A stilling well is sometimes a necessary addition to a flume or weir box system, as it can help slow down the flow at certain points. This is to ensure that the flow reaches criticality at the opportune moment to allow for accurate flow rate measurements. While there are some potential problems that come with them, stilling wells continue to be among the most popular accessories for a flow measurement system. Here’s everything you need to know about stilling wells.
The Two Styles
You’ll find two primary options available when choosing a stilling well for your flow channel: attached and detached wells. Attached wells are physically connected to the flume or weir at the side through welding, while detached wells are connected through tubing. Attached wells are more common among flumes in which the side walls are vertical, but they aren’t very common with flume styles like trapezoidal. The general rule is that the well must be away from the flume the same distance as the top of the flume’s width.
Sizing the Opening
In general, a stilling well’s opening is about two inches, but that can vary based on the conditions of the flume or weir and the flow. The opening of a stilling well is the main factor affecting the lag, or the length of time between a change in the primary channel and the change in the stilling well. A detached well is going to have a greater lag since it’s further away, so a larger opening can help account for that difference.
Sedimentation Considerations
One of the most important rules when it comes to stilling wells is that you cannot use them with sanitary solids. Solids will deposit themselves in a stilling well and will have to be frequently removed if you want the well to function as normal. Sediment buildup in the well could alter its function and, in turn, throw off your measurements. Unless you’re prepared for the constant maintenance, it’s generally best to abandon the idea of stilling wells from solid-heavy flows altogether.
If the solids in the flow are suspended high, it can still cause serious problems for a stilling well. As the name suggests, the water in a stilling well is still. That means the high-suspended solids have nowhere to go. While still a potential problem, removing high-suspended solids is easier than removing accumulated solids on the bottom of the well. If you don’t have a problem with removing high-suspended solids, you can connect a line to pulse water into the well and force solids to the surface where they can be extracted.
Preventing Ice
Stilling wells in cold weather can form ice, which will alter the well’s functioning since all the water within needs to be still with the potential to freely flow. Fortunately, there are several solutions. First, you can utilize an electric immersion heater. You just have to have a power source available. Alternatively, you can put a layer of oil on the surface of the water in the well. Just make sure that the inlet to the flume is low enough that the oil won’t go through even during low flow conditions.
Stilling Wells From Tracom
If you’re looking to get your hands on a quality stilling well or a flume with one already attached, Tracom is happy to help. Our fiberglass creations are reliable and customizable, so you can find the perfect solution for your flow measurement needs. Contact our team today to get started.
