Chemical Engineering July 2013 - 49

Solids Processing
Rotary Valves in Pneumatic
Conveying Systems
Because of their wide application in pneumatic
conveying systems, it is important to understand
how rotary valves are designed and used
Amrit Agarwal
Pneumatic Conveying Consulting
R
otary valves are used almost universally
in pneumatic conveying
systems. They serve three main
functions: 1) to provide a pressure-seal
(airlock) between two adjacent
processes, 2) to provide solids metering
(feeding) and 3) to provide a combination
of solids metering and a pressure-seal
for feeding solids into a pneumatic conveying
system.
Rotary valves are common devices
for feeding solids into a pneumatic
conveying pipeline, and they function
in different ways in different circumstances.
For instance, rotary valves
can function as follows:
* As an airlock at locations where an
air-seal is needed, such as at the end
of a pneumatic conveying system
where the conveyed solids are discharged
from a receiving vessel into
a storage hopper, bin or silo
* As a feeder when they are used
to discharge a fixed or a variable
volumetric flow of solids from
an upstream process to a downstream
process
* As a combination airlock and a
feeder when they meter solids into a
pneumatic conveying pipeline
Key aspects of how rotary valves
are designed and used are discussed
below.
Types of rotary valves. Rotary valves
are generally made in the following
three varieties:
1. Drop-through type
2. Off-set or side-entry type
3. Blow-through type
In a drop-through valve (Figure 1), the
solids inlet and outlet are vertically
inline and are generally of the same
size. When the rotor starts
to turn, the empty rotor
pockets are filled by the
solids flowing vertically
down from a hopper above
the valve. These valves
can have square, rectangular,
or round inlets and
outlets; the square shape
is more common because
it provides a larger opening
area compared to a
round-shaped opening.
In a side-entry valve
Line
injector
Gas
inlet
Gas and
solids
outlet
FIGURE 1. In a drop-through valve, the solids inlet
and solids outlet are typically the same size and are
vertically aligned. As the rotor turns, the empty rotor
pockets are filled by solids that flow vertically down
from a hopper located above the valve
FIGURE 2. In a side-entry rotary
valve, the solids inlet and
solids outlet are typically offset
from the vertical flow line by
30-45 degrees. This allows the
upcoming empty rotor pocket
to be filled only partially as the
(Figure 2), the solids inlet
is offset from the vertical,
solids-gravity-flow line by
30 deg. This offset allows the upcoming
empty rotor pocket to fill only partially
when the rotor turns past the
valve inlet. This partial filling minimizes
shearing of the solid particles
that sometimes get trapped between
the rotor and the valve housing. Partial
filling also prevents jamming or
seizing of the rotor by solid particles
that sometimes becometrapped between
the rotor and the valve housing.
The volumetric fill efficiency of the
empty pocket depends on the solid's
flow properties, but it is generally
about 60%. These valves should have
an adjustable slide plate in their inlet
section to allow for the increase or decrease
of pocket filling.
Blow-though valves are similar to
drop-through valves except that they
are installed directly in the conveying
line without any intermediate device.
In this design, the conveying gas enters
from one end of the rotary valve,
blows through the emptying rotor
rotor turns, helping to minimize
shearing and jamming of solid
particles during operation
pocket, and carries the solids to the
opposite end of the rotary valve and
directly into the conveying line. These
valves are used for materials that are
sticky and have difficulty in flowing
out from the rotor pocket.
Rotary valve construction. Rotary
valves have three main components: A
cylindrical body with both ends closed,
a horizontal rotor that rotates inside
this body, and a drivetrain that drives
the rotor. These components are described
below:
Rotary valve body. The valve body is
a horizontal cylinder with a top inlet
and a bottom outlet, and with vertical
plates to close both sides of the cylinder.
The body is generally cast from a
metal such as cast iron, carbon steel,
stainless steel or aluminum, although
other materials are used for special
applications, such as very high temperatures
or highly abrasive solids.
All internal surfaces of the cylindrical
body are made smooth by polishCHEMICAL
ENGINEERING WWW.CHE.COM JULY 2013 47
Bulk solids
inlet
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Chemical Engineering July 2013

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