Process/Instrument Suitability

Chapter 19 - Introduction to Continuous Pressure Measurement

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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:

technology & principle & & flow \cr & potential-kinetic energy exchange & $\sqrt{\Delta P}$ & (some) \cr & potential-kinetic energy exchange & $H^n$ & no \cr & a vaned wheel & linear & yes \cr & Coriolis effect & linear & yes \cr & specific heat of fluid & linear & no \cr

Flow measurement Operating Linearity & 2-way Differential pressure Fluid mass self-acceleration,     Laminar Viscous fluid friction linear yes
Weirs \ flumes Fluid mass self-acceleration,     Turbine (velocity) Fluid velocity spinning     Vortex von K\'arm\'an effect linear no
Magnetic Electromagnetic induction linear yes
Ultrasonic Sound wave time-of-flight linear yes
Coriolis Fluid inertia,     Turbine (mass) Fluid inertia linear (some)
Thermal Convective cooling,     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 plate.

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.