Throughout my time in the industry, the one question that always gets brought up is, “With all of the options and technologies available, what is the best way to measure water and wastewater flow?”
This is a tough question to answer because there are so many options and each has their pros and cons. As government requirements and regulations are becoming more stringent, it is important to know the advantages and disadvantages of both because there is a great need to measure the flow rate throughout not only in the water/wastewater and irrigation industries but in all industries.
As your available options, you can choose from closed pipe flow and open channel flow before getting into the specific types. Let’s begin with closed pipe flow.
Closed pipe flow
In this type of application, there are several types of products and technologies that can be used to measure flow. The options for measuring flow include the use of magnetic and ultrasonic flow meters, pressure transmitters and other less practical and/or frequently used technologies such as capacitance, Time Domain Reflectometry (TDR ), and paddle wheel technologies to measure and totalize the flow.
Magnetic flow devices are one of the most commonly used technologies for measuring flow. The technology is accurate and well understood, and it offers a non-intrusive design because it has no mechanical moving parts. Magnetic flow meters are durable and some models can be submerged or buried without issue. Magnetic flow meters are usually not affected by solids that may be in the flow stream; however, they can be affected by air in the pipe and can give false readings if the pipe is not kept full. They can also be affected by stray electrical currents from Variable Frequency Drives (VFDs) that interfere with the magnetic field of the flow meter. Proper grounding helps to mitigate external influences. That is why it is key to use the best installation methods to ensure that the proper operation and readings are received from the magnetic flow meter.
Ultrasonic flow meters are another common means for measuring flow in a closed pipe. They are easy to install and can be installed on large enclosed pipes without interrupting the process. They are accurate and, with the proper installation, can yield accuracy in the 1% range of flow. This technology offers the ability to measure the flow of liquid in either an in-line or a non-intrusive clamp-on design. With both Transit Time and Doppler designs this technology can be used on clear liquids or liquids with entrained gases or solids. However, ultrasonic flow devices based on Transient Time technology can be affected by solids in the flow and in such applications, a magnetic flow meter is the right choice where suspended solids are present.
Pressure transmitters are an economical way to measure flow. The technology is widely used through all industries and easily deployed. Volumetric flow is obtained by measuring the pressure at two different points in a tube or pipe and this is done using a differential pressure transmitter. There are a variety of primary elements like orifice plates, pitot tubes, Venturi tubes, etc. to cover pretty much any application. They have readily available communication interfaces and are easy to calibrate in the field. Unfortunately, pressure devices are not always the most accurate compared with
other technologies and they do not have the turn-down capability of other devices. Pressure measurement devices can suffer from calibration drift and biological buildup, causing errors in the reading if they are not maintained over time.
On the other hand, open channel flow is a popular choice for water/wastewater applications. Why is it so popular for these applications?
Join me for my next blog and find out! We’ll be diving into open channel flow in more depth.
What pros and cons have you found while using closed pipe flow technologies?