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Why Measuring Flow is a Difficult Task: Damping the Signal (Part 2 of 2)

By David W. Spitzer and Walt Boyes

E-Zine June 2014

Click here to read "Introduction"
Click here to read Part 1

Another alternative is to implement damping in the flowmeter and in the control system. The amount of damping installed in the flowmeter should be relatively small to attenuate the higher frequency flow noise. The signal should be of sufficient quality to use for recording purposes and might still “paint” a small band on a chart. A small amount of additional damping may be added in the control system. An advantage of locating some damping in both locations is that the flow signal is responsive (but not overly so) and can be observed (and adjusted) in the control system. Disadvantages include additional adjustment and documentation associated with the two damping locations.

In control loops, flow noise can cause unwanted control valve movement because the controller flow noise contributes directly to the controller error calculation. Such error affects the controller output. An almost universal technique to reduce the effect of flow noise in flow control loops is to eliminate the use of the derivative (rate) in flow controllers. The derivative (rate) adjusts the control valve based upon the rate of change of the error. Because the process variable directly affects the error, flow noise (that generally changes relatively rapidly) can cause the controller output to change rapidly. In addition to process problems, this can result in rapid valve movement and excessive valve wear.

Click here to read "Profile Effects"

From Flow Control (August 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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