David W. Spitzer, P.E.

Most flowmeters are sensitive to the upstream and downstream piping in which they are installed. They are especially sensitive to the upstream piping, so much so that many discussions only focus on the upstream piping, even though both can be important. In some installations, it is possible to improve flowmeter performance by configuring the upstream piping such that it produces less distortion of the velocity profile. This is because some configurations require less upstream straight run than others to achieve a good velocity profile at the flowmeter.

Assuming that the flowmeter is sensitive to a poor velocity profile, which of the following common upstream piping configurations would typically result in the worst flow measurement problems?

A. One elbow
B. Two elbows
C. Two elbows out of plane
D. Two elbows out of plane – close coupled

Commentary
With one elbow upstream of the flowmeter (Answer A), there is a relatively long straight run followed by an elbow upstream of the flowmeter. The velocity profile entering the elbow is relatively good. As the fluid turns through the elbow, more fluid moves to the outside of the elbow than to the inside of the bend. This distorts the velocity profile at the exit of the elbow such that more flow is on one side of the pipe than the other side.

With two elbows upstream of the flowmeter (Answer B), there is a relatively long straight run followed by one elbow and then another elbow upstream of the flowmeter. The velocity profile distortion caused by the first elbow is still present when the fluid encounters the second elbow and is distorted again. The velocity profile exiting the second elbow will usually be more distorted than the velocity profile exiting a one elbow. As a result, a longer straight run is usually needed to dissipate the distortion.

As with the one elbow installation, two elbows out of plane (Answer C) consists of a relatively long straight run followed by one elbow and then another elbow upstream of the flowmeter. However, the second elbow is not in the same plane as the first elbow. What this means is that one elbow may turn the flow into this page, and the second elbow turns the flow to be parallel to the surface of this page. The velocity profile distortion caused by the first elbow is still present when the fluid encounters the second elbow and is distorted again. However, in addition to velocity profile distortion, the flow stream will also swirl because the elbows are out of plane. As compared to an installation with two elbows in plane, even longer straight run is usually needed to dissipate the distortion and swirl.
In general, an installation with two elbows out of plane that are close coupled (Answer D), that is installed within a few diameters of one another, creates even more swirl than an installation with two elbows out of plane. It generally requires more upstream straight run.