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Open Channel Flowmeters

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Open channel flow occurs when liquid flows in a conduit or channel with a free surface -- like rivers, streams, canals, and irrigation ditches -- as opposed to the closed pipes common in industrial and process control environments. Other examples of open channel flow include flow in water treatment plants, storm and sanitary sewer systems, industrial waste applications, sewage treatment plants, and irrigation systems.  

Many towns and municipalities that monitor drinking water and wastewater treatment flows use open channel measurement. Some industrial applications also measure open channel flow.

Open channel vs. closed-pipe flow

What is slightly confusing about the term open channel is that that the flow of liquids in partially filled pipes -- when not under pressure -- is also considered open channel flow. Water flowing through a culvert running underneath a street is considered open channel flow, for example even though the channel is not actually "open." Likewise, flow in sewers and tunnels are classified as open channel flows, along with other closed channels that flow partly filled. 

One way to understand the difference between open channel and closed-pipe flow is to think of it as the difference between gravity-induced and pressurized flow: 

  • Flow in uncovered channels like irrigation ditches depends on gravity.  Likewise, flow in partially filled closed conduits, such as culverts and drain pipes, also is gravity-dependent.  

  • Flow in closed pipes for industrial applications, however, occurs under pressure.  

So open channel flow might be called gravitational flow, while closed-pipe flow could be called pressurized flow. This explains why flows in both uncovered conduits and partially filled pipes are considered to be open channel flow: They are both examples of gravitational flow.

How to measure open channel flow

There are two main ways to measure open-channel flow.  

Hydraulic structure

A very common method of open-channel flow involves the use of a hydraulic structure such as a weir or flume as a primary device -- a restriction placed in an open channel that has a known depth-to-flow relationship. 

A flume is a specially shaped portion of the open channel with an area or slope that is different from the channelís slope or area. The velocity of the liquid increases and its level rises as it passes through the flume. To determine flowrate, liquid depth is measured at specified points in the flume. 

A weir resembles a dam placed across an open channel. It is positioned in such a way that the liquid can flow over it. Weirs are classified according to the shapes of their openings. Types of weirs include: V-Notch; rectangular; and trapezoidal.
Water depth is measured at a specific place upstream from the weir.

Charts correlate various water depths with flowrates, taking into account different types and sizes of weirs and flumes. They associate an equation for determining flowrate with each type of weir: 

Velocity method

Flow can also be measured without a hydraulic structure. In the area velocity method, the mean velocity of the flow is calculated at a cross-section, and this value is multiplied by the flow area. Normally, this method requires that two measurements be madeóone to determine mean velocity and another measurement to determine depth of flow. The area velocity method is used when it is not practical to use a weir or flume and for temporary flow measurements -- influx and infiltration studies and sewer measurements, for example. 

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Opportunities in open channel flow

While pressurized, closed-pipe flow has grabbed much of the attention in the flowmeter market, open-channel flow measurement is an exciting area that is worth a closer look. Custody transfer of water, for example, is a critical application. Environmental regulations and the increasing importance of water as a natural resource are generating growing interest in open channel flow. 

In addition, the variety of open flow technology types provides room for new methods. Recent developments, for example, revolve around electronic enhancements and improvements in communication. There is also room for truly novel developments involving new or improved sensor technologies o, including Doppler technology.

For more information about open channel flow, check out our comprehensive studies on the world flowmeter market.

Flow Research, Inc. | 27 Water Street | Wakefield, MA 01880 | (781) 245-3200 | (781) 224-7552 (fax) | (800) 245-1799 (from the USA) | info@flowresearch.com

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