OPUS

Specialty Item Flow Measuring Devices

Image provided by Watts Water Technologies

Flow-measuring devices are used in a piping system to determine accurate flow rates. Depending on their use, they might be called a flow meter, flow indicator, flow gauge, flow rate sensor, or liquid meter; and they can be constructed of metal (e.g., stainless steel, brass, aluminum), or plastic (e.g., PVC, nylon, PVDF). These meters use some method of determining the velocity of the fluid. Velocity and the internal cross sectional area of the pipe are used to determine flow.

Differential pressure devices are based on Bernoulli’s Principle, which states that the pressure drop across some constriction is proportional of the square of the flow rate. Differential pressure flow meters use laminar plates, a venturi, nozzles, or an orifice to cause a constriction, and measure the pressure drop across that constriction to determine flow. These meters are rugged and work well with clean liquids. They have an inherent disadvantage of restricting flow to create a pressure drop. They can, however also measure temperature and pressure.

Electromagnetic flow meters (megameters) operate on the principle of Faraday’s Law, which states that voltage is induced in a conductive liquid (like water) as it flows at a right angle through a magnetic field. The induced voltage is proportional to the speed of the fluid. Megameters create the necessary magnetic field using an electromagnet and determine flow rate using the induced voltage to determine velocity, along with the internal area of the meter. These meters feature no moving parts and a low pressure drop.

Rotameters (or variable area flow meters) consist of a tapered vertical tube, a float, and a calibrated scale. Water flow upward through the tube and causes the float to rise. The tube area increases with height, so the more water flow, the higher the float. The position of the float is read on the scale to determine flow rate.

Turbine or Paddle Wheel flow meters place a turbine or paddle wheel in the path of the flowing liquid. The liquid causes the turbine or wheel to spin, and the rate of spin is proportional to velocity of the liquid. Strainers should be installed upstream to prevent debris in the fluid from damaging the components.

Ultrasonic flow meters use sound waves to measure the velocity of the fluid. Because sound waves pass through solids, these meters can be mounted external to the pipe with zero pressure drop. Further, because everything is external, there is no fouling or damage to any components. With no moving parts, these are reliable meters.

Vortex flow meters require an obstruction (bluff body) to be installed in the flow stream. This causes downstream vortices which form alternately on either side of the bluff body. The vortices form high and low pressure zones that oscillate and frequencies that are proportional to the fluid’s velocity. These meters have no moving parts and are accurate over a long period of time.

Coriolis mass flow meters are based on the Coriolis effect. That is, the deflection of moving objects from the perspective of a rotating reference frame (think of throwing a ball while on a merry-go-round – the ball will appear to fly in a curved path although it is actually flying straight). The Coriolis effect is why hurricanes form a spiral. These meters cause a Coriolis acceleration and measure the forces generated, which are proportional to the velocity of the fluid. Coriolis meters use this technique to measure flow, but they can also measure density, mass flow, and temperature. Thus, they can be used to measure the total energy being transported through the pipe. These meters have very high accuracy.

 



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