Many things can go wrong and cause a flowmeter to measure improperly. Typical errors cause the flowmeter to measure incorrectly by a certain percentage of flow rate, a fixed flow rate or a certain amount of signal. The problem can usually be fixed or corrected once it is identified and analyzed. This process typically requires specialized knowledge and is not necessarily simple or straightforward.
However, some problems are more difficult to include quantifying and correcting for the effects of velocity profile on the flow measurement. The performance of many flowmeters can be adversely affected by a velocity profile that is not sufficiently developed before entering the flowmeter. In other words, measurements can be affected by varying velocity profiles because insufficiently developed velocity profiles can vary with flow rate and composition to affect the measurement in different ways at different flow rates (and sometimes) at different times. Identifying this problem is often difficult — especially in larger pipe sizes where velocity profile distortion can propagate well beyond published straight run requirements.
This instability problem is often difficult to find and diagnose because the actual and measured flow rates often cannot be compared. It sometimes takes a "sixth sense" to suspect that this type of instability is even occurring — let alone how to correct for it.
Even flowmeter suppliers and users who know their instruments and processes, respectively, are at a distinct disadvantage when it comes to diagnosing this type of instability, so it helps when both work together. Fortunately, this type of instability does not happen often, but you should be aware that it can bite you — hard.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC offering engineering; seminars; strategic marketing consulting; distribution consulting; and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. Spitzer has more than 40 years of experience and has written more than 10 books and 300 articles about flow measurement, instrumentation and process control. He may be reached at 845-623-1830 or at spitzerandboyes.com.
