E-Zine April 2013
When first exposed to flowmeters, I was told that flowmeters require straight run consisting of 10 diameters upstream of the flowmeter and 5 diameters downstream of the flowmeter. Myths such as this, and other similar rules of thumb, abound in a world where details are things to avoid because they just seem to get in the way.
Is straight run required for flowmeter installations? Why?
A. Yes, to generate a fully developed velocity profile.
B. Yes, because it is recommended by the manufacturer.
C. Yes, to provide proper supports for the flowmeter.
D. No, straight run is not required.
Flowmeters are typically designed to operate properly with homogeneous single-phase fluids that have a fully developed non-swirling velocity profile entering the flowmeter. This can be accomplished by installing sufficient straight run. In Answer A, a fully developed velocity profile can be generated using sufficient straight run. However, this is not the only way that a fully developed velocity profile can be developed.
Depending upon the design, flow conditioners remove swirl, velocity profile distortion or both, from the fluid stream. Installing a flow conditioner upstream of the flowmeter to remove swirl and/or distortion can generate a fully developed velocity profile without straight run per se. In other words, development of an adequate velocity profile that may require a long straight run of pipe might be accomplished in but a few diameters by applying a flow conditioner. In a way, straight run could be considered to be a type of flow conditioner, albeit a rather long one.
When velocity profile distortion can affect flowmeter operation, manufacturers should provide a recommendation for straight run (Answer B). Note that this information is often not found in the flowmeter specification, but rather in the installation manual. Again, the object of the recommendation is to provide a fully developed velocity profile at the flowmeter inlet. This can also be accomplished in a shorter piping section by using a flow conditioner.
As previously stated, most flowmeters that require straight run do so to generate a fully developed velocity profile at the flowmeter inlet. However, sometimes the purpose is mechanical, such as the case of a Coriolis mass flowmeter that requires rigid supports located a prescribed number of diameters upstream and downstream of the flowmeter. This installation effectively becomes a straight run (Answer C), but the straight run has more to do with the proper functioning of the flowmeter, and not much to do about generating an adequate velocity profile.
One of the “tricks of the trade” when sufficient straight run is not available is to consider installing a flowmeter that does not require straight run (Answer D) and avoid the velocity profile issue altogether.
Additional Complicating Factors
The question implies that a fully developed velocity profile is required for the flowmeter to operate properly. However, the operating principles of some flowmeters are such that they are not affected by velocity profile and have no straight run requirements. Other flowmeters are designed to be a combination of a flow conditioner and a flowmeter that has minimal or no straight run requirements. When one gets into the details, there can be literally thousands of choices.
From Flow Control (Nov/Dec 2002)