CN113295244B - Cryogenic fluid flow measurement and calibration method - Google Patents
Cryogenic fluid flow measurement and calibration method Download PDFInfo
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- CN113295244B CN113295244B CN202110557858.8A CN202110557858A CN113295244B CN 113295244 B CN113295244 B CN 113295244B CN 202110557858 A CN202110557858 A CN 202110557858A CN 113295244 B CN113295244 B CN 113295244B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F25/00—Testing or calibration of apparatus for measuring volume, volume flow or liquid level or for metering by volume
- G01F25/10—Testing or calibration of apparatus for measuring volume, volume flow or liquid level or for metering by volume of flowmeters
- G01F25/14—Testing or calibration of apparatus for measuring volume, volume flow or liquid level or for metering by volume of flowmeters using a weighing apparatus
Abstract
The invention relates to a method for measuring and calibrating the flow of a low-temperature fluid, which can ensure that a medium is in a pure liquid state before entering an element to be measured and a weighing system through two designed subcoolers, and provides a measuring and calibrating device with simple structure and high precision for the field of deep low temperature by combining a standard table comparison method and a weighing method. The device can lead the calibration working condition to be consistent with the actual working condition, and the calibration process does not need to carry out working condition conversion, for example, the actual working condition of the flowmeter is 0.6MPa, the flowmeter works under 83K, and then the calibration system can be directly adjusted to be 0.6MPa, and the flowmeter and the like are measured and calibrated under 83K working conditions. The device has two functions, can test, measure and calibrate the flowmeter and the cryogenic pump respectively, can run once to test and calibrate the flowmeter and the cryogenic pump simultaneously, and has high efficiency and low cost. The structure of the invention can realize the selection of a plurality of paths and reduce the calibration cost as much as possible.
Description
Technical Field
The invention relates to the technical field of flow calibration, in particular to a method for measuring and calibrating low-temperature fluid flow.
Background
In the fields of scientific research and industrial deep low temperature, liquid nitrogen is frequently applied as a conventional deep low temperature cold source; in the whole low-temperature liquid nitrogen system, the accuracy of flow measurement has extremely important significance; in other words, the liquid nitrogen flow meter used therein is calibrated to the precision required by us. In a commonly used flow meter calibration device, the basic principle is generally performed by a volume method, a weighing method, and a standard meter method, wherein the weighing method and the standard meter method are basically used in a system with a large flow rate.
The principle of the standard table method is shown in fig. 1. The method includes determining a precisely calibrated flowmeter, connecting the precisely calibrated flowmeter and a measured flowmeter in series on the same pipeline, measuring the flow values of fluid continuously passing through the two flowmeters in the same time interval, further calculating the measurement error, and after the error is corrected, giving corresponding evaluation to the measurement performance of the measured flowmeter. The calibration method has the advantages that: the calibration personnel can greatly improve the calibration efficiency through simple and reliable operation, but the calibration method is limited by the precision grade of the standard flowmeter.
The weighing method is based on the principle shown in fig. 2. Calculating to obtain the liquid flow according to the liquid mass collected by the weighing container in a certain time, the required time and other related data, and further evaluating the metering performance of the measured flowmeter; specifically, the method can be divided into a stop method and a dynamic method.
The stopping method is that fluid enters a weighing and collecting container from a measured flowmeter within a certain time, after the time is timed for a certain time, the total mass of the liquid entering the container through the flowmeter within the time period is measured on the premise that the liquid flowing into the collecting container completely stops flowing in, the current density of the liquid can be checked, the volume of the liquid entering the container is further calculated, the calibrated flow is finally calculated, and the calibrated flow is compared with the display value of the flowmeter for evaluation;
the dynamic method is characterized in that under the condition of steady-state flow of liquid, the increased mass of the liquid in the weighing and collecting container within a period of time is recorded, and then the current steady-state flow can be calculated to be compared and evaluated with the display value of the flowmeter;
comparing the above-mentioned stopping method with the dynamic method, the error of the stopping method is that the residual part of liquid in the pipeline and the pipe between the flowmeter and the weighing container passes through the flowmeter but does not enter the weighing container, and the stopping method is only suitable for measuring the accumulated flow and the average flow and is not suitable for measuring the instantaneous flow. Therefore, two principles, namely a dynamic method and a standard table method, are commonly used in a calibration system;
based on the above two basic calibration principles, various calibration methods have appeared in the prior art, such as a calibration method and device for a low temperature flowmeter disclosed in application number CN201810233988.4, the method and device first control the low temperature fluid to flow through the low temperature flowmeter to be measured, then heat the low temperature fluid flowing out from the low temperature flowmeter to be measured to recover from low temperature to normal temperature, calibrate the normal temperature fluid after heating, and perform calibration evaluation on the low temperature flowmeter to be measured after converting the calibration data of the normal temperature fluid. In summary, the device calibrates the normal temperature fluid by heating the low temperature liquid to convert it into the normal temperature fluid, and then reversely deduces the calibration result of the low temperature fluid flow meter. Due to the existence of conversion errors, the device still has the problem of calibration precision.
As is known, low-temperature liquid such as liquid nitrogen, liquefied natural gas and the like can be vaporized due to heat leakage when flowing in a pipeline, the liquid in the low-temperature liquid can not be kept in a pure liquid state by using the conventional device, and high-precision calibration can not be completed by a standard meter method and a weighing method at normal temperature. For example, low-temperature propellants used in the aerospace field need to accurately master information such as the filling flow and the flow speed of the propellants; in the industrial field, the accumulated flow and the real-time flow of the liquid natural gas filling are directly related to the settlement accuracy. However, in the current scientific research and industrial deep low temperature field, an analog calibration method is adopted conventionally, namely, normal temperature water is used for calibration, and then flow conversion is carried out by analogy to the working condition at low temperature, and the method obviously cannot realize the calibration precision of 0.2 per mill.
Disclosure of Invention
The invention aims to provide a high-precision flow calibration device aiming at the deep low temperature field.
The invention solves the technical problems through the following technical means:
a measuring and calibrating method for low-temperature fluid flow comprises a liquid storage Dewar (1), a low-temperature pump (2), a first subcooler (3), a second subcooler (5), a test box (4) and a weighing system (6); the outlet end of the liquid storage Dewar (1) is connected with the inlet of the cryogenic pump (2), and the inlet and the outlet of the test box (4) are respectively connected with the outlet end of the cryogenic pump (2) and the inlet of the weighing system (6); an outlet and an inlet of the first subcooler (3) are respectively connected to a pipeline between the cryogenic pump (2) and the test box (4), and an outlet and an inlet of the second subcooler (5) are respectively connected to a pipeline between the test box (4) and the weighing system (6); a low-temperature valve (8) is arranged between an inlet and an outlet of the first subcooler (3) in a pipeline between the low-temperature pump (2) and the test box (4), and a low-temperature valve (8) is arranged between an inlet and an outlet of the second subcooler (5) in a pipeline between the test box (4) and the weighing system (6); a low-temperature valve (8) is arranged on an outlet pipeline of the first subcooler (3), and low-temperature valves (8) are arranged on an inlet pipeline and an outlet pipeline of the second subcooler (5); and a to-be-tested element (42) is arranged in the test box (4).
The calibration of the flow meter includes the following steps,
step 1, installing a flowmeter to be tested on a station in a test box (4), and selecting a low-temperature pump (2) meeting the requirement to be installed to a set position;
and 6, after the flow is stable, closing a liquid discharge port of the weighing system (6) and simultaneously starting timing, measuring the weight of liquid nitrogen flowing into the weighing system within a certain time, and further comparing the reading of the flowmeter to be measured for calibration, and meanwhile, comparing the reading difference between the standard flowmeter and the flowmeter to be measured for real-time instantaneous flow calibration.
The invention can ensure that the medium is in a pure liquid state before entering the element to be measured and the weighing system through the two subcoolers, provides a measuring and calibrating device with simple structure and high precision for the deep low temperature field by combining a standard meter comparison method and a weighing method, realizes the measurement and calibration under the actual working state in the real sense, can ensure that the calibration working condition is consistent with the actual working condition, and the calibration process does not need to carry out working condition conversion, for example, the actual working condition of a flowmeter is 0.6MPa and the flowmeter works under 83K, so that the calibration system can be directly adjusted to be 0.6MPa and the flowmeter and the like to carry out measurement and calibration under the working condition of 83K. The device is small in size, simple in structure and convenient to build. The device has two functions, can test, measure and calibrate the flowmeter and the cryogenic pump respectively, can also calibrate the flowmeter and the cryogenic pump simultaneously by one-time operation, and has high efficiency and low cost. The structure of the invention can realize the selection of a plurality of paths, and different paths are selected according to different media, thereby reducing the calibration cost as much as possible.
Further, the test box (4) comprises a standard element (41) mounting position and a to-be-tested element (42) mounting position; the standard element is installed at the standard element (41) installation position, the element to be tested (42) is installed at the element to be tested (42), and the low-temperature liquid sequentially passes through the standard element (41) and the element to be tested (42) or sequentially passes through the element to be tested (42) and the standard element (41).
Further, the weighing system (6) comprises a force measuring element (61), a weight measuring tank (62), a mounting bracket (63) and a protective cover (64); the mounting support (63) is fixed at the bottle mouth of the liquid storage Dewar (1), the fixing support is fixed at the top of the force measuring element (61), the weight measuring tank (62) is hung on a pull rod at the bottom of the force measuring element (61), the protective cover (64) is reversely buckled at the bottle mouth of the liquid storage Dewar (1) for sealing and fixing, and the force measuring element (61), the mounting support (63) and the weight measuring tank (62) are positioned in a sealing cavity formed by the protective cover (64) and the liquid storage Dewar (1); the outlet end of the test box (4) is connected with the inlet of the weight measuring tank (62), and the liquid outlet of the weight measuring tank (62) is communicated with the inner cavity of the liquid storage Dewar (1).
Further, the weight measuring tank (62) is hung in the inner cavity of the liquid storage Dewar (1); the bottom of the weight measuring tank (62) is higher than the liquid level in the liquid storage Dewar (1); the bottom of the weight measuring tank (62) is provided with a liquid outlet (67), and the liquid outlet (67) is provided with a pneumatic valve.
Furthermore, a standard weight block (65) can be detachably mounted on a pull rod of the force measuring element (61), and the pull rod can be used for calibrating the force measuring element before calibration every time, so that the calibration precision of the system is increased.
Furthermore, the outlet end of the low-temperature pump (2) is also communicated with the inlet of the liquid storage Dewar (1) through a return pipe; a backflow control valve (9) is installed on the backflow pipe, and when the whole system performs small-flow circulation calibration, the backflow pipeline can be used for bypass backflow.
Furthermore, the device also comprises an automatic pressure control device; the automatic pressure control device comprises a pressurization tank (10), a heating element (11) and a controller; the controller controls the heating element (11) to heat the pressurizing tank to vaporize the liquid in the pressurizing tank so as to control the operating pressure of the whole circulating system; the pressurization tank (10) comprises an exhaust port, and the exhaust port is communicated with the inner cavity of the liquid storage Dewar (1).
Furthermore, the pressure boost jar (10) is located stock solution dewar (1) inner chamber, and the gas vent has been seted up at the jar body top of pressure boost jar (10).
Furthermore, the automatic pressure control device also comprises a controller and a pressure sensor; the pressure sensor is installed at the bottleneck of stock solution dewar (1), measures stock solution dewar (1) internal pressure, pressure sensor and controller communication connection, controller control heating element (11) are opened and close.
Further, the first subcooler (3) and the second subcooler (5) have the same structure; the first subcooler (3) comprises a subcooler Dewar (31), a heat exchange tube (32) and a vacuum pump (33); liquid nitrogen is stored in the subcooler Dewar (31), the heat exchange tube (32) is positioned in the subcooler Dewar (31), the inlet end of the heat exchange tube (32) is connected with the outlet end of the low-temperature pump (2), and the outlet end of the heat exchange tube (32) is connected with the inlet end of the test box (4); and the vacuum pump (33) is connected with an exhaust port of the subcooler Dewar (31) through a pipeline.
The invention has the advantages that:
the invention can ensure that the medium is in a pure liquid state (namely a supercooled state) before entering the element to be measured and the weighing system through the two supercoolers, provides a measuring and calibrating device with simple structure and high precision for the field of deep low temperature by combining a standard contrast method and a weighing method, realizes the measurement and calibration under the actual working state in the real sense, can ensure that the calibration working condition is consistent with the actual working condition, does not need to carry out working condition conversion in the calibration process, for example, the actual working condition of a flowmeter is 0.6MPa and the actual working condition of the flowmeter is 83K, and can directly adjust the calibration system to be 0.6MPa and 83K to carry out the measurement and calibration on the flowmeter and the like. The device is small in size, simple in structure and convenient to build. Simultaneously, two functions are provided, the flowmeter and the cryogenic pump can be calibrated respectively, the flowmeter and the cryogenic pump can be calibrated simultaneously by one-time operation, the efficiency is high, and the cost is low. The structure of the invention can realize the selection of a plurality of paths, and different paths are selected according to different media, thereby reducing the calibration cost as much as possible.
The weighing system provided by the invention has the advantages that the weighing tank is suspended in the liquid storage Dewar, the requirement on the weighing tank is reduced by utilizing the heat insulation performance of the liquid storage Dewar, the requirement on the weighing tank can be reduced by only using a single-layer tank body, the size of the whole device can be reduced, the cost can be saved, multiple purposes can be realized, the sealing difficulty is reduced by adopting a protection cover mode, and the internal pressure and temperature of the liquid storage Dewar are ensured.
By adopting a plurality of weight blocks, the weighing element can be calibrated before the system runs, and the precision of the final result is ensured.
Through the design of return line, the accessible adjusts cryogenic pump converter frequency and the aperture of returning the liquid valve, realizes under the different pressure conditions, and the regulation of different liquid nitrogen flow can select a large-traffic cryogenic pump to be suitable for multiple operating mode, has reduced use cost.
The liquid nitrogen in the subcooler can be cooled by decompressing and evacuating the subcooler, so that the liquid nitrogen in the pipeline obtains cold energy, the pure liquid state of the liquid nitrogen is ensured, and the subcooler is also suitable for other low-temperature fluids such as LNG and the like.
Based on the device, the test, the measurement and the calibration of the flowmeter, the cryogenic pump, the flowmeter and the cryogenic pump can be realized, and the calibration method is simple and easy to operate.
Drawings
FIG. 1 is a schematic diagram of a calibration by a standard table method introduced in the background of the invention;
FIG. 2 is a schematic diagram of calibration by a weighing method introduced in the background of the invention;
fig. 3 is a schematic structural diagram of a low temperature flowmeter measuring and calibrating apparatus in an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 3, the present embodiment shows a cryogenic fluid flow measurement and calibration device, which can be used for measuring and calibrating other cryogenic fluid flow meters such as liquid nitrogen, liquefied natural gas, and the like. The device comprises a liquid storage Dewar 1, a cryogenic pump 2, a first subcooler 3, a second subcooler 5, a test box 4 and a weighing system 6; the outlet end of the liquid storage Dewar 1 is connected with the inlet of the cryogenic pump 2, and the inlet and the outlet of the test box 4 are respectively connected with the outlet end of the cryogenic pump 2 and the inlet of the weighing system 6; an outlet and an inlet of the first subcooler 3 are respectively connected to a pipeline between the cryogenic pump 2 and the test box 4, and an outlet and an inlet of the second subcooler 5 are respectively connected to a pipeline between the test box 4 and the weighing system 6; a low-temperature valve 8 is arranged between an inlet and an outlet of the first subcooler 3 in a pipeline between the cryogenic pump 2 and the test box 4, and a low-temperature valve 8 is arranged between an inlet and an outlet of the second subcooler 5 in a pipeline between the test box 4 and the weighing system 6; a low-temperature valve 8 is arranged on an outlet pipeline of the first subcooler 3, and low-temperature valves 8 are arranged on an inlet pipeline and an outlet pipeline of the second subcooler 5. The test box 4 mounts a device under test 42 therein. Through opening and closing a plurality of low temperature valves 8 on the regulation pipeline, can realize 4 at least routes:
route 1: the method comprises the following steps of (1) storing a Dewar 1, a cryogenic pump 2, a test box 4 and a weighing system 6;
route 2: the method comprises the following steps of (1) storing a Dewar 1, a cryogenic pump 2, a first subcooler 3, a test box 4 and a weighing system 6;
route 3: the method comprises the following steps of (1) storing a Dewar 1, a cryogenic pump 2, a test box 4, a second subcooler 5 and a weighing system 6;
path 4: the method comprises the following steps of (1) storing a Dewar 1, a cryogenic pump 2, a first subcooler 3, a test box 4, a second subcooler 5 and a weighing system 6;
the above 4 paths are selected according to actual test requirements. Taking liquid nitrogen as an example, in order to ensure a single phase of the liquid nitrogen in the whole path, the path 2, the path 3, and the path 4 may be selected, and for the sake of safety, the path 4 is selected in this embodiment, so that the liquid nitrogen before entering the test box 4 and the weighing system 6 is pure liquid, and the measurement and calibration accuracy is improved. Of course, the whole device is provided with a plurality of low-temperature valves 8, thermometers 7 and pressure gauges which are all conventional arrangements, and the details are not described.
In this embodiment, the test box 4 includes a standard component mounting position and a component to be tested mounting position; namely, installation positions of a standard element 41 and an element to be measured 42 are reserved on the pipeline, the standard element 41 is installed at the installation position, the element to be measured 42 is installed at the installation position of the element to be measured 42, and the low-temperature liquid sequentially passes through the standard element 41 and the element to be measured 42 or sequentially passes through the element to be measured 42 and the standard element 41.
In this embodiment, the weighing system 6 includes a force measuring cell 61, a weight measuring tank 62, a mounting bracket 63, and a protective cover 64; the mounting bracket 63 is substantially in an inverted U shape, a flange is mounted at the bottleneck of the liquid storage Dewar 1, and the mounting bracket 63 can be fixed on the flange through bolts or welding. The load cell 61 may be a tension meter or a spring balance or the like. A pull rod at the top of the force measuring element 61 is fixed on a cross beam of the mounting bracket 63, the weight measuring tank 62 is hung on the pull rod at the bottom of the force measuring element 61, the protective cover 64 is reversely buckled on a bottle opening of the liquid storage Dewar 1 and is fixed with a flange in a sealing manner, a through hole is formed in the middle of the flange, the pull rod at the bottom of the force measuring element 61 extends into an inner cavity of the liquid storage Dewar 1 through the through hole, and the weight measuring tank 62 is hung in the inner cavity of the liquid storage Dewar 1; the bottom of the weight measuring tank 62 is higher than the liquid level in the liquid storage Dewar 1 so as to avoid the influence of buoyancy on weighing; the bottom of weight measuring tank 62 is provided with liquid outlet 67, and pneumatic valve is installed to liquid outlet 67. The weighing system 6 is in flange sealing connection with the bottle mouth of the liquid storage Dewar 1 through the protective cover 64, a single-layer tank body can be directly adopted as the weight measuring tank 62 and hung in the inner cavity of the liquid storage Dewar 1, the heat insulation and heat preservation functions of the liquid storage Dewar 1 can be utilized, liquid nitrogen in the weight measuring tank 62 is guaranteed not to be vaporized, and the cost can be reduced; through the setting of pneumatic valve, can make the liquid nitrogen of surveying in the heavy jar 62 arrange to stock solution dewar 1 fast in, reduce the pipeline route, reduce the loss of liquid nitrogen, practice thrift and mark the cost. The mounting mode of the flange and the bottle mouth of the liquid storage Dewar 1 is a conventional structure, and detailed description is omitted. In order to reduce the influence of shaking caused by the liquid nitrogen impacting the weight measuring tank 62 on the measurement accuracy, the present embodiment extends the liquid inlet pipe of the weight measuring tank 62 to the bottom of the weight measuring tank 62, and sets the flow damper 66 at the bottom end of the liquid inlet pipe.
In order to ensure the accuracy of the load cell 61, the present embodiment also has a detachable standard weight block 65 mounted on the tie rod at the bottom of the load cell 61. The standard weight block 65 is a plurality of blocks, the weight of the weight block 65 is constant and known, and the precision of the force measuring element 61 can be detected by adding or subtracting the weight block 65. If a 10kg weight block 65 is reduced, the reading of the load cell 61 is reduced by 10kg, or the error is within a certain range, the accuracy of the load cell 61 is in accordance with the requirement. The detachable assembly structure of the weight block 65 and the pull rod is a conventional structure, for example, a stopper is arranged on the pull rod, a notch is arranged on the weight block 65, and the notch of the weight block 65 is matched with the pull rod and then placed on the stopper, so that the weight block 65 can be fixed, similar to a weight of a scale. Typically, the weight block 65 is located above the weigh tank 62.
The cryogenic pump 2 is a power source of a calibration circulating system of the whole device, but since the calibration of different flow meters has different requirements on the flow range and the lift of the cryogenic pump 2, in order to expand the application range, the cryogenic pump 2 with larger flow and lift can be selected, and the flow can be adjusted through the return pipe. Specifically, the outlet end of the cryogenic pump 2 is communicated with the inlet of the liquid storage Dewar 1 through a return pipe; a return control valve 9 is mounted on the return pipe. When the system is used, after the system is started, valves on a pipeline are opened, then the backflow control valve 9 and the low-temperature valve 8 on the upstream of the test box 4 are adjusted, the flow of liquid nitrogen entering circulation can meet the requirement, and redundant liquid nitrogen flows back to the liquid storage Dewar 1 from the backflow pipe.
In this embodiment, an automatic pressure control device is further provided, which includes a pressure boost tank 10, a liquid supplement valve, an exhaust valve, a heating element 11, a pressure guide pipeline, and a controller; the heating element 11 is positioned in the pressurization tank, and the controller controls the heating element 11 to heat the pressurization tank; the pressurizing tank 10 is suspended in the liquid storage Dewar 1 through a suspension piece, a suspension rod can be fixed on the lower surface of the flange in a specific suspension mode, and the pressurizing tank and the suspension rod are fixed. The air exhaust port is arranged on the pressurization tank 10, so that high-pressure air can be directly exhausted into the liquid storage Dewar 1 to be pressurized. All the pipes of the booster tank 10 can be passed out through a hole made in the flange. Certainly, for automatic control, all be provided with a plurality of pressure gauges on the pipeline and on stock solution dewar 1, pressure gauge and controller communication connection, controller and heating element 11, fluid infusion valve, discharge valve communication connection, the controller is according to current pressure information, judges whether will carry out the pressure boost to stock solution dewar 1, if needs, starts heating element 11 and heats the pressure boost jar, then inputs the vaporized nitrogen gas into stock solution dewar 1, until the pressure meets the requirements and stops the gas transmission. Through suspending pressure boost tank 10 in midair in stock solution dewar 1, both can reduce the volume of whole device, can utilize the heat insulating properties of stock solution dewar 1 itself to guarantee the temperature of liquid nitrogen in the pressure boost tank again, can reduce the technical requirement to the pressure boost tank, generally adopt the individual layer jar body can, practice thrift the cost.
In the present embodiment, the first subcooler 3 and the second subcooler 5 are identical in structure; the structure is described by taking the first subcooler 3 as an example:
the first subcooler 3 comprises a subcooler Dewar 31, a heat exchange tube 32 and a vacuum pump 33; liquid nitrogen is stored in the subcooler Dewar 31, the heat exchange tube 32 is positioned in the subcooler Dewar 31, the inlet end of the heat exchange tube 32 is connected with the outlet end of the cryogenic pump 2, and the outlet end of the heat exchange tube 32 is connected with the inlet end of the test box 4; the vacuum pump 33 is connected to an exhaust port of the subcooler dewar 31 through a pipe. The temperature of liquid nitrogen in the subcooler Dewar 31 is reduced by vacuumizing, so that the temperature of the liquid nitrogen in the heat exchange tube 32 is reduced, and the pure liquid state of the liquid nitrogen in the heat exchange tube is ensured. The second subcooler 5 ensures that the liquid nitrogen entering the weighing system 6 has a single phase, and the calibration precision is improved.
In this embodiment, stock solution dewar, weighing system, subcooler and heat exchanger, liquid nitrogen infusion line, flowmeter test box, cryopump all integrate and form wholly on sled dress platform. The product is small in size and convenient to move.
The present embodiment also provides a method of use for the above-described apparatus, the method comprising calibration of the flow meter and test calibration of the cryopump 2. The calibration of the flow meter comprises the following steps, selecting a suitable path from the 4 paths according to different media, taking path 4 as an example in the embodiment:
step 1, mounting a component 42 to be tested on a station in a test box 4, and selecting a low-temperature pump 2 meeting requirements to be mounted on a set position;
and 6, after the flow is stable, closing a liquid outlet of the weighing system 6 and starting timing, measuring the weight of liquid nitrogen flowing into the weighing system within a certain time, and then calibrating by comparing the reading of the element to be measured 42, and meanwhile, performing real-time instantaneous flow calibration by comparing the reading difference between the standard element 41 and the element to be measured 42.
Taking path 4 as an example, the calibration of the cryopump 2 includes the following steps:
step 1, mounting a low-temperature pump to be tested to a station, and selecting a corresponding standard flowmeter to mount to a corresponding station;
and 6, after the flow is stable, closing a liquid discharge port of the weighing system 6 and starting timing, measuring the weight of liquid nitrogen flowing into the weighing system within a certain time, and calibrating by comparing the reading of the standard flowmeter to determine whether the flow accords with the set flow of the cryopump to be measured. Meanwhile, the corresponding rotating speed is recorded, and the whole flow characteristic curve of the cryopump can be drawn.
Of course, the flow meter and the cryopump may be simultaneously calibrated by the following method:
step 1, mounting a low-temperature pump and a flowmeter to be tested to a station, and selecting a corresponding standard flowmeter to be mounted to the corresponding station;
and 6, after the flow is stable, closing a liquid discharge port of the weighing system 6 and starting timing, measuring the weight of liquid nitrogen flowing into the weighing system within a certain time, calibrating by comparing the reading of a standard flowmeter, and drawing a flow characteristic curve if the set flow of the cryopump to be measured and the flow of the flowmeter are met. Meanwhile, the reading difference between the standard element 41 and the element to be measured 42 is compared, and real-time and instant flow calibration is carried out.
In the test process of the technology of the embodiment, the selection criteria of each component are as follows:
1. dewar with liquid storage
The liquid storage Dewar is the core of the whole calibration system, not only takes charge of storing the liquid nitrogen of the whole circulation system, but also integrates a plurality of important parts, and mainly comprises an upper cover plate assembly and a Dewar cylinder body; the upper cover plate component consists of an upper cover plate flange, a weighing system, an automatic pressure control device, a low-temperature valve, a pressure and temperature measuring sensor and other safety components. The weighing system comprises a high-precision force transducer (with the measuring range of 0-200kg and the precision of 0.02% F S, the use temperature of-30-150 ℃), a weighing tank, a standard weight block and a liquid injection and discharge valve, wherein the weighing tank is hoisted on an upper cover plate flange through the force transducer to measure the weight of the added liquid nitrogen, and the standard weight block is used for calibrating the force measuring element before calibration.
The Dewar cylinder adopts a vacuum double-layer heat insulation mode, and is in a wide-mouth Dewar shape; the design pressure of the inner container is 1.2MPa, the material is SUS304, the inner diameter is phi 1050mm, the height is 1945mm, the thickness of the inner wall is 6mm, the bottom is a seal head structure, the inner container belongs to a pressure container, and a special welding process and a pressing test are required; the interlayer is vacuum, the outer liner bears external pressure of 0.1MPa, the material is SUS304, the size of the outer cylinder is phi 1160mm, the height is 2225mm, the thickness is 4mm and the bottom is also in a sealing head structure through design calculation and ANSYS simulation.
2. Automatic pressure control device
The cryogenic pump is only used for overcoming the flow resistance of the system and providing power for the whole circulating system; therefore, the pressure increase and control of the system are required to be carried out through the pressure control device, and the calibration of the flowmeter is carried out on the premise that the pressure and the temperature in the liquid storage Dewar are well matched with the element to be calibrated.
The automatic pressure control device can automatically increase and control pressure for the whole circulating system, so that the circulating system is stabilized at the required calibration pressure;
the automatic pressure control device keeps a certain pressure which is higher than the saturated vapor pressure of the reflux liquid nitrogen, so that the reflux liquid nitrogen is further ensured to be supercooled and no bubble is generated. The automatic pressure control device is internally provided with a heater, the automatic pressure control device is kept at a higher pressure by heating liquid nitrogen in a container, and the automatic pressure control device is communicated with the main liquid storage Dewar so as to transmit the pressure to the circulating system, wherein the pressure range is 0.15-1.0 MPa (gauge pressure), and the saturation temperature of the corresponding liquid nitrogen is 86-96K.
During the circulation process, the liquid nitrogen in the pressure control device will slowly decrease due to certain system leakage. When the liquid nitrogen in the automatic pressure control device is reduced to a set value (monitored by a liquid level meter), the liquid nitrogen storage tank is prompted to supplement the liquid nitrogen into the automatic pressure control device.
The working pressure range of the pressure control container is 0.15-0.5 MPa, the common working pressure is 0.6MPa, and the design pressure is 1.0MPa internal pressure. The material was SUS304, designed to have a capacity of 28L.
3. Cryopump selection
The cryogenic pump provides a power source for the whole flow calibration circulating system, and when the selection matching is carried out, the flow range and the lift of the pump are mainly considered, namely the flow characteristic curve of the pump meets the requirements of the whole system as much as possible;
according to the design requirements of the whole system, the liquid nitrogen is required to be regulated within the range of 10-100L/min (0.6MPa), a frequency converter is arranged for a liquid nitrogen pump to regulate the flow, and the flow range which can be realized by the liquid nitrogen pump covers the regulation range as much as possible;
the lift of the cryogenic pump is mainly used for overcoming the flow resistance generated when the liquid nitrogen flows, namely the lift of the pump corresponding to the flow is more than or equal to the pressure drop of the whole circulating system;
ΔP＝ΔPS+ΔPN+ΔPf
namely, it is
In the formula: Δ P-total pressure drop, kPa, of the piping system; delta PS-static pressure drop, kPa; delta PN-velocity pressure drop, kPa; delta Pf-friction pressure drop, kPa; z1、Z2-the elevation, m, of the beginning and the end of the piping system, respectively; u. of1、u2-the fluid flow rates, m/s, at the beginning and at the end of the pipe system, respectively; u-average flow velocity of fluid, m/s; p-fluid density, kg/m3；hf-energy of friction loss in the tube, J/kg; l, Le-the length of the pipeline and the equivalent length of the valves, pipes, etc., m, respectively; d is the inner diameter of the pipeline, m.
According to design requirements, the maximum flow of liquid nitrogen is 100L/min, the total pressure drop of the whole system can be estimated to be 584.53KPa according to the maximum flow, and the height of a liquid nitrogen column is about 72m in a conversion mode, so when a liquid nitrogen pump is selected, when the maximum flow reaches 100L/min, the head is larger than 72 m; searching related data of the low-temperature liquid nitrogen pump, and selecting a submerged liquid type liquid nitrogen pump produced by Hangzhou New Asia low-temperature science and technology limited company;
the submerged liquid type liquid nitrogen pump is selected for later-stage system expansion and use, and is provided with an independent pump pool, so that the performance of the calibrated flowmeter can be tested in the later stage, and the liquid nitrogen pump can also be tested;
4. subcooler and evacuation pump
In order to ensure that the liquid nitrogen entering the flow meter and the weighing tank is in a single liquid phase and needs to be kept in a supercooled state, two groups of supercoolers are arranged and respectively arranged in front of the inlet ends of the flow meter and the weighing tank, the liquid nitrogen in the supercoolers is reduced to 75K in a decompression and evacuation mode, and the high-pressure circulating liquid nitrogen of a circulating system obtains cold energy from the supercoolers by using a coil pipe heat exchanger;
the subcooler mainly comprises a Dewar cylinder, a low-temperature valve, a subcooled heat exchanger, a decompression pump, a connecting pipeline and other temperature and pressure safety measurement components. The Dewar cylinder adopts a vacuum double-layer heat insulation mode, and is in a wide-mouth Dewar shape; the design pressure of the inner container is 0.2MPa, the material is SUS304, through design calculation and ANSYS simulation, the inner diameter is phi 850mm, the height is 1742mm, the thickness of the inner wall is 3mm, and the bottom is of a seal head structure; the interlayer is vacuum, the outer liner bears 0.1MPa of external pressure, the material is SUS304, the size of the outer cylinder of the cold box is phi 940mm, the height is 1825mm, the thickness is 4mm, and the bottom is also in a sealing head structure.
The experiment hoped that the liquid nitrogen in the subcooler reached 75K, at which temperature the maximum total heat load of the system was calculated to be 5.2 kW. LN2 had a latent heat of vaporization of 201.1J/g, and since LN2 had a molecular weight of 28, a thermal load of 5.2kW required a vaporization rate of 0.92mol/s to maintain 75K
The saturated vapor pressure P of LN2 corresponding to 75K is about 0.017 MPa. In order to maintain this pressure, the pump pumping rate at the corresponding pressure must be met.
If the vacuum pump is at room temperature (295K), then the volumetric pumping rate is V:
from the gas state equation.
V＝nRT/P＝≈0.133m3/s
Where R is the gas constant and T is the gas temperature within the pump.
The air extraction capacity is not lower than 133L/s (478 m) under the corresponding 0.017MPa3H), take 478m3H, providing one for each of the two subcoolers250m3A/h single-stage rotary vane pump.
5. Heat exchange tube
The inside of the heat exchange tube is flowing high-pressure super-cooling circulating liquid nitrogen, the outside of the heat exchange tube is 75K low-pressure liquid nitrogen, and the liquid nitrogen inside and outside the tube exchanges cold energy through the heat exchanger, so that the heat exchange tube needs to bear certain pressure and has good heat conduction performance. Therefore, in the calibration system, the heat exchange tube is made of a red copper tube into a coil heat exchanger, and the red copper tube with the specification model of phi 32 multiplied by 2 is calculated and selected to be manufactured according to the maximum pressure bearing of 1.2MPa of the design requirement.
The physical parameters of the liquid nitrogen under the pressure of 1.0MPa and the average temperature of 77K are shown in Table 3. According to design requirements, the heat exchange temperature difference of the heat exchanger is planned to be, an inlet 86K, an outlet 83K and a subcooler temperature 75K of the heat exchanger are set during calculation.
TABLE 10 thermophysical parameters of liquid nitrogen at a pressure of 1MPa and an average temperature of 80K
Calculation formula for heat exchange area and other parameters required by heat exchange tube[5]The following were used:
in the formula: l is the length m of the copper pipe required by the heat exchange pipe; f-heat exchange area, m, required by heat exchange tube2(ii) a Pr-prandtl number, Pr is greater than or equal to 0.7 for liquid nitrogen, the above formula holds; Re-Reynolds number; q-cooling system thermal load, W, Q ═ 5.2 KW; k-heat transfer coefficient of heat exchange tube, W (m)2·k)-1(ii) a Delta T-heat exchanger logarithmic mean temperature difference, K; Δ t1-a large temperature difference, K, across the heat exchanger; Δ t2-a small temperature difference, K, across the heat exchanger; d1，d2Inner and outer diameters m, d of heat exchanger tubes1＝0.028m， d2＝0.032m；α1，α2Coefficient of heat transfer between fluid inside and outside pipe, W (m)2·k)-1And, natural convection; r is the curvature radius of the coil pipe, m; omega1-flow velocity in coil, m.s-1；qmMass flow rate in coil, kg.s-1。
The pipe length was chosen to be 25m, taking into account a 1.3 margin. When the spiral diameter of the heat exchange tube is 800mm, the required number of turns is 11 turns, and the final coil length is 27.6 m.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. The method for measuring and calibrating the flow of the cryogenic fluid is characterized in that a measuring and calibrating device comprises a liquid storage Dewar (1), a cryogenic pump (2), a first subcooler (3), a second subcooler (5), a test box (4) and a weighing system (6); the outlet end of the liquid storage Dewar (1) is connected with the inlet of the cryogenic pump (2), and the inlet and the outlet of the test box (4) are respectively connected with the outlet end of the cryogenic pump (2) and the inlet of the weighing system (6); an outlet and an inlet of the first subcooler (3) are respectively connected to a pipeline between the cryogenic pump (2) and the test box (4), and an outlet and an inlet of the second subcooler (5) are respectively connected to a pipeline between the test box (4) and the weighing system (6); a low-temperature valve (8) is arranged between an inlet and an outlet of the first subcooler (3) in a pipeline between the low-temperature pump (2) and the test box (4), and a low-temperature valve (8) is arranged between an inlet and an outlet of the second subcooler (5) in a pipeline between the test box (4) and the weighing system (6); a low-temperature valve (8) is arranged on an outlet pipeline of the first subcooler (3), and low-temperature valves (8) are arranged on an inlet pipeline and an outlet pipeline of the second subcooler (5); an element to be tested (42) is arranged in the test box (4);
the calibration of the flow meter includes the following steps,
step 1, installing a flowmeter to be tested on a station in a test box (4), and selecting a low-temperature pump (2) meeting the requirement to be installed to a set position;
step 2, vacuumizing the whole device, wherein the device comprises a liquid storage Dewar, a first subcooler (3), a second subcooler (5), a weight measuring tank (62) and a device pipeline;
step 3, filling liquid nitrogen into the first subcooler (3), the second subcooler (5) and the liquid storage Dewar (1), and reducing the liquid nitrogen temperature of the first subcooler (3) and the second subcooler (5) to a set temperature;
step 4, adjusting the pressure in the liquid storage Dewar (1) to a design pressure;
step 5, starting the cryogenic pump (2) to drive liquid nitrogen in the liquid storage Dewar (1) to start circulation, enabling the liquid nitrogen to obtain cold energy from the first subcooler (3) and keep a single liquid phase to enter an element to be calibrated, enabling the cold energy to flow out and enter the second subcooler (5) so as to keep the single liquid phase to enter the weighing system (6), and returning the single liquid phase to the liquid storage Dewar (1) from a liquid outlet at the bottom of the weighing tank to finish the whole circulation;
and 6, after the flow is stable, closing a liquid discharge port of the weighing system (6) and simultaneously starting timing, measuring the weight of liquid nitrogen flowing into the weighing system within a certain time, and further comparing the reading of the flowmeter to be measured for calibration, and meanwhile, comparing the reading difference between the standard flowmeter and the flowmeter to be measured for real-time instantaneous flow calibration.
2. A cryogenic fluid flow measurement and calibration method according to claim 1, wherein the test box (4) comprises a standard component (41) mounting location, a component to be tested (42) mounting location; the standard element is installed at the standard element (41) installation position, the element to be tested (42) is installed at the element to be tested (42), and the low-temperature liquid sequentially passes through the standard element (41) and the element to be tested (42) or sequentially passes through the element to be tested (42) and the standard element (41).
3. A cryogenic fluid flow measurement and calibration method according to claim 1 or 2, wherein the weighing system (6) comprises a load cell (61), a load cell tank (62), a mounting bracket (63), a protective cover (64); the mounting support (63) is fixed at the bottle mouth of the liquid storage Dewar (1), the fixing support is fixed at the top of the force measuring element (61), the weight measuring tank (62) is hung on a pull rod at the bottom of the force measuring element (61), the protective cover (64) is reversely buckled at the bottle mouth of the liquid storage Dewar (1) for sealing and fixing, and the force measuring element (61), the mounting support (63) and the weight measuring tank (62) are positioned in a sealing cavity formed by the protective cover (64) and the liquid storage Dewar (1); the outlet end of the test box (4) is connected with the inlet of the weight measuring tank (62), and the liquid outlet of the weight measuring tank (62) is communicated with the inner cavity of the liquid storage Dewar (1).
4. A cryogenic fluid flow measurement and calibration method according to claim 3, wherein the weigh tank (62) is suspended in the interior of the reservoir dewar (1); the bottom of the weight measuring tank (62) is higher than the liquid level in the liquid storage Dewar (1); the bottom of the weight measuring tank (62) is provided with a liquid outlet (67), and the liquid outlet (67) is provided with a pneumatic valve.
5. A cryogenic fluid flow measurement and calibration method according to claim 3, wherein a standard weight block (65) is also removably mounted on the tie rod of the load cell (61).
6. A cryogenic fluid flow measurement and calibration method according to claim 1 or 2, wherein the outlet end of the cryogenic pump (2) is further in communication with the inlet of the reservoir dewar (1) via a return line; a return control valve (9) is mounted on the return pipe.
7. The cryogenic fluid flow measurement and calibration method according to claim 1 or 2, further comprising an automatic pressure control device; the automatic pressure control device comprises a pressurization tank (10), a heating element (11) and a controller; the controller controls the heating element (11) to heat the pressurizing tank to vaporize the liquid in the pressurizing tank; the pressurization tank (10) comprises an exhaust port, and the exhaust port is communicated with the inner cavity of the liquid storage Dewar (1).
8. The cryogenic fluid flow measurement and calibration method according to claim 7, wherein the pressurization tank (10) is located in an inner cavity of the liquid storage dewar (1), and an exhaust port is formed in the top of the pressurization tank (10).
9. The method for measuring and calibrating the flow of cryogenic fluid of claim 8, wherein the automatic pressure control device further comprises a controller, a pressure sensor; the pressure sensor is installed at the bottleneck of stock solution dewar (1), measures stock solution dewar (1) internal pressure, pressure sensor and controller communication connection, controller control heating element (11) are opened and close.
10. A cryogenic fluid flow measurement and calibration method according to claim 1 or 2, wherein the first subcooler (3) and the second subcooler (5) are structurally identical; the first subcooler (3) comprises a subcooler Dewar (31), a heat exchange tube (32) and a vacuum pump (33); liquid nitrogen is stored in the subcooler Dewar (31), the heat exchange tube (32) is positioned in the subcooler Dewar (31), the inlet end of the heat exchange tube (32) is connected with the outlet end of the low-temperature pump (2), and the outlet end of the heat exchange tube (32) is connected with the inlet end of the test box (4); and the vacuum pump (33) is connected with an exhaust port of the subcooler Dewar (31) through a pipeline.
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