What are the advantages and disadvantages between a mechanical flow meter and a magnetic flow meter?
With the 2015 WEFTEC Expo coming up at the end of the month, there has been a lot of discussion on using mechanical meters versus magnetic meters for the measurement of conductive liquid. While both flow meters offer benefits, is one more efficient and effective for you to use? The purchasing price is certainly a consideration, but it’s not the only thing that you should consider when looking at upgrading your system.
What are the differences between the two flow meters?
The primary difference between a mechanical meter and a magnetic meter is that a mechanical meter has moving parts and a mag meter doesn’t. Typical examples of mechanical meters are target meters, where liquid hits a spring loaded plate and the amount of movement of that plate relates to velocity, and turbine meters, where liquid flowing over a set of angled blades causes those blades to rotate and that velocity is used to determine flow rate. Another example would be a positive displacement (PD) or oval gear meter where a set amount of material moves through chambers and the amount of material that moves through these chambers as they rotate is counted and converted to flow rates. There are other mechanical designs, but you get the picture.
A mag meter works by having a conductive liquid flow through a lined flow tube and of the flow passing through a magnetic field generated as part of the flow tube design causes a voltage to be created. This voltage increases or decreases based on velocity. So, as long as the material meets the minimum conductivity level, there are no moving parts (mechanical motion) in the flow meter itself required to measure the flow rate through the meter.
Okay, now that we have a basic concept of each flow meter, what are the advantages and disadvantages?
Let’s start with the most basic aspect of a mechanical flow meter, and the fact that it is mechanical.
Disadvantage: One of the biggest disadvantages is that mechanical parts wear. As these parts wear, there are a variety of effects that the wear can have on your process. When parts wear, they do not work as well as they did when they were first designed or installed. Look at a pair of scissors, or a knife. When they were new, they cut easily and efficiently but as they are used, even if they are used properly, they lose their factory fresh sharpness and their ability to cut as effectively.
Solution: The solution is to sharpen them (maintenance) to reinstate their original performance and eventually you will need to get a new cutting tool (replacement). The same analogy can be used in mechanical flow meters.
Let’s look at the three mechanical examples that I provided at the beginning of this article.
A target meter is spring loaded and needs a certain level of resistance in order to work to the design specifications. As the spring compresses and releases over time, the tension changes and, so does the accuracy. The target itself also has a specific surface area that is necessary for best performance and when the edges wear down, that surface area changes and so does the performance.
A turbine meter has blades that are specifically configured to rotate at a given speed for a given flow velocity. As the blades wear, that geometry changes and so does performance. Also, the ability of the blades to spin is related to the friction, or lack thereof, of the bearings that are part of the blade assembly. When the bearings wear, they require more energy to turn at the same rate. Again, we are talking about maintenance and eventual replacement.
In our last example, I discussed PD or oval gear meters. These are heavily geared meters with a lot of moving parts that work in mesh with each other to move material through them. The gears and bearings that move these gears are prone to wear and again, need maintenance and eventual replacement.
Magnetic flow meters, on the other hand, do not require as much maintenance as mechanical meters do. Now, that’s not to say that they never need maintenance or never need to be replaced. Any flow measurement device should be checked from time to time to make sure it is working properly and providing accurate flow measurement. What I am saying is that without the mechanical issues associated with moving part flow meter designs, mag meters can have more extended maintenance cycles and an overall longer lifespan at the expected accuracy, without the need for replacement.
This is the first of a two part blog series on mechanical flow meters versus magnetic flow meters. In part two, I will discuss issues such as mounting, straight run pipes, accuracy and the types of process that can be run through mag meters versus mechanical meter designs. So stay tuned…
In the meanwhile, check out Siemens’ booth at the WEFTEC Expo, booth #1212. Sadly, I won’t be able to make it, but a lot of my colleagues who work in the water/wastewater industry and flow instrumentation will be there!