The 36-Inch Parshall Flume

The 36-inch Parshall Flume is equally at home measuring water rights, sanitary flows, and surface waters.

36-inch stainless steel parshall flumes for Iraq

Applications

The 36-inch Parshall Flume is useful in a number of applications:

Dewatering of mines
Measuring surface waters
- Creeks
- Canals
- Streams
WWTP
- Influent
- Effluent
- Lagoons
Water rights

Configurations

Three standard configurations of the 36-inch Parshall Flume are available:

Plain (for field formed inlet / outlet transitions)
End adapters (allowing the flume to be connected to pipes / flanges)
- Stubs / flanges up to 36-inches [91.44 cm]
Wing Walls
- Inlet / outlet wing walls for rectangular channels

Flume Accuracy

Research has shown that the Parshall Flume be accurate to within +/-2% under controlled, laboratory conditions. Field conditions differ however and the free-flow accuracy shouldn't be expected to be better than +/-5% (ASTM D1941).

Should the flume submerge or shift out of position, corrections for submergence and settling have been developed.

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 — 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)

36-inch Parshall flume:

Minimum Head (ft)	0.15	Minimum Head (m)	0.0457
Minimum Flow Rate (cfs)	0.6151	Minimum Flow Rate (l/s)	17.42
Maximum Head (ft)	2.50	Maximum Head (m)	0.7620
Maximum Flow Rate (cfs)	50.39	Maximum Flow Rate (l/s)	1427
Equation (cfs, ft)	12*H^1.566	Equation (l/s, m)	2184*H^1.566

Discharge Table

The free flow discharge table for the 36-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 36-inch Parshall Flume in four different materials:

Aluminum
- Lightweight and abrasion resistant
Fiberglass (FRP)
- Lightweight and easily customized
Galvanized steel
- Most economical
Stainless steel
- Greatest resistance to abrasion and corrosion

Submergence Transition

Submergence in a flume is when the downstream conditions are such that they restrict the flow out of the flume. As submergence increases, a point is reached where the impact on the flow rate is large enough that the discharge equation overstates the flow out of the flume. This point is termed the submergence transition (S_t).

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 (S_t) for the 36-inch Parshall Flume is 70%. At 70% or above and the indicated discharge needs to be corrected to reflect the actual discharge.