The 18-Inch Parshall Flume

The 18-inch Parshall Flume is mainstay at wastewater treatment plants. It can also be found out in the field helping to measure water rights.

Applications

The 18-inch Parshall Flume is used for:

Industrial pre-treatment
- High volume users (i.e. paper mills and large regional breweries)
Mine dewatering
Surface waters
- Creeks
- Streams
- Springs
Wastewater treatment plants (WWTPs)
- Incoming flows at headworks
- Treated discharge
- Interplant flows
Water rights allocation

18-inch parshall flume with end adapters

Configurations

Openchannelflow's 18-inch Parshall Flume is available in several configurations:

Standard (transitions by others)
End adapters (used to connect to pipes and flanges)
- Stubs and flanges up to 36-inches [91.44 cm] can be accommodated
Metering manholes (factory integration of a flume into a fiberglass manhole)
- A 60-inch [1.53 m] manhole barrel is required to integrate an 18-inch Parshall Flume
- Two-piece manhole construction is common when burial depths are greater than 8-feet [2.44 m]
Wing Walls
- To span rectangular channels

Flume Accuracy

The laboratory accuracy of Parshall Flumes has been found to be +/-2%.

In the field, variances in approach conditions, minor errors in the installation, and typical construction tolerances mean that the user should more realistically expect an accuracy of +/-5% (ASTM D1941).

Should the flume become submerged due to downstream conditions, a universal flow equation is available to correct the flow rate. Also, a correction for settling has also been developed.

Flume Dimensions

Always remember that Parshall Flumes are not scale models of each other.

Free-Flow Equation

The general free-flow equation for the Parshall Flume 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)

The values for the 18-inch Parshall Flume are:

Minimum Head (ft)	0.10	Minimum Head (m)	0.0305
Minimum Flow Rate (cfs)	0.1738	Minimum Flow Rate (l/s)	4.923
Maximum Head (ft)	2.50	Maximum Head (m)	0.7620
Maximum Flow Rate (cfs)	24.56	Maximum Flow Rate (l/s)	695.5
Equation (cfs, ft)	6*H^1.538	Equation (l/s, m)	1056*H^1.538

Discharge Table

The free flow discharge table for the 18-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's 6-inch Parshall Flume is available in several materials:

Of the materials, fiberglass is the most customizable.

Submergence Transition

The Submergence Transition (St) of a flume is that point at which the downstream conditions sufficiently reducer the discharge out of the flume that the indicated flow rate needs to be corrected.

Equation — Flume Submergence Transition

S = submergence ratio
St = submergence transition
Ha = depth at the point of measurement (feet / meters)
Hb = depth at the point of measurement (feet / meters)

The Submergence Transition (St) for the 18-inch Parshall Flume is 70%.