REYNOLDS NUMBER EQUATION
7741.92 x ID x V
Cst
Where:
ID = the pipe inside diameter in inches
V = the flow velocity in feet per second
Cst = the fluid viscosity in centistokes
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To maintain accuracy, the Reynolds number must be above 4000 over the
entire flow range. Unlike variable area meters, paddlewheel meters are
not affected by changes in the fluid’s specific gravity. Special
calibrations are not necessary.
Another compelling reason people are turning to paddlewheel style
Flowmeters is their ability to have a remote readout. Paddlewheel
Flowmeters easily permit the panel mount readout to be installed in the
most desirous area, while the actual sensor is installed in an obscure
area of the pipe system. For example, in any application, the paddle
sensor can be installed in the proper location and the digital display
can be remotely located in a more desirable location in the system such
as on a centralized control panel.
Maintenance
The meter’s paddle and axle are in direct contact with the fluid. Since
the paddle will spin at a velocity that is directly proportional to the
rate of flow, these components will wear over time. Meters which are
operated at the high end of their calibrated flow range will tend to
wear more than units operated at the low end. Because every fluid has
different characteristics, it is difficult to estimate the life
expectancy of these components. Some water (DI water), may be very
aggressive while some water may be exceedingly hard or full of
abrasives. The resistance of the components to the chemical being
measured should also be considered. Axles and paddles are easily
replaceable. A neglected paddlewheel Flowmeter will in time have
degraded accuracy.
The
maintenance factor should not in any way deter using paddlewheel meters,
just remember, turbine meters have the similar problems; rotameters also
have to be maintained. This is a case where the benefits far
outweigh the cost.
Maximize
paddlewheel performance
If your flow falls below one foot per second you should expect
some accuracy challenges. Avoid using paddlewheel Flowmeters for
measuring very dirty fluid, or liquids with rocks or pebbles that could
break or damage the paddle or axle. Follow the manufacturer’s
installation recommendations regarding straight lengths of pipe.
If you cut corners, do not expect optimum performance from your meter.
In summary
Paddlewheel Flowmeters should be considered by anyone who requires
above average metering accuracy where cost is a consideration. These
meters are also recommended because they are easier than most meters to
install, can measure flow in either direction and serve the intended
purpose of measuring flow. Considering the overall cost of
ownership, it is very difficult not to consider the paddlewheel
Flowmeter.
Following are some terms you might find useful. The more you understand
how and why a Flowmeter works, the greater success you will have with
your selection.
K-Factor –
The number of signal pulses generated per unit volume of flow. Example:
225 pulses per gallon.
Reynolds Number –
A dimensionless number that combines the effects of viscosity, density,
and flow velocity which is used to identify either a turbulent or
laminar flow profile.
Specific gravity-
(relative density) The ratio of the density of a fluid, at it’s
temperature, to the density of water at a specified reference
temperature.
Feet per second of flow –
The velocity of fluid flow in a pipe expressed as the number of linear
feet of flowing fluid passing a given point in a pipe, per one second of
time.
