Flow Measurement

What is a velocity flow meter?

The flow rate can be calculated by measuring the velocity of the flow by using a velocity flow meter. These flow meters are used widely because they would provide the proper measurement of the fluid velocity. The velocity flow meters are really sensitive to the flow conditions. The operation of this flow meter is dependent upon the constant flow velocity so in case if the flow is not constant then the velocity flow meter won’t give any proper measurements.

What is the source of error in flow measurement?

  • The error could happen due to the variation in the fluid properties such as temperature, viscosity, and density
  • Meter orientation
  • Pressure
  • Damages in the moving parts
  • Flow disturbances which can be caused by valves, elbows, etc

What are the types of velocity flow meters?

Turbine flow meter

In this type of flow meter, a multiple-bladed rotor would be mounted with a pipe and it would be perpendicular to the liquid flow. The rotor of this meter would rotate when the liquid pass through it and this rotation can be used to determine the flow rate. The turbine flow meter is very economical, these meters have the disadvantage that it would clog in dirty water and it won’t work properly at low flow rates. A magnetic pick-up will be mounted on the turbine wheel and there will be a sensor to record the voltage pulses, this voltage pulse would be converted into a flow meter reading. We can use a turbine flow meter to do the measurement of natural gas and also the liquid flow

Advantages of turbine flow meter

  • Proper accuracy
  • Pressure drop is low
  • Maintenance and installation is really easy
  • It can be used with a wide range of flow rates
  • Quick response

Disadvantages of turbine flow meter

  • It could create a large pressure drop
  • It would need a certain straight length of pipe in the upstream and downstream

Electromagnetic flowmeter

The operation of this type of meter is based on the principle of electromagnetic induction. So according to this principle voltage will be created if an electrically conducting material passes through a magnetic-filed. So based on the movement of the conductor voltage will be created.

So in this measurement process, the liquid is the conductor and the energized coils which are outside the flow tube will create a magnetic field. We can determine the flow rate by calculating the produced amount of voltage. A pair of electrodes that are placed in the pipe wall would detect the voltage and the voltage is measured by a secondary element.

These flowmeters are mostly used to measure the process water flow and they can be used with most of the liquids and slurries the fluid which is to be measured should be conductive and that’s the major requirement. These flow meters are directly mounted to the pipe, the pressure drop across the meter will be the same as it is through the equivalent length of the pipe because of the fact that there is no flow obstruction and also there are no moving parts.

Advantages of electromagnetic flowmeter

  • Its measurement won’t be varied with the change in  temperature, pressure, density, and viscosity
  • Measurement of dirty fluids and corrosive fluids
  • Very small pressure drop
  • Less maintenance
  • Low cost
  • Forward and reverse flow can be measured
  • It can be used with pipes of varying sizes

Disadvantages of electromagnetic flowmeter

  • It can only do liquid measurements
  • The fluid should be conductive
  • Interference can be caused by electromagnetic noise
  • It won’t work in partially filled pipes
  • If the electrodes are wetted then it would affect the operation
  • Straight pipe-length is needed in the upstream and also in the downstream
  • The sludges or greasy fluids would flow slow and due to this there will be buildup and because of this accuracy of the measurement is affected

Vortex flowmeter

When a fluid moves by passing an object swirls will be formed an example for this will be river water passing the rock swirls are vortices. So in this type of flow measurement, a natural phenomenon is used. A blunt, non-stream line body will be placed in the stream of the flow in a pipe. So when the flow would pass this body then vortices will be created and fluid swirls will be formed when it flows downstream. So the vortices will be formed according to the flow velocity, which means that the flow velocity is directly proportional to the number of vortices which is formed. Due to the formation of the vortices pressure differential is created and this will be sensed and transmitted as voltage signals. The frequency of the voltage pulse will be proportional to the frequency of the vortices.

Advantages of vortex flowmeter

  • There is no moving part
  • Quick response
  • It can be used with a wide variety of chemicals, and slurries
  • It can also handle liquids with entrained particles and also viscous material
  • It is suitable to measure the liquid, gas, and steam.
  • Vortex flowmeters measurements are not affected by changes in density, temperature, pressure, and viscosity when the flow is turbulent

Disadvantages of vortex flowmeter

  • High cost
  • It needs to be mounted inline
  • The flow must be turbulent
  • Ineffective for slurries

Ultrasonic flowmeter

This type of flow meter would use ultrasonic waves to measure the flow velocity. The operating principle of this type of flow-meter is by considering the variation between the transmitted and the received sound waves and this measurement will be done as a result of the flow. The ultrasonic flow-meter would transmit sound waves through a stream by using a transducer and the returning sound wave would have the information about the flow rate of the steam. There are clamp-on ultrasonic flowmeters that can be placed outside the pipe and it would send signals through the pipe walls, so because of this feature, it can be used to measure the flow of large pipes.

Types of ultrasonic flow meter

Transit time flow meter

This type of ultrasonic flow meter is most commonly used for liquids and gases. This flow meter would measure the time difference between the ultrasonic pulse that is sent in the flow direction and the ultrasonic pulse which is sent on the opposite of the flow direction. This difference in time will be the velocity of the fluid.

Doppler flowmeter

This type of ultrasonic flow-meter can be used with slurries and gases, it requires sound-reflecting particles for the measurement and because of this, it can’t be used with clear liquids. The flow measurement is based on the fact that the sound wave would vary its frequency if they are reflected by moving particles or gases

Advantages of ultrasonic flow meter

  • There is no pressure drop because they are non-invasive
  • It would act quickly if there are any changes in the velocity of the fluid
  • It can measure slurries
  • It would remove the mechanical and electrical noise and because of that it can be used in many locations
  • It can handle flow in both directions
  • Installation is really easy
  • It can be used with different sizes of pipes and flow conditions
  • It can be used independently of temperature, pressure, density, electrical conductivity, and viscosity
  • Maintenance is less because there are no moving parts and also the sensor fouling is less
  • It won’t disturb the flow pattern

Disadvantages of ultrasonic flowmeter

  •  A steady flow is required so it won’t be able to handle rapid fluctuations
  • If there is excess solids or gases in the flow then it will affect the measurement

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