Differential Pressure Flowmeters |
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Differential pressure (DP) flowmeters are among the most traditional of flowmeters. Their large installed base means that they still exert a powerful force on the flowmeter market. Two new studies cover significant aspects of the DP market: The World Market for Pressure Transmitters, 5th Edition, covers DP flow transmitters. The World Market for Primary Elements, 3rd Edition, covers the primary elements used in DP flow. A DP flowmeter consists of a differential pressure transmitter integrated with a primary element. In the past, pressure transmitter companies sold DP transmitters and users ordered their primary elements separately. Now companies are selling their DP transmitters already integrated with a primary element, such as an Annubar or an orifice plate. When a DP pressure transmitter is integrated with a primary element and has the capability of calculating flowrate based on difference in pressure, it is a DP flowmeter. The DP flowmeter market appears to be holding its own, and possibly gaining ground, when other traditional technology flowmeters such as turbine meters are losing market share. There are several reasons for this:
Measuring DP flow — not so straightforwardDetermining the market size for a flowmeter can be fairly straightforward when it is clear what is included in the flowmeter. Most flowmeters have a sensor and a transmitter, along with an amplifier, transducer, and display. Typically, the transmitter generates one or more output signals that represent flowrate, and can represent other values such as pressure or temperature. DP flowmeters, however, are different because of the complexity of the measurement. A complete statement of the DP flowmeter market includes:
DP flowmeters require a DP transmitter to provide a reading of the difference in line pressure upstream and downstream from the constriction. DP flowmeters also require some type of constriction in the flowstream, created by a primary element. The primary element is essential to making the DP flow measurement. But often end-users order the primary element separately from the DP transmitter, and from a different supplier, so tracking primary element sales values can be a challenge. Primary elements are an important part of the picture, not only because of their cost, but also because they greatly add to the versatility of DP flow measurement. Types of primary elements include orifice plates, Venturi tubes, averaging Pitot tubes, flow nozzles, laminar flow elements, and wedge elements. There are many different types of orifice plates, for example, that have different shaped orifices with different locations. Flow nozzles are widely used for steam flow measurement. And Venturi tubes are suited for measuring high-speed gas flows, especially in the larger pipe sizes. The costs associated with these products should be included in the total value of the DP flowmeter market. When the value of primary elements is included in the DP flowmeter market, it is among the largest of any flowmeter market in terms of revenues. When to choose a DP meterDP meters excel at measuring clean liquids, steams, and gases when:
Many end users of DP transmitters today are facing a dilemma. Should they upgrade to new-technology flowmeters or stick with tried-and true-DP meters? Of course, some end users at large plants may choose to upgrade some of their DP meters, and stick with DP technology elsewhere in the plant. Instead of replacing all their DP meters, they may make this decision on a case-by-case basis. How they workDP flowmeters rely on a constriction placed in the flow line that creates reduced pressure in the line after the constriction. They also require a means to detect the difference in upstream vs. downstream pressure in the flow line. While this can be done with a manometer, today’s DP flowmeters use differential pressure transmitters that sense the difference in pressure, and then use this value to compute flowrate. The theory behind DP flowmeters is that energy is conserved when flow passes across or through a constriction in the pipe. A more exact statement of this theory is known as Bernoulli’s principle, which states that the sum of the fluid’s static energy, kinetic energy, and potential energy is conserved across a constriction in the pipe. One form of Bernoulli’s principle for incompressible fluids is as follows: V2/2g + z + P/ρg = constant V is the velocity of the fluid, g is the acceleration constant, P is pressure, ρ is density, and z is the elevation head of the fluid. The equation of continuity formulates a relation between fluid flowrate and velocity for fluids that are incompressible. It can be formulated as follows: Q = A1 x v1 = A2 x v2 Q is volumetric flowrate. The product of A1, the amount of an incompressible fluid that crosses the area at point 1 over some set amount of time, times v1, the velocity at point 1, equals the product of A2, the amount of fluid that crosses the area at point 2 over the same set amount of time, times v2, the velocity at point 2. DP flow in ancient Rome?The history of DP flow measurement goes back to at least the 17th century, though the measurement of flow using nozzles actually goes back to Roman times. At the beginning of the 17th century, Torricelli and Castelli arrived at the concepts that underlie differential pressure measurement today — that flowrate equals velocity times pipe area, and that the flow through an orifice varies with the square root of the head. In 1738, Bernoulli developed his famous equation for flowrate calculation. The development of primary elements for use in measuring DP flow also began about this time. Pitot presented a paper on the use of the “Pitot tube” in 1732. Venturi published his work on the Venturi principle for measuring flow in 1797. However, Venturi’s work was not developed for commercial application until 1887 when Clemens Herschel used Venturi’s work to develop the first commercial flowmeter based on it. In 1898, Herschel published his paper, “The Venturi Water Meter.” Max Gehre received one of the first patents on orifice flowmeters in 1896. The first commercial orifice plate flowmeter appeared in 1909, and was used to measure steam flow. Shortly thereafter, the oil and gas industries began using orifice plate flowmeters due to ease of standardization and low maintenance. Here to stayDP flow is important because of its large installed base, its versatility, and because it is one of the most studied and best understood flow technologies. DP flowmeters can be used to measure gas, steam, and liquid flows. They have been around for more than 100 years, and have been studied by groups such as the American Gas Association and the American Petroleum Institute. While some DP flowmeters are being replaced by new-technology meters, customers who prefer to stay with the tried and true are continuing to order DP flowmeters. And companies such as Emerson, Honeywell, Foxboro by Schneider Electric, Siemens, and ABB are continuing to bring out new and improved pressure transmitters. DP flowmeters are here to stay, and will continue to provide stiff competition to new-technology flowmeters. And the market might be even stronger if suppliers would look more seriously at primary elements. What the world needs now is some new and improved primary elements to go with those highly stable and accurate pressure transmitters. Only when suppliers work to improve primary elements as they have worked to improve pressure transmitters will we know the true potential of the DP flowmeter market. StudiesTwo new studies cover significant aspects of the DP market: The World Market for Pressure Transmitters, 5th Edition, covers DP flow transmitters. The World Market for Primary Elements, 3rd Edition, covers the primary elements used in DP flow.
We also published these classic studies: The World Market for DP Flow Transmitters (September 2007) The World Market for Differential Pressure (DP) Flowmeters and Primary Elements (January 2007)
Articles on DP flowClick here for a list of articles on DP flowmeters and pressure transmitters.
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