Considerations for Selecting Rotary Airlocks

David Donohoe
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Rotary airlock valves are fairly basic machines. They contain four main components: body, rotor, and two end-covers. These valves operate via a chain-driven or direct-coupled gearmotor. The main functions of the valves are as an airlock, a feeder, or combination of both. In all three instances, material is transferred from the inlet to the outlet, making the material characteristics an integral part of the valves’ overall performance. Due to their perceived simplicity, some individuals may consider rotary airlocks easily accessible commodities, capable of being utilized between any number of material handling systems. Although the construction makeup may be simple, the differing material characteristics that valves are subjected to require them to take on a greater degree of complexity. This can typically be addressed through additional features, either supplied initially with the valve, or possibly retrofitted in the field. The following are just a few different material characteristics that can be encountered by rotary airlocks and some features to consider.

Many rotary airlocks are required to work with larger material particulates, or pellets, which are susceptible to bind or jam a rotary airlock valve. This is particularly detrimental when dealing with non-friable products. The suggestion would be to utilize an offset rotary airlock in this application. Offset rotary airlocks contain a rotor arrangement which is offset from the inlet. It allows the valve to continually catch the material on the rotor upswing, which reduces pocket fill and allows material to continually fall to the following pocket. The valve’s intent is to decrease the chopping effect of material when meeting the leading edge of the valve inlet. This decreases product shear and lessens the chances of shock loads, which can adversely affect the balance of the rotor and the integrity of the drive components. Other options for reducing shear on standard rotary airlocks would be inlet meter baffles and inlet shear plates (space permitting).

Some materials may have a tendency to pack or hang up in the V-shaped pockets of a rotary airlock. One means of addressing this problem would be a scalloped or concaved pocket arrangement. Although this feature will reduce capacity, it allows sticking product to easily drop from the rotor by eliminating any areas for packing. Another option to dislodge hung-up product from the rotor would be by utilizing drilled and tapped end cover purge ports (only suitable for open rotors). This allows a steady stream of air to aid in releasing packed product. A third choice would be covering the rotor surface area with a PTFE coating, giving it a non-stick surface.

Abrasive materials will not only wear a valve’s materials of construction, it will increase the tight tolerances between the blades and the body. Valves coming in contact which such materials should be suited with additional wear-resistant material coatings, such as hard chrome or tungsten carbide. Additionally, the valves should be fitted with rotors with hardened AR400 steel rotors or replaceable AR400 rotor tips. Another beneficial option is to utilize a rotor that can adjust internal tolerances via external adjustment to atone for rotor wear, such as a tapered adjustable rotor. Air purge gland seals would also be suggested to create an air barrier for searching abrasive product in the seal and shaft contact areas.

Lastly, material and process temperatures must be taken into account since airlocks operate on tolerances typically ranging from 0.004-0.005 in. If temperatures are going to increase above ambient, rotor clearances must be adjusted on the valve either initially at the manufacturer or in the field. If not adjusted, thermal expansion of the rotor will cause the valve to bind and seize. Again, an external adjusting clearance-style tapered rotor could be a viable option for adjusting clearances to atone for thermal expansion.

These are just a few of the different material characteristics that can be encountered by rotary airlock valves. A small time investment in properly sizing rotary airlocks to meet your specific material characteristics should help ensure your valve will run more efficiently and effectively.

David Donohoe has worked in the power transmission & material handling field for the past 15 years. He has been a sales engineer with Bush & Wilton Inc. for the past 10 years.