Combustible dust is present in many manufacturing and process industries. Dust explosions are a plant's worst enemy. A dust deflagration is a subsonic combustion created when a heat source moves over and ignites cold materials. For this to happen, five conditions must be present.
- Combustible dust, which becomes the fuel
- Cloud of dust suspended in sufficient concentration
- A confined area surrounding the suspended dust cloud
- Oxygen
- Ignition Source such as friction, an overheated material or electrical discharges
So What Does This Have to Do With
Rotary Valves?
Depending on how they are built and maintained, rotary valves can reduce fire risks in your facility.
How?
Rotary valves act as isolation devices by stopping or slowing the spread of flames along a conveying line. They can also reduce the amount of oxygen feeding the fire by regulating fuel and ignition sources.
On the flip side, without regular repair and maintenance, rotary valves can increase fire risks. For instance, material leakage becomes a source of combutible dust.
Maintaining Rotary Valves for Fire Safety
- Rotor vanes. A minimum of eight vanes is preferable to keep flames contained within the rotor pockets in the event of an explosion. That way, at least two vanes can stay in contact with each side of the housing at all times. The vanes should be at least 3 mm thick.
- Rotor tips. Rotor tips should be mild, stainless, or AR400 steel, depending on your application. Neither rubber nor plastic rotor tips can withstand the heat of a deflagration.
- Shaft seals. Worn shaft seals can leak, providing a source of combustible dust and increasing the risk of fire. Replace shaft seals and packing regularly.
- Rotor-to-housing tolerance. Regularly check your valve’s rotor-to-housing clearances, or tolerances. Make sure they stay at or below 0.0079 inches. If they get any larger than that, it’s time to replace your rotor. To check your rotary valve clearances, Meyer has a good tutorial here.
