The Incredible Expanding Flowmeter

By David W. Spitzer, P.E.

It is toward the end of a shutdown and the process engineers determine that the steam flow to a piece of process equipment needs to be measured. The steam in the steam header is saturated and operates at approximately 200 PSIG and 200 C. There is no time to order a new flowmeter, so you go to the storage shed (every plant seems to have one), and find a two-inch, 316 stainless steel vortex shedder in apparent working condition.

A check of the flowmeter specifications reveals that the range of the instrument is suitable for the application and the temperature limit of the flowmeter will not be exceeded. What other temperature-related factor(s) should be addressed?
A. K-factor correction
B. Reynolds number correction
C. Viscosity correction

Commentary
Metals expand and contract as their temperature changes. In particular, stainless steel expands approximately 0.5 percent per 100 C. As a result of this phenomenon, flowmeters can expand and contract based on the operating temperature of the fluid.

This expansion is generally not a concern when the operating temperature is near the reference temperature of the flowmeter calibration. Common reference temperatures used by manufacturers include 15 C and 70 F (among others). However, significant bias errors can result when the operating temperature is significantly different from the reference temperature.

In this application, the vortex flowmeter may have a specified accuracy of 0.75 percent of rate. Examination of the manufacturer information for this flowmeter indicates that it was originally designed to measure the flow of water at ambient temperature. However, in the new steam application, it will operate at approximately 200 C, so the flowmeter will expand in this service. The amount of expansion will be approximately (200-15)*(0.5/100), or 0.925 percent. Therefore, the flow measurement will be biased by almost 1 percent — an amount that is greater than the stated accuracy of the flowmeter. In this application, the K-factor of the flowmeter (Answer A) should be recalculated to compensate for expansion.
Viscosity compensation (Answer C) and Reynolds number correction (Answer B) are generally minor concerns due to the low viscosity and high Reynolds numbers associated with gas and vapor flows.

Additional Complicating Factors
The steam header pressure will likely be lower than the nominal 200 PSIG, so the steam pressure in the pipe to the piece of equipment where the flowmeter is located will likely be lower as well. Because the pressure in the flowmeter will likely be less than 200 PSIG and will fluctuate somewhat, pressure compensation of the steam flow measurement will likely be necessary for accurate measurement. Be sure to install a tap for the pressure transmitter before the shutdown ends so you can install the pressure transmitter and program the compensation algorithms after startup.

About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. 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.

For More Information: www.spitzerandboyes.com