Measuring Pipe Flows with Trapezoidal Flumes

Originally developed to provide both low-end sensitivity and wide flow, Trapezoidal flumes have expanded beyond the agricultural applications they were designed to measure a wide range of pipe flows.

Trapezoidal flumes can be found on pipes measuring:

Dam seepage
Industrial effluents
Small municipal plant sewages

Trapezoidal flumes can measure pipe flows above grade (free standing or factory installed in an enclosure), below grade (in existing concrete manholes / vaults), or as part of a factory integrated fiberglass Packaged Metering Manhole).

Trapezoidal Flume Design

Their cross-sectional area is that of either a sharp V (60º series of flumes) or a truncated V (SRCRC, CSU, and WSC Trapezoidal flumes).

v shape cross section of a fiberglass 2-inch 30 degree CSU Trapezoidal flume

Their flat-bottomed design also mean that they provide better head conservation than flumes requiring large slopes up to and out of the flume (i.e. Parshall). The flat-bottomed design also passes debris easily and makes the flume, with staged end adapters, an easy installation in existing concrete manholes.

End Adapters and Pipe Stubs

End adapters are specially formed transitions that connect to the inlet or outlet of a flume on one end and then a pipe on the other. Pipe stubs are molded into the end adapter and can be left plain (for use with a compression coupling), flanged, or oversized to act as a caulking collar.

a fiberglass trapezoidal flume with end adapters measuring pipe flow in a concrete vault

Pipes of varying sizes can be molded into the end adapter. The limiting factor is the width of the end adapter opposite the flume. For pipe stubs larger than the flume is deep, the stub simply extends above the flume / end adapter.

pipe stub on inlet end adapter of a fiberglass Large 60 degree Trapezoidal flume

Staged End Adapters for Existing Manholes

A variation on the end adapter design is the curved, staged end adapter. These end adapters take the general design of the standard pipe connection end adapter, remove the pipe stub, and curve the inlet / outlet wall. The curved wall does away with the need to make a connection with a pipe stub and the existing line and instead allows the staged end adapter to be bolted directly to the manhole wall.

staged inlet end adapter on a large Trapezoidal flume used to measure pipe flow in a concrete manhole

As the quality of the incoming flow is a great factor in overall measurement accuracy, the inlet staged end adapter is bonded to the inlet of the flume.

The outlet staged end adapter ships loosely bolted to the outlet of the flume. That way, if conditions in the concrete manhole are less than ideal, the less critical outlet staged end adapter can be omitted if spacing or obstructions require a modification to the flume.

As with the pipe stub end adapters, Trapezoidal flumes can be used on pipes as large as the end adapter is wide.

Note that where the inlet and outlet pipe elevations are not the same, the outlet staged end adapter can be omitted and downstream channel formed to transition flow out of the flume.

Trapezoidal Flumes in Packaged Metering Manholes

For some below grade applications, a concrete manhole or vault may not be available or suitable for installing a Trapezoidal flume. Here, factory integration of the flume into a fiberglass Packaged Metering Manhole is the best option.

fiberglass packaged metering manhole with factory integrated trapezoidal flume

Packaged Metering Manholes integrate a primary device (here a Trapezoidal flume) into an engineered access structure. The resulting unit is a single, watertight, and corrosion resistant structure that is ready for immediate installation and which requires little in the way of line downtime. While somewhat more than a flume / concrete manhole installation, a Packaged Metering Manhole is considerably quicker to install, requires less on-site labor / equipment, and provides a turnkey solution that is preferred by engineers and end-users.