Page 113 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& Oscillating piston flow meters are available in sizes
from 11.25 to 75 mm (half in to 3 in.) sizes, and can
generally be used between 6.9 and 10.3 barg (100 and
150 psig).
& Some industrial versions are rated to 103 barg
(1500 psig). They can meter flow rates from
3
0.2273 to 15 m /h (1–65 gpm) in continuous service
3
with intermittent excursions to 22.73 m /h
(100 gpm).
& Meters are sized so that pressure drop is below
2.5 barg (35 psig) at maximum flow rate.
FIGURE 4.32 Rotating vane positive displacement flow meter. & Accuracy ranges from 0.5 % for viscous fluids to
2% for nonviscous applications. Upper limit on
viscosity is 10,000 cP.
. What is an oscillating piston positive displacement flow
& Reciprocating piston meters are probably the oldest
meter?
positive displacement meter designs.
& Figure 4.33 illustrates an oscillating piston positive
& They are available with multiple pistons, double-
displacement flow meter.
acting pistons, or rotary pistons. As in a reciprocating
& The measurement chamber is cylindrical with a par-
piston engine, fluid is drawn into one piston chamber
tition plate separating its inlet port from its outlet port.
as it is discharged from the opposed piston in the
& The piston is also cylindrical and is punctured by
meter.
numerous openings to allow free flow on both sidesof & Typically, either a crankshaft or a horizontal slide is
the piston and the post.
used to control the opening and closing of the proper
& The piston is guided by a control roller within the
orifices in the meter.
measuring chamber, and the motion of the piston is & These meters are usually smaller (available in sizes
transferred to a follower magnet that is external to the down to 2.5 mm (0.1 in.)) diameter and are used for
flow stream. The follower magnet can be used to measuring very low flows of viscous liquids.
drive either a transmitter, a register, or both. The
& A single piston reciprocating positive displacement
motion of the piston is oscillatory (not rotary) since it
flow meter is shown in Figure 4.34.
is constrained to move in one plane.
. What are metering pumps?
& When a quantity of fluid enters the chamber it causes
& Metering pumps are PD meters that also impart
a piston to rotate on its shaft. As it does so, a specific
kinetic energy to the process fluid.
volume of fluid is moved through the meter and
& There are three basic designs: Peristaltic, piston, and
discharged at the outlet port. Each revolution of the
piston corresponds to the movement of a fixed vol- diaphragm.
ume of fluid through the meter. A sensing system, & Peristaltic pumps operate by having fingers or a
typically magnetic or optical, senses a pulse each cam systematically squeeze a plastic tubing
time a portion of a revolution occurs. against the housing, which also serves to position
& Oscillating piston flow meters typically are used in the tubing.
viscous fluid services such as oil metering on engine ➢ This type of metering pump is used in laboratories,
test stands where turndown is not critical. in a variety of medical applications, in the majority
& These meters also can be used on residential water of environmental sampling systems, and also in
service and can pass limited quantities of solids, such dispensing hypochlorite solutions.
as pipe scale and fine ( 200 mesh or 74 mm) sand, but ➢ The tubing can be silicone rubber or, if a more
not large particle size or abrasive solids. corrosion-resistant material is desired, PTFE
& The rate of flow is proportional to the rate of oscil- tubing.
lation of the piston. & Piston pumps deliver a fixed volume of liquid with
& The internals of this flow meter can be removed each out stroke and a fixed volume enters the cham-
without disconnection of the meter from the pipeline. ber on each in stroke.
& Because of the close tolerances required to seal the & Check valves keep the fluid flow from reversing. As
piston and to reduce slippage, these meters require with all positive displacement pumps, piston pumps
regular maintenance. generate a pulsating flow. To minimize the pulsation,