Flow Meter Tutorial - Differntial Pressure - Flow Control Ju

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Differential Pressure Run New Search by: Steven Rogers Pages: 32-37; July, 1999 Differential Pressure is the oldest and most widely used method of measuring flow in a pipe. According to recent market studies1,2, in 1997, about half of all industrial flow measurements were made with some type of differential producer and an accompanying DP measurement. The next most common flow technology is used in less than 15 percent of flow measurements3. Over the past few years, DP flow has been threatened by newer flow technologies. Vortex, electromagnetic, Coriolis, and ultrasonic flowmeters experienced significant growth since reliable meters have become available. But if new technologies are so much better, why have DP flowmeters remained so popular? If the newer technologies are as good as we hear, why haven't the orifice plate, nozzle, venturi, and averaging pitot tube markets dried up?

•A •C •F •F •G •L DP flow continues to be so popular not because the flowmeter industry is entrenched in tradition, or process engineers are resistant to change, it is likely • M because of value. There are a number of reasons why -- application flexibility, •P standardization, accuracy, rangeability, flexible permanent pressure loss, meter •P interchangeability, and ease of calibration. •P •R F

Application Flexibility With a wide variety of primary elements available, DP flowmeters can be engineered to fit a larger percentage of flow measurements than any other flow technology. Other flowmeters have restrictions on line size, process fluids, or temperature, making them impossible to use for all flow measurements.

•S •S •S •S •T •T •V

One of the most common pieces of equipment to be found in any process plant is a boiler. A boiler normally requires flow measurements to be made on large combustion air inlet lines, smaller natural gas supply lines, high-temperature steam lines, and condensate returns. Even something as common as an industrial boiler would be difficult to instrument using any other single flow technology. Another flexibility advantage unique to DP flow is the ability to customize the flow range without changing the meter size. By changing the bore size of an

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orifice, the flowmeter maximum can be adjusted over a range of 17 to one, without changing the pipe size or the DP measuring device. This advantage is common in applications where the flow rate changes seasonally over a wide range. It is also useful in experimental applications and pilot plants where process lines are frequently converted for use in a variety of applications.

Standardization The use of orifices, nozzles, and venturis is standardized by at least three organizations: American Society of Mechanical Engineers (ASME), International Organization for Standardization (ISO), and American Gas Association (AGA). The industry standards play an important role in establishing flowmeter accuracy and reducing metering costs. In order to meet the accuracy requirements of the market, all flowmeters rely on some type of empirical testing to improve accuracy. For vortex, electromagnetic, Coriolis, and ultrasonic flowmeters, this testing is done for each unit separately in a flow laboratory. Due the large body of data available on orifices, nozzles, and venturis, equations have been developed, which describe the empirical correction factor for all primary elements manufactured in accordance with the standard. Because consumers can rely on these industry standards to guarantee the accuracy of DP flowmeters, they are seldom calibrated in a flow lab. The accuracy of the flowmeter is verified by the standard rather than a flow lab. This results in a significant cost reduction for the consumer. Standardization serves as the basis for many custody transfer measurements. All of the installation, calculation, and uncertainty variables are outlined in the industry standard. The buyer and seller can agree that the metering is to be done per a specific industry standard rather than spelling out all of the details, reducing costs.

Accuracy It has been a common misconception that the technology of primary and secondary elements has not seen any new advances since the pneumatic DP transmitter of the 1950s. Most concerns expressed are about orifice plates because they are, by far, the most common primary element. A good portion of consumers can recite that the accuracy of an orifice flowmeter system is one to three percent of full scale. That may have been true in 1955, but technology has come a long way since then. Today, a system accuracy of 0.5 to one percent of flow rate is possible in most applications. This can be achieved using a multivariable transmitter that is capable of full dynamic flow calculation. For a complete discussion of how this accuracy is accomplished, see the Wiklund/Engelstad paper in the references.4 Another popular perception is that the inaccuracy comes from the orifice itself, not the secondary instrumentation. In fact, the orifice plate is the most accurate, uncalibrated primary element available. According to ASME or ISO, the uncertainty of the orifice plate is 0.6 to 0.75percent uncalibrated.5,6 The AGA standard gives 0.44 to 0.72 percent accuracy for an uncalibrated orifice at pipe Reynolds numbers over 20,0007. The orifice itself is certainly accurate enough to achieve an overall flow accuracy in the range of 0.5 to one percent

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of flow rate. DP flowmeters also have an inherent accuracy advantage over flowmeters that measure velocity. In gas flow measurements, changes in fluid density with static pressure and temperature variation are a significant source of flowmeter error. In order to save metering costs and reduce complexity, many gas flow measurements are not compensated for these changes and density is assumed to be constant. This assumption causes additional error for any velocity-based measurement (vortex, magnetic, turbine, or ultrasonic), DP flowmeter measuring mass, or standard volume flow. The advantage of DP flowmeters is that the error caused by density variation will only be half that of a meter that measures velocity. This is because the density term is under the square root in the flow equation.8,9

Rangeability Rangeability is usually stated as the ratio of maximum to minimum flow. There are two definitions of maximum flow, which are commonly used in specifying the rangeability of a flowmeter. It is important to understand the difference between the two methods of defining the maximum flow. It can mean the difference between a flowmeter that can measure the flow range of interest, and one that cannot. The first way to define maximum flow is the maximum flow rate that the flowmeter can measure. This is how the newer flowmeters specify rangeability in their sales literature. The other way to define the maximum flow is the maximum flow rate that actually occurs in a particular application. This is how most DP flowmeters are specified. A distinction is critical because the meter maximum is often two to three times the application maximum. Vortex, Coriolis, magnetic, and ultrasonic flowmeters are designed to measure maximum flow rates at 20 to 30 ft/sec. in liquids. The optimum economic flowing velocity for most liquids is about five to six ft/sec.10, and they rarely flow over 10 ft/sec. This means that a flowmeter with a 20 or 30 to one rangeability specified from meter maximum is likely to measure less than a 10 to one flow range in a typical process. Meter maximum flows are similarly high for gas flows, although there is much more variation in the maximum due to the wider variation in gas density. The only way to achieve the advertised rangeability in a typical application is to reduce the pipe size, which increases the velocity. Reducing the pipe size also increases installation costs and permanent pressure loss, which may not be acceptable for the user. For this reason, flowmeter rangeability should be considered from application maximum, not the specified maximum flow. It is also important to understand how the maximum to minimum flow ratio relates to percent of the flow range. Figure 2 shows the relationship between rangeability and percent of maximum flow. Although 20 to one rangeability seems like twice as much as 10 to one, it actually only measures five percent more of the flow range, down to five percent of maximum, rather than 10 percent. Since a meter with a high rangeability can significantly increase cost, care should be taken to specify no more rangeability than will actually be required in a particular application.

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The orifice plate has traditionally been regarded as having a maximum rangeability of three to one or four to one. As with accuracy, this rangeability limitation is perceived to come from the orifice plate itself, not the secondary instrumentation. This limitation was a result of the DP transmitters of the day and is long outdated. With a properly selected orifice and a modern DP transmitter, a rangeability of five to one to as high as eight to one is certainly possible. With a multivariable measurement and full dynamic compensation, typical rangeability is from six to one to 15 to one with better than one percent of rate accuracy over the entire range11. Combining this multivariable rangeability with the ability to interchange orifice plates, a flow range of 200 to one can be measured without changing the pipe size or the transmitter.

Permanent Pressure Loss Permanent pressure loss is always considered by vendors when comparing their flowmeters, yet permanent pressure loss may not have any impact at all on cost savings or process efficiency. When considering the permanent pressure loss of different flowmeters, the first question to ask is "Will the pressure be lost somewhere else in the process?" In some applications, there is a regulator or pressure regulating valve downstream, or a flow control valve where line pressure will drop to maintain the desired flow or pressure. In these cases, there is no energy saved. If a flowmeter with a lower permanent pressure loss is used, the permanent pressure loss through the regulating device must be higher to maintain the set point. For applications where there is a real energy savings in lowering the permanent pressure loss, DP flow offers a variety of solutions that can meet the requirements. Venturis and averaging pitot tubes have a permanent pressure loss among the lowest available. Only magnetic flowmeters and ultrasonic flowmeters have lower losses. The permanent pressure loss of the orifice is not among the lowest available, but it is also not as high as often portrayed. Besides, the permanent pressure loss of an orifice plate is adjustable. It can be made as low or as high as required, although a lower permanent pressure loss generally means lower rangeability. In days gone by, practically all orifice plates were sized to produce 100 in H2O DP. This was done for convenience of calculating square roots, for ease of calibration, and because higher DP's were necessary to be measured accurately by transmitters of the day. This practice resulted in a permanent pressure loss of 46 to 96 in water (1.7 - 3.5 psi). With advances in smart transmitter technology and characterization to enhance accuracy, sizing to 100 in water is no longer necessary. With today's transmitters, orifices can be sized to produce 20 to 50 in water DP and still achieve better accuracy than in the past. These lower differential pressures will cause only nine to 48 in water (0.3 - 1.7 psi) permanent pressure loss, an acceptable loss in most applications.

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For other flowmeters, a reduction in line size is often recommended by flowmeter suppliers in order to achieve the desired rangeability. In these cases, it is important that the permanent pressure loss through the reducer, expander, and the length of smaller pipe be included when comparing the permanent pressure loss of such a meter to one that does not require a line size reduction. The permanent pressure loss due to a line size reduction is almost never included in the sizing calculations for a flowmeter. Many times, the loss through the reduced section is as much as the loss through the meter itself.

Meter Interchangeability All DP flowmeters share one piece of equipment in common -- differential pressure measuring devices. In industrial flow measurements that device is almost always a DP transmitter. This DP transmitter is completely interchangeable across all flowmeters of approximately the same DP range. By stocking just a few different ranges of a transmitter, spares for all of the DP flowmeters in the facility are on hand. These spares can also be combined with the spares for many DP level and other pressure measurements to reduce inventory and maintenance training costs even further. By contrast, the transmitter portion for any of the newer flow technologies is specific to the manufacturer and sometimes to a certain model by that manufacturer. The primary element of DP flowmeters can yield significant inventory savings. General primary elements are reliable and long lasting because of their simplicity. The useful life of a venturi, nozzle, or averaging pitot tube is practically endless. Orifice plates are reportedly prone to wear over a short period of time, but excessive wear is normally due to misapplication. Dirty, abrasive, or corrosive processes are probably better suited to another primary element. Orifice plates may eventually wear out. In a clean process, this takes a very long time. If they do wear, it causes only an accuracy shift, not a loss of the measurement. Because of their popularity and simplicity, orifice plates can be purchased and shipped in very short order, reducing inventory requirements. For critical processes, spare orifice plates can be kept on hand at a much lower cost than Coriolis, magnetic, vortex, or ultrasonic primary sections.

Calibration Almost all instruments need to be calibrated to maintain product quality, ISO certification, or custody transfer verification. Although newer flowmeter technologies may require calibration less frequently than a DP flowmeter, calibration of a DP flowmeter is by far the least expensive. Because the accuracy of primary elements is verified by industry standards, a few dimensional measurements or even a visual inspection can verify that they are still within the specified accuracy. The measurements of DP, static pressure, and temperature can be verified by equipment found in most instrument shops, often without even removing the instrument. For a more thorough calibration the transmitter can be sent back to the manufacturer and re-calibrated. Even a

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thorough, NIST traceable calibration normally costs a fraction of the price of a flow lab calibration. All of this is not to say that other flowmeters do not have their advantages. All flowmeters have unique advantages that make them well suited for particular applications. Other flowmeters should be selected on their own merits, not because DP flow is misrepresented. DP flow is certainly one of the oldest methods of measuring flow, but with modern innovations it certainly isn't obsolete. References World Sensor Technology Assessment: Pressure, Flow and Level, Frost & Sullivan, 1996. Ultrasonic Flowmeter Worldwide Outlook: Market Analysis and Forecast through 2001, Automation Research Corporation, August 1997. World Sensor Technology Assessment, ibid. Wiklund, David E. and Engelstad, Loren M., Improving Flow Measurement by Real-Time Flow Calculation in Transmitters Having Multiple Process Variables, Advances in Instrumentation and Control vol. 50, part 2, International Society for Measurement and Control, pp. 653-662, Oct. 1-6, 1995. ASME/ANSI MFC-3M-1989, Measurement of Fluid Flow in Pipes Using Orifice, Nozzle, and Venturi. ISO 5167-1, Measurement of fluid flow by means of pressure differential devices, Part 1 Orifice plates, nozzles and venturi tubes inserted in circular cross-section conduits running full, Dec. 15, 1991. AGA Report No. 3, Orifice Metering of Natural Gas and Other Related Hydrocarbons, Part 1 General Equations and Uncertainty Guidelines, Third Edition, Oct. 1990, American Gas Association. ASME MFC-3M, ibid. AGA Report No. 3, ibid. Capps, R.W. Selecting an Economic Optimum Pipe Size, Flow Control, Apr. 1998. Wiklund, ibid. About the Author Steven B. Rogers has a BS degree in Mechanical Engineering from Brigham Young University and is a member of ASME and ISA. He is currently an application engineer at Rosemount Inc. (Eden Prairie, MN; [email protected]) specializing in flow measurement and providing companywide technical assistance. Subscribe | Email | Site Map | Search | FAQ | Advertise

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