Every now and then I stumble across information that raises my eyebrows and makes me think of that classic adage: believe only half of what you see and nothing of what you hear. Some time ago, I read a series of responses to a problem involving a Parshall flume used for regulatory compliance that was measuring low. The questioner wanted to know how to calibrate the flume. The suggestions from a number of sources included:
• Rebuilding the entrance box
• Installing an ultrasonic level transmitter
• Performing proper level measurements for free flow or submerged conditions
• Measuring flow with a Pitot tube
• Using a linear relationship between its level meas. and flow
• Using dilution techniques to determine the actual flowrate
• Making sure the level sensor is clean.
Most of these solutions were likely a reflection of the problems those that offered them had had with their respective flumes. Some quick comments to some of these responses might read:
• No details about the entrance box were given, so how do you know the entrance box needs to be rebuilt?
• No details about the level transmitter were given, so how do you know the level transmitter needs to be replaced?
• How do you characterize the flow profile through the flume at different flowrates and levels so as to properly locate the Pitot tube and determine the relationship between its level measurement and flow?
• When did the relationship between level and flow for a Parshall flume change from a nonlinear to a linear relationship?
About half of these suggestions are reasonable, such as performing proper level measurements, checking with dilution techniques, and making sure the level sensor is clean. But what ever happened to the concept of reading the instruction manual and checking the installation to ensure the flume is installed correctly? What is the accuracy of the flume flow measurement system? Are engineering specifications available? What were the original and current operating conditions? How accurate is the methodology used to determine the actual flowrate? Why do you think the measurement is in error? Is the level transmitter functioning properly? What is its maintenance history?
The answers to these and other similar questions will usually illuminate a path to follow. That path may not be straight or fast, but it should provide logical steps that lead to a resolution.
While some of the suggestions provided in this case were believable, they were incomplete … As always, the best way to solve a performance problem is through a full investigation of the entire installation, rather than by making hasty conclusions based on limited information.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
