01 July 2004
Flow only goes 2 ways
By Paul Murrill
More than 75% of all transmitter sales are for measuring flow.
As you might expect with such a huge market, flow-meters come in all imaginable types, shapes, and sizes.
All flow measurement devices operate using one of two approaches—either energy additive or energy extractive.
The oldest is the energy extractive approach. Within this approach the differential-pressure-producing flowmeters such as the orifice plate, the flow nozzle, and the venturi tube are the most common.
The orifice meter is probably the most commonly found and used flowmeter today. It has two fundamental elements: the orifice plate and the differential-pressure transmitter.
The orifice plate itself is a thin plate with a hole bored in it. It acts to restrict—extracting energy—the flow of fluid through the pipe at the very point where the meter sits.
As the fluid flows along the pipe through the orifice plate, the fluid's velocity must increase because the cross-sectional flow area decreases, from the diameter of the pipe to the diameter of the hole. The energy necessary to increase the fluid velocity comes by reducing the static line pressure. By measuring this change in static line pressure with a differential-pressure transmitter, which has taps on the upstream side (high-pressure side) and the downstream side (low-pressure side) of the small hole in the orifice plate, one can infer the flow rate.
Both the flow nozzle and the venturi tube work on the same principle as the orifice meter. Only the restriction shape and flow pattern are different.
Behind the byline
Paul Murrill is an ISA member. The content for this piece comes from his book Fundamentals of Process Control, ISA Press. Nicholas Sheble (nsheble@isa.org) edits the Control Fundamentals department.
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