Parshall Flume Design
When viewed from the top, the Parshall flume has an hourglass shape and, like the Venturi flume upon which it is based, is composed of three distinct sections:
- Uniformly converging inlet
- Short and narrow throat section
- Uniformly diverging discharge
The floor of the Parshall flume – one of Dr. Parshall’s more radical inventions – is flat in the converging section (where the primary point of measurement is), drops through the throat (creating supercritical flow), and then rises in the diverging section. Although higher than the lowest portion of the throat, the outlet of the diverging section is always lower than the floor of the converging section.
The flat floor of the converging section (where the free flow point of measurement is) means that all that is required to determine the flow rate is to gauge or “stick” the water level at the point of measurement - water surface to flume floor. Unlike Palmer-Bowlus or RBC flumes, there is no ramp level to deduct from the reading.
Unlike Palmer-Bowlus or Trapezoidal flumes, the side walls of the Parshall flume are vertical. Vertical walls mean that when a staff gauge is used, there is no need to check that the gauge has been calibrated to read true vertical depth.
The flat converging section floor and the straight vertical sidewalls make the Parshall flume one of the easiest flumes to use accurately.
Parshall Flume Dimensions
The dimensions for many Parshall flume sizes were developed by Dr. Parshall during his investigations. Additional smaller and larger Parshall flume sizes were added after Dr. Parshall’s investigations had ended.
The dimensions, and tolerances, of the Parshall flume have been standardized in a number of national / international standards and authoritative publications:
- ASTM D1941-91 (Reapproved 2013) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall Flume
- ISO 9826:1992 Measurement of Liquid in Open Channels - Parshall and SANIIRI Flumes
- Water Measurement Manual, 3rd Edition, United States Department of the Interior, Bureau of Reclamation
- The Parshall Flume: Instructions for Installation and Table of Discharge, State Engineer of Colorado
- JIS B7553 Parshall Flume Type Flowmeters
It's critically important to note that although Parshall flumes share a common layout, they are not scale models of each other – they are empirical devices for which investigations have been made to determine their specific flow rates.
The use of non-standard flume sizes is HIGHLY discouraged and the dimensions and flow accuracies of such devices should be considered suspect.
ASTM D1941 requires that Parshall flumes have dimensions within +/-2% of nominal, while JIS B7553 requires tolerance of +/- 1 to 1.5% depending upon the flume size.
Parshall flumes with dimensions outside of these ranges should be considered to be non-conforming and should be either replaced for field rated.