Assessing Flowmeter Needs First Will Save Time and Money Later
Waters universal importance makes the close monitoring of its production, distribution, and consumption imperative. Designers responsible for systems that monitor this precious fluid face a daunting task in the selection and installation of reliable, accurate meters. Steve Huth, president of Water Specialties Corp. of Porterville, CA, the worlds largest manufacturer of propeller meters, offers insights into meter design, installation, and application for engineers and specifiers who wish to avoid career-limiting pitfalls associated with flowmeter selection.
Tip No. 1: Know the relative strengths and weaknesses of available meter types.
The first step for the designer in simplifying and "goof proofing" the meter selection process is understanding that meters fall into two general categories: 1) meters whose measuring instruments are located in the process stream, 2) meters offering an unimpeded flow path. Common examples of these instruments include propeller meters and magnetic flowmeters, respectively.
Tip No. 1: Know the relative strengths and weaknesses of available meter types.
"The propeller meter is one of the most durable mechanical meters on the market and is available at a relatively low cost," Huth said. "The propeller is the meters only moving part and it is cone-shaped to harmlessly deflect sticks, leaves and other material. The propeller is ideally made of sturdy injection-molded plastic that resists bending and assures accuracy at high flow rates."
Tip No. 1: Know the relative strengths and weaknesses of available meter types.
The magnetic flowmeter is even simpler. Coils inside the meter create an electromagnetic field across the meters flow tube. Its indicated output is proportional to the velocity of the water passing through the induced field. It has no moving parts and its flow tube is completely free of obstructions.
Tip No. 1: Know the relative strengths and weaknesses of available meter types.
"The advantage of the magmeter is that its non-intrusive. If you look through a magmeter its just like looking through a pipe," Huth said. "Its a very good meter for pipes that contain a lot of debris or sludge, like those in wastewater treatment facilities or plants that handle industrial process fluids."
Tip No. 2: Buy the level of accuracy required by the application.
To avoid "gold plating" the system by paying a premium price for unnecessarily precise (and expensive) meters, the designer has to determine 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.
Tip No. 2: Buy the level of accuracy required by the application.
"In a production environment ?2 percent of indicated accuracy may by fine, but for water sales a range of ?1.5 percent may be mandatory," Huth said. "In other applications where water consumption is closely monitored, an accuracy approaching ?1 percent or better may be desired. If accuracy is the designers major consideration, he should know that the accuracy of a propeller meter is typically ?2 percent and the accuracy of the magnetic meter is ?1/2 percent."
Tip No. 3: Know your flow rate
In some applications there can be huge daily, hourly, or even minute-to-minute variations in the volume of water being measured; in other scenarios, flow rates remain relatively constant. Awareness of flow rates is vital to the designer in selecting the most accurate, least costly meter for his business. If widely varying flow conditions are anticipated, the designer must specify a meter that is accurate over the entire anticipated flow range.
Tip No. 3: Know your flow rate
"Magmeters have a top end of 34 feet per second (fps) and can still measure accurately at flow rates as low as 0.2 fps," Huth said. "On the top end, a propmeter is normally sized for 10 to 12 fps and is accurate down to 1 fps.
Tip No. 3: Know your flow rate
"On the other hand, if the designer knows that the application involves a fairly constant flow rate, he can order a propeller meter and have it factory calibrated to ?3/4 percent accuracy," Huth said. "Hes getting the accuracy of a magmeter at a much lower cost."
Tip No. 4: Learn whats in the water.
If accuracy were the only criteria, the designers search for the perfect flowmeter would stop there. However, water can vary widely in both content and consistency. Drinking water, ideally, is clean and free of debris; irrigation water can contain abrasives like sand or twigs; and industrial wastewater and sewerage can be thick and sludgy.
Tip No. 4: Learn whats in the water.
These are major factors that determine the life and efficiency of the meter. By taking the time to determine if the water to be measured is fresh, gritty, or even highly viscous, the designer can avoid creating a "career memorial" by selecting an accurate, low maintenance meter.
Tip No. 4: Learn whats in the water.
"If the designer wants to reduce his equipment costs by using a propeller meter in dirty water conditions, he must first make sure it can stand up to the job. For example, our propeller meter is durable in even the dirtiest water due to its unique water-lubricated, one-piece, ceramic bearing design," said Huth.
Tip No. 4: Learn whats in the water.
Large sewage handling facilities cant be shut down to just clean out plugged flowmeters. Therefore, magmeters are available in pipe diameters up to 48 inches.
Tip No. 4: Learn whats in the water.
"Since the magmeter doesnt project into the flow, its ideal for sewage, sludges, or extremely trashy water from industrial processing plants," Huth said. "It is also a very low cost meter to operate since there are no associated energy costs due to pumping head losses or flow element maintenance."
Tip No. 5: Location, location, location.
Since flowmeters should be located in non-turbulent flow areas to obtain maximum accuracy, the designer must field check the meters proposed location for the presence of upstream flow disturbances including els, tees, and control valves. Ideally, the meter should be located upstream from these disturbances.
Tip No. 5: Location, location, location.
If this is not possible, a meter should be placed no closer than five pipe diameters (5 x the pipes 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.
Tip No. 5: Location, location, location.
"Most of the time when a client complains about low accuracy, well take a look at their pipe layout and determine that the flowmeter and a valve or other source of turbulence needs to be flip-flopped. This is an expensive fix with larger line sizes," Huth said.
Tip No. 5: Location, location, location.
Even magnetic flowmeters can be influenced by turbulence and are not completely exempt from the "five diameter" rule.
Tip No. 5: Location, location, location.
"Some manufacturers have run turbulence tests on magmeters located only two pipe diameters from valves. I dont think that water this close has settled down enough to give an accurate reading," Huth said. "In this case, I think the magmeter would still be fairly accurate, but not down to its ?1/2 percent rating."
Tip No. 6: Replace only the equipment that really needs replacing.
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 instrumentation. To assure compatibility, this otherwise functional instrumentation 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 necessitates a line shut down or reduced plant production. If the old instrumentation can be retained, then meter replacement costs and lost production time can be greatly reduced.
Tip No. 6: Replace only the equipment that really needs replacing.
"Todays most commonly used signal output is 4-20 mA, but many older systems have different requirements. Designers can save money, downtime, and inconvenience by purchasing meters which are compatible with any type of current or past chart recording or instrumentation system," Huth said. "For example, we can design signal outputs for recording devices that are no longer made. So rather than make the customer discard his old recording system, we can build a transmitter with a signal to operate it."
Tip No. 7: Make sure theres power.
Water meters are often placed in remote areas that may lack power connections. When specifying water meters, the designer must check for the availability (or lack) of power.
Tip No. 7: Make sure theres power.
"If power is not readily available, the best choice may be a standard prop meter," said Huth. "They are mechanically driven and can give total flow readings without external power. Self-powered electronic prop meters are also available with batteries that have an eight-year service life."
Tip No. 8: Remember Murphys Law.
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 its only accessible to an operator the size of Shaquille ONeil.
Tip No. 8: Remember Murphys Law.
"The meter may be in the best possible position, flow-wise, but this doesnt mean much to the operator who has to read it above a ceiling, or twelve feet in the air," Huth said. "We probably see this on a monthly basis."
Tip No. 8: Remember Murphys Law.
When, in spite of the best planning, a meter must be put in an inaccessible location, or when design parameters change (or have been overlooked), the designers only hope for an inexpensive solution may rest on the meters ability to accept retrofits.
Tip No. 8: Remember Murphys Law.
"For the most flexibility in flowmeter location, remote readouts are available that can be ordered with (or retrofitted to) inaccessible meters," Huth said. "These readouts can communicate with the meter and can be conveniently mounted up to 300 feet from the main sensor."
