New Design Overcomes Rotary Valve Leakage
Guest article by Jim Sigmon of Sigco Technologies, LLC

Two types of leakage have long been associated with traditional rotary valves, static and dynamic. "Static leakage" is the leakage that continuously flows through the valve due to required running clearances between the outside of the rotor and the inside of the housing. Static leakage occurs whether or not the rotor is turning. "Dynamic leakage" is the leakage that occurs due to the “pumping action” of the rotor. Dynamic leakage occurs only when the rotor is turning.

The amount of static leakage depends on the “gap” between the rotor and the housing, and the newness of the rotary valve. Static leakage increases with use. That is as wear occurs with use, static leakage increases. The amount of dynamic leakage is dependent on the number of rotor pockets and pocket size. Dynamic leakage does not occur if the rotor is not turning, but it does occur with rotor turn and increases proportional to speed of rotor turn.

With traditional rotary valves, based on published manufacturer data, static leakage accounts for about 85% of total leakage, dynamic leakage accounts for about 15% of total leakage.

Static leakage can present major functional problems on both pressure and vacuum conveying systems. On pressure conveying systems, static leakage permits blow line air pressure to blow directly up into the storage vessel, if provision has not been provided to vent the large amount of static leakage. If provision is not made, static leakage can in fact blow material out the top of the vessel, and it can prevent proper filling of the rotor pockets. Perhaps worst of all, it wastes energy.

The problems with static leakage on vacuum convey systems are not as obvious. Static leakage or the cause there of can and often does result in uncontrollable (“wild flow”) of fine powders. It can also result in improper pocket fill.

Traditional rotary valves are sometimes referred to as rotary airlocks. Technically, this is not correct in that they have both inherit static leakage and inherit dynamic leakage; consequently, they do not perform the true function of an airlock, considering the fact that they do not provide an air-tight lock. That is, they do not provide an air-tight chamber, while transferring material from one pressure zone to another pressure zone.

Figure 1. Sigco Cylinder Feeder Airlock - Pressure Conveying

Cylinder feeder airlocks are now available that function as true airlocks in that they provide an air-tight chamber between two different pressure zones, preventing dilution of either pressure zone. This means that they also provide the function of a valve, no leakage (see Figure 1).

These feeder airlocks are not constructed with vaned rotors as are traditional rotary valves but are constructed with cylinder rotors. The cylinder is fitted with a matching radius sealing seat cage constructed of highly abrasion resistant non-metallic materials, such as UHMW-PE.

There is no “gap” between the rotor and the seat cage; the seat cage is maintained in compressive sealing engagement with the surface of the rotor. This results in an air-tight sealing system. The sealing seat cage self compensates for both thermal cycling and for wear. It also pressure energizes which makes sealing of the cylinder airlock proportional to blow line pressure.

With no leakage between the rotor and the seat cage, there is no static leakage with this cylinder feeder. Therefore, the cylinder feeder is not plagued with static leakage on either pressure or vacuum systems. Unvented, the cylinder feeder airlock does have dynamic leakage. Remember all rotary valves have dynamic leakage, but with the cylinder feeder airlock, it is easy to eliminate the dynamic leakage so that the airlock provides an air-tight chamber between two different pressure zones, no dilution of either zone.

To eliminate dynamic leakage (pressure systems), the cylinder airlock is equipped with small vent tubes in the sealing seat cage, one on each top side, the cylinder has multiple, helical pockets (see Figure 1).

As the pockets pass beneath the 12:00 position of the housing, they fill with material, as they pass over the 6:00 position, the material discharges from the pockets. As the empty pockets pass through the 9:00 position (assuming clockwise rotation, although it can be either), air picked up at 6:00 is trapped in each sealed pocket. As each pocket passes the vent tube ports, located on the top inside of the seat cage, the trapped pocket air is exhausted in puffs through less than a 1” diameter tube. A vent tube equipped with a filter sock or other filter catch means can handle the small amount of material carry-over. It is not a necessity to take the vent tube back into the top the storage vessel.

The advantages of no static leakage are obvious, no blow back, no unfilled rotor pockets, no wasted energy. The advantages of no dynamic leakage are not quite as obvious. One sure advantage is proper pocket fill. It has been shown on conveying of plastic pellets that if a pocket is not vented, it will not fill with pellets. But vented, it fills 100%. It can be argued both ways when handling powders. The trapped pocket air imploding into the bottom of the storage vessel does in fact aerate the powder, causing it to free flow into the pocket.

Figure 2. Sigco Cylinder Feeder Airlock - Vacuum Conveying

On vacuum systems, negative pockets passing beneath the powder result in filled pockets (see Figure 2).

The bottom line is that true leak tight cylinder feeders are now available that side step the age old problems of static and dynamic leakage of traditional rotary valves, and they can be used on both pressure and vacuum conveying systems. The new air tight cylinder feeders work especially well on conveying powders and other light materials, as there are no air streams through the feeder.

About Sigco Technologies, LLC

Sigco Technologies began making it's first airlock valves designed to be used on vacuum systems in 1987.  Today, Sigco products included the Cylinder Airlocks, the CA-Series and Cylinder Feeders, the CF-Series. Both airlocks are specially designed for use in pressure- and vacuum-tight applications like dust collectors, cyclone separators, process reactors and other applications that require air-tight sealing.  All Sigco products are patented and/or patent pending.  All designs and drawings contained in this article are copyright © Sigco Technologies, LLC.

About our author

Jim Sigmon is the CEO of Sigco Technologies, LLC. His undergraduate degree is from the University of Tennessee, Worcester Polytechnic Institute and his PhD work was at Kennedy-Western University. For more information contact our author at:

Mr. Jim Sigmon
Sigco Technologies, LLC
4625 Piedmont Row Drive
Charlotte, NC 28210
Telephone: 980-939-1705
Email: jsigmon@sigcollc.com
Web site: http://www.sigcollc.com/

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