This article explains how rotary airlocks work, what hazards are present, and what the standards are for safely guarding them. The experts at Robson Forensic have investigated many incidents involving these and other industrial system components.
Fundamentals of Rotary Airlocks - Expert Overview
What is an airlock?
Rotary airlocks are used in pneumatic conveying systems as a way to dispense material while maintaining system pressurization. The materials, which include pellets, nuggets, dust, shavings, cuttings and the like, are transferred from a bin, silo, or hopper to an adjacent operation in the manufacturing process. Constructed of rotating vanes sealed within a metal housing, the airlock allows material to flow through while preserving a slight negative pressure through the line. The function is similar to that of a revolving door, where people are allowed to pass through, but drafts (loss of pressure) are prevented. The close clearance of the rotary airlock vanes to the housing prevents or limits the loss of pressure between the bin, silo, or hopper and the adjacent operation, which are usually at different pressures.
ROTATING AIRLOCK SAFETY
When an airlock is turned on the vanes rotate within the housing, which allows material to fill the rotor pockets at the top of the housing and then be discharged by gravity as the material passes through the lower opening. In order to maintain proper system pressure, clearance allowances between the rotating vanes and the housing are very tight. These moving vanes present a pinch point and shear point hazard, which are defined by the National Safety Council’s Accident Prevention Manual:
- Pinch point – A pinch point is any place where a body part can be caught between the two or more moving parts.
- Shear point – A Shear point is a hazardous area created by a cutting movement of a mechanical part past a stationary point on a machine.
The Hierarchy of Hazard Analysis as established in the safety literature provides the following procedure that designers, builders, and sellers/distributors of equipment and machinery should follow:
- Identify each Hazard
- Design out the hazard (if possible)
- If the hazard cannot be designed out, isolate the hazard
- Guard the hazard
- Warn of the hazard, by signs and/or graphics
The moving components that are hazardous to an airlock are also necessary to its function: they cannot be designed out. Therefore, they should be guarded and warned against. While designers and manufacturers bear the initial and greatest burden of safety, the seller, system integrators, and employers also have a responsibility to protect people from hazards in their products, and to warn/instruct them about the dangers.
Airlocks are susceptible to jamming and require periodic servicing. In order to clear the accumulation or build-up of debris from the vanes, workers must access parts which are extremely dangerous while moving. Without the appropriate administrative controls and interlocking guards to shield the operator from the rotating mechanisms, an operator attempting to clear a jam is at risk of amputation or other injury.
MACHINE GUARDING ROTARY AIRLOCKS
To prevent users from exposing themselves to these hazards, industry practice and safety engineering design consensus has been to install an interlock guard/cover. A guard that is interlocked with the airlock will prevent the airlock from operating if the guard is removed or disabled.
Another hazard exists where the airlock discharges into a vessel or container attached to it, as with the discharge of a dust collector. The means of connecting the discharge of a rotary valve to another container should also have an interlock guard. This would prevent someone disconnecting that container to empty the discharged material from inadvertently coming into contact with moving parts.
The addition of an electrical interlocking of the guards is an added fail-safe that will prevent any current from flowing through the circuit unless all guards are in the proper position. That is to say, the entire conveying line will cease to function if any components are not guarded. For a system to be safely and properly engineered, all components should be safeguarded to prevent exposure to hazards. These guards should be accompanied by warnings in the form of signage, graphics, and/or indicator lights that illuminate when the airlock is powered on.
WORKPLACE SAFETY & MACHINE GUARDING INVESTIGATIONS
The industrial safety experts at Robson Forensic have designed, built, maintained, and operated a broad variety of industrial machinery. Moreover, our experts have investigated countless mishaps over the years involving all manner of equipment. By retaining Robson Forensic, you are securing the full weight of our collective experience, knowledge, and resources toward the resolution of your case.
Mechanical Engineer & Industrial Machinery Expert
Jack Green is an expert in industrial machinery, process systems and construction site safety. His expertise includes the design, operation, maintenance, and inspection of machinery; he has specific expertise in machine safety, including machine guarding, energy control (lockout/tag out), and associated instructions and warnings. For nearly 30 years, Jack worked as a rotating equipment engineer, project manager, and maintenance manager at a coal burning power plant. His experience assimilates the many machines, processes, and facilities involved in transforming harvested coal into electrical energy. The scope of his machinery expertise includes grinders and pulverizers, conveyors, turbines, pumps, compressors, fans, gearboxes, industrial boilers, and more. In addition to the equipment and machinery involved in crushing, moving, and burning coal, Jack also has extensive experience in the tools and equipment necessary to service and repair highly customized industrial machines and facilities.