The 2-Inch Parshall Flume

The 2-inch Parshall Flume is the second smallest of the Parshall Flumes. Like the 1-inch, the small size means that it is somewhat limited in use.

When used to measure sanitary flows, the flow entering the flume must be well screened, or it will clog. The 3-inch Parshall Flume is the smallest Parshall that can be used on unscreened sanitary flows.

Applications

Dam seeps
Clean industrial pre-treatment flows
Acid mine discharge
Smaller surface waters
- Creeks
- Furrows
- Ditches

Configurations

Openchannelflow's 2-inch Parshall Flume is available in different configurations:

Standard
- Transitions by others
End adapters
- Connecting to pipes and flanges
- Stubs and flanges up to 10-inches [25.4 cm] can be accommodated
- Staged end adapters to fit into existing manholes
Enclosures
- Factory mounted into an above-ground enclosure
Metering manholes
- Integrated into a fiberglass manhole
- 2-inch Parshall Flumes can be integrated into manholes 4-feet [1.22 m] and larger
Wing Walls
- Span rectangular channels

Flume Accuracy

Under lab conditions, a Parshall Flume can be accurate to within +/-2% but practically speaking, once approach conditions, installation errors, and construction tolerances are considered, the flume's free-flow accuracy usually is closer to +/-5% (ASTM D1941).

A universal flow equation is available to correct for the effects of submerged flow.

If the flume shifts out of position, a correction for lateral / longitudinal settling has been developed.

Flume Dimensions

No two Parshall Flumes are identical in scale. Openchannelflow recommends against using intermediate or untested sizes.

Smaller Parshall Flumes (1, 2, 3-inch sizes) do not have standardized sidewall heights. Openchannelflow's 1-inch Parshall has a standard sidewall height (at the primary point of measurement - H_a) of 9-inches [22.86 cm].

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)

For the 3-inch Parshall flume, the values are:

Minimum Head (ft)	0.05	Minimum Head (m)	0.0152
Minimum Flow Rate (cfs)	0.0033	Minimum Flow Rate (l/s)	0.0921
Maximum Head (ft)	0.70	Maximum Head (m)	0.2134
Maximum Flow Rate (cfs)	0.1945	Maximum Flow Rate (l/s)	5.507
Equation (cfs, ft)	0.338*H^1.55	Equation (l/s, m)	60.36*H^1.55

Discharge Table

The free flow discharge table for the 2-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 2-inch Parshall Flume is available from Openchannelflow in four different materials:

Of these materials, fiberglass is the most easily customized.

Submergence Transition

The Submergence Transition (S_t) 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 for the smaller Parshall Flumes (1, 2, 3-inch) is 50%. At this point and the discharge needs to be adjusted to reflect the actual flow rate.