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Powder Bulk Solids Staff
April 25, 2017
6 Min Read
Image courtesy of Vortex Global
Whether it be agriculture, chemicals, food, minerals, plastics, textiles, 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.
As Figure 1 shows, the rotary airlock serves as the most accepted component for feeding dry bulk material from a silo to the conveying line. 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. The consequences of air loss can have significant impact on the overall efficiency and performance of a conveying system.
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. 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.
Air loss through rotary airlocks 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.
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 occurs, rotary vanes and housing begin to deteriorate, creating larger tolerances in an airlock that allow greater air loss. Further, if the presence of fugitive material particles becomes severe, they may enter an airlock’s bearings and cause mechanical failures, heightening the need for costly system maintenance.
Solution One: Installing a Pneumatic Gate Valve 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 seen in Figure 2, when an airlock is not in operation, a 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 to these benefits, a gate valve can also act as a maintenance device to isolate a silo full of material, if an airlock needs maintenance.
Note that gate valves are not universal in nature, so it is important to select a valve that is designed to handle dry particulate, yet capable of sealing typical pressures associated with dilute phase systems. Other factors must be considered in selecting a gate valve for such installations, including a valve’s ability to close through material as it flows from a silo into the vanes of an airlock, a valve’s adaptability to connect to the outlet of a silo and the inlet of an airlock, and a valve design that does not obstruct particle flow.
Solution Two: Installing a Pneumatic Gate above and below a Rotary Airlock
Depending on the application parameters and system design, it may be worthwhile to consider every scenario the system may need to operate within. If there are multiple silos and rotary airlocks feeding material into the same conveying line, as shown in Figures 1 & 2, it may be advisable to install a manual maintenance gate above the airlock and a pneumatically actuated gate valve below.
As simulated in Figure 3, this approach helps to further prevent pressure drop by installing a gate valve closer to the conveying line, while offering the ability to continue operating other system silos as usual, if an airlock were to require out of line maintenance, rebuild, or replacement. A manual maintenance gate valve isolates a silo, eliminating the need to empty it if an airlock unexpectedly fails. As with the pneumatically actuated gate valve, it is important to select a maintenance valve that is made for purpose. For example, the maintenance valve should be able to seal up to 15 psig (1 bar) to atmosphere when in the open position. The design should also incorporate manual actuation, with a torque ratio to adequately close through a standing column of material.
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. The purpose of this article is to introduce 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.
Logan Cameron is content manager, Cory Downing is vice president of engineering, and Jeff Tibbetts is regional sales manager, Vortex, Salina, KS. Vortex is a manufacturer of dry bulk material handling components. For more information, visit www.vortexglobal.com.
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