When it comes to metering water flow — drinking water or wastewater — full-bore mag meters offer many advantages. While the underlying technology based on Faraday’s Law of electromagnetic induction (Figure 1) is shared among all styles of full-bore mag meters, specific implementations have impacts on longevity and accuracy. Here is what to look for when the time to choose arrives.
With sensing focused on the full cross-section of the pipeline, full-bore mag meters offer reliable, accurate performance while being relatively tolerant of disturbances in the flow. With no moving parts and high accuracy, they are often chosen to replace mechanical meters such as propeller meters.
Mag meters typically tolerate turbulence from elbows or Ts located within one or two pipe diameters (1d to 2d) upstream and zero to 1d downstream. Chemical injection ports or valves might cause turbulence requiring 2d of straight pipe upstream of the mag meter. Depending on meter features and cost, accuracy can range from + 1 percent to + 0.5 percent of the standard operating range. Operation is typically reliable over a long service life, although some regulated applications might still mandate periodic calibration, usually every one or two years.
The one Achilles heel can be the protective layer between the metallic body and the conductive liquid flowing through it that insulates the meter against short-circuiting. Where separately installed liner inserts are used to provide that protection, deterioration of those liners can be one of the biggest complaints about full-bore mag meter performance in water industry applications.
The most durable mag meter coating is one that is not simply layered on the inside of the tube, but one that completely envelops the entire assembly with thermoset powder coating epoxy. This epoxy bonding process provides 100 percent continuous coverage, enveloping the entire interior and exterior surface of the mag meter housing with uninterrupted protection. It provides excellent adhesion to resist corrosive soils, hydrocarbons, harsh chemicals, and even sea water and to avoid problems experienced with other types of interior “sleeve” linings, which can delaminate, separate, or collapse.
This type of coating has demonstrated its durability in the field by standing up to flows that include sand, rust, or harsh chemicals in the water — without cracking, peeling, or falling off. If those problems occur, particles of the liner can get into the process flow and the mag meters can produce grossly inaccurate readings.
The chemical compatibility for this NSF-61-approved dry-powder epoxy coating is extensive, exceeding most conditions typically experienced in water or wastewater treatment plants (Figure 2).
Full-bore mag meters with durable full-coverage epoxy coatings are available in several feature sets to meet different requirement levels, including options for visual readings at the meter or for remote reading and the ability to tolerate sand- or clay-laden well-water without liner damage.
Aside from sensing accuracy, variability in the water conveyed, and interior coating protection, there are other service factors to consider when evaluating mag meters for water industry applications.
Onsite customer service — when appropriate — can make the difference between instant problem resolution and an expensive shutdown to remove, replace, and return a meter for factory diagnostics or repair. One example of such an occurrence could be dealing with electronic interference in the plant environment that impacts meter reading accuracy. Another could include assistance in designing and sizing replacement mag meters to fit the specific space and configuration in an existing installation.
