The Parshall flume is an economical and accurate way of measuring the flow of water in open channels and non-full pipes. Originally developed to measure surface waters, water rights apportionment, and irrigation flows, Parshall flumes are now used to measure piped / plant sewage flows, industrial discharges, and dam seepage.
As a class of 22 standard sizes, the Parshall flume is the most commonly used and recognized flume for flow measurement.
Parshall Flume Materials
Openchannelflow manufactures Parshall flumes in a number of different materials:
- Fiberglass (FRP)
- Galvanized steel
- Stainless steel
Aluminum offers is great for portable flumes or remotes sites where it is difficult to bring in material. Light weight and robust of construction, aluminum does cost more than most other materials.
Fiberglass hits the sweet spot for many: it is light weight, cost competitive, and provides good corrosion resistance. For sewage applications, fiberglass is almost exclusively used.
When measuring surface waters where cost is the concern, galvanized steel is the material of choice. Although heavy, it is easy to repair and withstands abuse well.
Parshall Flume Accessories
- Piping / end connections
- Flow condition options
- Flow meter mounts
- Sampler / parameter mounts
- Custom configurations (nesting, extended / reduced sidewalls, etc.)
Parshall Flume Applications
The versatility of the Parshall flume has seen it used in a diverse number of applications, including:
- Sewage Treatment Plants
- Watershed Monitoring
- Edge-of-Field Runoff
- Dam Seepage
- Stream Gauging
- Industrial Discharge Monitoring
- Mine Discharge
- Irrigation Canals
- Spring Discharge Measurement
How a Parshall Flume Operates
A Parshall flume is a fixed hydraulic device that is placed in a flow stream and which accelerates flow by both contracting the parallel sidewalls and well as creating a drop in the floor elevation. The contraction and drop accelerates the flow from a slow, subcritical state to a supercritical one.
As a result, the flow can accurately determined in the upstream, converging section of the flume by taking a single depth reading at a specific point of measurement.
Short-throated flumes like the Parshall have only ONE point of measurement (Ha) at which the flow rate can be determined. A level reading taken upstream of the point of measurement will result in the flume over reading, while one taken downstream will result in the flume under reading (as the flow accelerates and the water surface draws down towards the throat).
Developing the Parshall Flume
While working at the U.S. Soil Conservation Service, Dr. Ralph L. Parshall saw a need for more accurate measurement of surface waters – in particular irrigation / water rights flows. At that time, flows were commonly measured with weirs or Venturi flumes – both of which had sizeable shortcomings. Starting in 1915 with the sub-critical Venturi flume, Dr. Parshall made a series of modifications, ultimately leading to his Improved Venturi flume.
The principal improvement over the sub-critical Venturi flume was the introduction of the drop in elevation through the throat. This drop in elevation allows for the creation of supercritical flow through the throat of the flume. As a short-throated flume only one measurement needed to be made to determine the flow rate through the flume – greatly simplifying the determination of the flow rate.
Six years after his work began, Dr. Parshall filed a patent for his new “Parshall Flume”.
Over the years, Dr. Parshall’s work has been extended to flumes both larger and smaller than his original investigations. Currently there 22 distinct size of Parshall flume standardized under ASTM D1941-91(2003) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall Flume and ISO 9826:1992 Measurement of Liquid Flows in Open Channels – Parshall and SANIIRI Flumes and other national and industry standards.
It is important to note that Parshall flumes are not scale models of each other, as such intermediate sizes (commonly the 30-inch and 42-inch sizes) should not be used, or if used they MUST be laboratory or field rated. The use of standard Parshall flume equations for non-standard sizes is not acceptable.