Choosing the method of gravity flow measurement in public sewers

Summary

The aspects of choosing the method of measuring the flow rate and volume of wastewater gravity flow, particularly in case when the measurement results are used in accounting procedures (commercial accounting) are considered. It is shown that before studying the methods of measurement possible replacing gravity flow with pressure flow shall be considered because pressure flow rate can be measured more accurately. This measurement option is optimal from the technical point of view; however, in case the pressure section in the gravity pipeline is not available additional expenses are required for the water main reconstruction. In case this option is unacceptable the following methods of gravity flow measurement shall be considered (in the order of application priority): «level-flow rate» with the use of standard orifices (weirs and flumes) according to MI 2406-97 regulatory document; «level-flow rate» without any orifices with preliminary calibration of the measuring section according to MI 2220-13; «velocity-surface area» with the use of ultrasound flow meters; studying the velocity profile of the cross-section of the water main; calculation-instrumental (balance). By the specific example of an ultrasound flow meter it is shown that the flow rate error subscribed by the technical documentation can be several times lower than the actual one.

Key words

flow rate , measurement error , wastewater , ultrasonic flowmeter , gravity flow , flow measuring flume , calibration of measuring section