Most of the flume styles currently used to measure the flow of water in non-full pipes were not originally conceived to handle such flows. While engineered transitions – end adapters – have been developed for some of these flumes, many installations have site-formed transitions – some good and some bad.
In Booklet 7: Flow Measurement Methods in Open Channels of Sampling Guide for Environmental Analysis, Centre d’ Expertise en Analyse Environmetale du Québec illustrates the three common problems associated with poorly transitioned flows:
- Setting the flume lower than the incoming pipe
- Excessive upstream pipe slope
- Insufficient upstream flow conditioning
Each illustration shows a Parshall flume – the flume most commonly used to measure the flow of water in open channels.
Setting the Flume Lower than the Inlet Pipe
For a flume to operate correctly, the approaching flow should be tranquil and without hydraulic jump or upstream disturbance for 10-20 throat widths. To help obtain this, the floor of the flume should be set level with or above the invert up the upstream channel or pipe.
Here we see a not untypical installation where the flume has been set lower than the incoming pipe. When this is done a jump will occur as the flow drops down. Depending upon where the jump occurs, the flume will either under or over-indicate the flow rate.
Excessive Upstream Pipe Slope
Another condition to using a flume to measure flow is that the flow must be subcritical as it enters the flume. This is because a flume operates by accelerating slower, subcritical flow in a defined manner to a critical state.
If the flow entering the flume is already supercritical – in this case because the pipe slope is too large – the level-to-flow relationship that the flume is designed to develop, never does and flow shoots through the flume. The result here is that the water level (and thus indicated flow rate) is lower than it would be for a given flow rate under subcritical conditions.
In general, the incoming pipe slope should not exceed 2% - decreasing as the line size gets larger.
Insufficient Upstream Flow Conditioning
As the flow approaches a flume it must be well distributed across the approach channel and possessing a normal velocity profile. If a pipe is placed too close to the converging section of a flume, the flow is never allowed to become well distributed or assume a normal velocity profile.
As a result, the flow is channelized as it passes through the converging section of the flume. With the level in the flume lower down the centerline of the incoming pipe than along the sidewalls. As with the conditions above, the is no ready correction that can be applied to the flow equation. To obtain accurate flows, the installation must be modified or flow conditioners used to develop more normal approach velocities / profiles.