Flow Control: Technology information & new products for fluid handling engineers

September 2006

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Quiz Corner: Organizing Your Flow Investigation
Identifying the Root Cause of Measurement Error

David W. Spitzer, P.E.

The plant is having operational problems, and you are informed that an existing process gas flowmeter with pressure and temperature compensation appears to be measuring higher than it should. What instruments would you investigate to determine the cause of the problem?

A.   Flow transmitter
B.   Pressure transmitter
C.   Temperature transmitter
D.   Flow computer
E.   All of the above
F.   None of the above

Commentary
You might consider verifying the information that you received before you charge out and do a lot of unnecessary work. Challenge the person(s) making the claim to provide data and other information to support their claim because the situation could be such that the (political) pressure has led the person(s) to redirect attention away from their diligent efforts to resolve the problem. Further, even if you investigate and find nothing wrong with the instruments, you run a good chance of being blamed for the plant problem. You might feel honored to help at first — but this drill gets “old” after a few repetitions.

Presuming that the claim of an incorrect measurement is legitimate, you might consider investigating all of the listed instruments (Answer E). But which of the four instruments should be checked first? I don’t know that there is a correct answer, but here are some of my thoughts.

If there is a history of problems in this flow measurement system, the most likely offender should be investigated first. For example, if the pressure transmitter taps are known to plug occasionally, they should be checked. Assuming this is the first time the claim has been made and assuming that the flow measurement system functioned properly in the past, you may want to consider checking the transmitter calibrations and associated installation to ensure that they are in proper working order and calibrated. The output of the flow computer can then be verified for correctness at the current (or simulated) operating conditions.

More investigation will be required if no problems are found. The flowmeter calculations performed by the supplier should be found and checked for conformance with the application. The design calibrations for the pressure and temperature transmitters should likewise be verified to be reasonable for the process.

The inputs to the flow computer should be verified for accuracy and its calculations should be verified for correctness. Verifying the calculations may be the most difficult and time-consuming part of this process because some study may be required to learn which equations should be used and how the flow computer is programmed to implement them.

Additional Complicating Factors
If the flow measurement system is found to be operating properly, you may have to bring this information to the person(s) making the error claims. Don’t be surprised though if the plant problem has been resolved and the person(s) are not that interested in your results.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to... 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.

www.spitzerandboyes.com

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