April 6, 1998 Design News

DESIGN APPLICATIONS From the regional editors

Flowmeter combines flume and Venturi tubes

?provides accurate measurements under full or partially full pipe flows

by David J. Bak, International Editor

Austin, TX--Municipalities and metropolitan areas require flow data when a) billing industries for wastewater disposal, b) estimating sewage treatment plant capacity, and c) performing inflow and infiltration studies. A patented flow tube, designed for all flow conditions, is said to yield more accurate flow measurement data than presently used level-only or velocity-area meters.

Because level-only meters such as flumes and weirs cannot be used for full-pipe flow, their use is limited to open channel applications. Velocity-area meters (flow = velocity x area) handle full-pipe flows. Accuracy, however, depends on thorough velocity profiling--a lengthy, unpleasant, and sometimes dangerous task performed at the bottom of a manhole.

The DataGatorr flowmeter, developed with grants from the U.S. Department of Agriculture, overcomes the shortcomings of competing designs. Three parts comprise the whole: flowtube, electronics, and software for communication between the electronics and a PC. Flowtube design is a combination flume and Venturi tube.

Under open-channel conditions, the flow tube functions as a Venturi flume, i.e., a constriction in an open channel so proportioned as to cause the occurrence of flow at critical depth. Under full-pipe conditions, the flowtube functions as a Venturi tube, i.e., a constriction in a closed conduit designed to increase the velocity of flow and decrease pressure.

A predictable relationship between flow depth in the flowtube throat and flow depth immediately upstream, says Marappan Natarajan, president and engineering director at Renaissance Instruments, permits simultaneous filling of the throat and upstream pipe. "This feature," he claims, "eliminates inaccuracies during the transition of flow from open-channel to full-pipe conditions." As a result, the DataGator system is good for both partially full and full-pipe flow metering.

Three pressure transducers--inlet, throat, and outlet--measure hydrostatic pressures. Flowtube symmetry accommodates reverseflow with accuracy equal to forward flow. In addition, profile independence and fixed flowtube dimensions for a given pipe size allow precalibration at the factory. This, in turn, reduces installation to approximately ten minutes--a welcome change for operators used to spending hours inside a manhole with velocity-area meters.

Additional details...Contact Renaissance Instruments, 2013 Wells Branch Parkway, Suite 117, Austin, TX 78728-6904, Tel: (512) 251-6647.

Other Applications

Clean water flow