Flow conditioning is an important aspect of any flow rate measurement effort, and fiberglass Parshall flumes are no exception. Flows must be subcritical before they pass through the throat only to achieve criticality at that point. Any other outcome, and you’re going to have inaccurate measurements. That’s why it’s important to make sure your approach flow has the proper velocity. Here’s how to condition flow in Parshall flumes.
The upstream conditions of any flow can be tough to alter, as flow channels are pretty much set in stone. Fortunately, there are still some solutions you can implement, and one of the best is installing an energy absorbing manhole. These installations are placed upstream and are designed to get rid of any excess energy by dissipating the flow stream.
Installing manholes is easier than you might think, so these are among the most popular solutions for upstream flow conditioning. You don’t have to worry about needing excess space like you would if you’re trying to enlarge an upstream weir pool. You just need space above the flow channel just upstream from the flume. Keep in mind that manholes can even alter conditions caused by high slopes.
In addition to conditioning the area upstream of the flume, you can make alterations to the flume itself to alter flow velocity. If you have a smaller sized flume, around three inches or so, you can implement baffle plates. These are perforated sheets of metal that are used in groups to break up the velocity profile of any flow going through the flume. Not only are they easily applicable to flumes, but they can be removed as well if your approach conditions ever change.
For larger flumes that are greater than six inches, you may want to use tranquilizer racks instead. They function similarly to baffle plates, but they’re more adept at tackling larger flows. These add-ons are relatively large, however, so don’t be surprised if you have to lengthen the inlet adapter of your flume to account for the racks, as you’ll need at least two to break up the flow then redirect it to the throat.
Manholes aren’t the only features that can absorb energy from supercritical flows. You can also opt for energy absorbers within the flume itself. It’s important to note, however, that these energy absorbers are much more controlling regarding the flow. While energy absorbing manholes allow water to splash all around, in-flume energy absorbers have to mediate surface tension as well as approach velocity, as they both have a major impact on measurement.
To account for surface turbulence, energy absorbers contain splash guards designed to keep any excess water in check. Additionally, these energy absorbers aren’t always used on their own. You may find it efficient to use them in conjunction with tranquilizing racks, but that almost always requires lengthening the inlet end adapter.
Condition Approach Flows With Tracom
Now that you know how to condition flow in Parshall flumes, you just need the proper equipment. That’s where Tracom can help. You can design a Parshall flume with our design team and choose from a wide selection of add-ons to ensure that your measurements are as accurate as possible. Contact us today to learn more!