Flowmeter Selection

In order to control something, it is necessary to first measure it.

In process industry too, measurement is a crucial factor. For accurate measurement of parameters, selection of the suitable instrument/technology is key. For flow measurement, the options available are plenty. However, a good percentage of the flow meters installed do not perform satisfactorily due to incorrect selection of technology for the given application.

To get the most accurate results through flow metering, many other factors besides price and popularity have to be taken into consideration. Selection of technology needs to be based on the application demand and process details.

Factors to be considered while selecting Flow Meter Technology :

Following are the essential process parameters which need to be considered while sizing the meters.

Properties of the liquid to be measured

• Measurement in terms of volume
• Measurement in terms of mass
• Totalised flow measurement
• Relay output in case of switching action based on application

Purpose of measurement

• Basic monitoring
• Custody transfer applications where billing is involved
• Process control based on flow measurement

Application demand

• Accuracy of the measurement; based on the criticality of flow measurement
• Availability of the straight length available for the flowmeter
• Process diagnostics
• Maximum allowable pressure drop across meter

Process details

• Density of the fluid
• Viscosity of the fluid
• Operating temperature
• Operating pressure
• Flow range
• Line size

Flow Metering Technologies

Electromagnetic flowmeters

These flowmeters can handle most liquids and slurries, including those with corrosive properties, provided the fluid has electrical conductivity. They can measure forward and reverse flow with equal accuracy. The major components are flow tube (primary element) and signal converter (secondary element). The flow tube mounts directly in the pipe. In this type of flowmeters, there is no moving part so pressure drop across meter is negligible.

The converter can be installed directly over the flow tube or remotely connected by a shielded cable. They are based on Faraday’s law of electromagnetic induction, and operate only when the fluid is conductive, hence cannot be used for gas measurement.

Vortex flowmeters

Vortex type flowmeters operate on the natural phenomenon of vortices that occurs when a fluid flows around a bluff/shedder object. Vortices are shed alternately at the downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the fluid flowing through the meter. Major components of a vortex flowmeter are bluff body, sensor and transmitter.

Vortex type flow meters can be used for applications like gases, steam, compressed air, low conductive liquids which do not contain the solid particles. They are not suitable for slurry and high viscous fluid measurements.

Variable area flowmeters

Variable area flowmeters or VA meters are floating element flowmeters consisting of a tapered tube and a float. They are constant differential pressure devices. When there is no fluid flow, the float rests freely at the bottom of the tube. As fluid enters the bottom of the tube the float starts to rise. The float is selected in such a way that its density is higher than the fluid density and the position of the float varies with the flow rate. Its exact position is at the point where the DP between the upper and lower part balance the weight of the float. The float is linked to the pointer, which denotes the flow reading on the scale.

VA meters are manufactured from metal, glass or plastic. VA meters can be used for liquid and gas applications, the only limitation being that they cannot be used for sticky fluids as chocking of float may occur.

Coriolis mass flow meters

Coriolis mass flow meters directly give mass rate of the flowing fluids. The biggest advantage of mass measurement is that mass does not change with change in temperature and pressure. Coriolis mass flow meters work on the natural phenomenon called the coriolis force. The rangeability of mass meter is 1:100. Even the accuracy point of view, mass meters are the most accurate meters available in the market. Mostly for custody transfer applications, mass meters are prefered more frequently. The disadvantages of mass meters are it is costly and cause a high pressure drop across meter.

Ultrasonic flow meters

Ultrasonic flow meters use ultra sound technology to measure the velocity of a fluid and calculate volume flow. They are classified as doppler meters and time of travel meters.

Doppler meters

These meters measure the frequency shift caused by liquid flow. Two transducers are mounted in the case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. The frequency of the returned pulse is shifted and this frequency shift is proportional to the fluid’s velocity.

Time of travel meters

These meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at 45 deg angle to the direction of the fluid flow. Here the speed of the signal traveling between the transducers increase or decrease with the direction of transmission and the velocity of fluid being measured.

Doppler meters are cheaper, but less accurate as compared to time of travel meters.

Time of travel meters have a limitation that the fluid measured must be free of entrained gas or solids to minimize signal scattering and absorption issues.

KROHNE MARSHALL offers a wide range of flow measurement instruments, from cost-effective mechanical indicators to advanced flow instrumentation. With a wide selection of measuring principles for liquids, gases and steam these instruments suit varied process applications, in almost every industry.

Refer to our application guide to select an appropriate flow meter from our range for your application.