The effect of shortening the feed section of Venturi flume on the error of measuring the liquid flow rate

Summary

The use of flow meters with Venturi flumes for measuring the flow rate of effluent before discharge into the water reservoir or of wastewater transported to the treatment facilities is reported. The new construction projects boost the flow of wastewater; the level of the upstream water rises, the relative length of the feed section expressed in upstream maximum depth units decreases and does not meet the regulatory requirements any longer. When the similar situation occurs in the throat of the pressure flumes the flow meters are admitted to service however with a higher effluent coefficient error. The procedure of estimating a complementary error at the reduction of the relative feed section length in case of measuring the liquid flow rate with Venturi flumes is suggested.

Key words

measurement error , wastewater , flow rate measurement , Venturi flume , throat

Measuring liquid flow rate with Venturi flume at free and submerged flow

Summary

Topical issues of establishing the facilities for measuring wastewater flow and volume to provide for fiscal metering in gravity wastewater pipelines and channels on the basis of the Venturi flume are considered. The method of measurement with the use of flumes has been set by the regulatory document MI 2406-97; however, the document considers only the flumes of critical depth that function provided the flow from the flume is free. In practice the flow patterns deviate from the design values and often free flow intersperses with the submerged one. At submerged flow the provisions of the document cannot be applied. The prescriptive form of the document neither discloses the physics of the hydraulic processes in the flume nor provides for summarizing the use of the flume for different conditions. In this context the detailed analysis of the flume operation is presented; the flow rate formula has been obtained; the physical meaning of the coefficients included into the formula has been revealed; the analogy of the flume of critical depth with critical gas nozzles is shown; the requirements to the channel downstream the flume to ensure the critical regime in the flume have been formulated. On the basis of the calculations of energy losses in the flume the criterion in the form of the recovery depth separating free and submerged flows has been determined. It is shown that at submerged flow a multichannel level meter shall be used to control the flow depth in different cross-sections of the flume. The formula of the flow rate at submerged flow including the relationship of the depth at the neck and at the flume inlet as a parameter has been obtained. The instrumental solution of the problem is presented with a structural scheme of a multichannel ultrasonic level meter – flow meter; the principle of its operation is described.

Key words

flow rate measurement , Venturi flume , gravity flow , free and submerged flow , measuring facilities , ultrasonic multichannel level meter