On the flow characteristic of a measuring weir of unconventional profile

Summary

The aspects of measuring gravity flows in water supply and sewer systems with the use of different profile measuring weirs – weir notch, wide chute, and practical profile are considered. The aspect of flowing around the weir crest of different design is covered. It is shown that designing rectangular weirs without contraction in cases when atmospheric air admission under the stream of the watertail is not provided the measurement error can increase by 20–25%. The best solution is designing if possible the measuring weir with small, around 2–3%, side contraction. By the actual example of the rectangular weir designed for one of the Moscow treatment plants it is shown that deviation from the stringent metrological requirements to the overflow wall profile makes the identification of the weir impossible and can cause up to 20% additional measurement error. It is recommended to observe rigidly the provisions of MI 2406-97 regulatory document even in case when no commercial water account is required and the readings of the meter are used for process monitoring and operative control.

Key words

gravity flow rate , measuring weir , dynamic pressure , hydrostatic head , upstream reach , weir crest profile