A piston displacement type liquid flow calibration device

Introduction

In recent years, with the progress of science and technology, flow measurement technology has developed rapidly, and the corresponding flow calibration technology has been increasingly valued by domestic and foreign experts, scholars and relevant departments.

There are two traditional liquid flow calibration methods: one is the volume (time) method; the other is the mass (time) method. Among them, the volume method is divided into static volume method and dynamic volume method, and the mass method is divided into static mass method and dynamic mass method. Although these four methods can meet the requirements of flow calibration within a certain range, due to the limitations of the level of technological development, they have many obvious disadvantages. These disadvantages are mainly:

(1) The device is relatively large in size, occupies a large working area, and requires more working medium.
(2) The different degrees of openness make the working medium easy to volatilize, evaporate and be contaminated, which not only affects the calibration accuracy, but also shortens the cycle of replacing the medium.
(3) Due to the limitations of its own structure, it is difficult to improve the overall accuracy.
(4) It is difficult to control with a computer.

In the early 1990s, the United States introduced a new type of liquid flow calibration device, which basically overcomes the four main shortcomings of the above traditional methods. The device adopts the "dynamic displacement-time method" measurement principle. Its basic measurement method is based on the measurement of volume and time, but it is completely different from the above static volume method and dynamic volume method. We spent about three years to successfully develop a set of lubricating oil flow calibration device based on this principle, and its main technical indicators have reached and exceeded the technical indicators of the American device.

1 Working principle of the device

The working principle of the device is as follows:

The motor drags the piston at a constant speed to discharge the oil from the cylinder and pass through the calibrated flow meter. If the volume of oil discharged by the piston movement within the time interval t is V, the actual (standard) flow qs is:

By comparing qs with the corresponding indicated flow q of the calibrated flow meter, the error of the calibrated flow meter can be determined, thereby achieving the purpose of calibrating the flow meter.

The ball scale in Figure 1 is used to measure the displacement of the piston. The motor control unit is used to receive computer instructions and control the motor speed. P1, P2 and T1, T2 are two pressure sensors and two temperature sensors respectively. P1 and T1 are used to measure the oil pressure and oil temperature at the outlet of the oil cylinder, and P2 and T2 are used to measure the oil pressure and oil temperature at the flow meter to be calibrated. These four quantities are used to correct the pressure and temperature of the standard volume. The microcomputer controls the entire system, analyzes and calculates the measured values, and finally outputs the calibration results. The

piston cylinder structure in Figure 1 is similar to the piston volume tube in form, but there are essential differences between them, as follows:

(1) The displacement-time method device calibrates the instantaneous flow rate, while the piston volume tube calibrates the cumulative flow rate.

(2) l is the displacement, and A is the annular inner cross-sectional area of ​​the cylinder. Since the processing of the cylinder ensures that A is a constant, the measurement of V is converted into the measurement of l. The displacement-time method device is a dynamic displacement measurement, while the piston volume tube is a static displacement measurement.

(3) The displacement-time method device requires the piston to move at a uniform speed, while the piston volume tube does not require the movement to be uniform, and even stop-and-go movement is acceptable.

2 Design of the calibration device

2.1 Main design technical indicators

Flow range: (0.1-10) m3/h
Device uncertainty: 0.05%
Device flow certainty: 0.02%
Temperature range: room temperature-150℃

2.2 Design of the main mechanical system

The piston and cylinder system is the core of the main system of the calibration device. The following four points are mainly considered in the design:

(1) According to the requirements of the total uncertainty of the device, the processing accuracy requirements assigned to the cylinder and the piston rod are calculated, that is, the parallelism and ovality requirements.
(2) The matching accuracy requirements between the piston, cylinder, guide rail, motor shaft, etc.
(3) The design should fully consider the convenience of adjustment during assembly.
(4) Sealing guarantee during the piston movement.

2.3 Hardware design

The hardware structure of the system measurement and control part is divided into three modules: data acquisition module, I/O communication module, and technology and logic control module. These three modules are all in the form of microcomputer PC bus.

Figure 2 Hardware structure diagram of the system measurement and control part [page]

2.4 Software Design

The system software mainly includes parameter setting, state measurement, temperature control, calibration control, data processing and document management, as follows:

Figure 3 System software structure diagram

2.5 Temperature system design

According to the requirements of the total uncertainty and temperature range of the device, the requirements for temperature control and temperature measurement accuracy and the requirements for the temperature gradient of each part of the whole system are estimated and determined.

3 The level achieved in the development

After the successful development of the device, after about a year of operation, testing, experimentation and modification, and after testing by the test team headed by the China Institute of Metrology, the two main technical indicators of total uncertainty and flow stability have reached the original design indicators, that is, the indicators of the world's leading level in the United States in the 1990s; and in terms of temperature range (up to 150°C), it exceeds the level of the United States (up to 66°C); and passed the head office level appraisal and was evaluated as: leading in China and advanced in the world.

4 Innovation and characteristics of the device

4.1 From the above principle part, it can be seen that the key to obtaining the standard flow of the device depends on the measurement of the dynamic displacement l. The usual method of displacement measurement is to use a grating. We did not use a grating, but used a British patented technology ball scale with certain risks. Compared with the grating, it has the advantages of strong resistance to vibration interference, strong resistance to pollution, good long-term reliability, and easy maintenance. We have overcome the dual difficulties of hardware and software, and successfully applied the new technology of ball scale to the flow calibration device in China for the first time.

4.2 This device successfully applied ISO7278/3 pulse insertion technology (commonly known as dual time measurement method) for the first time in calibrating flowmeters with pulse output (such as the most commonly used turbine flowmeter). This method uses two timers to measure the small value of the flowmeter pulse, thereby greatly improving the calibration resolution and accuracy. The success of adopting international standards is not only convenient for international integration, but also convenient for comparison with international standards.

4.3 The device can calibrate the flowmeter at any temperature within a wide temperature range of room temperature to 150℃. This has great theoretical and practical significance in the field of flow calibration. The specific explanation is as follows:

We know that there are thousands of liquid media (such as oils and chemicals) for flow measurement, and there are no less than 100 commonly used ones. Therefore, it is impossible to build a flow calibration device for each liquid medium. Only some necessary liquid flow calibration devices can be built. In addition, due to the complexity of the device, the difficulty of cleaning, the incompatibility of liquid media, and the high economic cost, the method of replacing the medium is not advocated or adopted at home and abroad to calibrate the flowmeter used for different media. Therefore, for most flowmeters used for liquid media, they can only be calibrated with media different from the medium used. This creates the problem of correcting the instrument coefficient K of the flowmeter when the calibration medium is different from the medium used. For the same turbine flowmeter, the main factors affecting its instrument coefficient K due to different media are the viscosity (v) and density (ρ) of the medium, and the more important one is the viscosity. The problem of how to correct the instrument coefficient K of a turbine flowmeter when the calibration medium is different from the medium used is a complex experimental and theoretical problem. It is a problem that many experts and scholars in the field of flow measurement at home and abroad are currently working on. The problem is still far from being fully solved.

The device we developed has the characteristic of large-range variable temperature calibration, which provides a very important experimental condition for solving the above correction problem.

The important uses of this device are as follows:

(1) Use this device and the "Variable Temperature Simulation Calibration Method" to study the influence of viscosity (v) on the instrument coefficient K of a turbine flowmeter.

As mentioned above, v and ρ have the main influence on K. ρ should be relatively fixed, that is, the calibration medium and the pseudo ρ of the medium used are required to be close. The medium used in our device is 4050 lubricating oil, which has a density close to 1 (close to water) and a high viscosity (58cst at 20°C). In this way, a turbine flowmeter with stable performance can be selected. First, the K value is calibrated with water at room temperature (water v: 1cst). Then, on our device, the K value is calibrated at different viscosities by changing the temperature to change the viscosity (v) of 4050 lubricating oil, and the specific influence of v on K can be obtained.

(2) Use this device and the "Variable Temperature Simulation Calibration Method" to study the effect of density (ρ) on the instrument coefficient K of the turbine flowmeter.

To study the effect of ρ on K, the v value must be relatively fixed. Select a turbine flowmeter with stable performance, and use the transformer oil flow calibration device of the China Institute of Metrology to calibrate the K value at room temperature (natural temperature, cannot be changed), such as at room temperature 26°C (at this time, the transformer oil v = 21cst, ρ ≈ 0.86) to calibrate the K value, and then calibrate it on our lubricating oil device at a lubricating oil viscosity of 21cst (at this time, the lubricating oil temperature should be raised to 45°C, because the viscosity-temperature curve of the lubricating oil is known, and the lubricating oil ρ ≈ 0.97), and obtain the K value. In this way, the effect of the same viscosity and different density on the flowmeter instrument coefficient K can be obtained.

(3) Further application of this device can also be used to study and more satisfactorily solve the effect of viscosity on the performance of turbine flowmeters, that is, the "Universal Viscosity Curve Method" proposed by Jones in recent years, which is superior to the traditional K coefficient test and theoretical analysis methods.

4.4 The device has high accuracy, compact structure, small size (small footprint, equivalent to 1/5 of the traditional device), high technical content, closed operation, safe and reliable.

5 The promotion and application value of the device

Due to the following two aspects of analysis, the device has great promotion value in China:

(1) All key technologies of the device, including the processing and development of the piston and cylinder system, the motor speed stabilization system, the dynamic displacement measurement system, the implementation technology of ISO7278/3, the temperature control system, etc., are all independently developed by us in China, and we will not rely on or be controlled by foreign countries in key technologies.

(2) The principle of the device is in principle applicable to all liquid flow calibration devices, especially to flow calibration devices for various oils and high-viscosity liquids; after the main system of the device is processed and installed, it can be used as a flow calibration device for any medium, and the total uncertainty of the device will not be affected by the different media.

Reference
1 Technical Research Report on "Calibration Device for Lubricating Oil Turbine Flowmeter". Zhang Baozhu, 1997
2 Determination of Turbine Meter Usable Turndown, Paul D. Olivier, Flow Dynamics, Ins (P739~745,1995)
3 Effect of Kinematic Viscosity on Performance of Turbine Flowmeters—Solution to the Problem, Frank E. Jones, Independent Consultant (P379~389,1995)(end)