How to measure the flow in piping system

Flow Mesaurement

Flow measurement is required for the determination of the quantity of a fluid, gas, liquid, or steam, that passes through a particular point. The quantity to be determined may be volume flow rate, mass flow rate, flow velocity, or other quantities related to the previous three.

There exist a number different types of flowmeters based on different operating principle. Some of them are:

  • Coriolis flowmeter
  • Differential Pressure flowmeters (Elbow , Flow Nozzle, Orifice, Pitot Tube , Venturi ,Wedge)
  • Magnetic flowmeters
  • Target flowmeters
  • Positive Displacement (PD) meters ( Nutating Disc, Oscillating Piston , Oval Gear , Roots )
  • Thermal flowmeters
  • Turbine flowmeters
  • Ultrasonic flowmeters (Doppler, Transit time)
  • Variable Area flowmeters (Movable vane, Rotameter, Weir)
  • Vortex flowmeters

Differential Pressure Flowmeters

Laminar flow meters use the pressure drop created within a laminar flow element to measure the mass flow rate of a fluid. A laminar flow element takes turbulent flow and separates it into thin channels. By reducing the diameter of the flow channel and affecting velocity, the flow becomes laminar through the channels. The decrease in pressure, or pressure drop, across the channel is measured using a differential pressure sensor. The D/P across the restriction has a square root relation with the flow.

How differential pressure flowmeters work
Differential pressure flow meters use Bernoulli’s equation to measure the flow of fluid in a pipe. Differential pressure flow meters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping routes the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow. This technology accounts for about 21% of the world market for flowmeters.

Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter.

Different geometries are used for different measurements, including the orifice plate, flow nozzle, laminar flow element, low-loss flow tube, segmental wedge, V-cone, and Venturi tube.

Coriolis flow Meter

The Coriolis flow meter uses the Coriolis Effect to measure the mass flow of a fluid. The fluid travels through single or dual curved tubes. A vibration is applied to the tube(s). The Coriolis force acts on the fluid particles perpendicular to the vibration and the direction of the flow. While the tube is vibrating upward, the fluid flow in forces down on the tube. As the fluid flows out of the tube, it forces upward. This creates torque, twisting the tube. The inverse process occurs when the tube is vibrating downward. The amount of twist in the tube is directly related to mass flow of the fluid through the tube.

Some common applications for Coriolis flow meters include:

• Applications with low to high flow rates
• Fiscal custody transfer
• Challenging liquid, gas and slurry applications

Principles of Coriolis flow meter
The basic operation of Coriolis flow meters is based on the principles of motion mechanics. As fluid moves through a vibrating tube it is forced to accelerate as it moves toward the point of peak-amplitude vibration. Conversely, decelerating fluid moves away from the point of peak amplitude as it exits the tube. The result is a twisting reaction of the flow tube during flowing conditions as it traverses each vibration cycle.

How coriolis meter works
A Coriolis meter is based on the principles of motion mechanics. When the process fluid enters the sensor, it is split. During operation, a drive coil stimulates the tubes to oscillate in opposition at the natural resonant frequency. As the tubes oscillate, the voltage generated from each pickoff creates a sine wave. This indicates the motion of one tube relative to the other. The time delay between the two sine waves is called Delta-T, which is directly proportional to the mass flow rate.

Applications Coriolis flow meters
Coriolis flow meters are used in a wide range of critical, challenging applications, in industries including oil and gas, water and wastewater, power, chemical, food and beverage, and life sciences.

Ultrasonic Flow Meters

Ultrasonic flow meters use sound waves to measure the flow rate of a fluid. Doppler flow meters transmit ultrasonic sound waves into the fluid. These waves are reflected off particles and bubbles in the fluid. The frequency change between the transmitted wave and the received wave can be used to measure the velocity of the fluid flow. Time of Flight flow meters use the frequency change between transmitted and received sound waves to calculate the velocity of a flow

 An ultrasonic flow meter (non-intrusive Doppler flow meters) is a volumetric flow meter which requires particulates or bubbles in the flow. Ultrasonic flow meters are ideal for wastewater applications or any dirty liquid which is conductive or water based. Ultrasonic flow meters will generally not work with distilled water or drinking water. Aerations would be required in the clean liquid applications. Ultrasonic flow meters are also ideal for applications where low pressure drop, chemical compatibility, and low maintenance are required.

How ultrasonic flowmeters works
Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.

Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe. At no flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave. When the fluid moves faster, the difference between the upstream and downstream times increases. The transmitter processes upstream and downstream times to determine the flow rate. They represent about 12% of all flowmeters sold.

Ultrasonic flowmeters do not obstruct flow so they can be applied to sanitary, corrosive and abrasive liquids. Some ultrasonic flowmeters use clamp-on transducers that can be mounted external to the pipe and do not have any wetted parts. Temporary flow measurements can be made using portable ultrasonic flowmeters with clamp-on transducers. Clamp-on transducers are especially useful when piping cannot be disturbed, such as in power and nuclear industry applications. In addition, clamp-on transducers can be used to measure flow without regard to materials of construction, corrosion, and abrasion issues.

 However attractive, the use of clamp-on transducers introduces additional ultrasonic interfaces that can affect the reliability and performance of these flowmeters. In particular, if not properly applied and maintained, attenuation of the ultrasonic signal can occur at the interfaces between the clamp-on transducers and the outside pipe walls, and between the inside pipe walls and the fluid.Ultrasonic flowmeters are available in sizes to 72 inches and larger.

How to use ultrasonic flowmeters
Ultrasonic flowmeters are commonly applied to measure the velocity of liquids that allow ultrasonic waves to pass, such as water, molten sulfur, cryogenic liquids, and chemicals. Transit time designs are also available to measure gas and vapor flow. Be careful because fluids that do not pass ultrasonic energy, such as many types of slurry, limit the penetration of ultrasonic waves into the fluid. In Doppler ultrasonic flowmeters, opaque fluids can limit ultrasonic wave penetration too near the pipe wall, which can degrade accuracy and/or cause the flowmeter to fail to measure. Transit time ultrasonic flowmeters can fail to operate when an opaque fluid weakens the ultrasonic wave to such an extent that the wave does not reach the receiver.

 Applications ultrasonic flowmeters

The industries in order of higher to lower are oil and gas, water and wastewater, power, chemical, food and beverage, pharmaceutical, metals and mining, and pulp and paper.

Variable Area Flow Meters

Variable area flow meters, or rotameters, use a tube and float to measure flow. As the fluid flows through the tube, the float rises. Equilibrium will be reached when pressure and the buoyancy of the float counterbalance gravity. The float's height in the tube is then used to reference a flow rate on a calibrated measurement reference.

The various factors which govern the selection of a particular flow meter are as follows:

  • The most important ones are fluid phase (gas, liquid, steam, etc.) and flow condition (clean, dirty, viscous, abrasive, open channel, etc.)
  • The second most important factors are line size and flow rate (They are closely related). This information will further eliminate most sub models in each flowmeter technology.
  • Fluid parameters like density (specific gravity), pressure, temperature, viscosity, and electronic conductivity. The state of fluid (pure or mixed) and the status of flow (constant, pulsating, or variable) also needs to be considered.
  • Environment temperature, the arrangements (e.g., corrosive, explosive, indoor, outdoor), the installation method (insertion, clamped-on, or inline), and the location of the flowmeter, the maximum allowable pressure drop, the required accuracy, repeatability, and cost (initial set up, maintenance, and training).

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