Flow sensing know-how

Jeff Deane -- Control Engineering, 9/1/1999

Flow measurement is one of the 'big four' need-to-know process parameters (others are temperature, pressure, and level).

Closed-channel flowmeters are categorized by their operating technologies and fall into the following categories:

Differential Pressure (DP)
This most common flowmeter technology includes orifice plates, venturis, and sonic nozzles. DP flowmeters can be used to measure volumetric flow rate of most liquids, gases, and vapors, including steam. DP flowmeters have no moving parts and, because they are so well known, are easy to use. They create a nonrecoverable pressure loss and lose accuracy when fouled. Flow measurement accuracy depends on accuracy of the pressure gage.

Positive Displacement (PD)
PD flowmeters measure the volumetric flow rate of a liquid or gas by separating the flow stream into known volumes and counting them over time. Vanes, gears, pistons, or diaphragms are used to separate the fluid. PD flowmeters provide good to excellent accuracy and are one of only a few technologies that can be used to measure viscous liquids. However, they create a nonrecoverable pressure loss and have moving parts subject to wear.

Turbine
Fluid passing through a turbine flowmeter spins a rotor. The rotational speed of the rotor is related to the velocity of the fluid. Multiplying the velocity times the cross-sectional area of the turbine provides the volumetric flow rate. Turbine flowmeters provide excellent measurement accuracy for most clean liquids and gases. Like PD flowmeters, turbine meters create a nonrecoverable pressure loss and have moving parts subject to wear.

Electromagnetic ('mag meter')
Velocity of a conductive liquid can be determined by passing it through a magnetic field and measuring the developed voltage. Velocity times area yields volumetric flow rate. Magmeters have no moving parts and do not obstruct the flow stream. They provide good accuracy with conductive liquids flowing into a full pipe. Magmeters can be used to measure the flow rate of slurries.

Ultrasonic
Transit-time sound velocity or Doppler frequency shift methods are used to measure the mean velocity of a fluid. Like other velocity measuring meters, volumetric flow rate is determined by multiplying mean velocity times area. Besides being obstructionless, ultrasonic flowmeters can also be non-intrusive if their sonic transducers are mounted on the outside of the pipe. Good to excellent accuracy can be obtained for almost all liquids, including slurries. Pipe fouling will degrade accuracy.

Vortex Shedding
The frequency of vortices shed from a bluff body placed in the flow stream is proportional to the velocity of the fluid. Again, velocity times area gives the volumetric flow rate. Vortex flowmeters provide good measurement accuracy with liquids, gases, or steam. They have no moving parts and are fouling tolerant. Vortex meters can be sensitive to pipeline noise and require flow rates high enough to generate vortices.

Thermal
Mass flow rate can be determined by measuring the temperature rise of a fluid ('heat gain') or the temperature drop of a heated sensor ('heat lost'). Thermal flowmeters have no moving parts or orifices and provide good gas measurement accuracy. Thermal is one of only a few technologies that measure mass flow rate; it is also one of the few technologies that can be used for measuring gas flow in large pipes, ducts, or stacks. Measurement of the fluid temperature is also provided by thermal technology.

Coriolis
Fluid flowing through a vibrating flow tube causes a deflection of the flow tube proportional to mass flow rate. Coriolis flowmeters can be used to measure the mass flow rate of liquids, slurries, gases, or vapors. They provide excellent measurement accuracy. However, the thin wall of the flow tube necessitates careful material selection to minimize corrosion or erosion effects. Measurement of fluid density or concentration is also provided by Coriolis technology.

An accurate comparison of technology differences is the first step in flowmeter selection for a given application. Once completed, device selection is aided by detailed comparison of product specifications/features and vendors' service and support policies.

Jeff Deane is director of engineering at
Fluid Components Intl., San Marcos, Calif.