2019-10-22 09:42:50

Flowmeter challenges

Traditional mechanical flowmeters, such as differential pressure flowmeters, positive displacement flowmeters, and variable area flowmeters, are already in the stage of popularization, with fierce price competition, decreasing profit margins, less technological innovation, and relatively mature markets. . Frost & Sullivan believes that achieving product differentiation and customized production is an important breakthrough for manufacturers in the fierce competition in mature markets. Based on Frost & Sullivan's analysis of the needs of industry users, the user community expects manufacturers to provide automated equipment that will bring tangible benefits to the production process. Users will have specific requirements in the application process, such as: application in the special environment of the petrochemical industry, the need for rugged design and explosion-proof certification; user demand for Coriolis flowmeter designed for straight pipe. How to effectively obtain the actual needs of users and improve the traditional products is a challenge to the manufacturer's differentiated and customized production process.

Guiding users to accept and use new technology flow meters, such as ultrasonic flow meters, electromagnetic flow meters, and thermal mass flow meters, is another challenge for manufacturers to make the market bigger and stronger. In fact, the new technology flowmeter mentioned above has been developed and applied more than ten years ago. How to make customers realize that using the new technology flowmeter can effectively improve the production efficiency is an important issue for manufacturers.

In addition, new technology flowmeters are constantly being introduced into various industries, and fast and efficient after-sales service is also crucial for manufacturers. In particular, the use of Flowfield based on Foundation Fieldbus and Profibus PA bus has certain requirements for software technology. Effective services can provide users with more suitable solutions and close to users.

Variable area flowmeter (equal pressure drop flowmeter)

The float placed in the upper and lower small tapered flow passages is moved by the force of the fluid flowing from the bottom to the top. When this force is balanced with the "display weight" of the float (the weight of the float itself minus the buoyancy of the fluid it receives), the capture is stationary. The height at which the float is stationary can be used as a measure of the flow rate. Since the cross-sectional area of ??the flowmeter varies with the height of the float, and the pressure difference between the upper and lower parts is equal when the float is stationary, the flowmeter is called a variable area flowmeter or an equal pressure dropmeter. A typical instrument of this type of flow meter is a rotor (float) flow meter.

Momentum flowmeter

A flowmeter weighing flowmeter that uses the momentum of a measuring fluid to reflect the flow rate. Since the momentum P of the flowing fluid is proportional to the density of the fluid and the square of the flow velocity v, i.e., p v2 , when the flow cross section is determined, v is proportional to the volumetric flow rate Q, so p Q2 . Set the proportional coefficient to A, then Q=A. Therefore, P is measured to reflect the flow rate Q. In this type of flowmeter, most of the flowmeters are used to convert momentum into pressure, displacement or force, and then the flow rate is measured. Typical meters for such flow meters are target and rotary wing flow meters.

Ultrasonic flowmeter measurement principle

When the ultrasonic beam propagates in the liquid, the flow of the liquid will cause a small change in the propagation time, and the change in the propagation time is proportional to the flow velocity of the liquid, and its relationship conforms to the following expression.

among them

θ is the angle between the sound beam and the direction of flow of the liquid

M is the number of linear travels of the sound beam in the liquid

D is the inner diameter of the pipe

Tup is the propagation time of the sound beam in the positive direction

Tdown is the propagation time of the sound beam in the reverse direction

ΔT=Tup –Tdown

Let the speed of sound in the stationary fluid be c, the velocity of the fluid flow be u, and the propagation distance be L. When the sound wave is in the same direction as the fluid flow direction (ie, the downstream direction), the propagation velocity is c+u; otherwise, the propagation velocity is cu. Two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) are placed at two places separated by L. When T1 is in the forward direction and T2 transmits ultrasonic waves in the reverse direction, the time required for the ultrasonic waves to reach the receivers R1 and R2 respectively is t1 and t2, then

T1=L/(c+u); t2=L/(c-u)

Since the flow velocity of the fluid in the industrial pipeline is much smaller than the sound velocity, that is, c>>u, the time difference between the two is ▽t=t2-t1=2Lu/cc. Thus, the propagation velocity of the acoustic wave in the fluid is known. When it is known, the flow rate u can be obtained by measuring the time difference ▽t, and the flow rate Q can be obtained. The method of measuring the flow using this principle is called the time difference method. In addition, a phase difference method, a frequency difference method, or the like can be used.