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5.2 Limitations.  Several limitations that may preclude application of a fan
type meter are as follows:
q Fan
type meters are restricted to clean fluids.
q Meter
installation should be horizontal and level.
q Temperature
range is from -267C (-450F) to +260C (+500F).
Pressure limit is 3,000 psig.
q
5.3 Accuracy and Reliability.  Fan type meters are typically not as accurate
as turbine or positive displacement meters.  They have an accuracy of %1.5
percent over the flow range.  Reliability is comparable to turbine meters. If
installation is proper and maintenance is performed as required, the meters
can be expected to have a 4-1/2 year mean-time-between-repair and a total life
expectancy of 10 to 25 years.
6.  INSTALLATION.  Installation of turbine and fan type flowmeters involves
placement of the sensing element directly in the flow channel.  The location
of the turbine or fan meter in the system is important.  Whenever possible, it
is preferable to locate the primary element in a horizontal line or a vertical
line with flow in the upward direction.  To ensure accurate flow measurement,
the fluid should enter the primary element with a fully developed velocity
profile, free from swirls or vortices.  Such a condition is achieved by the
use of strainers, straightening vanes, and/or adequate lengths of straight
pipe preceding and following the primary element. Using a beta ratio of 0.6,
typical ASME recommended lengths of such piping are shown in Figure 6-5, but
specific requirements should be obtained from the manufacturer.  The diagram
in Figure 6-5 that corresponds closest to the actual piping arrangement for
the meter location should be used to determine the required lengths of
straight pipe on the inlet and outlet.  These lengths are necessary to limit
errors due to piping configurations to less than %0.5 percent.  If minimum
distances are not observed, applying flow equations and calculations may
result in inaccurate data.  Full-bore meters require shutdown of the system
during installation, periodic inspection, and maintenance, but insertion type
meters can be hot-tapped and do not interrupt the system for installation or
removal. Turbine and fan flowmeter installations must also take into
consideration the following items:
q Reduce
turbulence by observing recommended straight length distances.
q Care
should be taken to place the meter as far from valves and other
turbulence-producing fittings as possible.  All regulator and control
valves should preferably be located downstream of the meter; if
upstream, beyond the recommended straight length distance.
q If
used, place straightening vanes and strainers upstream of the meter.
6-7





 


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