A flume is one of the best ways to take flow rate measurements, but there are quite a few different kinds of flumes to choose from. While they all have something special to offer, Parshall flumes are among the most common and well-researched. Being a staple for wastewater treatment and other industries where flow rate measurement is necessary, Parshall flumes have been a go-to design for decades.
The design of a Parshall flume is useful for permanent installations in surface water and irrigation flows. Before you get one for yourself, however, you’ll need to make sure that it can offer the kind of measurements you’re looking for. There are plenty of advantages and disadvantages to consider before committing to what should be your permanent flow rate measurement solution. Take a look at the advantages and disadvantages of a Parshall flume, and discover whether it would be the best fit for your needs.
How a Parshall Flume Works
Before you can grasp the advantages and disadvantages of Parshall flumes, you’ll need to learn how they work. The most important thing to remember is that they accelerate flow to a point of criticality where a measurement is taken to correspond with flow rate. This is accomplished thanks to its clever hourglass design shape. The parallel sidewalls contract and the floor drops in the throat section. This causes the flow rate to increase.
Once the flow rate has been increased properly, a measurement is taken at the Ha, which is the primary point of measurement. The depth of the flow at this point is the variable that determines flow rate. It’s important to remember that this point of measurement isn’t actually at the throat of the flume but rather in the converging section upstream of the throat. Specifically, you’ll find it at about two-thirds of the converging section’s total length.
Abnormal Flow Conditions
The flume only works properly when faced with normal flow conditions, which means a flow that is approaching with a uniform subcritical velocity. If the flow isn’t uniform or is already supercritical, then the measurement won’t be very accurate when determining flow rate. Fortunately, there are a few ways to condition the approach flow to make sure it has the right conditions, like the use of stilling wells or energy absorbing manholes.
Submergence is a potential problem for all flumes. Submergence isn’t only when the flume is completely underwater; it technically refers to any kind of conditions that restrict the free flow of water out of the flume. Parshall flumes are relatively unique, however, in that they can account for submergence. You’ll just have to take an extra measurement. For submerged flows, you’ll have to measure at the primary point of measurement and the secondary point expressed as Hb.
To find the second point of measurement, you’ll have to look at the throat or the narrowest part of the flume. Finding the point is just the beginning, however, as it can be tough to get an accurate reading here. Submerged flows can be quite turbulent at the second point of measurement, so you may want to implement a chamber or well to the side to ensure you’re getting accurate measurements. Alternatively, you’ll need to alleviate the submerged state, which typically means adjusting the downstream conditions to allow for free flow out of the flume.
Once you’ve taken the secondary measurement, you’ll have to calculate the submergence ratio, which is the secondary point of measurement divided by the primary point of measurement. See how this relates to the submergence transition ratio, or St, of your Parshall flume size. If the ratio you get is greater than or equal to the St, then the flume is confirmed submerged, and you’ll have to use the more complicated flow equation.
Parshall flumes are used for a wide variety of applications thanks to their near-universal compatibility with an array of flow channels. While there are certain scenarios in which a Parshall would be inappropriate, that’s likely not the case for your needs. They’re most often used in wastewater treatment plant flows, but you’ll also find them good for surface water measurement and irrigation runoff. Stormwater runoff is a common application as well.
These flumes are particularly adept at monitoring and discharge applications. Dam seepage and leachate monitoring are common, but you’ll also find uses in spring discharge and industrial pretreatment discharge scenarios. Mine dewatering is another application. There are other applications as well, so be sure to consult with flume experts before writing off the Parshall entirely for your flow rate measurement needs.
Advantages of Parshall Flumes
The advantages of Parshall flumes are numerous, so it’s important to keep them all in mind when choosing the right flume for your needs. You’ll find that most advantages center around their usability and extensive research allowing you to compensate for a variety of different conditions by adjusting equations and adding additional points of measurement.
Rectangular Cross-Section Design
Parshall flumes have a shape and design that’s known as a rectangular cross-section. This kind of setup also happens to be the standard shape of channels used for irrigation flumes and treatment plant operations. Because of the similar shape, Parshall flumes easily fit into these channels, making installation and operation easy. This works for both new and existing channels. At worst, you’ll have to alter the walls to ensure that they contract properly enough to direct the flow into the flume, but even that’s relatively trivial compared with some other lengths required for different flume styles.
The Parshall flume being the most popular design isn’t just a mark of its credibility. It also means that the vast majority of operators will already be familiar with how they function. With that kind of common knowledge on your side, there’s less of a chance that you’ll be dealing with measurement mistakes and other problems caused by human error.
Accountability With Submergence
Submergence can ruin any hope of getting accurate flow rate measurements for a lot of flume styles, but Parshall flumes sometimes let you compensate for it, especially if your flume is large. In fact, for particularly large Parshall flumes, the submergence transition ratio is 80%, which means there’s a lot of room for accurate measurements before submergence is even a problem. It’s not the highest ratio out of all available flume styles, but it is certainly one of the highest.
Every flume is being constantly researched to discover new applications and approaches to flow rate measurement. The Parshall is the most popular, so it’s gotten the most attention from researchers. Because of nearly a century of research, Parshall flumes have numerous corrections that can be made to account for certain flow conditions. These include unified free/submerged flow equations, corrections for lateral/longitudinal settling, and corrections of submergence itself.
Easy to Accessorize
The available accessories for Parshall flumes are expansive with even more options to make your measurement efforts easy and efficient. Many accessories are developed specifically with Parshall flumes in mind. Some of the available options include flow meter mounts, integrated structures both above and below ground, inlet flow conditioners and a variety of accessibility options.
Some flume designs and styles are more of a general concept than anything really set in stone. The Parshall isn’t like that. With this flume, you’ll find standardization in its design and the flow characteristics that it works with. Both national and international technical standards have been established and are maintained for all modern Parshall flumes, so you can be sure yours is ready to deal with open channel flows.
Accuracy is among the most important factors when it comes to flow rate measurement. Without good accuracy, the effort is mostly wasted because you won’t be able to get any useful information. Fortunately, the Parshall’s accuracy is reliable throughout all its various applications, coming in at +/-3%-5%. This is easily on par with weirs and other flume styles. While the Parshall is not technically the most accurate flume available, it maintains its accuracy no matter what kind of appropriate application you use it for.
Disadvantages of Parshall Flumes
The Parshall flume is a popular favorite and dominates the market, but it’s not the only option. If it was perfect, you wouldn’t need to consider anything else, but there are some disadvantages to keep in mind when using the Parshall. Its design isn’t always applicable for all scenarios, and you should always be using whatever it takes to get the best flow rate measurements possible.
Standardization has a lot of benefits to it, but there are some disadvantages to this too. Standardization makes it an empirical device with set flow equations and discharge characteristics. That means you’ll have to opt for a device that has been laboratory tested to ensure it meets the necessary standards that make the flume’s equations work properly. Any deviation from the standard and you technically don’t have a Parshall.
Sizes Don’t Scale Evenly
Parshall flumes come in many different sizes, and each size is a bit unique in the sense that they’re not all just scale models of each other. In order to keep the equations and functionality working properly, different parts of the flume have to scale at different rates. When you’re dealing with a nonstandard size, you’ll have to get it independently rated to ensure that it’ll fit your flow channel conditions and offer the accurate measurements you deserve.
One of the key features of a Parshall flume is that it utilizes a drop-floor design. This is a part of what accelerates the flow to transfer it from subcritical to supercritical as it passes through the flume. With a drop floor, however, the bottom of the flume is not flat. When it comes to installation, this can be relatively tricky to work with, especially in existing channels. You could set the flume above the channel as long as you secure it and direct the flow properly, or you could alter the dimensions of the channel itself. Either way, you’ll find increased flume installation costs.
Parshall flumes are generally quite large compared with a lot of other flume designs. Their hourglass shape and long throat take up quite a bit of space even if you opt for one of the smaller ones. If you’re dealing with a small channel, you may find it fairly difficult to install a Parshall within. Additionally, larger Parshall flumes often have to be manufactured in pieces before being put together rather than being constructed as a single piece. This can increase manufacturing costs.
Custom Sizes Aren’t Standard
Because of the Parshall’s standardization, it can be tough to find intermediate sizes. Custom sizes will have to be requested and manufactured specifically, and that can be more costly than simply buying a model that’s already manufactured and available. Alternatively, you can simply opt for the next available standard size and alter your channel to accommodate it. The tricky part is determining which approach would be better for your budget.
Parshall Flumes From Tracom
The advantages and disadvantages of Parshall flumes are important to keep in mind when you’re searching for a solution to your flow rate measurement needs. Every channel requires a unique perspective to find the right fit, and your priority should always be getting your measurements as accurate as possible without going over budget.
Fortunately, you’ll likely be able to use the Parshall given its wide range of applications. When you’re looking for Parshall flume solutions, Tracom has got you covered. You’ll find all the standard sizes available, but if you want something a bit more unique, our team will work with you to create and test a custom size to ensure you get the accurate measurements you need. Contact us today to get started!