Page 112 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 89

& The maximum allowable pressure drop across the & Meters with rubber disks give better accuracy
meter constrains the maximum operating flow in than metal disks due to the better sealing they
high-viscosity services. provide.
& Wear rapidly destroys their accuracy. For this reason, & Nutating disk meters are available in sizes from 1.6 to
PD meters are generally not recommended for mea- 5 cm (five-eighth to 2 in.). They are suited for 10 barg
suring slurries or abrasive fluids. (150 psig) operating pressures with overpressure to a
& The process fluid must be clean. Particles greater than maximum of 20 barg (300 psig). Cold-water service

100 mm in size must be removed by filtering. units are temperature limited to 50 C. Hot-water

units are available up to 120 C.
& In clean fluid services, their precision and wide
rangeability make them ideal for custody transfer & The accuracy of these meters is required to be 2%
and batch charging. of actual flow rate.
& They are most widely used as household water meters. & Higher viscosity can produce higher accuracy, while
lower viscosity and wear over time will reduce
& In industrial and petrochemical applications, PD
accuracy.
meters are commonly used for batch charging of
both liquids and gases. . Describe a rotating vane PD flow meter and illustrate it.
. What is a nutating disk flow meter? Illustrate with a & Rotating vane meters (Figure 4.32) have spring
diagram. loaded vanes that entrap increments of liquid be-
tween the eccentrically mounted rotor and the
& Nutating disk meters are the most common PD
casing.
meters. They are used as residential water meters.
& The rotation of the vanes moves the flow increment
& As water flows through the metering chamber, it
from inlet to outlet and discharge.
causes a disk to wobble (nutate), turning a spindle,
which rotates a magnet. This magnet is coupled to a & Accuracy of 0.1% of actual rate is normal, and
mechanical register or a pulse transmitter. larger size meters on higher viscosity services can
achieve accuracy to within 0.05% of rate.
& Because the flow meter entraps a fixed quantity of
fluid each time the spindle is rotated, the rate of flow & Rotating vane meters are regularly used in the pe-
is proportional to the rotational velocity of the troleum industry and are capable of metering solids-
3
spindle. laden crude oils at flow rates as high as 4000 m /h
(17,500 gpm).
& Figure 4.31 illustrates nutating disk flow meter.
& Pressure and temperature limits depend on the ma-
& The meter housing is usually made of bronze but can

terials of construction and can be as high as 175 C
be made from plastic for corrosion resistance or cost
and 70 barg (1000-psig).
savings.
& Viscosity limits are 1–25,000 cP.
& The wetted parts such as the disk and spindle are
usually bronze, rubber, aluminum, neoprene, Buna- & In the rotary displacement meter, a fluted central
N, or a fluoroelastomer such as Viton. rotor operates in constant relationship with two wiper
rotors in a six-phase cycle.
& Nutating disk meters are designed for water service
and the materials of which they are made must be & Its applications and features are similar to those of the
checked for compatibility with other fluids. rotary vane meter.

Nutating disk positive displacement flow meter.
FIGURE 4.31