Open Channel Flow meters

Open channel flow is defined as flow in any channel in which
the liquid flows with a free surface. Examples include rivers and
irrigation channels. Certain closed channels such as sewers,
when flowing partially full and not under pressure, are also
classified as open channels.
Open channels are used to conduct liquids in most
sewer systems, sewage treatment plants, industrial waste
applications, and irrigation systems.
There are three methods for automatically measuring open
channel flow.
• Hydraulic Structures
• Area Velocity
• Slope-Hydraulic Radius
Hydraulic Structures
The most common method of measuring open channel flow is
the hydraulic structures method. A calibrated restriction
inserted into the channel controls the shape and velocity of the
flow. The flow rate is then determined by measuring the liquid
level in or near the restriction.
The restricting structures are called primary measuring devices.
They may be divided into two broad categories-weirs and
flumes.
A weir (Figure 1) is an obstruction or dam built across an open
channel over which the liquid flows, often through a specially
shaped opening. Weirs are classified according to the shape of
this opening. The most common types of weirs are the
triangular (or V-notch) weir, the rectangular weir, and the
trapezoidal (or Cipolletti) weir.
The flow rate over a weir is determined by measuring the liquid
depth in the pool upstream from the weir.
Weirs are simple and inexpensive to build and install. Common
materials of construction include metal, fiberglass and wood.
However, they represent a significant loss of head, and are not
suitable for measuring flows with solids that may cling to the
weir or accumulate upstream from it.
A flume is a specially shaped open channel flow section
providing a restriction in channel area and/or a change in
channel slope. The flow rate in the channel is determined by
measuring the liquid depth at a specified point in the flume.
The most common flume is the Parshall flume (Figure 2). The
flow rate through a Parshall flume is determined by measuring
the liquid level one third of the way into the converging section.
Parshall flumes are designated by the width of the throat, which
ranges from one inch to 50 feet. The throat width and all other
dimensions must be strictly followed so that standard discharge
tables can be used. Also, note the drop in the floor of the flume,
which makes it difficult to install a Parshall flume in an existing
channel.
Top View
Level
measurement
point
2/3 A
W
Flow
P D Throat section
H
Diverging
C
section
section
Converging
A
R
Side View
M B T G
Water
surface
E
K
Slope 1/4
N
Figure 2. Parshall flumes are self-cleaning and measure a wide
range of flow rates.
Weir
Weir
Figure 1. Non-contacting ultrasonic sensors are often used to
measure the level upstream from a weir.
In contrast, the Palmer-Bowlus flume (Figure 3) is designed to
be installed in an existing channel with minimal effort. The flow
rate through a Palmer-Bowlus flume is determined by
measuring the liquid depth at a point one-half pipe diameter
upstream from the flume throat.
Palmer-Bowlus flumes are designated by the size of the pipe
into which they fit. Standard sizes range from four to 42 inches.
The dimensional configuration is not rigidly established for
each flume size. However, a Palmer-Bowlus flume with a
trapezoidal throat with a flat bottom has emerged as the
standard design for circular pipes.
Flumes are more expensive and more difficult to install than
weirs. Common materials of construction include fiberglass,
concrete and metal. However, flumes result in a lower head loss
and are self-cleaning, requiring less maintenance