FAQ
    Contact Us
    Position:Home > FAQ

    Magnetic Flowmeter Piping Requirements

    Date:2016-3-30 
    Always pipe an electromagnetic flowmeter (magmeter) so it is full of liquid. The pipe configuration should avoid a buildup or accumulation of any secondary phase (solids or air). Any entrained air should be carried out of the meter by flow, or by buoyancy (at zero flow). Likewise, any solids should fall from the meter by gravity under zero flow. You can install magmeters in horizontal lines, but best practice calls for installation in vertical lines with upwards flow (Figure 2).

    Pipe the magnetic flowmeter to remain full at zero flow. Otherwise the output can become erratic because of electrode exposure to air. If draining at zero flow is inevitable, use empty pipe detection options to prevent erratic output.


    Flowmeter Piping Requirements for Magmeter Installation

    Figure. When installing magnetic flowmeters, avoid downward flows. Horizontal configurations following an elbow require five pipe diameters of straight pipe upstream from the meter.

    Magnetic flowmeters are relatively insensitive to errors caused by nonsymmetrical velocity patterns or swirl. The general rule of thumb for straight piping is a five-diameter length of piping upstream and three diameters downstream from the meter (measured from the center of the tube). Independent testing has shown that magmeters may be affected by piping effects when the length of upstream straight pipe is less than three pipe diameters. Errors from piping effects generally run between 0.1 percent and 1.5 percent, depending on the exact configuration of piping and length of pipe run upstream of the meter.

    The magnetic field across the electrodes is very small, making the magmeter susceptible to stray ground currents in the pipeline. If the pipeline is made of nonconducting materials, such as plastic, or is lined with an insulating material, these stray ground potentials can cause significant measuring errors. Five or six millivolts of stray potential in the measuring section of the flowmeter can render the signal meaningless. The transmitter will confuse this stray potential with the actual signal and provide inaccurate flow readings.

    If the pipeline is made of an unlined conducting material, the process ground should be excellent, and no further measures are normally required. As a precaution, install grounding straps between the pipe flanges and magmeter flanges, and connect one pipe flange to a good ground.

    If the connecting pipe is not electrically conducting or is lined with insulating material, use grounding rings, disks, or electrodes. Strap the grounding disks or rings to the detector head flanges at either end. If the magnetic flowmeter contains grounding electrodes, the manufacturer will connect them to ground inside the magmeter casing.