermal power plant according to claim 7, characterized by, The normally working the deaerator liquid level automatic control module specifically comprises: when the actual condensate flow rate is less than the condensate flow rate limit value for preventing the feed water pump from being short of a cavitation margin or the increment of the condensate flow rate for preventing the feed water pump from being short of a cavitation margin is negative, the deaerator liquid level automatic control module operates normally.

9. A power plant feed water pump group NPSH control system, characterized by, including: an available net positive suction head calculation module for acquiring actual available net positive suction head of the feed water pump inlet; a deaerator pressure variation lower limit calculation module for determining lower limit of allowed deaerator pressure variation according to difference between the available net positive suction head and known necessary net positive suction head of the feed water pump; a transient time calculation module for acquiring time required for water flow in the deaerator to flow to the feed water pump inlet according to length and radius of the deaerator downcomer and feed water pump inlet flow corresponding to the unit load; The condensate flow increment calculation module is configured to determine the transient process mass-energy balance of the deaerator according to the deaerator pressure change lower limit and the time required for water flow in the deaerator to reach the feed water pump inlet, and obtain the condensate flow increment for preventing the insufficient NPSH of the feed water pump; and sum the condensate flow increment and the actual condensate flow corresponding to the unit load to determine the condensate flow limit value for preventing the insufficient NPSH of the feed water pump; The deaerator water level automatic control module is configured to increase a limiter on the deaerator liquid level automatic control module, so that when the actual condensate flow is greater than the condensate flow limit value, the deaerator liquid level automatic control module output is limited to increase; when the actual condensate flow is less than the condensate flow limit value or the condensate flow increment is negative, the deaerator liquid level automatic control module normally works.

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