![]() ![]() The derived equations above will always underestimate actual flow since both the derived C and n values are lower than their respective chart values.įor the Parshall flume equation used to calculate the flow rate, both empirical values C and n are known constants (with various values for each Parshall flume size) leaving Ha (depth upstream) as the only variable needing to be measured. In fact, when looking at the flume tables, n has a value equal to or slightly greater than 1.5, while the value of C is larger than (3.088 b 2) but still in a rough estimation. These final two equations are very similar to the Q = CH a n equations that are used for Parshall flumes. The free-flow discharge can be summarized in this equation: Q = C H n under free-flow conditions, the depth of water at a specified location upstream of the flume throat can be converted to a rate of flow. The Parshall flume accelerates the flow through a contraction of both the parallel sidewalls and a drop in the floor at the flume throat. Soil Conservation Service, the Parshall flume is a fixed hydraulic structure used in measuring volumetric flow rate in surface water, wastewater treatment plants, and industrial discharge applications. The Parshall flume is a modified version of the Venturi flume. Parshall flumes (right) alongside other types of flow meters Diagram of a Parshall flume, showing free-flow and submerged flow operating regimes The secondary point of measurement (Hb) for a Parshall flume is located in the throat, measuring Hb can be difficult as flow in the throat of the flume is turbulent and prone to fluctuations in the water level in which 90% is viewed as the upper limit for which corrections for submerged flow are practical. Submergence transitions for Parshall flumes range from 50% (1–3 inch sizes) to 80% (10–50 foot sizes), beyond which point level measurements must be taken at both the primary and secondary points of measurement and a submergence correction must be applied to the flow equations. The flumes are not patented and the discharge tables are not copyright protected.Ī total of twenty-two standard sizes of Parshall flumes have been developed, covering flow ranges from 0.005–3,280 cfs (0.1416–92,890 L/s). The design of the Parshall flume is standardized under ASTM D1941, ISO 9826:1992, and JIS B7553-1993. This article's tone or style may not reflect the encyclopedic tone used on Wikipedia. ![]()
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