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

January 2009

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Outside of the Process Line
The Effect of Ambient Conditions on Flow Measurement

David W. Spitzer, P.E.

The entire flow measurement system should be operated within its constraints in order to provide accurate flow measurements. This means that each component of the flow measurement system should be considered to ensure that it is operated properly.

For example, turbine flowmeters should be operated above a certain Reynolds number. Failure to do so will cause K-factor shifts that can result in measurement error. Some turbine flowmeters can use flow computer functionality to correct this error by calculating the Reynolds number in conjunction with a theoretical or experimental relationship between the Reynolds number and the flowmeter K-factor. However, the correction is not necessarily performed perfectly because there is an uncertainty associated with the parameters used to calculate the Reynolds number as well as an uncertainty associated with the K-factor calculation.

There are additional parameters that are typically ignored but should be considered. Process and ambient temperature can have a profound affect on the quality of measurement. For example, a flow transmitter designed to operate in ambient temperatures up to 60 C may operate properly in a boiler house during the winter when the temperature is 30 C. However, it can operate sporadically in the summer when the ambient temperature approaches 70 C in an enclosed space near the steam pipes in the middle of the afternoon on a sunny day.

Ambient conditions can also profoundly effect flowmeter operation. In one such instance, a flowmeter operated properly during the winter but would suddenly measure zero flow in the middle of the afternoon on warm summer days. The cause of the problem was traced to the sun warming the pipe to the extent that the liquid would vaporize and cause the flowmeter to stop operating. This was a major issue because zero flow would cause the reactor to trip. By the time the reactor was restarted a few hours later, the ambient temperature was lower and the flowmeter operated properly (until the next warm day).

Even the electrical power supply can cause problems, albeit not often. What if the power supply voltage is outside of its specifications or varies? Flowmeter operation and performance can suffer. Did you ever calibrate a low range differential-pressure transmitter outdoors in the sun and then notice a calibration shift a few minutes later when a cloud shaded the transmitter?

In general, we tend to concentrate on how the process affects flowmeter operation in the context of a given flowmeter technology. However, we should not forget that other seemingly trivial factors can influence the measurement.

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.

www.spitzerandboyes.com

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