Chemical Engineering July 2013 - 50

Solids Processing
Design
ing or chrome-plating because
smooth surfaces are needed to
maintain the required tight clearances
between the valve body and
the rotor. A surface finish of 2B is
desirable.
The valve rotor is of weldedsteel
or stainless-steel construction
with eight or more pockets.
Its horizontal shaft is also steel or
stainless steel.
The entire valve is designed
to withstand the maximum and
minimum pressures and temperatures
to which the valve will be
exposed. These include both process
and ambient conditions.
To maintain the required clearances,
3/4
in.
3/4 in.
clearance
Pipe tap for
purging
(2) each end
Design
clearance
Pipe tap for
purging
(2) each end
in locations where the
valve is exposed to extremely low
temperatures (such as -40°F), the
valve body is jacketed, heated and
insulated. The heating medium
is a heat-transfer fluid that is circulated
throughout the body to maintain
a constant and uniform temperature.
Alternatively, the valves can be installed
inside heated enclosures that
are provided with easy access for the
valve's inspection and maintenance.
In locations where temperatures
are not extreme, electrically heated
blankets placed over the valve body
can be used to maintain a uniform
body temperature.
The valve bottom may have an open
FIGURE 3. In an open-bottom rotary valve,
solids that may have entered the clearance
between the rotor ends and valve body are allowed
to drop out. This option is not viable for
feeding solids into positive-pressure conveying
systems, but they are suitable for vacuumtype
conveying systems, such as airlocks
or closed space between the rotor and
the valve body. As shown in Figure 3,
open bottoms allow solids that may
have entered the clearance between
the rotor ends and the valve body to
drop out. Open-bottom rotary valves
are unsuitable for feeding solids into
positive-pressure-type conveying systems,
because they allow the conveying
air to flow upward into the clearances,
thereby increasing the potential for
conveying air leakage. These valves
can be used in vacuum-type conveying
systems such as airlocks, or as a
feeder. In most pneumatic conveying
applications, closed-bottom rotary
valves, such as that shown in Figure 4,
are more commonly used because they
provide a better air seal between the
rotor edges and the valve body.
For feeding coarse particles such as
plastic pellets, drop-through rotary
valves are provided with a well-configured
inlet plow in their inlet section.
This plow prevents solids from entering
the clearance between the rotor
and the valve housing, thereby preventing
the resulting jamming or seizing
of the rotor. The plow is V-shaped,
is cast or welded into the downstream
side of the rotary valve inlet, and directs
the solids flow into the rotor
pocket.
Rotors. Rotors are of welded construction
with rectangular-shaped blades
that are welded to a shaft. Blades are
evenly spaced around the rotor, forming
triangular pockets. The bottom
of the pockets can be flat or curved,
depending on whether the solids are
free-flowing or sticky.
The number of blades is at least
eight for any size rotary valve. Large
size valves, such as thouse with 4 ft3/
rev. capacity or larger, can have have
10 or 12 blades.
Blade tips are generally hardened
with stellite or tungsten carbide to reduce
their wear. When handling coarse
solids, such as plastic pellets, tips are
generally relieved at a 45-deg angle on
their trailing edge to prevent clipping
of the pellets and the resulting binding
of the rotor inside the valve housing.
The two ends of the rotor can be
open or closed. In open-end rotors,
rotor pockets are fully open on both
ends. In closed-end rotors, rotor pockets
are fully closed by full-size plates
48 CHEMICAL ENGINEERING WWW.CHE.COM JULY 2013
FIGURE 4. Closed-bottom rotary
valves are widely used in pneumatic
conveying applications because they
provide a good air seal between the
rotor edges and the valve body
that are welded at each end. Blades
are welded to the shaft and also to
the two end-plates, thereby providing
strength and rigidity to the rotor.
Closed-end rotors are, therefore, more
rigid and sturdy, and are less prone to
flexing and bending under high differential
pressures than open-ended
rotors. They are used for a large variety
of materials. Open-ended rotors
cost less but are more susceptible to
bending and rubbing with the internal
surface of the valve housing, resulting
in its wear and erosion.
In closed-end rotors, the clearance
space between the end plates and the
valve housing is generally about ½
to ¾ in.
Rotary valve drive. Rotary valves are
generally driven by a gear-head motor,
instead of by a separate motor and a
gear box, because this method is more
economical. The gear-head motor reduces
the output speed to about 30
rpm. From this motor, the rotary valve
rotor is driven by chain and sprockets
to arrive at the valve speed that
is needed. This motor can be installed
either at right angles to, or parallel
to, the rotary valve. Parallel installation
with a chain-and-sprocket drive is
preferable because valve speed can be
changed easily by changing sprockets.
Right-angle installation is more difficult
because it requires changing of
worm gears to change the speed.
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Chemical Engineering July 2013

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