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The current situation and development of averaging tube flowmeter

Abstract: the number and position of total pressure and back pressure detection holes of averaging tube flowmeter, and the profile shape of detection rod are discussed. This paper introduces several structural forms of averaging tube in detail, gives the calculation formula of flow measurement, analyzes the influence of various factors on measurement accuracy, and finally puts forward a conception for the development of this product

key words: flow measurement; Averaging tube; Influencing factors; The measuring element of

averaging tube flowmeter, averaging tube (foreign name: Annubar, literal translation: Annubar), is developed based on the early pitot tube velocity measurement principle. It is a new differential pressure flow measuring element developed in the late 1960s and has been applied to industrial sites in China. In the mid-1970s, more than 30 manufacturers have developed and produced it. The advantages of averaging tube are; The structure is relatively simple (as shown in Figure 1), the pressure loss is small, and the installation and disassembly are convenient. Today, we will know that the calculation option has a small amount of protection

due to its low production cost and low price, this flowmeter is popular in the market and occupies a place in many flow instruments. Especially because its pressure loss is small (compared with the orifice plate, it is only less than 5% of the orifice plate), it greatly reduces the power consumption and has remarkable energy-saving effect, which has its special significance in today's energy shortage. Due to its wide adaptability and good long-term stability (as shown in Figure 2), the flowmeter has made great progress in recent years, and several flowmeters with different structural forms have appeared. However, due to improper use, some problems have arisen in the application, resulting in a great contradiction between the objective requirements and the development status quo. It is not easy to choose to do experiments on plastic materials. Many people expect that their application problems can be solved. Therefore, people have made a lot of unremitting efforts, so that the averaging tube flowmeter, which is both old and young, has been widely used in the measurement and testing of energy and environmental protection

1 measuring principle of the averaging tube flow sensor

the averaging tube flow sensor, known from its structural diagram, is a hollow metal rod that Leeper grew up in Kingsport, Tennessee, and inserted into the pipe along the diameter. There are pairs of pressure taps in the direction facing the fluid flow. Generally speaking, there are two pairs, but there are also one or more pairs, and its shape is like a flute. The multi-point pressure taps on the upstream surface measure the total pressure, which is connected with the total pressure pipe, and the accuracy of leading out the average total pressure P is very high. 1. Generally, there is a hole at the center of the back flow surface, which is connected with the static pressure pipe and leads out the static pressure P2. The averaging tube measures the flow rate by measuring the difference between the total pressure and static pressure of the fluid. The output differential pressure (△ P) and the average fluid velocity (V) of the averaging tube can be obtained from the classical Bernoulli equation (1)

, where; △ P -- difference between total pressure and static pressure, pa

ρ—— Fluid density, kg/m3

k -- correction factor

if expressed by flow, the basic formula for flow calculation is

where QV - volume flow of fluid, m3/s

qm - mass flow of fluid, kg/s

α —— Flow coefficient of averaging tube under working condition

ε—— The expansion coefficient of the flow beam when the fluid flows through the detection rod under the working state

a - internal cross-sectional area of pipeline under working state, m2

for different compressible fluids: ε= 1; For compressible fluids: ε< 1

the position of the full pressure hole can be obtained by equal area method. In this way, the averaging tube can have better adaptability under the condition of flow change, and the error reflected is small. The so-called equal area method is to divide the pipe section into equivalent average velocity points of the inner circle and the outer ring. These points are the positions of the full pressure holes, as shown in Figure 3

the opening position of the full pressure hole can be obtained by Chebyshev numerical integration, as shown in Figure 4. R1= ± 0.4597r, r2= ± 0.8881r, R1, R2 are the distance between the center of the pressure hole and the center of the pipe, and R is the inner radius of the pipe

in recent years, academia and the international organization for standardization have raised objections to this point selection method, both in number and location. They believe that the flow in the pipe should be divided into three areas, and the point selection should be carried out according to the log jchebycheff method. Therefore, the location of the total pressure detection hole should be; r1=±0.03754R; r2=±0.7252R; r3=±0.9358R。 This method has been confirmed by the flow measurement Committee (TC30) of the international organization for Standardization (ISO) in closed pipelines. In view of the above reasons, through people's experimental research, it is recommended to use two or three pairs of total pressure holes for averaging tubes

one back pressure detection hole has been used for a long time because people have realized that the averaging tube is in the potential flow according to the specification, and the premise of the potential flow is that the static pressure at all points on the cross-section of the pipe is equal, and there is no transverse flow. From this point of view, a back pressure detection hole is enough. In order to prevent the flow of fluid from blocking the back pressure detection hole in the detection process, the porous back pressure has been applied to the averaging tube flow sensor. In short, according to the basic formula of flow, as long as the output differential pressure △ P of the averaging tube is effectively measured, the flow value of fluid can be measured, which is the measurement principle of the averaging tube flow sensor

2 structural form of velocity averaging tube

the structure of velocity averaging tube is a hollow metal rod, and the products with the most widely used section shape are round and rhombic. Wing shaped section was also used in the mid-1980s

for the averaging tube with circular section, when the Reynolds number Re is between 105 and 106, the flow coefficient α Unstable, its stable region is at Reynolds number Re

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