Flumes are used to provide flow measurements to monitor a variety of wastewater flow rates. To obtain accurate readings, a flume has to measure a flow that meets certain factors. If those requirements aren’t met, the equations are thrown off and the measurement will be inaccurate. One of the most important factors is the pipe slope in a pipe flow system. Learn how pipe slopes affect flume measurements.
How Flumes Work
To understand the impact of pipe slopes, you must first grasp how flumes work in general. The layout and dimensions of a flume are designed to bring the flow to a critical state. Criticality is the point at which accurate measurements of flow rate can be taken with all the variables controlled. In order to reach this critical state, the flow must be at a subcritical state before reaching the flume. It is then accelerated to criticality.
After reaching criticality at the point of measurement, the flow will continue to accelerate until it becomes supercritical upon exiting the flume. This is similar to airflow going from a subsonic to supersonic state. Because you can’t get to the supersonic level when you’re already supersonic, you have to start from the subsonic state. The same is true for subcritical water flows.
Criticality Quantified
Criticality is essential for accurate measurements, but what is it exactly? Imagine airflow again. The Mach number determines whether an airflow is supersonic or subsonic. Flow rates are measured similarly using the Froude number. A critical flow rate will have a Froude number of 1. Anything less than 1 is subcritical, and anything greater than 1 is supercritical.
With that in mind, the ideal Froude number of a flow approaching a flume would be less than 1. To make measurements and necessary dimensions of a flume easiest to deal with, the number should be closer to 0.5 rather than anything greater. Numbers above 0.5, even if they’re less than 1, can cause surface waves to form on the flow, which can throw off flow measurements.
Pipe Slopes
With criticality and Froud numbers in mind, the effect of pipe slopes is apparent. If the pipe slope is too steep and the wastewater is too far accelerated, it will reach supercriticality before even reaching the flume, completely eliminating the possibility for an accurate measurement.
It’s important to remember, however, that there is no universally appropriate pipe slope. The general rule is that the larger the pipe size is, the more the Froude number increases. For example, a 4-inch pipe may work with a slope as high as 2.83, but a 60-inch pipe maxes out at a slope of just 1.16 because the larger pipe would allow the flow to reach supercriticality easier.
Find Flumes From Tracom
Now that you know how pipe slopes affect flume rates, it’s time to find the right flume for your needs. That’s where Tracom is happy to help. We can work with your specific needs to design a flume uniquely suited for your flow channel, complete with all the durability that fiberglass design comes with. Contact us today to get started.
