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

Hydraulic design of measuring weirs with a triangular profile chute

Summary

The method of designing measuring weirs with triangular profile chute is presented. In the regulatory document MI 2406-97 «GSI (State System for Ensuring Uniform Measurement). Liquid flow rate in water and wastewater gravity channels. Measuring methods with the use of standard weirs and flumes» that regulates the use of flow metering devices for measuring water flow rate in gravity water supply systems and sewers the design aspects have been condensed without actual examples. This article aims at filling this gap. The conditions that provide for the possibility in principle to use the method of measuring liquid flow with the use of standard orifices (weirs and flumes) are considered. Such condition, in particular, is the availability of a straight-line channel section of adequate length with free (without backflow) flow. The design procedure is presented with specific reference to flow measuring in a concrete rectangular channel 3.5 meters wide with a maximum flow rate of 56 250 m³/h. The procedure suggests sequential determination of the maximum weir head and chute height as well as verifying the conformity of the weir with a triangular profile chute to the regulatory requirements including possible weir submergence with calculating the maximum flow level in the channel downstream. The ratios between the chute width and maximum head, maximum head limits and Froude number are also verified. Further in the process of designing and basing on the hydraulic calculations the type of the flow-level meter and its metrological performance are chosen and the geometry of the chute is determined.

Key words

flow rate measurement , gravity flow , weir , triangular profile chute , hydraulic design

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