Water Hydraulics - Wastewater Operators - page 100

Piston Sliding vane Oval gear

Trirotor Birotor Disc

figure 2.33 Six common positive-displacement meter principles.

Simply, a positive-displacement flow meter is one that continuously
divides the flowing stream into known volumetric segments, isolates the
segments momentarily, and returns them to the flowing stream while
counting the number of displacements. A positive-displacement flow
meter can be broken down into three basic components: the external
housing, the measuring unit, and the counter drive train. The external
housing is the pressure vessel that contains the product being mea-
sured. The measuring unit is a precision metering element made up of
the measuring chamber and the displacement mechanism. The most
common displacement mechanisms include the oscillating piston, slid-
ing vane, oval gear, trirotor, birotor, and nutating disc types (see Figure
2.33). The counter drive train is used to transmit the internal motion of
the measuring unit into a usable output signal. Many positive-displace-
ment flow meters use a mechanical gear train that requires a rotary
shaft seal or packing gland that penetrates the external housing.

The positive-displacement flow meter can offer excellent accuracy,
repeatability, and reliability in many applications. It has satisfied many
needs in the past and should play a vital role in serving future needs.

2.13.9 open Channel flow Measurement *

The majority of industrial liquid flows are carried in closed conduits
that flow completely full and under pressure; however, this is not the
case for high-volume flows of liquids in waterworks, sanitary, and storm-
water systems that are commonly carried in open channels. Low system
heads and high volumetric flow rates characterize flow in open channels.
The most commonly used method of measuring the rate of flow in an
open channel flow configuration is that of hydraulic structures . In this

* This section is adapted from Grant, D.M., in flow measurement , D.W. Spitzer, Ed.,
Instrument Society of America, Research Triangle Park, NC, 1991, pp. 252-290.