In the first part of this blog, I talked about closed pipe flow and the benefits and disadvantages it can have. In part two, I’d like to focus on open channel flow and how this option can be used to measure water and wastewater flow.
Measuring flow through an open channel is a popular choice for water/wastewater applications. It’s easy to see why, with ease of installation and maintenance over time. In open channel flow, the use of a primary measurement device, such as a weir or flume, along with a non-contacting level measurement device is used to determine the flow. There are several choices for the primary measurement device as well as the level device. Let’s take a look at just a few of the choices for this type of flow.
The Primary Measurement Device (PMD) is a key component of open channel flow measurement. A great deal of thought should be put into the selection of the primary measurement device. The flow rate that you have today may not be the flow rate that you will have tomorrow and must be considered to avoid the purchase of an inadequate system. Other items to consider are the accuracy of the PMD, maintenance and the fluid to be monitored. Installation of the PMD is critical for the proper reading of the open channel flow meter selected to measure the head of the flow. Improper installation or sizing of the PMD will impact the overall performance of the system.
It’s not just flow.
The level measurement system should also be given a great deal of thought. There are a couple of options in technology here such as radar or ultrasonic, and both options have their positives and negatives.
Radar has come a long way in the last few years and is a proven technology. The cost of a radar device has decreased and performance has increased.
Radar level transmitters are easy to install and configure, and usually have a low cost of ownership once deployed. However, radar technology does have a few drawbacks when compared to other level technologies. Radar instruments do not have relay outputs, totalizers or displays that can be easily seen when installed over the weir or flume. Radar transmitters are mostly used for level applications and when used to determine flow, it is often required that a head vs. flow characterization curve is entered in the instrument itself or on a PLC. Radar also does not react as quickly to changing head levels as other technologies. Finally, the designs used for open channel flow measurement have a small horn antenna. This coupled with the operating radar transmitter frequency results in signal with a wide beam angle. This is an undesirable attribute when applied in narrow flumes and weirs.
Non-contacting ultrasonic level is one of the most popular technologies when it comes to open channel flow. When correctly applied and installed, an accuracy of ±1 mm can be achieved. For the highest accuracy, an external temperature sensor, separate from the transducer, is recommended to ensure the speed of sound is accurately adjusted for the varying temperatures of the day. Ultrasonic level transmitter devices usually have a variety of communication options and outputs to meet the needs of the end user. They are easy to configure via the user graphical interfaces and have many of the flow calculations for the various PMDs pre-programmed. Most ultrasonic level flow meters have a separate transducer and controller. The ultrasonic level controller can be mounted separately, hundreds of feet away from the transducer, to allow easy access for configuration, routine monitoring, and capture of the flow data.
So, as you can see, there are many options to choose from and it depends on your set-up. For volumetric flow in a closed pipe, you need to know if the liquid is conductive or not, a given in the water industry. If you are using the flow meter for clean or for water with significant suspended solids, you’ll need to consider whether a closed pipe or open channel is the right solution for you.
What is the right technology to use once I have decided on open or closed pipe?