Raising Parshall Flumes to Overcome Poor Entrance Conditions

In space constrained applications it can be difficult to get the upstream conditions necessary for accurate flow measurement with a Parshall flume. While flow conditioners - usually perforated plates or stainless steel fingers - can be used in some installations, they aren't appropriate for all of them. For installations where flow conditoners aren't appropriate, and where there is sufficient head available - it may be more practical to simply raise the flume above the approaching channel floor.

Raising the flume creates a backwater effect that slows and conditions the approaching flow. The flow is slowed by the upsteam pool and rises up before it enters the flume.

a parshall flume in a chlorine contact chamber application raised above the channel floor

To be effective, the Parshall flume should be set at least 2 times the height of the converging section. Lower and there is the chance of turbulence being generated as the flow transitions from a vertical rise to a horizontal run into the entrance of the flume.

elevated parshall flume at the exit of a chlorine contact chamber

By raising the flume, not only can straight run requirements be minimized, but bends, dips, and elbows - normally generators of turbulence and poor velocity profiles - can occur upstream without impacting the accurate operation of the flume. Likewise, the EPA recommendation of 5-20 downstream throat widths of straight channel is greatly reduced / eliminated.

Finally, when a Parshall flume is used in an elevated installation a shorter - Montana or USGS - style can be used. This can save not only space, but cost for the operator.