Civil Engineering Reference
In-Depth Information
What is the fluid being measured?
•
What is the flow range (minimum, average and maximum)?
•
Is the fluid likely to be contaminated?
•
What accuracy of flow measurement is required?
•
Is the fluid corrosive?
•
What is the available headloss that can be tolerated?
•
After these questions have been answered, one of the following meters can be selected.
For these meters, typically they should have a length of about 10 pipe diameters
upstream of the meter and three to five pipe diameters downstream of the flow element.
Differential Pressure Meters
Differential pressure meters include the Venturi-type meters such as the Dall tube,
Venturi insert, and conventional Venturi. They can handle a flow range of 10:1 and
have an accuracy of
1 percent. For best results, the flow range should be less than
10:1. These flow tubes should be installed such that they are always full of the fluid,
and do not permit a negative pressure to occur at the throat at maximum flow. These
units are typically used for filter rate control and raw-water and finished-water meter-
ing.
Orifice Meters
Another differential device is the orifice plate, described earlier in this chapter. Their
flow accuracy is rated at
2 percent with a flow range of only 4:1. However, these
units create a high headloss, ranging from 40 percent of the differential pressure to as
much as 90 for a small throat diameter to pipe diameter. These are sometimes used
for air flow measurement and occasionally for water. But due to their high headloss,
they are not favored. Other meters that are frequently used in water treatment plants
are described below.
Propeller and Turbine Meters
These units operate through the fluid causing a propeller or turbine to rotate. As they
rotate, a mechanism measures the rotation and sends out electric pulses. These flow
devices have an accuracy of about
2 percent and a flow range of 10:1. For these
units to operate efficiently, the minimum velocity should be about 1 ft / sec. These
meters are typically used on clean, uncontaminated water for billing purposes for
customer supplies.
Magnetic Flow Meters
These devices operate by the water changing a magnetic field as it passes through the
unit. The change in magnetic fluid creates a voltage that is proportional to the velocity
through the meter, and through continuity, the flow rate is computed (
Q
V
A
).
These meters have an accuracy of less than 1 percent for higher velocities (3 to 20 ft/
sec). At velocities less than 3 ft / sec, the accuracy deteriorates. These meters have
