Brief discussion on factors affecting the accuracy of Annubar flowmeter and their countermeasures

Introduction

Annubar flowmeter (also known as flute-shaped Annubar flowmeter) is a new type of differential pressure flow detection element developed based on the principle of pitot tube velocity measurement. It outputs a differential pressure signal and is used in conjunction with instruments for measuring differential pressure. It can accurately measure the flow of various liquids, gases and steam (superheated steam and saturated steam) in circular and rectangular pipes. It has gradually replaced orifice plates and other detection elements with its advantages of small pressure loss and easy installation. It has been widely used in industries such as electricity, petroleum, chemical industry, textile, and metallurgy. The most important thing is that Annubar flowmeter has solved many problems of other flowmeters in large-diameter pipeline measurement. However, Annubar flowmeter has very strict requirements from design, manufacturing to installation and use. As long as one of the links is not careful, it can cause a large error.

1 Structure and measurement principle

Figure 1 Annubar flowmeter structure

The structure of the Annubar flowmeter is shown in Figure 1. It has a hollow metal rod inserted into the pipe along the diameter. There are paired pressure measuring holes in the direction of the fluid flow. Generally speaking, there are two pairs, but there are also one or more pairs. The hollow metal rod looks like a flute. The multi-point pressure measuring holes on the frontal surface measure the total pressure, which is connected to the total pressure pipe and leads to the average total pressure P1. There is generally a hole in the center of the back surface, which is connected to the static pressure pipe and leads to the static pressure P2. The Annubar flowmeter measures the flow rate by measuring the difference between the total pressure and the static pressure of the fluid. It is a differential pressure detection rod flowmeter. The output differential pressure (△p) and the average velocity (v) of the fluid of the Annubar detection rod can be obtained according to the classic Bernoulli equation:

V＝ K×（2／ρ×△P）1/2Where

; △P—the difference between the total pressure and the static pressure, kPa;
ρ—the density of the fluid under working conditions Kg/m3;
k—correction coefficient.

If expressed in terms of flow rate, the basic flow rate calculation formula is

Qv = K × π/4 × D2 (2ΔP/р) 1/2

where: Qv - fluid flow rate flowing through the measuring tube m3/h;
D - measuring tube inner diameter mm.

From formula (2), it can be seen that the essence of the Annubar flowmeter system is the measurement of differential pressure △P. Its measurement principle is shown in Figure 2. This is a common feature of all differential pressure flowmeters. The technology is universal, that is, a differential pressure transmitter is used to convert △P into a corresponding mechanical signal or electrical signal.

Figure 2 Measurement principle diagram

2 Factors affecting the accuracy of flowmeters and solutions

① The flow rate at each pressure hole on the differential pressure detection rod of the Annubar flowmeter is different, and there is a certain pressure difference between each pressure hole. In this way, there is medium flow between each pressure hole, and the debris in the flowing medium will be buried and blocked, which will cause differential pressure loss over time. Although the Annubar manufacturer claims that the anti-blocking problem of the measuring sensor has been solved, due to the special circumstances of each production process, the author still recommends the installation of necessary auxiliary equipment such as debris filtering devices to ensure the normal operation of the instrument (the specific approach can be negotiated with the supplier). [page]

② From the basic flow formula, it can be seen that as long as the output differential pressure △P of the detection rod is effectively measured, the flow value of the fluid can be measured. For a long time, the back pressure detection hole of the detection rod has only used one measurement hole. People believe that the detection hole of the detection rod is in potential flow according to the specification requirements. The premise of potential flow is that the static pressure at each point on the cross section of the pipeline is equal and there is no lateral flow. From this point of view, one back pressure detection hole is sufficient. In order to prevent the flow of the fluid from blocking the back pressure detection hole during the detection process, a porous back pressure can be used to take the pressure, which has begun to be applied to the detection rod flow sensor.

③ The flow coefficient K is unstable, resulting in unstable flow. For the detection rod with a circular cross-section, when the Reynolds number Re is between 105 and 106, the flow coefficient K is unstable, and its stable area is between Reynolds number Re<105 and Re>106. This is mainly due to the "resistance crisis" of the resistance member of the circular cross-section itself. The pressure distribution caused by the fluid flowing through the circular tube is different due to different separation points when the circular tube faces the flow of the fluid, thereby causing the flow coefficient K to change. The use of a detection rod with a diamond cross-section can overcome this unstable area of ​​the circular cross-section. No matter how much the Reynolds number value Re of the diamond cross-section is, its separation point is fixed and unchanged, thus better solving the difficulty of the unstable area of ​​the detection rod flow sensor when detecting liquid and gas (steam) flow.

④ Annubar flowmeter is based on the principle of Pitot tube velocity measurement, and measures the total static pressure to calculate the flow rate. It is often used to measure the flow rate of liquid and gas (steam) in large-diameter pipelines. The differential pressure it generates is generally small, and the minimum may be only 20Pa~30Pa. For this reason, it is necessary to avoid using long pressure pipes as much as possible, and select high-precision micro differential pressure transmitters, such as Honeywell, EJA differential pressure transmitters, etc. The best solution is to use a direct installation method that integrates a detection rod, a three-valve group, and a differential pressure transmitter, such as the LG-A series integrated intelligent Annubar flowmeter, which can not only reduce the leakage caused by the use of pressure pipes, but also compensate for the distortion of differential pressure signals caused by changes in temperature and pressure.

3 Selection and installation

① The mechanical size of the Annubar flowmeter is tailored according to the size of the installation pipeline, and its measurement range is calculated and calibrated according to the flow data provided by the process. When selecting, be sure to provide the manufacturer with the correct flow data and pipeline size, which includes the outer diameter and wall thickness. The flowmeter must be installed according to the standard to ensure that the measurement error of the entire measurement system is within the allowable range.

② The front pipe of the detection rod sensor must ensure that there is a straight pipe length of 7D to 9D (this data is different for each manufacturer, ranging from 3D to 30D), so that the flow rate of the liquid and gas (steam) in the large pipe can be distributed symmetrically on the axis as much as possible. Only in this way can the flow rate flowing through the entire cross section be calculated by measuring the flow rate of several points, otherwise the flow rate distribution of the liquid and gas (steam) in the large pipe will be very complicated, the fluctuation of the flow coefficient K will be very large, and the measurement accuracy cannot be guaranteed. However, in actual process design, it is impossible to ensure a long straight pipe. The author believes that if a certain form of Annubar flowmeter is determined, it can be discussed with the supplier to shorten the straight pipe as much as possible while ensuring the repeatability of the system. According to relevant data, as long as the ratio of the measuring pipe diameter to the process pipe diameter is less than 1/10, the straight pipe length of the front pipe can be reduced to 3D to 4D.

③ The total pressure hole of the detection rod sensor must face the flow direction, and the deviation should not exceed 7°, as shown in Figure 3.

The detection rod should be inserted to the bottom along the pipe diameter direction, as shown in Figure 4.

For vertical pipelines, the detection rod can be installed at any position along the 360° circumference of the pipeline on the horizontal plane (Figure 5a). The high and low pressure pipes should be on the same plane; for horizontal pipelines, they should be installed tilted downward during measurement (Figure 5b).

Whether the flowmeter can accurately measure and its service life is directly related to whether it is installed correctly. Once the flowmeter is installed, the systematic error that affects the accuracy of the flowmeter will always be accompanied, and it is difficult to eliminate it from the software. The correct installation method can refer to the installation specifications of the differential pressure flowmeter, which will not be repeated here.

④ The detection rod is fixed to the pipeline through the joint. After tightening the joint, the detection rod must not loosen or leak. Pipeline vibration and fluid impact will cause the detection rod fixing head to loosen and fall off, causing the detection rod to break and damage, affecting accurate measurement and causing economic losses.

3 Calibration issues

In order to ensure the accuracy of the Annubar flowmeter, it is best to perform real flow calibration on site. Although each Annubar has been calibrated by the manufacturer in the wind tunnel of the China Meteorological Administration and the instrument coefficient has been given, it is difficult to implement accurate calibration in the downstream of the uniform flow field of the wind tunnel without providing a variety of specifications and a sufficiently long straight pipe section. In addition, due to the large diameter of the on-site pipe (DN1000mm or more), and the reference conditions, fluid types, environment, etc. of the pipeline during calibration are different from the actual measured fluid, especially the on-site straight pipe section length is not enough, the process conditions are complex, and the influence of pipeline roughness on the velocity distribution coefficient with the change of service time is difficult to grasp. The correction coefficient of the change of the instrument coefficient caused by the insufficient length of the straight pipe section between the resistance parts before and after the pipe section where the flow meter is located currently lacks mature test data. According to the requirements of relevant materials, it can generally be directly verified and determined on-site. However, there are many problems in on-site verification, which inevitably leads to additional errors in measurement. The measurement department can use the measurement data, production status, output and other factors of the newly installed and normal operation as a reference to sublimate the meter measurement test data into measurement settlement data.

5 Application precautions

① Pay enough attention to the structure and working principle of the flow meter, especially for a new instrument. Before use, you must fully understand its structure and working principle to install it correctly, and make accurate judgments and eliminate problems that arise during use to avoid blindness.

② Paying attention to the maintenance of flow meters is an important measure to improve the service life and accuracy of the meter. Meter maintenance and technical personnel must develop good operating habits.

References
1 Annubar Flowmeter Series Product Reference Manual.
2 Li Liangmao, et al. Practical Guide to Common Measuring Instruments. Beijing: China Metrology Press, 19.
3 Refer to ISO 3966-1997 (E) Fluid Flow Measurement in Closed Pipes - Velocity Area Method Using Pitot-Static Tube. (end)