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Differential Flowmeters: Simple Can Be Better

Dick Johnson, CONTROL ENGINEERING

Control EngineeringSeptember 1, 2000In 1502, Leonardo da Vinci noted, 'A river in each part of its length in an equal timegives passage to an equal quantity of water, whatever the width, the depth, the slope,the roughness, the tortuosity.' This simple observation provided the basis for simpleflow measurement-the principle of continuity.

However, it would not be until the 18 th century that the work of the likes of DanielBernoulli, Henri di Pitot, and Jean Charles Borda would develop into practicalapparatus for measuring flow. These basic instruments would evolve over time andendure into the 21st century.

Accordingly, dealing with flow situations does not always demand that controlengineers go the great length to find the most high-tech solution. Actually, many flowapplications in process industries still use simple differential pressure (dp) flowmetersto do the job.

Who's using what?

In a recent Control Engineeringsurvey of process industry users, of the eight mostpopular flowmeter types, orifice plate devices were the most widely used. Why dothese comparatively simple devices get the nod so often?

Orifice plate flowmeters along with most of the rest of the dp types share the followingadvantages.

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Selection processControl engineers who are involved in flowmeter selection must consider a minimumnumber of factors in the process. Although many applications have specializedrequirement, included in most are:

Compatibility with the process environment-fluid type, pressure,temperature, etc.;

Pressure loss incurred, level of swirl generation, or pulsation produced;

Accuracy of measurement required;

Long-term stability, repeatability, and frequency of calibration;

Durability/serviceability/maintenance requirements;

Compatibilty with existing equipment and control protocols;

Installation requirements;

Total cost of ownership; and

Adaptability to future needs.

For more information on flowmeter manufacturers, see flowmeter listings in theControl Engineering Buyer's Guide.

Types of differential pressure flowmeters

The most commonly encountered dp flowmeters are:

orifice plate

Venturi

flow nozzle

Pitot-static tube

elbow

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wedge

Other special designs of dp devices include:

'V-cone' flowmeter

spring-loaded variable aperture

laminar flow element

Dall tube

Elliot-Nathan flow tube

Dall orifice

Epiflo

c 2003, Reed Business Information, a division of Reed Elsevier Inc. All RightsReserved.