Ten tips for maximizing mag meter performance

Where flow measurement is required but the media is harsh -- such as in sewage, sludge or slurry -- the electromagnetic flow meter, commonly referred to as a "mag" meter, is preferred because of its non-intrusive design. This article details key maintenance concepts to maximize mag meter performance.

06/25/2001


FLUID HANDLING
MAINTENANCE


On-line exclusive: Ten tips for maximizing mag meter performance

David Vidrio, Water Specialties Corp., Porterville, CA

  • Mag meters are accurate and non-intrusive

  • Mag meters require little maintenance

  • Liquid to be measured must have conductivity


    • Where flow measurement is required but the media is harsh -- such as in sewage, sludge, or slurry -- the electromagnetic flow meter, commonly referred to as a "mag" meter, is preferred because of its non-intrusive design ( Fig. 1 ). In these environments, where suspended solids plug up conventional flowmeters, the mag meter offers reliability, low maintenance, and high accuracy. The mag meter also excels in the food processing and pharmaceutical industries since high performance translates into greater productivity and less waste.

      A mag meter is essentially a flow tube that measures an induced voltage created by liquid passing through an electromagnetic field, based on Faraday’s Law ( Fig. 2 ). It offers no flow restrictions and nothing plugs it up. With no moving parts, the mag meter is very low maintenance, and measures flow more accurately (

      Despite the mag meter’s rugged utility, proper planning and installation are essential to its overall performance. Here are ten simple tips to help prepare, install, and successfully use a mag meter.


      1. The flow media must be above the minimum conductivity level.

      The electromotive force can then drive the induced voltage and current to the microprocessor-based signal converter. The flow media must have a minimum conductivity of 5 microseimens per centimeter.

      2. The meter tube must be full at all times.

      If not, the mag meter’s readings may be inaccurate or erratic. Most mag meters operate with the flow tube full and cannot sense the difference between a full or partially full pipe.

      Methods to ensure full liquid flow include:

      • Positive head pressure from pumps

      • Uphill grades

      • Vertical installation

      • Positioning the meter in the lowest point of a plumbing trap.

        • 3. Maintain straight piping before and after the meter for accurate measurement

          Liquids traveling through obstructions and rapid changes in fluid conductivity (such as caused by chemical injecting and blending) may create process noise, leading to inaccuracy and erratic behavior. Chemical injecting should be done early enough so flow media and chemicals are thoroughly mixed before entering the measurement area.

          4. Ensure proper grounding and electrical connections

          If not, the mag meter may act erratically or not register at all. The mag meter relies on a potential to earth ground to operate, and it is imperative that the mag meter be well grounded, especially on pipelines that have cathodic protection. Stainless steel ground rings are typically used to achieve this. Taking a few minutes to read the installation manual and properly ground the meter will save hours of troubleshooting.

          5. Know the metering application to choose the proper converter orientation

          When the signal converter is integrally mounted on the flow sensor, the assembly is NEMA 4X rated. If the converter is remotely mounted, the sensor end is rated NEMA 6P, which is intended for limited submergence. If installation must exceed the NEMA 6P rating, then waterproof remote cabling should be used.

          6. If using a remote configuration, be precise on the remote cable’s length

          Many manufacturers use a unique cable that contains delicate inner conductors. Extending this cable with wire nuts or electrical tape is unacceptable. Use a special splice kit, offered by most manufacturers, to extend the cable length with no risk of lost accuracy or performance life. Also available are retrofit kits that convert a directly mounted signal converter to a remote mount configuration.

          7. Avoid heavy vibration

          The signal converter’s electronics may be damaged by vibration, causing meter failure. Gas or diesel-driven pump motors and liquid traveling through obstructions are two things that can create resonance vibration. To minimize vibration, create concrete or structural steel supports. Flex joints and special vibration suppressing fittings are also useful.

          8. Avoid wide temperature variations

          The mag meter’s sensor is robust, but its electronics are sensitive. The mag meter works well from 14 to 140 deg. F. But even within this wide temperature range it is a good idea to shade the signal converter with an awning or covering. If outdoor conditions are extreme, remotely mount the signal converter in a better environment such as a NEMA enclosure, pump house, or indoor facility. A signal converter can be mounted up to 300-ft away from the flow sensor, which helps when dealing with pit applications and confined-space laws.

          9. Avoid welding or extreme heat near the mag meter

          The liner and outer coatings can be damaged by heat. Weld before installing the flow sensor, or use compression and transition couplings to avoid welding altogether.

          10. DC-powered signal converters can be used when AC power is not available

          Mag meters have successfully been used with solar-powered systems and with uninterruptible power supplies (UPS) to prevent unmeasured water flow.

          Edited by Joseph L. Foszcz, Senior Editor,
          630-320-7135, jfoszcz@cahners.com





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