The 24-Inch Parshall Flume

The 24-inch Parshall Flume is a popular size of Parshall Flume for measuring surface water flows, water rights, and sanitary flows at treatment plants.

24-inch parshall flume with two stilling wells and custom end adapters

24-parshall-galvanized-stilling-well-cover-b

Applications

The 24-inch Parshall Flume can be useful in a number of applications, including:

Mine dewatering
Surface waters
- Creeks
- Canals
- Streams
Wastewater treatment plants
- Headworks - influent
- Treated effluent
- Interplant flows
- Lagoons
Water rights / apportionment

Configurations

Openchannelflow offers the 24-inch Parshall in four primary configurations:

Plain (for field formed inlet / outlet transitions)
End adapters (allowing the flume to be connected to pipes / flanges)
- Stubs and flanges up to 36-inches [91.44 cm].
Metering manholes (factory integration into a fiberglass manhole)
- A 24-inch Parshall Flume requires a 72-inches [1.83 m] manhole barrel
- Two-piece manhole construction is needed when burial depths are greater than 8-feet [2.44 m]
Wing Walls
- Radius / flat inlet wing walls
- Flat / flared outlet wing walls

Flume Accuracy

Although laboratory investigations have shown that Parshall Flumes can be accurate to within +/-2% (under free-flow conditions), once approach conditions, installation errors, and construction tolerances are accounted for, it is more practical to expect the free-flow accuracy to be +/-5% (ASTM D1941).

Should the flume submerge, either a dedicated submerged flow or a universal flow equation should be used.

The accuracy of a Parshall Flume can be affected if the flume settles out of position. Corrections to the flow equations have been developed to account for this.

Flume Dimensions

Parshall Flumes are not scale models of each other. The flumes are similar in layout, but they do differ in section lengths and convergence / divergence angles.

Free-Flow Equation

The general free-flow equation for Parshall Flumes is:

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)

Minimum Head (ft)	0.10	Minimum Head (m)	0.0305
Minimum Flow Rate (cfs)	0.4227	Minimum Flow Rate (l/s)	11.97
Maximum Head (ft)	2.50	Maximum Head (m)	0.7620
Maximum Flow Rate (cfs)	33.11	Maximum Flow Rate (l/s)	937.5
Equation (cfs, ft)	8*H^1.55	Equation (l/s, m)	1429*H^1.55

Discharge Table

The free flow discharge table for the 24-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

The 24-inch Parshall Flume from Openchannelflow are available in a range of materials:

Fiberglass provides the greatest flexibility out of the four materials when it comes to customization.

Submergence Transition

A flume is said to be submerged when the downstream conditions are such that they restrict the flow out of the flume. The submergence transition (S_t) is point at which the impact of submergence is large enough that the indicated flow rate needs to be correct to match the actual flow rate.

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 for a 24-inch Parshall Flume is 70%. If the submergence ratio is above this, the discharge needs to be corrected.