How Parshall Flumes are Affected by Approach Velocity

Approach conditions are an essential factor with any kind of measurement device, and that includes Parshall flumes, which are typically quite adaptable. After all, the Parshall is one of the oldest styles around, with decades of research for it and its various applications. Learn all about how Parshall flumes are affected by approach velocity and discover how you can optimize yours.

Need for Correction

When dealing with high approach velocities, there are some corrections that can be made to your channel and flume to account for the velocity increase under certain conditions. One of the most common is opting for a smaller throat width. This is the solution recommended by the researcher for whom the flume style is named, Dr. Ralph Parshall.

Additionally, you can use several techniques to slow down the approach flow, like energy-absorbing manholes and drop manholes. You can even utilize add-ons with the flume itself, like tranquilizing racks.

Upstream Pooling

When faced with accelerated approach conditions, you’ll need to alter your upstream pooling. One of the best ways to do this is with a nested flume, or a flume inside a flume. This is an easy way to retrofit an already established flume system with one that has a smaller throat width. You just have to make sure that the channel banks aren’t overtopped because the actual flow level will increase even if the flow rate stays the same.

Installing a nested flume can be either one of the easiest tasks in the world of wastewater measurement devices or downright impossible. It all depends on the size of the nested flume you’re looking to install. With 1- to 3-inch Parshall flumes, for example, the process is fairly easy, but the 6-inch and greater sizes may not even be an option if the manhole in a piped system or underground flume doesn’t offer enough access.

Inline Alterations

For inline systems, adding an energy-absorbing or drop manhole can help alter approach velocity to a desirable profile. When you place an energy-absorbing manhole upstream of the flume itself, you can alter the flow channel in a variety of ways whether you’re changing elevation, line size or slope. As the name suggests, this kind of change is designed to dampen the energy of the approaching flow.

When implementing a manhole, however, it’s important to give the flow time to settle out and become tranquil again. A slope change or elevation change can create a lot of turbulence, so it’s generally best practice to space the energy-absorbing manhole at least 30 times the maximum head of the flume upstream.

Flumes From Tracom

With knowledge about how Parshall flumes are affected by approach velocity in mind, you’re ready to get a flume of your own. At Tracom, we offer a wide variety of Parshall flumes along with a team that will work with you to create a custom design that’ll work for your unique flow channel conditions. We also offer Parshall flume options and accessories that can help you manage the approach velocity of your flow. Contact our team today to get started.

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