Characteristics and selection basis of various flowmeters

Commonly used flowmeter, flowmeter as water supply enterprises trade settlement zone measurement of important measuring instruments, in water supply enterprises played an extremely important role.Flowmeter is an important basis for the main operation indexes of water supply enterprises, such as the statistics of water volume sold, water production cost, pipe leakage and energy consumption.Flowmeter selection is more important, how to correctly use all kinds of flowmeters, it is worth thinking.

Characteristics of various flowmeters

 

Electromagnetic flowmeter

 

The principle of electromagnetic flowmeter is Faraday's electromagnetic induction law (right hand rule).

 

Advantages:

 

Can be used to measure conductive liquid or slurry;

 

Through the pipeline, no pressure loss;

 

The measuring range is large, with the pipe diameter ranging from DN2.5 to DN2600.

 

The effects of temperature, pressure, density and viscosity of the fluid are not involved in the measurement principle.

 

One problem to note here is that the water supply pipe structure and wear change the inner diameter size.We have done an experiment: for example, the change of 100mm pipe by 1mm will bring about a 2% error, indicating that the change of inner diameter has a great influence on the electromagnetic flowmeter.Therefore, the ex-factory water pipeline for water supply must do a good job in the internal structure maintenance of the ex-factory water pipeline, otherwise the measurement error deviation is very large, even some electrodes have dirt above a certain number, the electromagnetic flowmeter can not be measured.

 

Disadvantages:

 

Only the liquid flow rate of conductive medium can be measured;

 

When measuring volume flow and mass flow, fluid density and temperature should be corrected.

 

Installation and debugging are more complicated than other flowmeters, and the requirements are more strict.

 

In use, must exhaust the remaining gas in the measuring tube, otherwise it will cause a large measurement error.

 

Note that the transmitter and converter of the electromagnetic flowmeter must be used together. Different instruments cannot be used between them.When installing the transmitter, from the selection of the installation site to the specific debugging, the installation site must be strictly in accordance with the product manual, no vibration, no strong magnetic field.

 

Ultrasonic flowmeter

 

The ultrasonic flowmeter measures the flow rate by measuring the flow of fluid through the propagation of the ultrasonic beam (or ultrasonic pulse).

 

Advantages:

 

The fluid flow which is not easy to touch and observe can be measured;

 

It can measure the flow rate of highly corrosive and non-conductive media.

 

Through the pipeline, no pressure loss;

 

The measuring range is large, and the pipe diameter ranges from DN20 to DN5000.

 

The measured volume flow rate is not affected by the thermophysical parameters such as temperature, pressure, viscosity and density of the measured flow body.

 

Disadvantages:

 

The temperature measuring range is not high and the temperature is less than 200℃.

 

Poor anti-interference ability.It is easy to be interfered by bubble, scale and other sources.

 

Strict requirements for straight pipe section;

 

Short service life (general accuracy can only be guaranteed for two years)

 

The uncertainty of installation will bring great error to flow measurement.

 

Reliability, accuracy level is not high, repeatability is poor;

 

When measuring volume flow rate and mass flow rate, fluid density and temperature should be corrected.

 

Due to scaling, measurement accuracy will be seriously affected and significant measurement error will be brought about.

 

Ultrasonic flowmeter is to measure the fluid velocity to determine the volume flow, the liquid should be measured its mass flow, but the mass flow is greatly affected by temperature;

 

The price is higher.

 

Plug-in flowmeter

 

Classification:

 

Point velocity type: insertion vortex street, insertion turbine, insertion electromagnetic, pitot tube differential pressure flowmeter, etc.

 

Runoff velocity type: differential pressure flow sensor of isotrope (circular section, diamond section, bullet section, t-shaped section), thermal isotrope flow sensor, etc.

 

Advantages:

 

Installation without stopping water;

 

Cheap;

 

Small size and light weight;

 

Loss of pressure;

 

Off-line verification can be realized;

 

Insert type ultrasonic flowmeter: it is relatively simple to insert steel tube.

 

Insert turbine flowmeter: accuracy is about 2.5%, reliable and stable, good anti-interference and anti-electromagnetic performance, poor anti-pollution.

 

Vortex flow meter

 

Vortex street flowmeter is a flowmeter that places a non-streamlined vortex generator in a fluid and causes the fluid to separate alternately on both sides of the generator to release two series of regular staggered vortexes, and the vortex separation frequency is proportional to the flow within a certain range.

 

Advantages:

 

Simple structure, firm, easy installation and maintenance;

 

High accuracy, generally ±(1 ~ 1.5)%R;

 

The measuring range is wide and the calibre can be reasonably determined up to 20:1.

 

Small pressure loss, about 1/4 ~ 1/2 of throttling differential pressure flowmeter;

 

The output is directly proportional to the flow of the pulse signal, no zero drift;

 

Within a certain Reynolds number range, the output frequency is not affected by the physical property (density, viscosity) and composition of the fluid, that is, the meter coefficient is only related to the shape and size of the vortex generator and the pipe.

 

Disadvantages:

 

It is sensitive to the mechanical vibration of pipeline and is not suitable for strong vibration.

 

The larger the diameter, the lower the resolution, the general full tube flowmeter for DN400 below;

 

The requirements of the upstream and downstream straight pipe segment are too high. Generally, the requirements of the upstream straight pipe segment are ≥15D. If there is a 90° elbow, the requirements of the upstream straight pipe segment are even higher than 35D.

 

When the fluid has pressure pulsation or flow pulsation, the value is greatly high, which has a great influence, so it is not suitable for pulsating flow.

 

Note: use with caution on the following occasions:

 

Where vibration and interference are strong, and where stability is required;

 

Low static pressure and low velocity.