flow and level measurement - Omega Engineering

increases. The higher the viscosity,
the less slippage and the lower the
measurable flow rate becomes. As
viscosity decreases, the low flow
Piston
Cylindrical
Abutment
Piston
Hub
Figure 3-2: Piston Meter Designs
Inlet Port
Partition
Plate
Follower Magnet
Cylindrical Abutment
A) Oscillating
Measuring Chamber
performance of the meter deteriorates.
The maximum allowable pressure
drop across the meter constrains
the maximum operating flow
in high viscosity services.
• Liquid PD Meters
Nutating disc meters are the most
common PD meters. They are used as
residential water meters around the
world. As water flows through the
metering chamber, it causes a disc to
wobble (nutate), turning a spindle,
which rotates a magnet. This magnet
is coupled to a mechanical register
or a pulse transmitter. Because the
flowmeter entraps a fixed quantity
of fluid each time the spindle is
rotated, the rate of flow is proportional
to the rotational velocity of
the spindle (Figure 3-1A).
Because it must be nonmagnetic,
the meter housing is usually made of
bronze but can be made from plastic
for corrosion resistance or cost
Control Roller
Piston
Outlet Port
Magnet Assembly
Cover
Control Roller
Housing
Measuring
Chamber
savings. The wetted parts such as the
disc and spindle are usually bronze,
rubber, aluminum, neoprene, Buna-N,
or a fluoroelastomer such as Viton®.
Nutating disc meters are designed
for water service and the materials of
which they are made must be
checked for compatibility with other
fluids. Meters with rubber discs give
better accuracy than metal discs due
to the better sealing they provide.
Nutating disc meters are available
in L-in to 2-in sizes. They are suited
for 150-psig operating pressures with
overpressure to a maximum of 300
psig. Cold water service units are
temperature-limited to 120°F. Hot
water units are available up to 250°F.
These meters must meet American
Water Works Association (AWWA)
standards for accuracy. The accuracy
B) Single-Piston Reciprocating
3 Mechanical Flowmeters
of these meters is required to be
±2% of actual flow rate. Higher viscosity
can produce higher accuracy,
while lower viscosity and wear over
time will reduce accuracy. The AWWA
requires that residential water meters
be re-calibrated every 10 years.
Because of the intermittent use
patterns of residential users, this corresponds
to recalibrating L x I in
residential water meters after they
have metered 5 million gallons. In
industrial applications, however, these
meters are likely to pass this threshold
much sooner. The maximum continuous
flow of a nutating disc meter is
usually about 60-80% of the maximum
flow in intermittent service.
Rotating vane meters (Figure 3-1B)
have spring-loaded vanes that entrap
increments of liquid between the
eccentrically mounted rotor and the
casing. The rotation of the vanes
moves the flow increment from inlet
to outlet and discharge. Accuracy of
TRANSACTIONS Volume 4 35
Slide
Valve
Inlet
Piston