|David W. Spitzer|
I recently read an interesting e-mail exchange that was originated by a person who was involved with a large (over 30-inch) orifice-plate flowmeter. He stated that ISO 5167 required 20 diameters of straight run upstream, but that the installation only allowed for 10 diameters. Approximately 5 percent accuracy was required, and the writer wanted to know how much the shortened straight run would affect the flow measurement and if repeatability would be good.
The first response to the inquiry was that an insertion flowmeter might be more economical and still produce the desired accuracy. The responder also wanted to know if the fluid was a liquid or gas, leading me to believe that the orifice-plate flowmeter was in the process of design and had not yet been installed.
The originator responded by saying that this was a liquid application and that he could not change the technology because the flowmeter had already been purchased. In addition, the originator repeated that accuracy was not a big deal because the flowmeter would be used for “minimum flow control,” and reiterated wanting to know if the measurement would be repeatable and reliable with the shortened straight run.
I interpret “flow control” to mean that the flowmeter is one component in a flow control loop that includes a flowmeter, controller, and control valve. “Minimal” could mean either that the purpose of the control loop is to maintain a minimum operating flowrate, or that maintaining tight flow control is not really that important to the process. Given the relatively wide accuracy requirement, I suspect that the latter is more likely.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.