In 1502, Leonardo da Vinci noted, 'A river in each part of its length in an equal time gives 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 simple flow measurement-the principle of continuity.

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

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

In a recent Control Engineering survey of process industry users, of the eight most popular flowmeter types, orifice plate devices were the most widely used. Why do these comparatively simple devices get the nod so often?

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

• Simple construction;
• Relatively low cost;
• No moving parts;
• Low maintenance;
• Wide applicability to fluid types;
• External transmitters; and
• Ease of instrument and range selection

In addition, dp flow-meter technology is simple and easy to understand. Extensive product experience and performance databases exist for most types, application and selection guides are abundant, and standards and codes of practice are readily available.

There are some distinct disadvantages to this technology. Flow rate is a nonlinear function of the differential pressure. Dp flowmeters also have limited low flow rate rangeability with normal instrumentation

Head meters, as they are often called, consist of a primary element, a restriction in the flow line that induces the head, and a secondary element that connects to the differential head and measures it as a means of determining flow rate. Primary elements come in many forms. The most common are orifice plates, flow nozzles, and venturi tubes. Others include the pitot tube (the familiar device used for determining airspeed since the very beginnings of aviation) not commonly used in industrial flowmeters, pipe elbows, wedges, and V-shaped cones. There are other proprietary variations on many of these elements. In short, most resulting flowmeters take their name from their primary element, i.e. orifice plate flowmeter, V-cone flowmeter, etc.

Orifice plate flowmeters are usually concentric with their associated pipe. However, eccentric, quadrant, and segmental designs are available. They produce the best results when measuring turbulent flows of clean liquids. Accuracy depends on installation, orifice area ratio, and physical properties of the liquid being measured. Orifice plates are subject to erosion, which will produce readings on the low side for a given flow rate, and must be installed in a straight pipe run. Cost of these units is low and does not increase significantly with pipe size.

Venturi tube devices can handle large flows with a low-pressure drop and good accuracy. Higher in cost than other dp flowmeters, they are usually limited to high flow rate applications. Venturis can handle most liquids, including those with high (but nonsticky) solids content. They are not recommended for highly viscous liquids.

In many applications, flow nozzles are a good compromise between orifice plate flowmeters and Venturi tubes. At high flow velocities, they can accommodate approximately 60% greater flow than an orifice plate with the same pressure drop. Flow nozzles can handle large solids, high velocity, high turbulence, and very high temperatures.

Control engineers who are involved in flowmeter selection must consider a minimum number of factors in the process. Although many applications have specialized requirement, 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 the Control Engineering Buyer's Guide.