Why Not Use a Stilling Well on Sanitary Flows?

For many engineerings and end-users the default configuration for flume always includes a stilling well...and why not? When they work, they work great - providing a convenient area to measure the flow in a flume. When they don't work....well that's another matter.

Abandoned Stilling Well on a Free Flow Parshall Flume measuring Sanitary Flows

What is a Stilling Well?

A stilling well is a round or rectangular chamber that is isolated from, but connect to, the main body of flow in a flume. The sole connection between the stilling well and the main body of flow is through a small connector through the sidewall of the flume.

Detached Stilling Well on a Stainless Steel Extra Large 60Âº V Trapezoidal Flume from Openchannelflow

Waves or surges experienced in the flume are dampened in the stilling well so that the level in the stilling well reflects only the actual rise / fall of the level in the flume due to changes in the flow rate.

Problems with Stilling Wells and Sanitary Flows

While on the surface this sounds good, the problem with using stiling wells on sanitary flows is two fold:

The inlet to the stiling well throught the flume sidewall is small (2-inches [5.08 cnm] or less)
The floor of the stiling well is depressed below the floor of the flume (generally 3-inches [7.62 cm] to 6-inches [15.24 cm]

Abandoned Stilling Well on a Tracom Parshall Flume Measuring Sanitary Flows

For sanitary applications, the minimum recommended opening size for a flume is 3-inches (hence the reason the 3-inch Parshall is the minimum size recommended for sanitary flows). Since the standard stilling well connectioin is usually only 2-inches [5.08 cm] in diameter, there is a strong likelihood that the conenction will plug. The obviuous solution to this problem is increase the size of the opening...except for the fact that increasing the inlet size decreased the effectiveness of the stilling action in the well!

Even if the connection doesn't plug, solids will always deposit in the stilling well and cause a source of ongoing maintenance and potential source of error.

Summary

In the end, in applications were stilling wells are used to meaure sanitary flows, you can be assured that three things will happen:

The inlet will clog
The operators will need to perform regular / ongoing maintenance to keep the stilling well functioning (and probably curse you because of it)
...and the stilling well will be abandoned

Save yourself time, effort, and money....don't use stilling wells on santiary flows. It's better to fix the upstream hydraulics of an application than it to misapply the equipment only to end up abandoning it and still have the problem!