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Electromagnetic flowmeter caliber selection description

The flowmeter is not a choice of caliber, it is first to know the caliber, parameters, and then determine the flowmeter.

Liquid is one of the four major forms of matter. It is an undetermined shape and is often affected by the container. However, its volume is fixed under constant pressure and temperature conditions. In addition, the liquid exerts the same pressure on the sides of the container as the other physical states. This pressure is transmitted in all directions, not only does it decrease, but increases with depth (the deeper the water, the greater the water pressure).

How to choose the right diameter flowmeter?

1, electromagnetic flowmeter

The electromagnetic flowmeter is mainly used for measuring the volume flow of the conductive liquid in the closed pipe. The minimum flow rate of the fluid specified by the electromagnetic flowmeter is not less than 0.5 m/s, normal at 2~4 m/s, and the highest is not higher than 8 m/s. Therefore, when choosing the diameter of the electromagnetic flowmeter, we must fully consider the proper pipe size under the measurement accuracy of the electromagnetic flowmeter.

(1) The diameter of the selected sensor is the same as the diameter of the connected process pipe

The sensor usually uses the same diameter as the process pipe, which is easy to install. However, it is based on the premise that the flow rate of the fluid in the tube should be within the usual range. The general industrial electromagnetic flowmeter has a full-scale flow rate ranging from 1 to 4 m/s, which can be selected according to different measurement objects.
For example, when measuring clean water, the economical flow rate is 1.5-3m/s. When measuring the solution that is easy to crystallize, the flow rate should be appropriately increased, 3-4m/s is appropriate, and it can be self-cleaning to prevent adhesion and deposition. When the abrasive fluid such as pulp is used, the flow rate should be appropriately reduced, preferably 1-2 m/s to reduce the wear on the inner liner and the electrode. Practical applications rarely exceed 7m/s, and more than 10m/s is even rarer.

See the following figure for the relationship between flow rate, flow rate and caliber:

(2) The diameter of the selected sensor is different from the diameter of the connected process pipe.

This choice applies to the following situations:

A. The flow rate in the pipeline is low, and the process flow is relatively stable. In order to meet the requirements of the flow rate range of the instrument, the flow rate is selected locally in the instrument part to select the sensor diameter smaller than the diameter of the process pipe, and the different diameter pipe is added before and after the sensor;

B. Considering the price, for large-caliber electromagnetic flowmeters, the larger the diameter, the higher the price. For the case where the flow rate in the pipeline is low and the process parameters are stable, a sensor with a small diameter can be selected, which not only enables the instrument to operate under a good working condition, but also reduces the instrument cost.

(3) Problems that should be paid attention to when installing reducer

A. Selection of the taper angle of the reducer
In order to affect the flow rate distribution after installing the reducer, the accuracy of the electromagnetic flowmeter is not affected, and the central cone angle α of the reducer is required to be no more than 15°, and the smaller the better. This way the reducer can be considered part of a straight pipe section.

B. Adding a reducer will result in pressure loss (this pressure loss does not affect the measurement of the electromagnetic flowmeter). The total pressure loss consists of three parts:

The total pressure loss is:
Where: ρ is the density of the medium in kg/m3
Ξ1 ξ3 is the coefficient of the Reynolds number of the divergent tube
Ξ2 = 0.02 is the coefficient of the sensor measuring tube
V1 and V2 are the process pipes and the flow rate in the sensor measuring tube, respectively, in m/s.

Note: The pressure loss caused by installing the reducer does not affect the measurement of the electromagnetic flowmeter. The formula here is based on the pressure signal that the customer may have to collect downstream pipelines.

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