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Process/Instrument Suitability of Flowmeters

Every flow-measuring instrument exploits a physical principle to measure the flow rate of fluid stream. Understanding each of these principles as they apply to different flow-measurement technologies is the first and most important step in properly applying a suitable technology to the measurement of a particular process stream flow rate. The following table lists the specific operating principles exploited by different flow measurement technologies:

 

  Flow measurement technology  
  Operating principle     Linearity  
  2-way  flow  
Differential pressure

Fluid mass self-acceleration,

potential-kinetic energy exchange

 
 (some)
Laminar Viscous fluid friction  linear  yes
Weirs & flumes

Fluid mass self-acceleration,

potential-kinetic energy exchange

 Hn  no
Turbine (velocity)

Fluid velocity spinning

a vaned wheel 

 linear  yes
Vortex von K´arm´an effect  linear  no
Magnetic Electromagnetic induction  linear  yes
Ultrasonic Sound wave time-of-flight  linear  yes
Coriolis Fluid inertia, Coriolis effect  linear  yes
Turbine (mass) Fluid inertia  linear  (some)
Thermal

Convective cooling,

specific heat of fluid
 linear  no
Positive displacement Movement of fixed volumes  linear  (some)

A potentially important factor in choosing an appropriate flowmeter technology is energy loss caused by pressure drop. Some flowmeter designs, such as the common orifice plate, are inexpensive to install but carry a high price in terms of the energy lost in permanent pressure drop (the total, non-recoverable loss in pressure from the inlet of the device to the outlet, not the temporary pressure difference between inlet and vena contracta). Energy costs money, and so industrial facilities would be wise to consider the long-term cost of a flowmeter before settling on the one that is cheapest to install. It could very well be, for example, that an expensive venturi tube will cost less after years of operation than a cheap orifice plate1.

In this regard, certain flowmeters stand above the rest: those with obstructionless flowtubes. Magnetic and ultrasonic flowmeters have no obstructions whatsoever in the path of the flow. This translates to (nearly) zero permanent pressure loss along the length of the tube, and therefore. Thermal mass and straight-tube Coriolis flowmeters are nearly obstructionless, while vortex and turbine meters are only slightly worse.

 

1This is not always the case, as primary elements are often found on throttled process lines. In such cases where a control valve normally throttles the flow rate, any energy dissipated by the orifice plate is simply less energy that the valve would otherwise be required to dissipate. Therefore, the presence or absence of an orifice plate has no net impact on energy dissipation when used on a process flow throttled by a control valve, and therefore does not affect cost over time due to energy loss.

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