In Part One of this series, we started the discussion of magnetic flow meters versus mechanical flow meters. We talked about the differences in construction and how a meter with moving parts can affect the total cost of ownership through maintenance and reduced performance. In this article, I want to address some of the mounting, accuracy and process considerations of mechanical meters compared to mag meters. Let’s look at process considerations first.
Mag meters are suitable for the measurement of conductive liquids. What do we consider a conductive liquid? For conductive liquids, we are specifically looking at liquids with conductivity of 5 microsiemen/cm or greater as suitable. Mag meters are also suitable for liquids with either suspended solids or slurries. This makes them ideal for water-based products and most acids and bases as well as slurries like the ones you find in wastewater treatment, mining, and pulp and paper applications. Mag meters are also very flexible when it comes to viscosity, but they are not suited for hydrocarbon-based products, gases or steam. With hydrocarbon-based products, there is just not enough conductivity present for a mag meter to work.
On the other side of the spectrum, mechanical meters are not subject to conductivity restrictions. Because they rely on the process of making internal mechanisms move in order to measure, they may have a minimum velocity required to overcome internal resistance to motion. In most cases, there is an upper limit for the viscosity tied to the ability of the meter to physically operate and/or pressure drop across the mechanical aspects of the meter to allow a highly viscous liquid to even flow. Mechanical meters are also not suitable for slurries or liquids with suspended solids as these can cause either wear and/or build-up on the meter, and will require higher levels of maintenance or replacement of the device entirely.
Mag meters are accurate to 0.2% of flow rate with a 5 diameter upstream and a 3 diameter downstream of straight run from the same ID pipe. This requirement is from the measurement electrodes so the mag meter is part of the straight run requirement. Some mechanical technologies, such as PD/Oval Gear, have no requirements for straight run since they are moving the process from one chamber to another to totalize the flow going through the device. Other technologies, like turbine meters, require 10 to 20 diameters upstream and 5 to 10 downstream from the process connection of the meter in order to provide a flow profile that will allow for best performance.
Mechanical meter accuracy depends on the type of mechanical meter involved. Accuracies can range from 0.2% to 2+% based on the design and the precision of the manufacture of the device. Once a mechanical meter is installed, any mechanical wear will have an immediate effect of the performance of that meter. This means that for critical processes, the frequency of maintenance checks will be highly dependent on the performance expectation for the process being measured. These maintenance checks add cost of ownership and may affect the throughput of a process if routine downtime is required for maintenance. Mag meters with no moving parts and high tolerance of slurries and solids can significantly reduce the required maintenance if the materials of construction are selected with the process conditions in mind.
While all measurement technologies have their place, we encourage you to always weigh the cost of purchase with the overall cost of ownership whenever selecting a flow measurement technology for your application.