Chemical Engineering July 2013 - 51

Bin hopper
Bin hopper
Slide gate
Pipe sized to
match bin flange
and extend into
feeder throat
Sized to match
feeder flange
Cut on radius
about equal to
rotor radius
about 1/4 in.
clearance
from motor
Solids-injection
device
Conveying
line
Vent connection
use maximum
slope
Pipe sized to match
feeder flange
Insert sized to match
feeder throat. Cut on
approximate radius
of rotor. About 1/4 in.
clearance from rotor
Deflection of the rotor at the
midpoint of its axis due to this
differential pressure drop must
not exceed 0.001 in.
Line size smaller than feeder flange
Line size equal to or larger than feeder flange
FIGURE 5. This installation shows a rotary valve venting system for a dilute-phase conveying
system, to evacuate leakage between the rotor and valve housing. The solids-feed chute
extends into the rotary valve, as shown. The feed chute's bottom matches the outer contour
of the rotor with a space of 1/8-1/4 in. between it and the rotor. A leakage gas vent chamber is
installed above the rotary valve. The annulus between the feed chute and this vent chamber
is 2-in. minimum. Leakage gas flows into ths annulus and exits via a vent nozzle to a dust
collector and exhaust fan
From feed hopper
Vent line to
dust collector
Pocket
carry-over
gas vent line
Leakage and
carry over gas
accumulator
Leakage gas
vent chamber
Leakage gas
vent line
Vent nozzle
Solids
feed chute
Annulus,
2 in.
Rotary valve
pocket vent
minimum
FIGURE 6. Air leakage between
the rotor and the valve housing
must be vented so that it does not
interfere with the flow of incoming
solids. In this venting installation,
for a dense-phase conveying system,
a solids-feed chute extends
into the rotary valve, as shown. The
feed chute's bottom matches the
outer contour of the rotor with a
space of 1/8 in. between it and the
rotor. A leakage gas-vent chamber
is installed above the rotary valve.
The annular space between the feed
chute and this vent chamber is 2 in.
minimum. Leakage gas flows into
this annular space and exits via a
vent nozzle to the top of the leakage
gas and pocket carryover gas accumulator.
Carryover gas from the
rotor pocket and body vent is also
vented to this accumulator
The rotary valve shaft extends beyond
the side plates of the valve housing
using outboard, spherical-roller,
dust-sealed-type bearings between the
shaft and the housing. For shaft packing,
a long-wearing material such as
Teflon with Neoprene gaskets is typically
used. The normal speed range
of rotary valves is 15 to 22 rpm. For
rotary valve sizing, the typical speed
used is 16 rpm.
Rotary valves used in continuous
production operations are provided
with a zero-speed motion switch that
is installed on the driven shaft to detect
chain breakage and the resulting
valve stoppage.
Rotary valve pressure rating. For
pneumatic conveying systems, rotary
valves are designed so that the rotor
can withstand the maximum pressure
differential across the valve's inlet and
outlet. In most cases, this pressure is
15 psi for dilute-phase conveying systems.
However, instead of using 15 psi,
the differential pressure rating should
be based on the actual design pressure
of the conveying system.
The internal vacuum or pressure
rating of the rotary valve
housing should be 10% higher
than the maximum operating
vacuum or pressure to which
the housing will be exposed.
Most standard rotary valves
have a 150-psig housing design
pressure.
Rotary valve clearances.
Clearances between the rotor
and the rotary valve body must
be as small as possible, and they
must be concentric. Typically,
circumferential clearances are
between 0.004 and 0.008 in.,
and end-clearances are 0.006
to 0.0010 in. To minimize conveying
gas leakage, the design
clearances should be the lower
value of these ranges.
Binding or seizing of the
rotor inside the housing should be
prevented by maintaining these minimum
clearances at the highest and
lowest operating temperatures of the
incoming solids and of the ambient
conditions.
The rotary-valve vendor should perform
tests to measure the clearances,
and the air leakage as a function of
rotary valve speed, before the valve is
accepted for use. This test should be
run under the actual operating conditions,
such as ambient and process
temperatures and pressures. Hot air
may be needed to heat the valve when
it is operated under temperatures
higher than the ambient.
Leakage-venting methods. In pressure-type
conveying systems,
pressurized
conveying air that fills the
returning empty rotor pockets is carried
over to the inlet side of the rotary
valve. To prevent this air from
interfering with the flow of incoming
solids, this air is vented out from the
valve before it reaches the valve inlet.
This is done by providing a vent port
on the return side of the valve body.
To prevent increasing air leakage, this
vent port is located so that there are
at least two rotor pockets between the
valve bottom and the vent port. The
vent port size should be large enough
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