Whether it be agriculture, chemicals, food, minerals, plastics, or other industry sectors, the most common practice for handling dry bulk solids in a dilute-phase production process is to feed material from a silo through a rotary airlock and into a positive pressure pneumatic conveying system. This material conveying method is most logical across industries due to its efficiencies and relative affordability. As most pneumatic conveying systems are equipped to support pressures of no greater than 15 psig (1 bar), the airlock is a standardized mechanism that supports these benefits and possesses the versatility to handle most dry bulk materials, making it a popular option. However, while such a configuration is beneficial for the movement of material from multiple silos through a single conveying system, flaws in the system’s design may negatively impact the overall performance of a manufacturing process. Read more: http://www.vortexglobal.com/applying-gate-valves-rotary-airlocks-pneumatic-conveying/

2. Whether it be:
…or other industry sectors, the most
common practice for handling dry bulk
solid materials in a dilute-phase
production process is to feed material
from a silo, through a rotary airlock,
and into a positive pressure pneumatic
conveying system (as shown).
Agriculture Chemicals Food Minerals Plastics

3. This material conveying method is most logical across industries due to
its efficiencies and relative affordability. As most pneumatic conveying
systems are equipped to support pressures of no greater than 15 psig
(1 bar), the airlock is a standardized mechanism that supports these
benefits and possesses the versatility to handle most dry bulk
materials, making it a popular option.

4. However, while such a configuration is beneficial for the movement of
material from multiple silos through a single conveying system, flaws in
the system’s design may negatively impact the overall efficiency and
performance of a manufacturing process.

5. Despite the term “airlock,” it is a common misconception that the
equipment can positively seal against pressures generated from the
system’s positive displacement blower. Generally speaking, the more
rotary vanes an airlock has, the better its ability to seal against positive
pressure. But, because there are required tolerances between an
airlock’s rotary vanes and its housing
to prevent metal-on-metal friction,
air will leak past an airlock and up
into a silo (as shown).

6. Depending on particle characteristics, materials conveyed in a dilute phase system
have a minimum air velocity requirement for the material to remain suspended in
air flow. This interaction is often referred to as “saltation” or “pick up velocity.” If
too much air loss occurs at a singular or multiple points throughout a conveying
system, reductions in air velocity will result, causing material particles to fall out of
suspension and create plugs in a conveying line (as shown). This situation often
results in a manual process of decoupling pipes or tubes from the line to empty
built up material, which leads to expensive downtime, labor costs, and product
loss.

7. Air loss also impacts the energy efficiency of a positive displacement
blower. To overcome a reduction in air velocity, it may be required to
specify an oversized blower and run the blower at inefficient levels.
Thus, finding alternatives to reduce system air loss can lead to
significant energy cost savings, over time.

8. Pressure losses also serve as a vehicle for pneumatic-assisted
mechanical abrasion, where fugitive particles blow back upward
through gaps in a rotary airlock’s return-side and cause wear to the
airlock’s vanes and housing. If this abrasion persists, rotary vanes and
housing begin to deteriorate,
creating larger tolerances
in an airlock that allow
greater air loss. If the
presence of fugitive material
particles becomes severe,
they may enter an airlock’s
bearings and cause
mechanical failures, causing
costly system maintenance.

9. Solution 1: Install a Pneumatic Gate Above a Rotary Airlock
In order to counteract pressure loss across a rotary airlock, a pneumatically
actuated gate valve can be installed above it to serve as a barrier between the
conveying line and a system’s silo. As illustrated below, when an airlock is not in
operation, a pneumatic gate valve can be closed to reduce air loss from the conveying
line. This prevents line plugs, improves blower efficiency, and reduces pneumatic-
assisted mechanical abrasion. In addition, the pneumatic gate valve can duel benefit as
a maintenance
device to isolate
a silo full of
material, if an
airlock requires
maintenance.

10. Note that gate valves are not universal in nature, so it is important to
select a valve that:
• is designed to handle dry particulate.
• is capable of sealing typical pressures associated with dilute phase
systems.
• is able to close through flowing materials.
• is adaptable to connect with the outlet of a silo and the inlet of an
airlock.
• does not obstruct particle flow.
Pictured: Vortex
Clear Action Gate

11. Solution 2: Install a Maintenance Valve Above a Rotary Airlock &
a Pneumatic Gate Below
Depending on application parameters and system design, if there are multiple silos and rotary
airlocks feeding material into the same conveying line, this approach further prevents pressure drop
by installing a pneumatic gate closer to the conveying line. This allows the conveying line to
continue operating other system silos as usual, if an airlock requires out of line maintenance,
replacement, or rebuild (as shown). A maintenance valve isolates a silo, eliminating the need to
empty it if an airlock unexpectedly fails.

12. As with the pneumatic gate valve, it is important to select a
maintenance valve that is made for purpose. The maintenance valve
should:
• seal up to 15 psig (1 bar) to atmosphere when in the open position.
• Incorporate manual actuation.
• Possess a torque ratio necessary to adequately close through a
standing column of material.
Pictured: Vortex Maintenance Gate

13. Applying gate valves above or below rotary airlocks in dilute phase
pneumatic conveying systems can provide significant production
benefits and cost savings for dry bulk material processors. This
SlideShare introduces basic concepts on how gate valves can impact
efficiency and functionality of common conveying system layouts.
There can be many variances in system design and material
characteristics, so it is advised to always consult with experts on what
best approaches and methodologies should be used.
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