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 3.4 Accuracy and Reliability. The turbine meter is highly accurate through
q ost of its range. Guaranteed accuracy of %1.O percent is available.
Insertion meters are subject to the inaccuracies that result from locally
measuring an average velocity. Reliability of properly installed and
maintained turbine meters should yield a 4-1/2 year mean-time-between-repair
and a total life expectancy of 10 to 25 years.
4. SHUNT METERS. Shunt flowmeters are a special class of turbine meters
which use an orifice plate to control a bypass flow metered by the turbine.
4.1 Operating Principles. The turbine rotation is at a speed proportional to
a bypass flow controlled by an orifice plate, which in turn is proportional to
the main flow rate. A reduction train, that gears down the turbine, is
coupled to a driving magnet. The magnet influences another magnet that is the
counter. The two magnets operating in unison enable totalization of the
turbine rotation.
4.2 Meter Design. There are two types of shunt meters. One is external to
the main flow pipe, as shown in Figure 6-4. The other is mounted as an inline
meter within the main pipe. Neither type requires power for totalization.
4.3 Limitations. The limitations of a shunt meter are as follows:
Requires system shutdown to install.
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High maintenance and difficulty in calibration.
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q Costs
are moderate to high.
Turndown ratio is 7 to 1.
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Not recommended for steam below 30 psig or over 200 psig.
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Not recommended for fuel oils.
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Not recommended for air or natural gas below 5 psig.
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Permanent pressure loss due to orifice plate.
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4.4 Installation. The flow in the line must be turned off during
installation of a shunt meter. A diversion orifice plate with flanges must be
placed in the main line. Holes must be cut into the main line before and
after the orifice plate to accept the bypass piping which contains the turbine
and counter box (Figure 6-4).
4.5 Maintenance. Maintenance should be performed every 6 months. The
diversion orifice plate should be checked for wear and the turbine blades for
deformation. The counter should be checked for accuracy and calibrated if
necessary.
6-5
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