When it comes to measuring wastewater flows, there are probably two devices that come to mind: flumes and weir boxes. While they both work to help measure flow rate, they’re only applied on their own to flows on the surface. While this is adequate for a variety of scenarios, there are wastewater flows underground as well. For solutions to measuring underground wastewater flows, nothing works quite like a manhole.
How a Manhole Works
Generally speaking, a manhole is used to assist in flow measurement rather than doing the measuring itself. It’s a cylinder-shaped structure installed in the ground with the flume at the bottom and an opening at the surface for access. If it’s long enough, the manhole will come complete with a ladder so workers can get to the flume easily and safely.
These manholes can be made from many different materials, but fiberglass tends to be the most efficient. Not only is it resistant to wear and tear, but it requires virtually no maintenance. This leaves you more time to focus on the flow measurements themselves rather than working on keeping the manhole intact. After all, a severely damaged manhole could not only compromise your measurements, but it could also compromise the flow itself.
When it comes to installing a manhole, the primary factor to consider is whether there’s already a flume in place. Some underground wastewater flows feature flumes used for measurement located inside underground vaults. If you’re looking to add a manhole to one of these systems, it’s important to remember that the entire vault and the end connections have to fit inside.
If there’s currently no flume in place, manhole installation is notably easier. There are high-end manholes available that have a flume or other measuring device built-in. As long as the connections are aligned with the pre-existing open flow pipe, you can create an easily accessible measurement manhole in a single operation.
Controlling the Flow
In order for a flume to properly measure flow rate, the flow has to reach critical velocity when passing through the flume. If the flow slows upon entrance, it can be subcritical. If it’s going too fast, it can exceed the necessary velocity, becoming supercritical. Either result can throw off a flume’s measurements. When a flow is underground, it can be more difficult to control, so the best solution is another manhole.
An energy absorbing manhole is designed to slow down a fast flow, providing the best conditions for measurement. These kinds of manholes are installed a few feet upstream of the flume and contain an energy absorber plate. This device is oriented perpendicular to the flow direction, diverting the water around it through curved slopes and effectively slowing it down.
Finding a Manhole of Your Own
When you’re looking for solutions for measuring underground wastewater flows, a manhole from Tracom is your best bet. We have a wide variety of options available including energy absorbing manholes and manholes with grinders. Get in touch with our team today to learn more about our customizable manhole solutions.