Isolation doesn’t help much when you’re sad, but when it comes to safety in your bulk processing facility, isolation prevents a one-time event from escalating into a more dangerous situation.
A deflagration is a type of explosion that typically happens in a confined space. When a heat source ignites bulk materials, it can create a fast-moving fire that spreads down a conveying line, creating a chain reaction as it consumes the materials it meets along the way.
Deflagrations are difficult to control, but NFPA 69 outlines several types of active and passive isolation devices that can be used to prevent, control, and contain explosions in systems that process combustible dusts.
As the main transfer points between different pieces of bulk handling equipment such as bins, hoppers, and silos, rotary airlock valves are a critical part of explosion isolation.
Primary and secondary explosions
When a combustible dust cloud is ignited in a confined space, it can trigger a series of explosions. The first in the sequence is called the primary explosion. This can happen in a container, a room in your facility, or a piece of equipment.
The blast wave from the primary explosion can disperse more combustible dust that has accumulated in or around the system. When this dust becomes airborne and ignites, it causes the secondary explosion, which is often more destructive than the primary one.
It’s possible for the blasts to trigger multiple secondary explosions in a row, which is why isolation devices are so essential.
Active vs. passive isolation
Different types of active and passive isolation devices are typically installed throughout a processing facility to support different parts of the system. It’s not about choosing one or the other — it’s about making sure they’re used in complementary ways.
Active isolation devices usually have features or design elements that rely on external power sources and control systems, meaning they must be activated in some way to manage the isolation process. By contrast, passive isolation devices don’t require external input. Some provide isolation through mechanical triggers, while others isolate flames simply by virtue of their design.
Rotary airlock valves are passive isolation devices. They typically work by responding to pressure changes and restrict airflow to isolate the affected part of the system, preventing more fires or explosions from traveling further down the line.
What an isolation device doesn’t do
To be clear, isolation devices are not meant to prevent fires, explosions, or deflagrations from happening in the first place. They are safeguards installed throughout a conveying system to prevent flames from spreading further and affecting other parts of your machinery.
For the best guidance on keeping your facility safe and compliant, we recommend reading up on NFPA 69 for explosion-specific information and NFPA 660 for standards that apply to your industry.
You should also be testing new materials for combustibility, performing regular dust hazard analyses (DHAs), and staying diligent with housekeeping and maintenance.
What to look for in valves
To work effectively as isolation devices under NFPA 69 guidelines, a rotary valve must be designed according to either one of two concepts: material blocking or close clearances.
For material blocking, at least one foot (30 centimeters) of material should remain in the hopper to allow for wider clearances in the housing and limit oxygen.
The close-clearance approach means rotor-to-housing tolerances must stay under 0.0079 inches at all times. You’ll need to inspect and maintain them regularly to prevent them from increasing.
Here’s a quick list of other features rotary airlock valves need to stay NFPA compliant and function properly as isolation devices:
- Rotor vanes: Valve rotors should have a minimum of eight vanes with at least two vanes in contact with each side of the housing at all times. This will keep flames contained within the rotor pockets in the event of an explosion. Vanes should also be at least three millimetres thick with metal tips.
- Outboard bearings: Valves should have bearings located on the outside of the housing. This placement reduces their exposure to combustible dust that could fuel additional fires.
- Heat detection: You can install a temperature switch on outboard bearings to make the valve stop when excessive heat is detected. You can also prevent excessive heat by operating your rotary valves at a low speed (under 20RPM).
- Reduced friction: Use sleeve-style shaft seals made of Teflon® to reduce friction that could create heat or static electricity (two common ignition sources). You can also install a metal detector upstream of the valve to catch tramp metal that could cause further friction.
Get help with compliance
NFPA standards are your best pals for understanding all the different types of explosion protection systems you’ll need in your facility. When it comes to rotary airlock valves, you can count on us. Get answers to all your valve-related NFPA questions by contacting our team.
