A cryogenic pump inlet net positive pressure head measurement system and measurement method

Abstract

The present invention relates to the technical field of measuring the net positive head pressure at the cryogenic pump inlet, and more particularly to a system and method for measuring the net positive head pressure at the cryogenic pump inlet. The measurement system comprises: a storage tank, a cryogenic pump, a first measurement module, a second measurement module, a temperature measuring device, and a measurement and control cabinet. The measurement method comprises the following steps: filling the cryogenic pump with a medium, charging the cryogenic pump with a gaseous medium, precooling the cryogenic pump, and measuring the net positive head pressure at the inlet. A coil is provided at the cryogenic pump inlet, medium vapor is input into the coil, and a saturated two-phase equilibrium state is formed between the medium vapor in the coil and the liquid medium at the cryogenic pump inlet. The differential pressure transmitter of the first measurement module is then used to obtain the extreme operating state of the pump during cavitation. When the pump cavitation occurs, the differential pressure transmitter of the second measurement module is used to obtain, in real time, the difference between the inlet pressure and the saturated vapor pressure of the cryogenic medium at the pump inlet temperature. This is the net positive head pressure (NPSHr) of the pump under this operating condition.

Description

Technical Field

[0001] The present invention relates to the technical field of measuring the net positive pressure head at the inlet of a cryogenic pump, and in particular to a system and method for measuring the net positive pressure head at the inlet of a cryogenic pump. Background Art

[0002] When the centrifugal pump is working, the liquid at the inlet of the centrifugal pump will vaporize to produce gas because the inlet pressure is lower than the saturated vapor pressure corresponding to the inlet temperature (saturated vapor pressure of the liquid medium). The vaporized bubbles will erode the metal surface of the impeller and other parts under the impact movement of the liquid particles, thereby damaging the impeller and other metals. This is the cavitation phenomenon of the pump.

[0003] In order to avoid cavitation in the pump, during the operation of the centrifugal pump, it is necessary to ensure that the liquid pressure at the pump inlet is always greater than the saturated vapor pressure when the liquid vaporizes, thereby avoiding the vaporization of the liquid and the formation of cavitation.

[0004] Specifically, it is to ensure that the effective NPSHHa (also called the net positive pressure head at the inlet) of the pump is always higher than the NPSHr (also called the required NPSH) of the pump. The larger the NPSHa of the pump, the less likely it is to cavitate, and the smaller the NPSHr of the pump, the better the anti-cavitation performance. Therefore, a detection system and detection method for the net positive pressure head of the pump are produced. Through detection and calculation, the net positive pressure head NPSHa at the inlet when the pump is working, and the NPSHr when the pump cavitates, can be obtained.

[0005] In the Chinese invention patent application number 201710283136.1, a water pump NPSH automatic testing system and method are disclosed. Specifically, the flow rate and head data are fitted into a flow NPSH curve through a polynomial, and then a set of flow target operating conditions to be measured is substituted into the Q-NPSH curve expression to obtain a set of NPSH values. Among them, the flow rate-NPSH curve is based on the flow rate-head curve QH. The NPSH is calculated by inverting the flow rate and head data, and then the flow rate-NPSH curve is drawn. Then, the flow rate-NPSH curve is used as a standard to measure the NPSH of the pump, and the inlet positive pressure head of the pump is obtained.

[0006] In addition, in the Chinese invention patent application number 201410105625.4, a closed test device for the NPSH of a water-filled submersible pump and a well submersible pump is disclosed. The device uses a thermometer, an inlet pressure gauge, an outlet pressure gauge, a speed sensor, a speed measuring instrument and a flow meter to measure the key parameters of the NPSH of the submersible pump and then convert the NPSH into the NPSH.

[0007] Furthermore, in the Chinese invention patent application number 201810439296.5, a method and system for detecting the NPSH value of a water pump is disclosed. The automatic acquisition method is used to control the data acquisition of the entire detection process. When the cavitation of the water pump reaches the critical cavitation value, the automatic acquisition method is used to automatically complete the collection and calculation of the cavitation value of the water pump, and the accurate value of the NPSH of the water pump is obtained. The technical solution provides the following calculation process of the NPSH of the water pump: NPSH=H1-z D + (p amb -p v ) / (ρ1g), where H1 is the inlet head, unit: m; ZD is the distance relative to the reference plane, unit: m; pamb is the pressure, unit: pa; pv is the vapor pressure, unit: pa; ρ1 is the liquid density, unit: kg / m3; g is the acceleration due to gravity, unit: N / kg.

[0008] The above technical solutions all measure the liquid pressure at the pump inlet first, then obtain the saturated vapor pressure by consulting the material's physical property data sheet, using the steam equation, or experimentally determining it. The formula is then used to calculate the NPSH when the pump reaches critical cavitation.

[0009] However, saturated vapor pressure is strongly correlated with temperature: as temperature rises, the kinetic energy of liquid molecules increases, allowing more molecules to overcome the surface tension of the liquid and escape into the vapor phase, resulting in an increase in saturated vapor pressure. Conversely, when temperature decreases, saturated vapor pressure decreases. When a cryopump is operating, the pump inlet pressure and temperature will change due to the entry of external heat. Especially for cryopumps, even slight temperature changes will affect the saturated vapor pressure. Therefore, accurately obtaining the saturated vapor pressure value is very important during the detection process. Summary of the Invention

[0010] In response to the above problems, the present invention provides a system and method for measuring the net positive head pressure at the inlet of a cryogenic pump, which is aimed at measuring the net positive head pressure of the cryogenic pump. A coil is set at the inlet of the cryogenic pump, medium vapor is input into the coil, and a dynamic balance is formed between the medium vapor in the coil and the liquid medium at the inlet of the cryogenic pump, thereby obtaining the saturated vapor pressure of the cryogenic medium at the inlet of the cryogenic pump. Thereafter, a second differential pressure transmitter set at the inlet of the cryogenic pump directly measures the differential pressure between the real-time medium pressure and the saturated vapor pressure at the inlet of the cryogenic pump, which is the net positive head pressure NPSH at the inlet. When the inlet of the cryogenic pump is about to reach the critical point of cavitation, the differential pressure value displayed by the second differential pressure transmitter is the cavitation margin NPSHr. No conversion is required, and the detection is fast and accurate.

[0011] To achieve the above object, the present invention provides the following technical solutions:

[0012] A cryogenic pump inlet net positive head pressure measurement system, comprising:

[0013] Storage tank, cryogenic pump, first measurement module, second measurement module, temperature measuring device and measurement and control cabinet;

[0014] The storage tank stores a cryogenic medium, the storage tank is connected to the inlet of the cryogenic pump through a liquid infusion pipeline, and a pressure relief valve is provided on the top of the storage tank;

[0015] The outlet of the cryopump is connected to a drainage pipe, which is connected to the storage tank through a reflux pipe;

[0016] The first measurement module is provided at the inlet and outlet of the cryopump, and is used to measure the pressure difference DP between the inlet and outlet of the cryopump, and the first measurement module includes a first differential pressure transmitter, and the first differential pressure transmitter is connected to the inlet and outlet of the cryopump respectively;

[0017] The second measurement module is arranged at the inlet of the cryopump, and is used to measure the net positive pressure head NPSHa at the inlet of the cryopump. The second measurement module includes a coil and a second differential pressure transmitter. The coil is arranged at the medium inlet of the cryopump, and an opening A at one end of the coil passes through the side wall of the medium inlet of the cryopump and is connected to the top of the storage tank. The opening B at the other end of the coil is connected to the medium inlet of the cryopump. The second differential pressure transmitter is arranged on the pipeline from the spiral tube body of the coil to the opening B;

[0018] The temperature measuring device is connected to the drainage pipe and measures the temperature at the outlet of the cryopump;

[0019] The measurement and control cabinet is respectively connected to the cryogenic pump, the first differential pressure transmitter, the second differential pressure transmitter and the temperature measuring device. A data acquisition module and a data control module are integrated in the measurement and control cabinet, and a network output interface for communicating with an industrial computer equipped with a test program is provided on the measurement and control cabinet.

[0020] As an improvement, the first measurement module further includes a first pressure gauge and a second pressure gauge arranged on both sides of the first differential pressure transmitter, the first pressure gauge is arranged on the infusion pipeline, and the second pressure gauge is arranged on the drainage pipeline.

[0021] As an improvement, the second measuring module also includes a third pressure gauge and a fourth pressure gauge arranged on both sides of the second differential pressure transmitter, a first control valve is arranged between the third pressure gauge and the spiral tube body, a second control valve is arranged between the fourth pressure gauge and the opening B, a pipeline connection is provided between the first control valve and the second control valve, and a third control valve is provided on the pipeline.

[0022] As an improvement, a fourth control valve is provided on the pipeline connecting the coil and the storage tank.

[0023] As an improvement, a reflux vent valve and a reflux control valve are respectively provided on the reflux pipeline.

[0024] As an improvement, the infusion pipeline is sequentially provided with an infusion control valve, a safety valve, an inlet bellows and a filter;

[0025] An outlet bellows and an outlet control valve are sequentially arranged on the liquid discharge pipeline.

[0026] In addition, the present invention also provides a measurement method of the cryogenic pump inlet net positive head pressure measurement system based on any one of the above items, comprising the following steps:

[0027] Step 1: Filling the medium: inputting a low-temperature medium into the storage tank, wherein the low-temperature medium is liquid nitrogen;

[0028] Step 2: Filling the gaseous medium, opening the fourth control valve between the coil and the storage tank, and filling the spiral tube of the coil with nitrogen. After the filling is completed, closing the fourth control valve between the coil and the storage tank, and at the same time closing the third control valve, the pressure displayed on the third pressure gauge and the fourth pressure gauge is in a stable state, and then proceeding to the next step;

[0029] Step 3: Pre-cool the cryopump, open the infusion pipeline to connect the storage tank to the cryopump, open the return pipe between the cryopump and the storage tank to form a closed circulation flow pipeline between the cryopump and the storage tank, and start the cryopump after the outlet temperature of the temperature measuring device shows that the pre-cooling work is completed;

[0030] Step 4: Measure the net positive pressure head at the inlet. Close the return pipe, open the discharge pipe, and open the pressure vent valve on the storage tank. Continue running the cryogenic pump and gradually close the infusion control valve until the pressure difference ΔP1 of the first differential pressure transmitter decreases by 3%. Then, shut down the cryogenic pump and simultaneously record the indicated pressure P1 of the third pressure gauge, the indicated pressure P2 of the fourth pressure gauge, and the pressure difference ΔP2 of the second differential pressure transmitter. NPSHr at the cryogenic pump inlet = ΔP2 = |P1-P2|, unit: Pa.

[0031] As an improvement, in step one, the liquid level of the low-temperature medium in the storage tank after filling exceeds 2m.

[0032] As an improvement, in step 2, the storage tank is pressurized so that when the gaseous low-temperature medium is filled into the coil, the pressure applied to the storage tank is 6-7 bar.

[0033] As an improvement, in step three, the cryogenic medium circulates between the storage tank and the cryogenic pump for 18-22 minutes until the pump head of the cryogenic pump is completely frosted and the temperature at the outlet of the cryogenic pump reaches -130°C. The pressure difference ΔP1 displayed by the first differential pressure transmitter is in a stable state, and the cryogenic pump completes the pre-cooling treatment.

[0034] The beneficial effects of the present invention are:

[0035] (1) The present invention adopts a closed container method to set a coil at the inlet of the cryogenic pump, input medium vapor into the coil, and use the medium vapor in the coil to form a dynamic balance with the liquid medium at the inlet of the cryogenic pump, thereby obtaining the real-time saturated vapor pressure of the cryogenic medium at the inlet of the cryogenic pump. Then, the differential pressure value between the real-time inlet pressure of the cryogenic pump and the saturated vapor pressure is directly measured by a second differential pressure transmitter set at the inlet of the cryogenic pump, which is the inlet net positive pressure head NPSH. When cavitation is about to occur at the inlet of the cryogenic pump, the differential pressure value displayed by the second differential pressure transmitter is the NPSHr, which does not require conversion and is fast and accurate.

[0036] (2) When the present invention detects the cavitation margin of the cryogenic pump through the differential pressure value of the second differential pressure transmitter, it also rechecks the measured values ​​of the third barometer and the fourth barometer on both sides of the second differential pressure transmitter, thereby ensuring the accuracy of the detection result. At the same time, during the detection, the pressure state of the third barometer and the fourth barometer is an important indicator for determining whether the gas medium in the coil and the liquid medium at the inlet of the cryogenic pump are in dynamic equilibrium to form a dynamic saturated vapor pressure, thereby effectively ensuring the accuracy, stability and reliability of the detection;

[0037] (3) The present invention provides a first differential pressure transmitter and uses the first differential pressure transmitter to measure the inlet and outlet pressure difference DP of the cryogenic pump, thereby accurately determining the critical state of cavitation of the cryogenic pump and accurately measuring the NPSH at the inlet of the cryogenic pump, providing accurate reference data for the subsequent use and operation of the cryogenic pump, thereby improving the operation and service life of the cryogenic pump;

[0038] (4) The present invention measures the pressure difference between the inlet and outlet of the cryogenic pump by setting a first barometer and a second barometer, and compares and rechecks the pressure difference between the inlet and outlet of the cryogenic pump with the pressure difference between the inlet and outlet measured by the first differential pressure transmitter, thereby ensuring the accuracy of the measurement of the pressure difference between the inlet and outlet of the cryogenic pump DP. Then, the head of the cryogenic pump in this state is calculated based on the measured pressure difference between the inlet and outlet DP, providing reliable guidance parameters for the subsequent installation and use of the cryogenic pump.

[0039] In summary, the present invention has the advantages of real-time detection, no need for conversion, high detection value accuracy, stability and reliability, and is particularly suitable for the technical field of measuring the net positive pressure head at the pump inlet. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the measurement system structure of the present invention;

[0041] Figure 2 This is a schematic structural diagram of the second measurement module of the present invention;

[0042] Figure 3 Schematic diagram of the local structure of the measurement system of the present invention;

[0043] Figure 4 Schematic diagram of the measurement method of the present invention;

[0044] Figure 5 The figure shows the flow rate and NPSHR curve obtained after measurement in the present invention.

[0045] In the figure: 1. Storage tank, 11. Infusion pipeline, 110. Infusion control valve, 111. Safety valve, 112. Inlet bellows, 113. Filter, 12. Pressure vent valve, 121. Outlet bellows, 122. Outlet control valve, 13. Liquid level gauge, 2. Cryogenic pump, 21. Discharge pipeline, 22. Return pipeline, 221. Return vent valve, 222. Return control valve, 3. First measurement module, 31. First differential pressure transmitter, 32. First pressure gauge, 33. Second pressure gauge, 4. Second measurement module, 41. Coil, 411. Spiral tube body, 42. Second differential pressure transmitter, 43. Third pressure gauge, 44. Fourth pressure gauge, 45. First control valve, 46. Second control valve, 47. Pipeline, 48. Third control valve, 49. Fourth control valve, 5. Temperature measuring device, 6. Measurement and control cabinet. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0048] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0049] Example 1:

[0050] like Figures 1 to 3 As shown, a system for measuring the net positive pressure head at the inlet of a cryogenic pump comprises:

[0051] Storage tank 1, cryogenic pump 2, first measurement module 3, second measurement module 4, temperature measuring device 5 and measurement and control cabinet 6;

[0052] The storage tank 1 stores a cryogenic medium. This application is aimed at the measurement of a liquid nitrogen cryogenic pump. Therefore, the cryogenic medium in this application is liquid nitrogen, and the gaseous medium is nitrogen. All parameters are set based on liquid nitrogen. The storage tank 1 is connected to the inlet of the cryogenic pump 2 through a liquid infusion pipe 11, and a pressure relief valve 12 is provided on the top of the storage tank 1.

[0053] The outlet of the cryopump 2 is connected to a drain pipe 21, and the drain pipe 21 is connected to the storage tank 1 through a reflux pipe 22;

[0054] The first measurement module 3 is provided at the inlet and outlet of the cryopump 2. The first measurement module 3 is used to measure the pressure difference DP between the inlet and outlet of the cryopump 2. The first measurement module 3 includes a first differential pressure transmitter 31. The first differential pressure transmitter 31 is connected to the inlet and outlet of the cryopump 2 respectively.

[0055] The second measurement module 4 is provided at the inlet of the cryopump 2. The second measurement module 4 is used to measure the net positive head pressure NPSH at the inlet of the cryopump 2. The second measurement module 4 includes a coil 41 and a second differential pressure transmitter 42. The coil 41 is provided at the medium inlet of the cryopump 2. An opening A at one end of the coil 41 passes through the side wall of the medium inlet of the cryopump 2 and is connected to the top of the storage tank 1. An opening B at the other end of the coil 41 is connected to the medium inlet of the cryopump 2. The second differential pressure transmitter 42 is provided on the pipeline from the spiral tube body 411 of the coil 41 to the opening B.

[0056] The temperature measuring device 5 is connected to the drain pipe 21 and measures the temperature at the outlet of the cryopump 2;

[0057] The measurement and control cabinet 6 is respectively connected to the cryogenic pump 2, the first differential pressure transmitter 31, the second differential pressure transmitter 42 and the temperature measuring device 5. The measurement and control cabinet 6 is integrated with a data acquisition module and a data control module, and the measurement and control cabinet 6 is provided with a network output interface for communicating with an industrial computer equipped with a test program. The measurement and control cabinet 6 can directly and automatically control the cryogenic pump 2 through software, and can also automatically read the measurement data of the first differential pressure transmitter 31, the second differential pressure transmitter 42 and the temperature measuring device 5.

[0058] It should be noted here that, compared with the existing measurement system, the net positive pressure head measurement system of the cryogenic pump inlet of the present invention cleverly sets a coil at the cryogenic pump inlet, fills the coil with gaseous cryogenic medium, and uses the gaseous cryogenic medium in the coil to form a dynamic balance with the liquid medium flowing at the cryogenic pump inlet, thereby forming a group of saturated vapor pressures of the cryogenic medium at the connection between the coil and the cryogenic pump inlet, and this group of saturated vapor pressures can be automatically corrected and adjusted in real time according to the working state of the cryogenic pump, such as temperature. In particular, when the cryogenic pump inlet is in a cavitation critical state, the saturated vapor pressure at the coil is the most accurate and real-time saturated vapor pressure in this state, which is more direct and accurate than the three methods of obtaining saturated vapor pressure in the background technology.

[0059] The principle of forming saturated vapor pressure by filling the coil with gaseous medium (medium vapor formed after the liquid medium is vaporized) is that after the gaseous medium is filled in the coil, the gaseous medium is directly locked in the coil. Then, when the liquid medium circulates in the cryopump, the liquid medium and the gaseous medium in the coil just form a saturated two-phase equilibrium, and the coil and the cryopump inlet are filled with liquid medium, just forming a similar closed space. The saturated vapor pressure of the liquid medium is formed at the connection between the coil and the cryopump inlet. Then, when the operating temperature of the cryopump changes, the saturated vapor pressure formed in the coil will change in real time.

[0060] Then, in conjunction with the second differential pressure transmitter 42, the pressure difference between the liquid pressure and the saturated vapor pressure at the cryopump inlet is detected in real time, thereby directly and accurately obtaining the NPSH at the cryopump inlet. Therefore, the NPSH at the cryopump inlet measured by the present invention is more direct and accurate than that of existing NPSH detection systems, eliminating the influence of calculations on the detection results, and is more suitable for detecting the net positive head pressure at the cryopump inlet, which can eliminate the influence of temperature changes during the operation of the cryopump on the measurement results.

[0061] In addition, the calculation formula for the net positive pressure head at the cryogenic pump inlet is:

[0062] When calculated in pressure units, NPSH = pressure at the pump inlet - saturated vapor pressure of the liquid at the current temperature, unit: Pa. Therefore, when the second differential pressure transmitter 42 of the present invention is in the cavitation critical state, the pressure difference measured is NPSHr;

[0063] When calculated in units of length or height, NPSH = (pressure at the pump inlet - saturated vapor pressure of the liquid at the current temperature) ÷ (liquid density × acceleration due to gravity), unit: m. This is recorded in the article "People in the Pump Industry: Several Things You Must Know About Cavitation" published by Anhui Southern Chemical Pump Industry Co., Ltd., in which the technical process disclosed in the Chinese invention patent application number 201810439296.5 in the background technology is consistent with this calculation formula. It should be noted that the pressure at the pump inlet has already calculated the pressure formed by the liquid level in the storage tank at the pump inlet. Therefore, the NPSH calculation formula of this application no longer repeats the calculation of the vertical height from the tank liquid level to the center of the pump inlet.

[0064] But if Figure 3 As shown, the present application may still provide a liquid level gauge 13 on the storage tank 1, the zero scale of the liquid level gauge 13 being level with the central horizontal plane at the inlet of the cryogenic pump 2, for measuring the vertical height from the center of the pump inlet to the liquid surface during cavitation state, and then comparing the NPSH with the measured liquid level. Since the pressure at the pump inlet in the present application is greater than the tank liquid surface pressure, the NPSH calculated by the present application should be greater than the liquid level displayed by the liquid level gauge when displayed in height units.

[0065] In the specific setting, the first measurement module 3 also includes a first pressure gauge 32 and a second pressure gauge 33 arranged on both sides of the first differential pressure transmitter 31. The first pressure gauge 32 is arranged on the infusion pipe 11, and the second pressure gauge 33 is arranged on the discharge pipe 21. The first pressure gauge 32 and the second pressure gauge 33 respectively display the real-time hydraulic pressure at the inlet and outlet of the cryogenic pump. The pressure difference formed by the first pressure gauge 32 and the second pressure gauge 33 can be compared with the differential pressure DP on the first differential pressure transmitter 31 to determine the accuracy of the detection result of the first differential pressure transmitter 31.

[0066] In addition, the differential pressure DP on the first differential pressure transmitter 31 can be used to directly calculate the head data H of the cryopump. The specific calculation method is: H=DP / (ρ*g), where ρ is the density of the liquid medium and g is the acceleration of gravity, thereby providing a reference for the head data for the subsequent operation of the cryopump.

[0067] Moreover, since the measurement of the differential pressure transmitter is very sensitive, when there is a slight change in the hydraulic pressure inside the cryopump, the value on the differential pressure transmitter will change accordingly. Therefore, when the first differential pressure transmitter is used to judge the working state of the cryopump, it will not be very accurate. When the indicated pressures on the first pressure gauge 32 and the second pressure gauge 33 tend to be stable, it means that the cryopump is working normally. The so-called indicated pressure tends to be stable means that when the cryopump is working, the pressure values ​​on the first pressure gauge 32 and the second pressure gauge 33 do not change by more than 2%. As for the 2% setting standard, it is because when the cryopump is working, it is not ruled out that the liquid medium itself contains bubbles. When the bubbles reach the cryopump, it will cause the cryopump to stop working. The inlet and outlet pressures change, and at this time, the cryogenic pump is working normally. Once the pressure difference on the first pressure gauge 32 and the second pressure gauge 33 drops by more than 3%, that is, the inlet and outlet pressure difference DP of the cryogenic pump measured by the first differential pressure transmitter 31 drops by 3%, it is assumed that the cryogenic pump has cavitation. The 3% setting standard is recorded in the article "Numerical Simulation and Experimental Analysis of Cavitation Characteristics of Compound Centrifugal Pumps" published in the journal China Space Science and Technology on June 25, 2019, "That is, when the pump head drops by 3%, it is usually considered to enter the cavitation state." The technical method of the head and the pump inlet and outlet pressure difference DP has been explained above. Therefore, when the inlet and outlet pressure difference DP of the cryogenic pump drops by 3%, the cryogenic pump enters the cavitation state.

[0068] The second measurement module 4 also includes a third pressure gauge 43 and a fourth pressure gauge 44 arranged on both sides of the second differential pressure transmitter 42. A first control valve 45 is provided between the third pressure gauge 43 and the spiral tube body 411, and a second control valve 46 is provided between the fourth pressure gauge 44 and the opening B. A pipeline 47 is provided between the first control valve 45 and the second control valve 46, and a third control valve 48 is provided on the pipeline 47. The third pressure gauge 43 is provided close to the spiral pipeline of the coil, and the fourth pressure gauge 44 is provided close to the coil and the cryogenic The connection port of the pump is set, the third pressure gauge 43 displays the saturated vapor pressure in the current state, and the fourth pressure gauge 44 displays the hydraulic pressure at the inlet of the cryogenic pump. Therefore, when the cryogenic pump is about to cavitate or is at the critical point of cavitation, the saturated vapor pressure displayed by the third pressure gauge 43 and the hydraulic pressure at the inlet of the cryogenic pump displayed by the fourth pressure gauge 44 will be infinitely close. Therefore, the indicated pressures of the third pressure gauge 43 and the fourth pressure gauge 44 will serve as another criterion for determining the critical point of cavitation of the cryogenic pump in this application, and are used to determine whether the pressure difference between the inlet and outlet of the cryogenic pump drops by 3%, that is, whether the default assumption that the cryogenic pump enters the cavitation state is accurate.

[0069] In addition, it should be noted that since the present application can directly form the saturated vapor pressure of the liquid medium at the pump port, it is easier and more accurate to determine whether the cryogenic pump has entered the critical state of cavitation by combining the third pressure gauge 43 and the fourth pressure gauge 44 with the standard of a 3% drop in the pressure difference between the inlet and outlet of the cryogenic pump, and thus the measurement of NPSH is more accurate.

[0070] A fourth control valve 49 is provided on the pipeline connecting the coil 41 and the storage tank 1. After the storage tank 1 is filled with the low-temperature liquid medium, due to temperature and pressure factors, even if insulation measures are provided on the storage tank 1, the low-temperature liquid medium inside the storage tank 1 will change with the temperature and pressure, and a part of the gasified medium will be quickly formed in the storage tank 1. Therefore, in this application, the gasified medium in the storage tank 1 is directly flushed into the coil through the fourth control valve 49 to form a saturated vapor pressure.

[0071] Before the cryogenic pump works normally, it is necessary to pre-cool the cryogenic pump so that the temperature of the cryogenic pump is infinitely close to the temperature when the cryogenic pump works normally. Therefore, a reflux vent valve 221 and a reflux control valve 222 are respectively provided on the reflux pipe 22, so that before the cryogenic pump works normally, the cryogenic liquid medium circulates back and forth between the cryogenic pump and the storage tank, and the temperature of the cryogenic medium is used to cool the cryogenic pump. The reflux control valve 222 is used to control the opening and closing of the cryogenic pump and the storage tank, while the reflux vent valve 221 is used to vent the residual cryogenic medium in the pipeline.

[0072] The infusion pipeline 11 is provided with an infusion control valve 110, a safety valve 111, an inlet bellows 112 and a filter 113 in sequence;

[0073] The discharge pipe 21 is provided with an outlet bellows 121 and an outlet control valve 122 in sequence;

[0074] The infusion control valve 110 is used to control the on-off of the infusion pipeline 11, and the safety valve 111 is used to protect the cryogenic pump. When the pressure in the infusion pipeline 11 exceeds 10 bar, the safety valve 111 will trip and block the connection of the cryogenic pump. The inlet bellows 112 and the outlet bellows 121 are used to regulate the infusion pipeline 11 and the drainage pipeline 21 respectively. When the pressure in the infusion pipeline 11 and the drainage pipeline 21 is too high, the inlet bellows 112 and the outlet bellows 121 can be extended. The filter 113 is used to filter impurities in the medium, and the outlet control valve 122 is used to open and close the drainage pipeline 21.

[0075] Example 2:

[0076] like Figure 4 As shown, referring to Example 1, a method for measuring the net positive pressure head at the inlet of a cryogenic pump according to Example 2 of the present invention is described, comprising the following steps:

[0077] Step 1: Filling the medium. A cryogenic medium is input into the storage tank 1. The cryogenic medium is liquid nitrogen. Specifically, the storage tank 1 has been vacuum-treated before being put into use. The liquid level of the cryogenic medium in the storage tank 1 after filling exceeds 2m to avoid the liquid level being too high. A too high liquid level will delay the occurrence time of the cavitation critical point, resulting in a too long measurement time. A too low liquid level will cause the medium temperature to change too much when pre-cooling the cryopump, affecting the measurement results.

[0078] 4. When the pressure gauge 43 and the pressure gauge 44 are stable, the pressure on the storage tank 1 is 6-7 bar, preferably 6.5 bar. If the pressure gauge 43 and the pressure gauge 44 are not stable, there is a leak, and the airtightness of the measurement system needs to be rechecked to ensure that there is no leak, and then the gas medium is filled again.

[0079] Step 3: Precool the cryopump. Open the infusion pipe 11 to connect the storage tank 1 with the cryopump 2. Open the return pipe 211 between the cryopump 2 and the storage tank 1. A closed circulating flow pipeline is formed between the cryopump 2 and the storage tank 1. Start the cryopump 2. The cryogenic medium flows in the circulating flow pipeline to precool the cryopump 2. The cryogenic medium circulates between the storage tank 1 and the cryopump 2 for 18-22 minutes, preferably 20 minutes, until the pump head of the cryopump 2 is completely frosted and the temperature at the outlet of the cryopump 2 reaches -130°C (the storage temperature of liquid nitrogen) as measured by the temperature measuring device 5. At -196°C, therefore, after pre-cooling, the temperature at the outlet of the cryopump 2 reaches -130°C, and the cryopump operates normally, so there will not be a large temperature difference, thereby avoiding the influence of the temperature difference on the measurement result). The pressure difference ΔP1 displayed by the first differential pressure transmitter 31 is in a stable state (the stable state means that the gas medium in the coil forms a dynamic equilibrium with the liquid medium at the inlet of the cryopump, thereby forming a saturated vapor pressure of the liquid medium. The criterion for determining the formation of saturated vapor pressure is that when the temperature measured by the temperature measuring device 5 is constant, the pressure values ​​of the third pressure gauge 43 and the pressure values ​​of the fourth pressure gauge 44 do not change with time, then the saturated vapor pressure is formed). Then, the cryopump 2 completes the pre-cooling process;

[0080] Step 4: Measure the net positive pressure head at the inlet, close the return pipe 211, open the discharge pipe 21, open the pressure vent valve 12 on the storage tank 1, keep the cryopump 2 running, and gradually close the infusion control valve 110 to resolve the cavitation critical state at the cryopump inlet until the pressure difference ΔP1 (i.e., DP) of the first differential pressure transmitter 31 decreases by 3% (at the same time, the pressure value of the third pressure gauge 43 is infinitely close to the pressure value of the fourth pressure gauge 44, which is considered to be the cryopump entering the cavitation critical state), turn off the cryopump 2, and record simultaneously. At this time, the scale h of the liquid level gauge 13, the indicated pressure P1 of the third pressure gauge 43, the indicated pressure P2 of the fourth pressure gauge 44, and the pressure difference ΔP2 of the second differential pressure transmitter 42, where ΔP2=|P1-P2|, the NPSHr at the cryopump inlet=ΔP2 (unit: Pa), after conversion to height, the NPSHr at the cryopump inlet=ΔP2÷(ρ*g), where ρ is the density of the liquid medium and g is the acceleration of gravity. If the NPSHr measured after re-testing does not contain any error, it means that the net positive head pressure measurement work of the cryopump inlet is completed.

[0081] In addition, during measurement, the pressure vent valve 12 on the storage tank 1 is opened to reduce the pressure in the storage tank 1, so that the hydraulic pressure at the inlet of the cryogenic pump eliminates the influence of the pressure in the storage tank, and the cavitation criticality at the inlet of the cryogenic pump is achieved more quickly. Moreover, when measuring the net positive pressure head at the inlet of the cryogenic pump when the flow rate changes, a flow meter can be set on the cryogenic pump to cooperate with the adjustment of the infusion control valve and the outlet control valve to perform measurements.

[0082] like Figure 5 As shown in the figure, it is a schematic diagram of the flow rate and NPSHr curve obtained after measurement in this application. It can be seen from the curve in the figure that the NPSHr-Q curve relationship of the cryogenic pump is very different from that of the normal temperature centrifugal pump. For example, the flow rate-NPSH Q-NPSH curve of the normal temperature centrifugal pump recorded in the comparative document with patent application number 201710283136.1 shows that the NPSH of the normal temperature centrifugal pump gradually increases with the increase of flow rate, and the corresponding functional relationship between NPSHr and flow rate is also the same. However, since the cryogenic pump transports low-temperature medium, the NPSHr-Q curve relationship of the cryogenic pump measured by this application is not as general as the comparative document. Figure 5 It can be seen that with the increase of flow rate Q, the NPSHr of the cryopump will gradually decrease. The reason for the decrease is that when the flow rate is small in the early stage, the flow velocity is slow, and the cryogenic medium will absorb heat, causing the NPSHr at the cryogenic pump inlet to gradually decrease. When the flow rate reaches a certain level, the cryogenic medium will tend to be saturated when absorbing heat from the outside. On the contrary, with the increase of flow rate, the flow velocity becomes larger, and the friction coefficient between the cryogenic medium and the cryogenic pump inlet becomes larger, which causes the NPSHr at the cryogenic pump inlet to gradually increase.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A cryogenic pump inlet net positive pressure head measurement system, characterized in that: include: Storage tank (1), cryogenic pump (2), first measurement module (3), second measurement module (4), temperature measuring device (5) and measurement and control cabinet (6); The storage tank (1) stores a cryogenic medium. The storage tank (1) is connected to the inlet of the cryogenic pump (2) via a liquid delivery pipe (11), and a pressure relief valve (12) is provided on the top of the storage tank (1). The outlet of the cryogenic pump (2) is connected to a drainage pipe (21), and the drainage pipe (21) is connected to the storage tank (1) via a return pipe (22); The first measuring module (3) is arranged at the inlet and outlet of the cryogenic pump (2), and is used to measure the inlet and outlet pressure difference DP of the cryogenic pump (2). The first measuring module (3) includes a first differential pressure transmitter (31), and the first differential pressure transmitter (31) is connected to the inlet and outlet of the cryogenic pump (2) respectively. The second measuring module (4) is arranged at the inlet of the cryogenic pump (2), and the second measuring module (4) is used to measure the net positive pressure head NPSH at the inlet of the cryogenic pump (2), and the second measuring module includes a coil (41) and a second differential pressure transmitter (42), the coil (41) is arranged at the medium inlet of the cryogenic pump (2), an opening A at one end of the coil (41) passes through the medium inlet side wall of the cryogenic pump (2) and is connected to the top of the storage tank (1), and an opening B at the other end of the coil (41) is connected to the medium inlet of the cryogenic pump (2), and the second differential pressure transmitter (42) is arranged on the pipeline from the spiral tube body (411) of the coil (41) to the opening B; The temperature measuring device (5) is connected to the liquid discharge pipe (21), and the temperature measuring device (5) measures the temperature at the outlet of the cryogenic pump (2); The measurement and control cabinet (6) is respectively connected to the cryogenic pump (2), the first differential pressure transmitter (31), the second differential pressure transmitter (42) and the temperature measuring device (5). A data acquisition module and a data control module are integrated in the measurement and control cabinet (6), and a network output interface for communication with an industrial computer equipped with a test program is provided on the measurement and control cabinet (6).

2. A cryopump inlet net positive head pressure measurement system according to claim 1, characterized in that: The first measurement module (3) further includes a first pressure gauge (32) and a second pressure gauge (33) arranged on both sides of the first differential pressure transmitter (31), the first pressure gauge (32) being arranged on the infusion pipe (11), and the second pressure gauge (33) being arranged on the discharge pipe (21).

3. The cryopump inlet net positive head pressure measurement system according to claim 2, characterized in that: The second measuring module (4) further comprises a third pressure gauge (43) and a fourth pressure gauge (44) arranged on both sides of the second differential pressure transmitter (42); a first control valve (45) is arranged between the third pressure gauge (43) and the spiral tube body (411); a second control valve (46) is arranged between the fourth pressure gauge (44) and the opening B; a pipeline (47) is provided between the first control valve (45) and the second control valve (46), and a third control valve (48) is provided on the pipeline (47).

4. The cryopump inlet net positive head pressure measurement system according to claim 3, characterized in that: A fourth control valve (49) is provided on the pipeline connecting the coil (41) and the storage tank (1).

5. The cryogenic pump inlet net positive head pressure measurement system according to claim 4, characterized in that: The reflux pipeline (22) is provided with a reflux vent valve (221) and a reflux control valve (222).

6. The cryopump inlet net positive head pressure measurement system according to claim 5, characterized in that: The infusion pipeline (11) is provided with an infusion control valve (110), a safety valve (111), an inlet bellows (112) and a filter (113) in sequence; An outlet bellows (121) and an outlet control valve (122) are sequentially arranged on the liquid discharge pipe (21).

7. A measurement method based on the cryogenic pump inlet net positive head pressure measurement system according to claim 6, characterized in that: The following steps are involved: Step 1: Filling the medium: inputting a low-temperature medium into the storage tank (1), wherein the low-temperature medium is liquid nitrogen; Step 2: Filling the gaseous medium, opening the fourth control valve (49) between the coil (41) and the storage tank (1), and filling the spiral tube body (411) of the coil (41) with nitrogen. After the filling is completed, closing the fourth control valve (49) between the coil (41) and the storage tank (1), and closing the third control valve (48) at the same time, the pressure displayed on the third pressure gauge (43) and the fourth pressure gauge (44) is in a stable state, and proceeding to the next step; Step 3: pre-cool the cryopump, open the infusion pipe (11), connect the storage tank (1) with the cryopump (2), open the return pipe (211) between the cryopump (2) and the storage tank (1), and form a closed circulation flow pipeline between the cryopump (2) and the storage tank (1). After the outlet temperature of the temperature measuring device (5) indicates that the pre-cooling work is completed, start the cryopump (2); Step 4: Measure the net positive pressure head at the inlet, close the return pipe (211), open the discharge pipe (21), open the pressure vent valve (12) on the storage tank (1), keep the cryogenic pump (2) running, and gradually close the infusion control valve (110) until the pressure difference △P1 of the first differential pressure transmitter (31) decreases by 3%, then turn off the cryogenic pump (2), and simultaneously record the indicated pressure P1 of the third pressure gauge (43), the indicated pressure P2 of the fourth pressure gauge (44), and the pressure difference △P2 of the second differential pressure transmitter (42). NPSHr at the cryogenic pump inlet = △P2 = |P1-P2|, unit: Pa.

8. The measuring method according to claim 7, wherein: In the step 1, the liquid level of the low-temperature medium in the storage tank (1) after filling exceeds 2m.

9. The measuring method according to claim 7, wherein: In the second step, the storage tank (1) is pressurized so that the gaseous low-temperature medium is filled into the coil (41), and the pressure applied to the storage tank (1) is 6-7 bar.

10. The measurement method according to claim 7, wherein: In the step 3, the cryogenic medium circulates between the storage tank (1) and the cryogenic pump (2) for 18-22 minutes until the pump head of the cryogenic pump (2) is completely frosted, and the temperature at the outlet of the cryogenic pump (2) reaches -130°C, and the pressure difference ΔP1 displayed by the first differential pressure transmitter (31) is in a stable state, then the cryogenic pump (2) completes the pre-cooling treatment.

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