As often as we use Parshall flumes for open channel flow measurement, it is important to understand “free versus submerged flow” conditions; as it greatly impacts the accuracy of the installation…or the lack thereof.
While any primary device could be affected by submerged flow conditions, there isn’t much data to be found on the topic. Luckily, the Parshall flume is the exception. The bulk of the research on submerged flow in Parshall flumes comes from Water Research Laboratory at Utah State University. We use their findings to identify submergence, determine the impact on flow accuracy, and to help with a remedy.
Discharge through a Parshall flume is considered “free flow when the tailwater (or downstream) conditions do not affect the flow conditions upstream and the discharge may be evaluated by measuring only one depth of flow, the upstream depth” (Ha). Submergence occurs “when downstream conditions are such that the flow conditions upstream are affected”… ¹
When flow cannot exit the flume quickly enough, water pushes back upstream; potentially to the free flow level measurement location known as Ha. If the flume is in submergence, most flow meters will detect an increase in level, but not the decrease in velocity; causing it to over-report.
We often hear of plants where the total flow is “out of balance”; with more total influent than total effluent; or the other way around. On several inspections, we’ve found a flume operating in submerged conditions; some continually …others just during prolonged periods of peak flow.
When a plant’s total influent and effluent delta is much more than can be explained in the treatment process, we begin to suspect submergence. A submerged flume problem can quickly drive daily totals out of balance. Too, this error tends to be cumulative over time; often adding up to huge differences over a week/month/year.
Identifying Submergence in Parshall Flumes
While most of the flumes we visit operate under free flow conditions, those cases of submergence we have found were almost always unidentified; much less compensated for, or remedied. Submergence in Parshall flumes is identified with a ratio of two level measurements; expressed as: xx % = Hb/Ha.
The point at which the flow changes from free flow to submerged flow is called transition submergence and is expressed as a percentage, which is the ratio Hb/Ha. Parshall Flumes with a throat section (W) from 1 inch to 8 feet, this ratio varies from 55% to 78%. In all Parshall Flumes above 10 feet, transition submergence is 80%.
Submergence % = Hb/Ha
Here, we find the flume completely submerged. Hb/Ha = 100%!
In this case, even as actual flow approaches zero, a typical flow meter is sensing peak flow; unaware of the downstream condition.
Remedies for Submerged Parshall Flumes
Simple changes in operation are often enough to minimize frequency, or magnitude, of submergence. (equalization, pump cycles, gate/valve position, et al). When that doesn’t work, we look for ways to fix the hydraulics. One cure is known as a ‘magic bottom’, or ‘false floor’; fiberglass inserts which raise the flume’s upstream floor to increase Ha water elevation.
A false bottom slides easily into the top of the flume. You can add as much floor as you can stand to lose in freeboard.
Another remedy is to install a flow meter capable of measuring free and submerged flume flow such as the Krohne Optisound VU31. This meter uses two ultrasonic sensors, at Ha & Hb, monitors the submergence ratio, and switches, when needed from a free flow equation, to the submerged flow equation.
FOR MORE INFORMATION ON PARSHALL FLUMES, SEE OUR RESOURCES SECTION!
¹ – Excerpts from:
“Design and Calibration of Submerged Open Channel Flow Measurement Structures – Part 2 – Parshall Flumes” Utah Water Research Laboratory College of Engineering Utah State University Logan, Utah.