Q free = the open channel flow rate through the Parshall flume under free flow conditions, cfs for U.S.The free flow equation for Parshall flume discharge calculation is Q free = C H a n, where The values of H b/H a for free flow and for submerged flow, for several ranges of throat width from 1″ to 8′ are as follows:įor 8’ 0.8 Excel Formulas for Free Flow Parshall Flume Discharge Calculation The ratio between head measurements at the two locations, H a and H b, as shown in the diagram at the left above, can be used as a quantitative criterion to differentiate between free flow and submerged flow. For flow situations where downstream conditions cause the flow to back up into the throat, the hydraulic jump isn’t visible, and the flow is said to be “submerged flow” rather than “free flow.” For free flow, a hydraulic jump will be visible in the throat of the Parshall flume. X = 2 inches, Y = 3′ Free Flow and Submerged Flow in Parshall Flume Discharge Calculationįor “free flow” through a Parshall flume, the flow rate through the throat of the flume is unaffected by the downstream conditions. For the range of throat widths in the table, the other dimensions in the diagram are constant at the following values: A 1 inch flume will carry a flow of 0.03 cfs at 0.2 ft of head, while a 50 ft Parshall flume will carry 3,000 cfs at a head of 5.7 ft. Such a range of sizes covers a very wide range of flow rates. Similar information is available for throat widths down to 1 inch and up to 50 ft. The table at the right below, shows the standard dimensions for Parshall flumes with throat widths ranging from 1 ft to 8 ft. The width of the throat is typically used to specify the size of a Parshall flume. The diagram at the left shows the general configuration of a Parshall flume with a plan and elevation view. Image Credit: City of Batavia, Illinois Flume Configuration and Dimensions for Parshall Flume Discharge Calculations A Parshall flume must be constructed with prescribed dimensions as shown in the next section. Flow rate through a Parshall flume can be calculated based on a measured head, using equations that will be discussed in a later section. A Parshall flume, however, also has prescribed variations in the channel bottom slope as shown in the diagram in the next section. As seen in the picture at the right, the plan view of a Parshall flume is similar to that of a venturi flume, with a converging section, a throat, and a diverging section. They are especially good for flows containing suspended solids, as for example the flow in wastewater treatment. Parshall flumes are used for a variety of open channel flow measurement. Read on for information about Excel spreadsheets that can be used for Parshall flume/open channel flow measurement calculations. Obtain a convenient, easy to use Parshall flume discharge calculation spreadsheet at a reasonable price. 0.0350 - 3.Where to find a Parshall Flume Discharge Calculation Spreadsheetįor a Parshall flume discharge calculation Excel spreadsheet to make open channel flow measurement calculations, click here to visit our spreadsheet store.Galvanized Parshall flumes should never be used on sanitary flows. Smaller surface water, irrigation, and edge-of-field flows are also good uses for the flume. The 6-inch Parshall flume is useful in measuring small water rights.
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