Vol. 5 No. 5 Subject: Flow Measurement News Date: 8-5-03 From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ ******************************************* This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field. For More information on our complete line of flowmeters go to: http://www.mccrometer.com ******************************************* McCrometer Newsletter - July 2003 Special Notice - McCrometer is pleased to announce the addition of guest authors for our monthly newsletter. This month we feature Walt Boyes and David W. Spitzer, PE. Walt has more than 25 years of experience in the controls and instrumentation industry both for process instruments and environmental controls, and David has over 25 years experience in many facets of instrumentation, process control, electrical, and utility engineering, including conceptualization, estimating, design, construction, startup, operation, troubleshooting, and teaching. Together, they own and operate Spitzer and Boyes LLC, serving both the profit and not-for-profit business sectors. Flowmeter Startup Blues By Walt Boyes and David W. Spitzer, PE Starting up flowmeters is often a gamble. You never know what may come up. There are dozens of horror stories. Flowmeters have been installed immediately between two elbows, because that was the only place the contractor could make it fit. Flowmeters have been installed upside down. Flowmeters have been installed backwards, or insertion flowmeters have been installed sideways. In most cases, you don't get to see the flowmeters until after they've been purchased and installed. This means that you are coming into the project at the very last minute. If you can get involved at the design stage, you can make sure that some of the more difficult problems are reduced. You can, for example, make sure that the flowmeter specified will work for the service. You can make sure that the drawings show the flowmeter installation correctly and that notes show things the contractor must not do, like installing a multijet water meter upside down, or sideways so that the display can only be read from below the pipeline. You can make sure that there is always sufficient straight run upstream and downstream from the installation point of the flowmeter. If you get involved early enough during the installation, you can check the ongoing installation of the flowmeters against the design drawings, and spot where the contractor has deviated from the drawings, either in procuring a different flowmeter or by installing the flowmeter where it was convenient for the contractor, instead of where the drawings showed it to go. Some of the more serious problems occur well before the startup stage. Significant problems can be caused by improper selection of the flowmeter. For example, one of the most common selection problems is the selection of a flowmeter for cooling tower makeup water. Many building cooling towers are relatively small, and have a 2" inlet. So, many times, a 2" cold-water meter is selected, with a pulse contact head, as the makeup water flowmeter. On the face of it, this makes some sense. However, cold-water meters are velocity-limited by design. Most cooling tower makeup lines are into open discharge, and the control valve is usually situated before the flowmeter in the pipeline. The valve slams open, the water hits the meter's internal assembly, and the bearings of the meter are shot. Why do people keep doing this? What needs to happen is to use a 2" magnetic flowmeter, or a 2" turbine or propeller flowmeter, which will stand the shock of the water surge. Since repeated water surge isn't good for the turbine or propeller meter, either, the correct solution includes some re-piping. While a 2" magnetic flowmeter will easily handle 300 gallons per minute, a 2" cold-water meter will not handle more than about 200 gallons per minute. But the cost differential between a pulse-equipped cold-water meter and a magnetic flowmeter, or a turbine flowmeter, or even a propeller meter, is large. So, what can be done, at the startup stage, to solve this problem? Move the flowmeter upstream of the control valve so it is always full of water Replace the snap-opening solenoid valve with a slow-to-open, slow-to-close valve Size the flow control valve so that it limits the flow to less than the rated maximum flow rate of the meter. Another typical design issue is the use of flowmeters where there is not adequate straight run, either upstream or downstream of the meter. Designers tend to try to use the same flowmeter throughout a project. There are valid reasons for this, among which are ease in installation of common designs, ease in maintenance, lower spare parts requirements, and ease in operation when all the flowmeters are the same. The problem, however, is that while it is often possible to use all the same flowmeters, the performance of these flowmeters may vary widely depending on where they are installed. A paddlewheel flowmeter may approach its rated accuracy of 2% of full scale when installed with 20 upstream straight run diameters and 10 downstream. Installed even 10 diameters after a butterfly valve, or after an elbow or two, and the meter may be accurate to 20% of full scale. But at startup, it isn't going to be possible to change the straight pipe length in the metering run. In the design phase, it often isn't possible to change the piping configuration. After all, if there isn't enough space, there isn't enough space. That's why it is incumbent on the designer to use the flowmeter that best fits the installation: In tight quarters, use flowmeters like magnetic flowmeters, V-Cones®, swirlmeters, fluidic meters, and positive displacement meters because they don't require long distances of straight pipe run. At startup, you are very severely limited when you face a flowmeter that you know is simply not going to be accurate as installed. Replace the flowmeter with one that requires limited straight run Install a flow conditioner, which may, itself require straight run, but less than the flowmeter without the flow conditioner Try to come up with a calibration that will reduce the error of the meter, if you can. The bad news is that sometimes you may have to throw up your hands and walk away. Sometimes you just can't make it work right at startup. ******************************************* About the Authors After receiving his MSEE in optimal control, David W. Spitzer PE obtained over 25 years experience in many facets of instrumentation, process control, electrical, and utility engineering, including conceptualization, estimating, design, construction, startup, operation, troubleshooting, and teaching. He has worked for United States Steel, Mobay Chemical, and Nepera Chemical and has consulted for numerous other companies worldwide. Walt Boyes has more than 25 years of experience in sales, sales management, marketing, and product development in the controls and instrumentation industry both for process instruments and environmental controls, including Executive Committee experience in three companies. He is an expert at business transformation for small to medium-sized companies. He provides strategic planning, organizational development, business re-organization and electronic business re-engineering services for companies from roughly $5 million to roughly $100 million in revenues per year. Spitzer and Boyes LLC serves both the profit and not-for-profit business sectors. To find out more about McCrometer's line of flow meters, including the V-Cone(r), Wafer-Cone(r), the V2-Municipal(tm), Water Specialties Propeller Meter(tm), Mc(r) Propeller, Ultra Mag(r), and SK(r) Variable Area Meters, please contact McCrometer at 951-652-6811 or visit our website at www.mccrometer.com ******************************************* Copyright 2003 McCrometer If you have comments or suggestions for newsletter topics you would like to see, please address them to: Bob Peters or Richard Steven mailto:newscomment@mccrometer.com ********************************************************************************* Know anyone looking for a steady dose of flow measurement information? ********************************************************************************* To subscribe send e-mail to mailto:news-request@mccrometer.com with a keyword SUBSCRIBE in the subject field. 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