Open Channel Flowmeters

Any conduit or channel in which the fluid or liquid is flowing with a free surface open to the atmosphere is referred to as an “open channel”. For instance, tunnels, nonpressurized sewers, partially filled pipes, canals, streams, and rivers, all these are Open channels. There are various techniques which can be used to monitor an open-channel flow. However, depth-related techniques are the most widespread. These techniques are based on the conjecture that one can instantaneously determine the flow rate of liquid by measuring its depth (or head). Two most commonly used primary devices for measurement of open-channel flows are:

  1. Weirs and
  2. Flumes

These two are specifically designed channel shapes which illustrate the flow of water. Selection of a flume or weir type is based upon following factors:

  • Type of application
  • Flow rate
  • Shape of the Channel
  • Solids content existing in the water

Open channel flowmeters are typically required for applications such as wastewater treatment and irrigation flows.

Weirs

These devices are based on the working principle that “an obstruction in a channel will cause water to back up, creating a high level (head) behind the barrier. The head is a function of flow velocity, and, therefore, the flow rate through the device”.1
Weirs are constructed of vertical plates having sharp crests. The term crest usually means the top surface of the weir above which the water flows. This top surface i.e. crest of the plate can be made either straight or notched. The height of the water flowing above the top surface of the weir is referred to as the head over the weir. It is denoted by H and used to calculate the flowrate.

Types of Weirs

According to the shape of the notch, weirs are classified in following two types:

Sharp Crested

Sharp crested weirs usually have a sharp edged crest or top made up of thin metal plates. These types of weirs are commonly found in following three shapes:

  1. Rectangular
  2. V-notch
  3. Triangular

Major application areas of sharp crested weirs include water supply, wastewater and sewage systems.

Broad Crested

Broad crested weirs have a broad flat surface at the crest which is made up of wide timber or concrete. These types of weirs normally find their application in dam spillways where the broad surface lies underneath the water surface across the complete streamline. In case of broad-crested weirs, accurate flow measurement installations can be done only if they are properly calibrated.
Apart from the above two major types, some other types of weirs are also existing like the trapezoidal i.e. Cipolletti weir, Sutro i.e. proportional weir and compound weirs which is a combination of multiple weir shapes.

Flumes

These devices are most often employed in areas where it is recommended to keep the head loss at minimum, or in cases where the flowing fluid has large quantity of suspended solids in it. Use of Flumes in open channels is equivalent to use of venturi tubes in closed pipes. Unlike a weir which puts in obstruction in the form of a vertical plate, a flume consists of a constriction in the channel width. This constriction results in change of water level which can be further correlated with flow rate calculation.
Most common types of flumes in use are:

Parshall Flume

This type of flume includes

  • a converging upstream section
  • a throat
  • a diverging downstream section

Walls of the flume are vertical and the floor of the throat is inclined in downward direction. Head loss through a Parshall flume is comparatively lower than other types of open-channel flowmeters like weirs. High velocity of flow assists in making possible the flume self-cleaning. Via a Parshall flume, precise measurement of flow can be done even under a wide range of conditions. These types of flumes are frequently employed in wastewater treatment plants for measurement wastewater flow rates.

Palmer-Bowlus Flume

This type of flume consists of a trapezoidal throat of uniform cross section. Its length is almost equal to the pipe diameter in which it is mounted. Its accuracy is as good as a Parshall flume. Besides, it has the capability to pass debris without cleaning. As compared to Parshall flume, this type of flume can be installed easily in existing circular conduits, since a rectangular approach section is not needed.

Level Measurement Involved

Since discharge by means of weirs and flumes is a function of level, level measurement methods are supposed to be employed with the device in order to calculate flow rates. For this purpose following systems can be used:

  • Staff gages
  • Float-operated units
  • Different electronic sensing, totalizing, and recording systems
  • Recently developed systems using ultrasonic pulses

In latest ultrasonic systems, measurements are done by transmitting sound pulses from a sensor to the liquid surface, and noting the time of the echo return. There is a Linearizing circuitry which transforms the height of the liquid into flow rate. Further, a strip chart recorder is used to log the flow rate, and a digital totalizer for registering the total gallons.

 

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