Going with the Flow

McMillan and Weiner Continue Their Interview With Ram Ramachandran. They Discuss What Flow Sensor Works Best in Various Applications

By Greg McMillan, Stan Weiner

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Greg: Flow measurements offer opportunities for feed-forward, identification of valve backlash and stick-slip, on-line metrics and diagnostics, rejection of pressure disturbance, linearization, modeling and tracking down disturbances as outlined in "Secondary Flow Loops Offer a Primary Advantage." Flow is by far the most manipulated process input for controlling process variables, optimizing process efficiency and setting throughput.

Stan: To get insights on what flow sensor works best in various applications we continue our interview with Ram Ramachandran the principal engineer at Systems Research Int'l. Inc.

Differential head measurements using differential pressure [DP] transmitters tend to have the lowest accuracy and rangeability due to the square root relationship; the highest noise due to sensitivity to velocity profile, and the highest maintenance cost due to impulse lines. If the velocity profile and density are constant (no opening and closing of valves or changes in composition), the repeatability is good. While the hardware cost may be low, the lifecycle cost is high from installation and winterization of impulse lines and maintenance to prevent line plugging, and to keep a constant phase and fluid composition in the lines. Furthermore, the increased process variability is a hidden cost. Most people don't realize density affects even the volumetric flow through the Bernoulli equation. Then there is the poor turndown leading to loss of flexible manufacturing to minimize inventories, and match fluctuating energy costs and market demands. I stopped using them [differential head measurements] in the 1980s. Why are differential head meters used?

Ram: Many of these installations are legacy systems, still with us in large chemical plants and refineries in the United States. For high temperatures and large pipe lines in mature continuous processes (e.g., refining, chemical intermediates and pigments and dye manufacturing) with infrequent and relatively limited production rate changes, the differential head meters have been traditionally used. Elbow flowmeters with purged taps have been used when the process fluid is too hazardous, too corrosive or erosive, or pressure drop can't be tolerated, and the accuracy required is only 5%. For much better accuracy and rangeability and less noise and sensitivity to piping configuration, Venturi tubes can be used with a minimal permanent pressure drop. Flow nozzles are a lower cost solution to Venturi tubes, particularly for large lines. The lowest cost flow sensor is an averaging pitot tube. Some manufacturers offer a retractable design.

Greg: If a wireless DP transmitter is used with an averaging pitot tube, the flow measurement is portable, enabling the exploring and prototyping of process control improvements, diagnostics and metrics. The rangeability can be extended to 10:1 if a second low-range transmitter is added, and measurement noise is incredibly low from strict attention to piping straight run requirements. These meters can justify a more permanent and accurate installation.

Stan: What can you do to achieve greater accuracy, rangeability and maintainability in large lines?

Ram: Turbine meters can provide incredible accuracy (e.g. 0.05%) and rangeability (e.g. 50:1). Bearing life is a problem for non-self-lubricating liquids, such as anhydrous ammonia. EMI has a sleeve bearing design that enables use in steam and gases. Insertion turbine meters and vortex meters are used for large pipelines. Vortex meters are the lower maintenance alternative, but at the expense of 5x deterioration in accuracy (e.g. 0.25%) and rangeability (e.g. 10:1) and greater straight run requirements. Repeatability stops at low flow. Improvements in bluff body and sensor design and temperature compensation have greatly increased vortex meter performance since their introduction. However, the meter coefficient is a still a function of kinematic viscosity.

Greg: Where is volumetric displacement used for flow measurement?

Ram: The oil and gas industry uses the technology of pulsating disc flowmeters for their huge pipe lines. The measurement of reciprocating pump stroke is used for toxic and lethal fluid flow measurement. Rotating disc flowmeters are used for natural gas and water flow measurement. Since these tend to read low as they age, there is an initiative to replace these with non-mechanical meters for large users.

Stan: Magmeters offer quite a performance advantage at low flows since their accuracy is a percentage of rate rather than a percentage of full scale like most other flowmeters. Magmeters also have a very short straight run requirement and can measure reverse flow. What do you see as some of the application considerations?

For much better accuracy and rangeability and less noise and sensitivity to piping configuration in flow measurement, Venturi tubes can be used with a minimal permanent pressure drop. Flow nozzles are a lower-cost solution.

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