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 5.2 Insertion Meter Installation. Insertion electromagnetic flowmeters are
available as hot tap or permanent types. Both types require installation of a
tee or welded fitting on the pipeline for access. Insertion meters are
generally restricted to lower temperature and pressure conditions.
Installation of an insertion electromagnetic flowmeter requires consideration
of the following items!
q The
meter is subject to electromagnetic fields and must be shielded
from large electric motors, transformers, communications equipment,
and other large electrical devices to avoid electromagnetic
disturbances.
q The
meter must be positioned so that the pipe is full flowing
approaching and exiting the u eter.
q Location
of the insertion electromagnetic flowmeter in the system is
important. Whenever possible, it is preferable to locate the primary
element in a horizontal line. To ensure accurate flow measurement,
fluid should enter the primary element with a fully developed velocity
profile, free from swirls or vortices. Such a condition is best
achieved by use of adequate lengths of straight pipe, both preceding
and following the primary element. Minimum recommended lengths of
piping are shown in Figure 6-5. The diagram in Figure 6-5 that
corresponds closest to the piping arrangement for a u eter location
should be used to determine required lengths of straight pipe on the
inlet and outlet. These lengths are necessary to limit piping
configuration errors to less than --
+0.5%.
6. MINIMUM AND MAXIMUM FLOW RATES. Minimum and maximum flow rates must be
established to maintain accurate flow measurements. Wear to the flow tube,
buildup of coatings, and deposition of solids can be reduced if recommended
flow rates are maintained.
6.1 Minimum Flow Rate. The minimum flow rate is usually determined by the
rate that produces the lowest acceptable voltage requirement of the signal
transmitted. If the minimum, rate causes excessive sedimentation, the diameter
of the tube may have to be decreased to increase velocity through the meter.
This provides a self-cleaning action.
6.2 Maximum Flow Rate. The maximum recommended flow rate is arbitrary, but
is based on wear patterns experienced with different flow tube u aterials
interfacing with a specific fluid. A maximum recommended flow rate of 30 feet
per second is common.
7. ACCURACY AND RELIABILITY. Although it is possible to calibrate
electromagnetic flowmeters to measure flow within +0.25%, a more
representative accuracy is 1.0% of its rated capacity. The downturn of
electromagnetic flowmeters can be up to 40:1. If recommended maximum flow
conditions are not exceeded (causing excessive flow tube lining wear), and
hydraulic shocks (water hammer) are avoided, electromagnetic flowmeters are
very reliable. Electrode corrosion greater than 0.002 inches per year is
excessive and indicates a need for investigation to determine the cause.
6-18
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