Flow Measurement

What is the importance of flow measurement in process control?

The measurement of flow is an important part of the process control, it could be the frequently measured variable in an industrial process plant. The importance of the flow measurement is that we would be able to determine the purchased material and also the sold material, and due to this very accurate flow measurement is needed. In a process industry, most of the process includes the measurement of the flow and it could be the measurement of the various flow, such as gas, liquid, or vapor. This measurement must be accurate and it will be recorded for the process applications.

What are the different types of flow that can be seen in a process?

Laminar flow

In this type of flow, the fluid will travel parallel to the pipe walls and the radial movement in the fluid will be comparatively low and the fluid could be in different layers. These layers could have different concentrations.

Transient flow

In this type of flow, there won’t be any layer and the different layers will be joined together.

Turbulent flow

In this type of flow, the entire fluid layers can’t be seen and there will be only radial flow. Mostly an industrial plant is composed of turbulent flow.

How flow meters are categorized?

The flowmeters are used in a process plant to measure the flow there are many types of flow meters and to select a flow meter we must consider the nature of the fluid and also the process condition in which it is measured.

There are many types of flow meters and they are grouped into four types

  • Inferential type flow meters
  • Velocity flowmeters
  • Volumetric flowmeters
  • Mass flowmeters

Inferential flowmeters

The inferential flow meter is a type of flow meter in which the flow is not directly measured but it would do the measurement by using other properties like pressure and temperature. So it would measure the flow by considering the drop in static pressure, in a pipe. It would also measure the flow by checking the rotation of the rotor or the impeller. So the details which are received by checking these properties would be converted into velocity.

Types of inferential flow meter

Head type flow meter

By measuring the differential pressure we can determine the flow rate.

Variable area meter/ Rota-meter type

In this type of inferential meter, the flow rate is determined by the displacement of an object.

Turbine flow meter

In this type of meter, the mechanical force of the fluid is used to rotate the rotor in a flowing stream. So the flow rate is measured with the help of a velocity proportionality factor.

Target meter

In this flow meter, the flow rate is calculated with the help of force measurement.

Thermal flow meter

In this type, the flow rate is determined by the change in thermal conductivity.

Swirl meter

In this type, the flow rate is determined from the temperature or the pressure oscillations.

Sonic meter

Flow rate is calculated with the help of noise level.

What are the advantages of an inferential flow meter?

  • The rotor speed is high
  • Low cost
  • The pressure loss is low

What are the disadvantages of an inferential flow meter?

  • Low accuracy
  • These flowmeters won’t suit low flow rates

What are the applications of inferential flow meter?

  • It can be used to measure and control the flow of gas
  • It can be used to determine the total flow and also the flow rates
  • It can be used to determine the performance of pumps, engines, valves, etc.

Velocity flowmeters

The velocity meter would calculate the stream velocity to measure the volumetric flow rate. There are many types of velocity flow meters like a paddle wheel, turbine, vortex shedding, and ultrasonic flow meters. So in this flow meter, the speed is calculated by measuring the speed in one or more points in the flow and the flow speed will be integrated into the flow area. This type of flow meter is widely used for generating applications of electric power, wastewater treatment, chemical industry, and refinery. These flowmeters can be directly connected to the pipelines so that they can operate linearly according to the flow rate. The velocity flow meter is widely used because of the fact that it can provide the direct measurement of fluid velocity. These flowmeters would operate properly for a constant flow, so in the case of the laminar regime the flow measurement would be inaccurate and this would be a big disadvantage of this type of flow meter. The inaccuracy of this meter is due to the fact that there will be a big difference in the fluid velocity at the wall and at the center of the pipe. The advantage of the velocity flow meters is fixed pressure loss at all flow rates. These flowmeters have good accuracy when used with low viscous liquids. It would also provide good operation over a wide range.

Volumetric flow meter

The volumetric flow meter would measure the flow volume and it would measure the volume of the fluid which passes per unit time. By using a volumetric flowmeter we can control the volume or mass of flow into a process. The volumetric flow is the measurement that shows a volume per unit time. The volume can be read as cubic feet, gallons, or liters. The time can be read per unit of time such as seconds, minutes, or hours. In most cases the volumetric flow is not measured directly instead of that an inferred measurement is carried out. The inferred measurement can be explained as it would do the measurement of some other variable like flow pressure or the fluid mechanical force and this measurement will be converted into required reading. The volumetric flow meter can calculate the flow by measuring the velocity at which it flows through the pipe. There are many types of volumetric flow meters such as ultrasonic meters, vortex meters, magnetic meters, turbine meters, etc.

Mass flow meter

This type of flow meter is used in a mass-related process and they are capable to measure the force that results from the acceleration of the mass. This type of meter would measure the force per time and not volume per time. These flow meters are also called inertial flow meters, the mass flow rate is the mass of the fluid that travels past a fixed point as per unit time. The mass flow meter is the right choice if precision is really important. This flow meter won’t measure the volumetric flow instead of that it would measure the mass flow in a pipeline. This type of flow meter can be used with fluids that would vary their viscosity according to the velocity. The major advantage of this type of flow meter is that it is reliable and can operate properly over a wide range of pressures and temperatures and also it has good accuracy. These flow meters are of two types and they are Coriolis and thermal type mass flow meter.

What are the requirements of a flow meter?

  • It should be able to calibrate
  • It must be capable to do the flow fluctuation integration
  • The flow meter should have the ability to merge with the piping system
  • Good accuracy is required
  • A good turndown ratio is needed
  • It should be less sensitive to dirt particles
  • Pressure loss must be low
  • No moving parts
  • It must be immune to corrosion

What are the applications of flow meters?

  • Medical
  • Food and beverage
  • Oil and gas transport
  • Petrochemical
  • Power generation
  • Pulp and paper industry
  • Distribution
  • It is used for mining and metallurgical applications too

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