Flow Measurement

Working Principle of Transit Time Flowmeter

Working Principle of Transit Time Flowmeter

Working Principle of Transit Time Flowmeter1
  • The working of the transit time flow meter is based on the time difference between flow sound propagation intervals occurring at upstream flow and downstream.
  • Transit Time Flowmeter works by calculating the transit time of ultrasonic sound pulses transmitted from one transducer to another transducer through the fluid.
  • The received rate of the ultrasonic pulses is then converted into the flow rate.
  • Depending on the type of mounting either Vertical or Horizontal, the signal may intersect the pipeline only once, twice, or four times.
  • The flow meter evaluates the time duration between transmitted and received signals.
  • These Ultrasonic signals are propagated from the upstream transducer and then the downstream transducer functioning as transmitters and receivers.
  • The transit time flow meters measure the flow rate of fluid nevertheless the direction of the flow.
  • The transit time of the signal in the flow direction is always greater than the transit time against thefluid flow.
  • This transit time Flowmeter can accurately compute the flow rate by comparing the difference with precision timing circuits.
  • Because this ultrasonic signal is forced to cross the pipe to determine the average of the flow profile.
  • So compensation between laminar flow and turbulent flow is automatic.

Mounting of Transit type Flowmeter

  • The choice of V, Z, or W mounting method depends on the application and diameter of the pipeline.
  • V-Mounting is the most common method in all process industries..
  • Z-Mounting is used for larger pipe diameters or for fragile signal applications.
  • W-Mount is most suitable for or smaller pipes.
  • Generally, “V-mode” installation is recommended installation method because this method gives the highest trade-off between propagation time values, and signal strength when compared to remaining “W-mode” and “Z-mode”.
  • These transducers can be installed on both vertical and horizontal pipelines.
  • The fluid flow inside the pipe must be full.

Describe various types of Transmit time flow meters

Based on their installation these transit time flow meters are classified as

Working Principle of Transit Time Flowmeter2

1. V – Type Transmit time flow meters

  • In a V-shaped transit time ultrasonic flow meter, the receiver, and transmitter are fixed on the same side of the pipeline so that this ultrasonic wave from one meter travels down to the pipe and bounces back to another meter in the form of a V-shape.
  • In this case, both flow meters are installed one near the other with some spacing between them.

2. W – Type Transmit time flow meters

  • The Two Transmit time flow meters are fixed on a pipeline so that this ultrasonic wave from one meter to another meter propagates in the form of a W-shape.
  • Here, an ultrasonic wave bounces at the bottom side of the pipe, back to the top side of the pipe, and then travels and again bounces at the bottom side of the pipe before reaching the receiver.
  • In this case, both flow meters are installed one near the other with some spacing between them.

3. Z – Type Transmit time flow meters

  • The two Transmit time flow meters are fixed on a pipeline so that this ultrasonic wave from one meter to another meter propagates in the form of a Z-shape.
  • In this case, one flow meter is installed at the top of the pipeline and another one at the bottom of the line.

What are the factors to be considered while selecting a transit time flow meter?

1. Safety: If the flow rate is not controlled, Temperature and Pressure may reach dangerous levels.

2. Product Integrity: Flow plays a role in ensuring the right amount of blended materials is constantly present.

3. Efficiency: A process’s efficiency can be assessed by comparing the amount of each input that went into a product to the amount generated.

4. Process Variable Control: The flow rate is measured and controlled during energy transfer applications.

5. Understanding the Process: Before specifying a flow meter it is important to understand the entire process. Involve plant operators during designing and discuss the issues of maintenance, and calibration.

6. Process Media:

  • When selecting a flow metre, the temperature and pressure conditions of an application must also be addressed.
  • Temperature and pressure characteristics, like flow rate, determine the meter’s material capabilities to withstand the effect of thermal energy and the pressures imposed by the flowing fluid.

7. Process Parameter:

  • When selecting a flow metre, the temperature and pressure conditions of an application must also be addressed.
  • Temperature and pressure characteristics, like flow rate, determine the meter’s material capabilities to withstand the effect of thermal energy and the pressures imposed by the flowing fluid.

8. Output Indication:

  • Whether flow measurement data is needed locally or remotely and how the meter will translate the flow rate into a usable data form need to be considered in the selection process.
  • These are established by the purpose of the data, such as billing, regulatory reporting or monitoring, or process control.
  • The remote indication can be transmitted via analog, digital, or shared through protocols such as HART, FOUNDATION Fieldbus, or Modbus.

When or where to use transit time flow meters?

  • Transit time flow metres are the industry standard for measuring cryogenic liquids at temperatures as low as -300°C, and they are also used in molten metal flow metering.
  • Transit time flow meters are usually used in domestic and various industrial sectors to measure the volume or mass of a liquid or gas, depending on the application.
  • Regarding flow meter installation for gas or liquid, the flow meter should always be placed in a position where it is filled with fluid, even if there is no flow. There should be a way out for the second undesirable phase as well.

Advantages of Transit Time Flow Meters

  • Installation is easy.
  • Easy to calibrate.
  • High resolution.
  • Applicable to higher flow velocities.
  • No moving parts to wear out.
  • No pressure drop.
  • Functions better for clean and ultrapure fluids.
  • No leakage potential.
  • Insensitive to temperature, viscosity, density, or pressure variations of flowing fluid.

Disadvantages of Transit Time Flow Meters

  • Its performance may endure pipe wall interference.
  • These Flow meters do not operate on dirty, bubbly fluids.
  • Accuracy and Repeatability problems may arise if there is a space gap between the pipe wall and flowing fluid.

Applications of Transit Time Flow Meter

  • The clean water flow rate in water treatment plants.
  • Hot or cold water in power plants.
  • Pure and ultra-pure liquids in all pharmaceutical, food, and beverage industries.
  • Liquid to medium crude oils in the petroleum refining industry.
  • Water distribution systems are used in agriculture and irrigation.

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