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The 12-Inch Parshall Flume
The 12-inch Parshall Flume is one of the most popular sizes of the Parshall Flume family. The flume is equally at home measuring sanitary wastewater at a municipal treatment plant as it is measuring water rights on The Western Slope.
Applications
The flume is regularly used to measure the flow of water in a number of applications, including:
- Dam seepage
- Industrial discharge monitoring
- Mine discharge
- Surface waters
- Creeks
- Smaller streams
- Wastewater treatment plants
- Influent
- Effluent
- Interplant flows
- Water rights / apportionment
Configurations
The 12-inch Parshall Flume is available in several configurations:
- Plain (inlet / outlet transitions are by others)
- End adapters (for connections to pipes or flanges)
- Stubs and flanges up to 36-inches [91.44 cm] can be accomodated
- Metering manholes (factory intergration into a fiberglass manhole)
- When integrating a 12-inch Parshall Flume into a metering manhole, the manhole barrel must be at least 60-inches [1.5 m] in diameter.
- Wing Walls
- Standard wing walls span channels up to 5-feet 6-inches [1.67 m] wide.
Flume Accuracy
Under laboratory free-flow conditions, Parshall Flumes have shown to be accurate to within +/-2%.
ASTM D1941 more practically suggest that the flume's free-flow accurate is +/-5% once approach conditions, the installation of the flume, and construction tolerances are considered.
For submerged flows, the observed discharge can vary greatly from the indicated discharge. If submergence is suspected, either a submerged flow equation or a universal flow equation should be used.
Flume accuracy can also be affected by settling. Corrections for the indicated flow rate have been developed to correct for this effect.
Flume Dimensions
It is important to remember that Parshall Flumes are not scale models of each other. While the flumes are similar in layout, they do differ in section lengths and convergence / divergence angles.
Free-Flow Equation
The free-flow equation for Parshall Flumes is:
Equation — Parshall Flume Free-flow
Q = free flow rate (cfs / m3/s)
K = flume discharge constant (varies by flume size / system of units)
Ha = depth at the point of measurement (feet / meters)
n = discharge component (dimensionless, depends upon flume size)
K = flume discharge constant (varies by flume size / system of units)
Ha = depth at the point of measurement (feet / meters)
n = discharge component (dimensionless, depends upon flume size)
For the 12-inch Parshall Flume the values are:
| Minimum Head (ft) | 0.10 | Minimum Head (m) | 0.0305 |
| Minimum Flow Rate (cfs) | 0.1202 | Minimum Flow Rate (l/s) | 3.405 |
| Maximum Head (ft) | 2.50 | Maximum Head (m) | 0.7620 |
| Maximum Flow Rate (cfs) | 16.13 | Maximum Flow Rate (l/s) | 456.9 |
| Equation (cfs, ft) | 4*H1.522 | Equation (l/s, m) | 690.9*H1.522 |
Discharge Table
The free flow discharge table for the 12-inch Parshall is available for download.
The table provides:
- Top-down view of the flume with the Ha (primary point of measurement) shown
- Discharge equations in SI and Imperial units
- Flume's accuracy
- Submergence Transition (St)
- Source for the table's information
Materials
Openchannelflow offers the 12-inch Parshall Flume in a range of materials, including:
The availabilty of accessories does vary among materials - fiberglass construction being the most customizable.
Submergence Transition
The submergence transition (St) is the point at which downstream conditions sufficiently reducer the discharge out of the flume that the indicated flow rate needs to be corrected.
Equation — Flume Submergence Transition
Q = free flow rate (cfs / m3/s)
K = flume discharge constant (varies by flume size / system of units)
Ha = depth at the point of measurement (feet / meters)
n = discharge component (dimensionless, depends upon flume size)
K = flume discharge constant (varies by flume size / system of units)
Ha = depth at the point of measurement (feet / meters)
n = discharge component (dimensionless, depends upon flume size)
For the 12-inch Parshall flume, the submergence transition occurs at 70%. Above this and the indicated discharge needs to be corrected.
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