How to select flowmeters for water-based applications

Meter design, installation, and application can make selecting flowmeters a challenge. Here are some recommendations that can make selections easier from Steve Huth, president of Water Specialties Corp., a manufacturer of propeller meters. Many of the common-sense tips also apply to other liquid flow applications.

1. Know the merits of available meter types. Meters fall into two general categories: those with measuring instruments located in the process stream and those offering an unimpeded flow path.

2. Buy the level of accuracy required by the application by determining a minimum acceptable system accuracy. A meter used at a well head may not need to be as accurate as a meter that monitors sales or is used for billing purposes.

“In a production environment, 2% of indicated accuracy may be fine, but for water sales a range of 1.5% may be mandatory,” explained Mr. Huth. “In other applications where water consumption is closely monitored, an accuracy approaching 1% or better may be desired.”

3. Know your flow rate. In some applications there can be huge daily, hourly, or even minute-to-minute variations in the volume of liquid being measured; in other scenarios, flow rates remain relatively constant. Accuracy can vary over the flow range.

4. Learn the water. Water varies widely in content and consistency. These are major factors that determine the life and efficiency of the meter.

5. Location. Since flowmeters should be located in nonturbulent flow areas to obtain maximum accuracy, field check the meter’s proposed location for the presence of upstream flow disturbances including els, tees, and control valves. Ideally, the meter should be located upstream from disturbances. If this is not possible, it should be placed no closer than five pipe diameters (5 x the pipe’s I.D.) downstream from the nearest source of turbulence. Failure to properly locate the meter can result in reduced accuracy, reduced profits, and large reworking costs.

“Most of the time when a client complains about low accuracy, we’ll take a look at the pipe layout and determine that the flowmeter and a valve or other source of turbulence needs to be flip-flopped. This is an expensive repair with larger line sizes,” adds Mr. Huth.

6. Replace only what’s needed. Since meters are immersed in the process flow stream, they usually wear out and require replacement long before chart recorders and related field- or control room-mounted instrumentation. To assure compatibility, this otherwise functional instrument is often replaced when a newer or different brand of meter is specified. This adds immensely to the overall cost of meter replacement and often requires shutting a line or reducing plant production.

7. Ensure there’s power. Unfortunately, water meters are often placed in remote areas that may lack power connections. However, some meters can operate for years on batteries.

8. When planning fails, think retrofit. Even when meters are properly located in areas with perfect flow dynamics, things can still go wrong. The perfect location may not be useful if it’s only accessible to an operator the size of Shaquille O’Neil. “For the most flexibility in flowmeter location, remote readouts are available that can be ordered with (or retrofitted to) inaccessible meters,” explains Mr. Huth. Some displays can be mounted 300 feet from the main sensor.

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