A: Jason Brame, Rotolok Valves says:
Screeching sounds can come from one of three sources on a rotary valve: the bearings or drive, the rotor scraping against the body, or the material passing through the valve. Since you’ve noted this is a new unit, I’ll assume that the bearings and drive, also known as the gearbox, aren’t the source of the screeching. This leaves the rotor or the material as the source of the noise.
The first question to answer is whether the valve makes any noise while running with no material passing through it. If there’s no noise at that point, then we know the material is causing the problem. However, if the rotor screeches without material passing through, then something is wrong with the valve’s mechanics. Maybe the rotor is set too close to one end cover or maybe one or more of the rotor tips is out of place (if the rotor tips are adjustable). In this case, you need to remove the valve from service and check all the clearances between the vanes and body to make sure they match your valve’s specifications. One last possibility is that the rotor has been damaged by either the material or something foreign in the system, such as a stray bolt. In this case, you’ll need to replace the damaged rotor or tips. If the material caused the damage, you need to contact your valve manufacturer to discuss other options.
Let’s look at a situation where the rotor doesn’t make noise when empty but does start screeching when material is added. There are three possible scenarios causing this noise.
First, the material may be problematic. Perhaps the material is a bit sticky and is coating the valve’s body. In that case, you may need to add beveled rotor tips or even scraper blades to remove the buildup from the bore. Another possibility is that the material is making the rotor chatter as it passes around the bore. Again, this may be helped by using beveled rotor tips or possibly making the rotor gaps larger. Sometimes a material can pack in the rotor pockets and not release well. This leaves the pockets full on the upswing with more material trying to drop into the pockets but no place for it to go. You can resolve this situation by adding pocket purge. A pocket purge is created when small ports are drilled into the valve’s end covers or body to shoot compressed air into the rotor pockets as they pass the outlet to dislodge stuck material.
Second, the material could be hot. The gap between the rotor vanes and the body casting for an ambient temperature valve is usually just a few thousandths of an inch. This is what allows these valves to act as an airlock and not just a feeder. For high-temperature applications, the manufacturer must machine the rotor down so the gaps are larger to allow for thermal expansion at the maximum temperature the material could reach. If this isn’t done, the rotor vanes can expand to a point where they hit the end covers or bore. If they barely hit, they’ll screech. If the expansion is large enough, the rotor will lock up inside the valve. In a perfect world, the manufacturer will have been advised of the maximum operating temperature during the quoting process. However, with multilevel purchasing processes where application data has to travel from the end user to a purchasing department and then possibly to an engineering firm or supplier and so on, information can get lost. The good news is that if the material is hot, you should be able to simply have the rotor machined down to the proper size to alleviate this issue.
Third, the material could be moving the rotor axially in the valve. This problem isn’t very common, but some materials tend to push the rotor toward one end or another. Each manufacturer has its own way of mechanically centering the rotor between the end covers. Some of these methods use a spring tension against an adjustment bolt to hold the rotor in position. If this is the case with your valve, some materials can push hard enough to overcome this tension and move the rotor so that it scrapes against an end cover causing screeching. In this case, you may need to center the rotor without using tensioning. I’ve seen valves that use shims or an adjustable nut with a locking ring. Most manufacturers should have a way of achieving this, because the tensioning method isn’t acceptable for high-temperature applications.
Rotolok Valves, Monroe, NC, supplies rotary valves and airlocks, diverters, and slide-gates and control valves.