Flumes come in many different shapes and sizes, and that allows them to fit in a wide variety of open channel systems. That includes pipes as well, provided the water flowing through isn’t pressurized or filling the entire interior of the pipe. Flumes can be a useful method of measurement for flows through pipes, but there are a few things that can go wrong. Here are the common problems with pipe-to-flume transitions.
Setting the Flume Too Low
The transition between pipe and flume must be calm and steady. If the flume is set too far beneath the pipe, there will be a hydraulic jump with the flow. Because of this jump, any effort to measure the flow will be either under or over the actual flow rate depending on the point of measurement and how far the drop between the pipe and the flume is.
If possible, it’s best to set the flume completely level with the incoming pipe. That way, you won’t have to worry about any hydraulic jump at all. A hydraulic jump is considered a type of upstream disturbance, but you must also eliminate any kind of upstream disturbance for about 10 to 20 throat widths before the start of the flume.
Pipe Slope Too Steep
Most pipe channel systems are at a slight incline by design to ensure that the flow is always going the correct direction. If that incline is too steep, however, it could compromise the integrity of your flume and any measurement efforts from it. Like any other open channel, the upstream pipe conditions must allow for a subcritical flow that can enter a critical state at the point of measurement in the flume.
If the pipe’s slope causes the flow rate to become supercritical before reaching the flume, the water level in the flume will be too low for accurate measurements. This will lead to readings far lower than the actual flow rate, rendering the flume pointless. The general rule is that the incoming pipe slope should be no more than 2%, though that figure decreases even further depending on how large the line size is.
Lack of Upstream Flow Conditioning
Incoming flows must have certain characteristics for a flume to measure flow rate properly. With some flows, that means conditioning their approach to create a normal velocity profile and even distribution among the channel. This can be tough if the pipe is placed too close to the converging section of a flume preventing the flow from evenly distributing itself throughout the channel.
Approach velocity and profile are essential for accurate measurements. Keep in mind that this is a matter of installation and not a simple correction after the fact. There is no alteration to the measurement equation you can implement to account for the error. The only solution is to ensure that the flow is evenly distributed across the channel at the appropriate velocity.
Find a Flume With Tracom
If you’re looking to install a flume in a pipe channel, Tracom is happy to help. You know the common problems to avoid with pipe-to-flume transitions, so now it’s time to get a customized flume perfect for your unique situation. Our experts will work with you to craft a fiberglass masterpiece that fits perfectly. Contact us today to get started.
