How Rotary Airlock Valves Are Used in Plastic Processing & Recycling
Rotary valves are essential to any industry that processes dry bulk solids, including in the plastic manufacturing sector. Large-scale processing of raw plastic involves many of the same pieces of equipment – including rotary airlock valves – that are used to produce other bulk materials. This is particularly true for recycled plastics, which often require processing into granulated, pelleted, or powdered form prior to their reuse.
The Use of Rotary Airlock Valves in the Plastics Industry
Plastic is widely used in various modern industries, including construction, electronics, healthcare, packaging, transportation, and other sectors. A highly versatile material, plastic can be adapted into various shapes and sizes for innumerable applications. At the same time, its lighter weight and lower cost make it preferable to other materials for many applications. Not every type of plastic is the same, though plastics exhibit properties like chemical resistance, durability, water resistance, and other physical characteristics that make them extremely useful.
While not used in injection and other molding systems that produce products directly from plastic, rotary airlock valves are essential devices for handling and processing raw bulk material from granulated, pelleted, and powdered plastic. Additionally, rotary valves are particularly important for processing plastic recovered through recycling efforts. As with other bulk materials, the rotary valve designs need to reliably process bulk plastic to prevent jamming or shearing of material, which requires that rotary valve bodies and rotors are precision engineered.
How Rotary Airlock Valves Are Used in Processing Plastic
Rotary valve airlocks are used at various points during the processing of bulk solids to control material flow. The plastic industry's use centers around the discharge of dry bulk plastics from upstream processes into equipment downstream. Rotary valves also regulate the discharge of this raw plastic from silos and other bulk storage containers, along with dust collection equipment like baghouses and cyclones. Additionally, rotary valves are used to feed into and discharge from other processing equipment, along with feeding pneumatic conveying systems.
Storage Silos
Like siloes used to store bulk solids like cement and grain, storage silos are often used to store plastic in bulk prior to processing. Rotary valves are positioned at the outlet port and mounted underneath the storage silo’s cone. They then feed the bulk raw plastic into bins, conveyor belts, feeders, pneumatic conveying systems, screw conveyors or other downstream equipment through the rotary valve. Other bulk containment systems and hoppers often use mounted rotary valves to control material flow as well.
Dust Collecting Cyclones
Also known as centrifugal separators, cyclones collect and separate dust during the processing of plastics. These machines use centrifugal force to remove potentially harmful particulates into the atmosphere by forming a vortex from air blown through the cyclone. As the airstream is redirected, the particulates within the air are separated through their inertia, with coarser particles directed downwards, at which point rotary valves are used to discharge them. Meanwhile, cyclones are less efficient at capturing finer dust, carried upwards and often filtered out via other dust collection devices.
Baghouse Dust Collectors
Acting in its capacity primarily as a rotary airlock, valves are attached to what are commonly called baghouse dust collectors, also referred to as reverse jet collectors or pulse jet collectors. These dust collectors use bags made from fabric as filters, which are held by a metal cage attached to a tube sheet above the collector. As dust enters, the collector deposits materials along the outside of the bags, with brief bursts of compressed air periodically removing this dust. The bags flex with each burst of air, resulting in a cascade of dust particles downwards, which are then discharged via a rotary airlock valve. Using rotary valves in this system decreases the chance of unplanned downtime during processing while minimizing the need for an operator to intervene to clear away the dust.
Feeding Into & Discharging from Processing Equipment
As with other dry bulk solids, plastic processing requires control over feeding and discharging the raw plastic into and out of various material handling equipment. Ball mills, centrifugal sifters, cone mills, conveying systems, hammer mills, hoppers, and other machinery require that material flows at a steady rate to function most effectively. Rotary valves control feeding into and discharging from such equipment to ensure efficient operation.
Feeding Pneumatic Conveying Systems
Rotary valves are often used to feed plastic in raw form from silos, hoppers, and other bulk storage systems into pneumatic conveying systems. Used to prevent air leakage, rotary airlock valves help maintain a balanced and continuous feed of dry bulk materials.
Recycling Plastics with Rotary Airlock Valves
Advanced equipment is required to turn waste plastic into raw material that can be transformed into automotive components, bottles, boxes, carpets, textiles, and various other useful items. However, the fragile plastic flakes produced from these initial recycling efforts tend to meld together, forming irregularly shaped clumps that can jam conveying equipment. Preventing downtime by keeping equipment operational and in good repair is one reason why rotary valves are often used in plastic recycling operations. For recycling operations, rotary valves require a design that improves the material flow while configuring to handle plastic flakes. They are constructed to ensure a longer life.
Material Flow
Recycled plastic generally presents different characteristics to other forms of plastic. Since it has a relatively low bulk density and irregular shapes, care must be taken to control material flow. Recycled plastic flakes can cause jamming between a rotary valve’s housing and rotor, and such bottlenecks can result in the rotary valve jumping, which in turn can damage the rotor shaft to the point that it would require replacement.
Configuring Rotary Airlock Valves
They need to handle recycled plastic pellets and flakes better to keep rotary valves from jamming. This requires that rotary valves be properly configured and designed. For example, rotors with closed ends prevent plastic flakes from building around the rotary valve’s outboard bearings, inhibiting the development of blockages. The use of air purge kits also augments material flow by blowing errant flakes away from bearings and shaft seals, instead returning them to the rotor pockets to provide a more continual flow. Another accessory to consider for rotary valves used to process recycled plastic is shear protectors, which prevent material from damaging the rotor shaft due to obstructions.
Extending Rotary Valve Lifespan
Abrasion-resistant rotary valves and conveying equipment work best when dealing with recycled plastic, due to the irregular shape of the flakes. For this reason, cast iron construction for rotary valves used for plastic recycling applications is often chosen. Additionally, wear-resistant coatings are frequently applied to handle abrasive plastics, enhancing durability and performance. However, stainless steel rotary valves are preferred for wet plastic flakes or sanitary applications, which often use hygienic coatings on the housing and rotor.
Case Study: Prater Rotary Airlock Valves for Vinyl Flooring Manufacturer
Prater Industries has been at the forefront of producing material handling equipment for bulk solids for a century, including various types of rotary airlock valves. Prater continues to provide innovative solutions to material handling issues through the problem-solving efforts of our skilled engineering team. For example, a flooring manufacturer had an issue with their handling system for vinyl powder, so they approached Prater’s engineers. Their setup included transferring finely powdered vinyl from rail cars into large silos, which were then pneumatically fed into smaller silos and mixing stations as necessary.
The problem involved the use of a competitor’s rotary airlock valve, which fed from one of the smaller silos onto a pneumatic conveying line. The manufacturer found that the composite shaft seals within the rotary airlock valve blew out after several weeks in service, causing downtime and loss of raw product. They already used our rotary airlock valves elsewhere in the plant, so they turned to Prater. Our engineering team identified the issue and recommended that the customer replace the rotary airlock valve with our heavy-duty PAV model. The fix resolved the seal leakage issue, with a complete return on investment in just over six months.
Prater Rotary Airlock Feeders for Plastic Processing & Recycling
While not all our rotary valves are designed to handle the rigors of plastic processing and recycling operations, Prater makes rotary airlock valves that can handle higher pressures. Additionally, our quick-take-apart models allow for easier maintenance and cleaning, which makes them useful for industries working with powdered plastic.
Prater rotary airlock valve designs for handling plastic processing and recycling include:
- Blow-Through Rotary Airlock Valves: Used to discharge powdered plastics into pneumatic conveying lines, these rotary valves feature larger vanes to allow greater throughput at lower speeds.
- Heavy-Duty PAV Series Rotary Airlock Valves: Used for handling dry, free-flowing plastic powders in applications involving pressure differentials between processes, such as with the above case study.
- Quick-Take-Apart Rotary Airlock Valves: Designed to allow safe, quick, and easy access to rotary valve internal components for cleaning and maintenance without the need for tools.
- Quick-Take-Apart Rotary Airlock Valves On Rails: Like the other QTA models sans rails, this further eases access to internal rotary valve components with a rotor that self-centers and rail design to protect the rotor from dropping.
To learn more about our rotary airlock valves and other equipment designed to handle the rigors of plastic manufacturing and recycling, contact the material handling experts at Prater today.
