Environmental Engineering Reference
In-Depth Information
•
Ease of instrument and range selection
•
Extensive product experience and performance database
Disadvantages include:
•
Flow rate being
a nonlinear function of the differential pressure
•
Low low rate rangeability with normal instrumentation
2.13.4.1 Operating Principle
Differential pressure flow meters operate on the principle of mea-
suring pressure at two points in the flow, which provides an indication
of the rate of flow that is passing by. The difference in pressures between
the two measurement locations of the flow meter is the result of the
change in flow velocities. Simply, there is a set relationship between the
flow rate and volume, so the meter instrumentation automatically trans-
lates the differential pressure into a volume of flow. The volume of flow
rate through the cross-sectional area is given by:
q
=
a
×
v
where:
q
= the volumetric flow rate.
a
= flow in the cross-sectional area.
v
= the average fluid velocity.
2.13.4.2 Types of Differential Pressure Flow Meters
Differential pressure flow meters operate on the principle of devel-
oping a differential pressure across a restriction that can be related to
the fluid flow rate.
Note:
Optimum measurement accuracy is maintained when the flow meter
is calibrated and installed in accordance with standards and codes of
practice, and the transmitting instruments are periodically calibrated.
The most commonly used differential pressure flow meter types
used in wastewater treatment are:
1. Orifice
2. Venturi
3. Nozzle
4. Pitot-static tube
2.13.4.2.1 orifice
The most commonly applied type, the orifice flow meter is a thin,
concentric, flat metal plate with an opening in the plate (see Figure 2.23)
installed perpendicular to the flowing stream in a circular conduit or
