Open channels are significant for irrigation systems. They provide water for agriculture and household use. Information about the rate of flow of water in open channels helps us to ensure the safety of the channel and control how water is distributed. There are many ways to measure the flow of water. Using a weir plate could be a better option.
What is a weir plate?
A weir plate is an obstruction across an open channel over which water flows. They can provide flow measurement with enough accuracy. Such information is very valuable for engineers. There are portable and fixed weir plates. There are also weir boxes and weir channels. Both of them have weir plates. Each one has a different use case scenario.
The difference between flumes and weir plates is water flows through the structure in flumes but flows over the structure in weir plates. We can expect 5 to 15% accuracy from weir plates for most fields.
How does a weir plate work?
Water flows over the weir plate. It begins to accelerate at the crest (The bottom edge of the notch) and falls into downstream flow. There is a relationship between upstream level, flow rate, and this acceleration. There are derived equations for the relationship.
How a weir plate is made?
A fixed weir plate should not be loose in dimensions over time. It should stay without swelling and be susceptible to crest damages. The affordable material that fulfills all these requirements is stainless steel. That is why weir plates are made of T-304 stainless steel. But sometimes we use weir plates with only the crest made of stainless steel.
Portable weir plates also have the same requirements but they come as a set of weir plates and a weir carrier. Normally we use four sets of portable weir plates to take measurements. Weir carriers are usually made of aluminum but T-304 stainless steel is the material for weir plates.
The notch of the weir plate takes different shapes on purpose. There are rectangular, trapezoidal, and Triangular (V-notch) notches. Each shape has different shaped crests. Triangular weirs have a pointed crest. This shape is very important when we do the calculations.
Major concerns for manufactures
The body of water that flows over the crest is called a nappe. If the nappe collapsed we will get measurements that are far from accurate. But we can prevent it by choosing the correct thickness for the crest.
The recommended crest thickness for a spring clear of nappe is 1 to 2 millimeters. Any crest thicker than that will face nappe collapses against the downstream face of the weir. Insufficient aeration leads to under-indicated flow rate values.
Theoretically thinner crest must increase the accuracy but there are too hard to maintain. Sharp and burr-free crests always spring clear flow of it.
How to calculate?
Equations are different for each one. Consider a rectangular suppressed weir plate. Bureau of Reclamation in their Water Measurement Manual recommends the below equation for SI units.
Q = 3.33 B H3/2
Here Q is the water flow rate in cubic meters per second, B is the length of the weir in meters and H is the head over the weir in meters.
To get accurate results from the above equation H/P should be less than 0.33 and H/B should be less than 0.33. P is the height of the weir crest above the bottom of the channel. Use Q = 3.33 B H3/ for imperial units (Q is in ft3/s and other variables in ft). Be is also equal to the channel width.
This is only one equation for a special weir plate. The suitable equation varies with the type of it and the use case. Also, conditions like temperature make the equation less accurate.
Range of weir plate measurements
There are limitations for weir plate uses. The recommended temperature at which a weir plate can give accurate results is between 3.90C to 390C. Ice formation can happen on the crest at below 3.90C temperatures. It can affect flow readings significantly.
Water free from debris, trash, or any other high solids contents is better for weir plate measurements. Mentioned content can cling to the crest. That affects the acceleration of the flow passing the crest.
Weir plates are not by any means for the pressurized conduits just like any other open channel primary device. But that does not mean pipes running full into the weir pool or box is not ideal for weir plate measurements. It is acceptable unless it is pressurized.
What is a weir box?
A weir box is a pre-engineered structure. It includes the weir box and a weir plate. The weir plate is for the same purpose but the weir box is an interesting structure here. Box has its body usually made with aluminum, fiberglass, or stainless steel. The inlet baffle and weir plate carrier are fixed into the box. The interchangeable weir plate is usually made with aluminum or stainless steel.
Weir boxes are in a variety of places. It could be dam seepage and treated industrial discharges. Also, above-ground or shallow burial installations are taken as possible use cases. You can customize a weir box for a special application using weir box accessories. Flow condition options, piping/end connections, Sampler/parameter mounts, flowmeter mounts, and custom configurations are some accessories.
We can specify a weir pool to generate a proper-sized weir pool. But weir boxes failed in doing it. More sadly, weir boxes also face the limitation of what a weir plate can do. But weir boxes can accommodate multiple flow streams and measure them independently.
There are three standard weir box sizes. Manufacturers offer different v-notches for each size.
Summery
A weir plate is a structure we use to measure flow in an open channel. There are many types of weir plates and other structures and accessories associated with them. However, all of them work by the same principle.
Now you know what a weir plate is. Stay with us for more articles like this. Be curious.
