Transitioning Piped Flows into Parshall Flumes

When using a Parshall Flume to measure pipe flow, it is important that sufficient upstream distance be provided to allow the flow to become tranquil and to assume a normal velocity profile as it enters the flume.

The most effective method of transitioning pipe flow into a Parshall flume is through the use of an end adapter. End adapters are specially designed structures that are attached to the end of a flume to transition flow into and out of a flume in a controlled manner.

In the video below, a contractor has decided not to use an end adapter and instead nearly butt the flume up to the incoming pipe. The result is that there is insufficient space for the flow to spread and evenly fill the inlet channel before it entering the flume. The flow is past the point of measurement before it spreads to the width of the inlet pipe.

To avoid expensive, post-installation retrofits to try and correct a poor installation, end adapters should be used when possible. End adapters are designed to transition the flow from a round pipe into the rectangular channel of the Parshall flume in a controlled manner.

Ideally, when using a Parshall flume to measure piped flow, the invert of the inlet pipe should be set lower (or no higher) than the invert of the converging section of the flume (where the point of measurement is located). The flow should rise up into the flume at a 1:4 slope (rise:run).

Best results have been shown to occur when the flow is expanded into a channel wider than the entrance of the flume and then directed into the entrance of the flume with smooth, radius wing walls. The expansion and subsequent contraction helps to more fully develope a uniform velocity profile.