A flume can be one of the best ways to measure flow rate, but accurate measurements depend on proper installation. At the very least, any deviation from a proper flume installation can result in inaccurate measurements. At most, the flume could be rendered effectively useless, offering measurements that don’t even remotely reflect the reality of the flow. Here are the most common flume installation errors to watch out for.
Off-Centered Flow or Flume
For a flume to operate properly, both the flow and the flume have to be properly centered in the channel. Any deviation is going to throw off the measurements. If the flume itself isn’t centered in the flow stream, for example, it won’t fully develop across the flume’s entrance. That means the head measurement on one side of the flume will differ from measurements on the other side when they should be the same. An off-centered flume could additionally cause the incoming flow to go from subcritical straight to supercritical.
Centering a flow involves much more than ensuring that the approaching flow travels in a straight line. It must be centered in all dimensions, meaning an evenly distributed velocity profile with no turbulence or surging. Every part of the flow must have the same subcritical velocity as it approaches the flume, or the measurement will be inaccurate. If you’re having trouble achieving these conditions with your flume location, there are upstream conditioners available that could be a solution.
Supercritical Flow Approach
A flume provides accurate measurements by accelerating a subcritical flow to a supercritical flow, with the flow reaching criticality at the point of measurement. In order for that to work, however, the flow needs to be at a subcritical state before it enters the flume. When the flow approach has already achieved supercriticality, the flume’s function is negated as the flow isn’t properly developed. That will leave the results much lower than they should be.
Fortunately, addressing the root cause of a supercritical flow approach is relatively easy. In most cases, the upstream pipe and/or channel slope is simply too steep. By adjusting this slope in accordance with your flow and flume setup, you can alter the flow to achieve subcriticality before it reaches the flume. It must be free-flowing, however, as a pressurized flow compromises the open channel conditions necessary for a flume to provide accurate measurements.
For a flume to function properly, it must be completely level. This is typically something that is finalized during the initial installation, though Parshall and Cutthroat flumes can be corrected at a later date. Not only does a flume have to be level from side to side, but it must also be level from front to back. Regardless of the flume style you opt for, there is no exception to this rule.
Failing to Consult the Experts
If you want to avoid the common flume installation errors, your best bet is to consult the experts at Tracom. We’re happy to offer customized solutions to all your flow measurement needs, regardless of your particular conditions. Get in touch with our team today, and discover how we can get your customized flume built and installed in no time.