Flowmeters clean and dirty

CONTROL contributor David W. Spitzer provides a look from a process automation perspective at how magmeters and Venturi meters are vying with ultrasonics for clean and dirty water applications.

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FlowBy David W. Spitzer

WATER AND wastewater flowmeters have much in common. They are typically large with many applications that operate over a wide range of flows. These flowmeters are used in plants categorized as potable water, wastewater and industrial facilities, so they must cover a variety of applications for both clean and dirty water.

Water for potable water systems is typically drawn from a lake, river or wells and treated to remove impurities that make it suitable for drinking. Flowmeters are often used to measure the plant influent flow and are sometimes to calculate payments and taxes. Permitted influent flow limits are often enforced using these measurements. The water treatment plant may include additional procedures, such as chlorination and fluoridation. These and other treatment operations typically require flowmeters to measure water and/or chemical flows to ensure smooth operation and efficacy.

In addition, flowmeters are used throughout the water distribution system to measure the output of the treatment plant and the water flows to various parts of the system. These flowmeters are usually located in large pipes that can operate at relatively low flow rates for significant periods of time. The measurements are typically used to account for water in the system, but they may also be used to calculate payment from local water purveyors. Flowmeter performance is extremely important, especially when the flowmeter is used to calculate payments.

Wastewater flow measurements include measurements throughout the collection system that may be used for monitoring and/or billing purposes. The plant influent flow usually is measured to enforce permit limits and sometimes to calculate payments and taxes. Wastewater treatment plants usually are designed such that their hydraulics reduce the number of instruments required, however there are still a number of flowmeters used within the plants to measure intermediate flow rates and chemical additives. Additionally, flowmeters that can be made portable are often used for infiltration and inflow, sewer sizing and stormwater monitoring studies.

Industrial plant influent flow is usually measured and is often used to enforce permit limits. Flowmeters are installed throughout the plant water distribution system for monitoring and control in addition to calculating water balances. Plant effluent flowmeters generally are used to measure the water leaving the plant and are often used to enforce permit limits and calculate payments when the effluent is treated by another entity.

Common to the water, wastewater and industrial water industries is the need to measure water in large pipes over relatively wide flow ranges. In general, the water is relatively clean in industrial plant influent streams and potable water systems; whereas wastewater treatment plants generally contain dirty water streams. Industrial water streams may be clean and/or dirty, depending upon the plant.

Venturi Flowmeter

Venturi flowmeters commonly are used to measure potable water flow in large pipes.

Clean water streams can be measured using many flowmeter technologies. Venturi and magnetic flow measurement systems (See Figures 1 and 2) are commonly used to measure potable water flow in large pipes, but other technologies, including ultrasonic, are also in use. Insertion flowmeters can be applied to provide cost-effective (but lower accuracy) flow measurements in larger pipe sizes. Generally, dirty water streams are measured using magnetic flowmeters, especially in the wastewater industry, although Venturi meters are often found, especially in older installations. Open-channel measurements are often used where the pipe is not full or in specially constructed metering locations with a primary device like a weir or flume.

Water treatment and distribution systems are relatively large, so flowmeter accuracy is a prime concern because the flowmeters often are used to generate invoices for the various water purveyors, and even a 1% error can have a significant impact on the bill. In addition, operation of the water system requires flow measurements that can be used to account for the water entering the system. Failure to do so on an ongoing basis can result in water losses and can reduce the effective capacity of the water system. Typically, accounting for the distribution of the incoming water is accomplished using a series of water balances around various portions of the system. Increasing “unaccounted for” water losses can prompt work to improve the quality of flow measurement for balance and billing purposes.

Magnetic Flowmeter
Expanded diagnostics available with some magnetic flowmeters allow some in-service accuracy testing, although it is no substitute for taking the meter offline for service.

It isn’t always easy to measure these flows. Dick Furness, consultant at JDF and Associates, Tirley, Gloucester, U.K., says, “The structure of the flow in large pipes is not hydraulically similar to that found in small pipes. There are many reported instances where flow turbulence and swirl in pipe sizes 6-inch and under are attenuated within a few diameters of straight run upstream of the flowmeter. The influence of the pipe walls on the flow in the center of pipe decreases as pipe size increases until the center flow becomes effectively decoupled from the wall and its frictional effects. In one such documented case, a large flowmeter located 1000 diameters downstream of a complex pump injection point was affected by swirl. Recent work in Brazil, Mexico and Libya on large pipes indicates that the straight run requirements to mitigate the effects of bulk swirl from upstream fittings may be far from adequate.”

Figures 3 and 4 below show predicted and actual flows downstream of a single-bend piping configuration. Note that the higher fluid velocity near the pipe wall will result in measurement of an improper differential pressure and cause flow measurement error.

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