What Turndown Means In Terms of Flowrate

Last month we discussed how claims of high turndown imply a wide range of flows. In particular, a turndown of 1000-to-1 for a magnetic flowmeter implies measurements between velocities of 0.01 and 10 meters per second. However, most applications tend to require turndowns of approximately 10-to-1 or less. Turndowns above 10-to-1 would be better, but the following questions come to mind.

• What do the higher turndowns mean in terms of flow (velocity)?
• Will the process ever operate at these flowrates (velocities)?

Flowmeters are often sized such that the liquid velocity at full-scale flow is approximately 2-3 meters per second. A turndown of 10-to-1 implies that a typical flowmeter will operate properly from (say) 0.25 to 2.5 meters per second. Increasing the turndown to 100-to-1 implies the flowmeter will improve operation from (say) 0.025 to 2.5 meters per second. This turndown (100-to-1) is 10 times better, but the actual difference is the coverage of the velocities between 0.025 to 0.25 meters per second. This is a relatively small flow range, but it could be important in some applications. Further improving the turndown to 1000-to-1 would allow improved measurement between 0.0025 and 0.025 meters per second. This represents another improvement, but it similarly applies to a small part of the flow range.

Will the process ever operate at the flowrates (velocities) covered by the improved turndown specifications? In some applications, the answer is absolutely "yes." However, in most applications, the process does not operate in these relatively small flow ranges of increased turndown … and if it does, flowmeter accuracy is not the prime concern. This is not to say that turndown is not important – it is. But accurate measurement may not be the biggest problem when the process operates in upset conditions only (say) 0.1 percent of the time.

Let’s integrate turndown and accuracy next month.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845-623-1830.