Flumes work by measuring flow rate at the point of criticality as water passes through. For these measurements to be accurate, the flow has to reach criticality in the flume. Being subcritical or supercritical will throw off any measurements. While there are numerous factors that go into facilitating proper flow conditions, pipe slope is among the most influential. Learn how pipe slope affects flume operation.
The Froude Number
To understand pipe slope and flumes, one must first understand the Froude number. This number represents the criticality state of a flow channel. If the number is less than 1, it’s subcritical, and if it’s greater than 1, it’s supercritical. Of course, that means criticality is achieved with a Froude number of 1.
Flumes work by measuring flow at the point of criticality, so the incoming flow would need to be subcritical, or have a Froude number that’s less than 1. Pipe slope is among the most influential factors when it comes to the approach velocity of any flow toward a flume. In general, that means the pipe slope needs to be configured in such a way that the approach velocity has a Froude number less than 1 with enough time to reach 1 by the time it reaches the point of measurement.
The slope of a pipe is essentially the angle at which the pipe is set as it connects to the flume. A Froude number will always be generated by a pipe slope, and that number is key to determining the best slope for your particular flow channel conditions. Theoretically, any Froude number between 0.5 and 0.99 would work, but it’s generally best to keep your Froud number as close to 0.5 as possible.
This is because anything greater can lead to surface turbulence. When the flow isn’t tranquil going into the flume, it becomes difficult to measure the actual head of the flow, throwing off flow rate measurements even if the flow reaches criticality at the appropriate point.
Not all pipes are created equal. In fact, the size of any given pipe plays a major role in how the Froude number is calculated. The general trend is that the bigger the pipe is, the more the Froude number increases in relation to the slope. That being the case, the slope would have to be less in a larger pipe compared to a smaller pipe.
A large pipe with a large slope can lead the flow to become supercritical before it reaches the point of measurement, or it can create excessive surface turbulence due to the flow being subcritical on the high end of the spectrum. Both scenarios can render measurements inaccurate. Always remember that the pipe slope decreases as pipe size increases.
Flumes From Tracom
You know how pipe slope affects flume operation, so now it’s time to put it to work. At Tracom, we can help you get a fiberglass flume that works perfectly with your unique flow channel conditions, and our experts can ensure an installation that works with an appropriate pipe slope. Contact us today to get started!