The oxy-fuel cutting process involves the use of fuel gases and oxygen to cut metals. Although it is an efficient technique, it is also potentially dangerous due to flashbacks, which can cause equipment damage, fires, or explosions. To prevent these risks, specialized safety valves are used.
What Can Cause a Flashback?
A flashback occurs when combustion travels in the opposite direction of the normal gas flow. The main causes include:
- Insufficient gas pressure: If the oxygen or fuel gas pressure drops below the required level, the gas mixture may ignite inside the hose or torch.
- Blockages: A damaged torch, metal particles, or dirt in the nozzles can create turbulence that promotes flashback.
- Damaged or leaking hoses: Leaking or kinked hoses can facilitate flame flashback.
- Overheating: Excessive heat in the nozzle or valve can trigger a flashback.
- Valve failures: Malfunctioning gas control valves or safety valves (flashback arrestors) may allow flashback to occur.
- Operational errors: Incorrect opening of valves or use of improper gas mixtures.
Types of Flashback and Examples
1. Flashback
This is the most dangerous phenomenon. The flame travels back inside the hoses or even into the gas supply equipment.
Example: During cutting, the operator hears a sharp hissing sound and the flame disappears from the nozzle—the combustion has travelled back into the hose.
2. Burnback
Occurs when combustion continues to burn inside the torch or hose after a flashback.
Example: The torch keeps getting hotter even after the gas valves have been closed.
3. Backfire
A small, momentary flashback that produces a “puff” or popping sound, but the flame does not continue inside the equipment.
Example: During cutting, a small popping sound is heard, but the operation continues normally.
Safety Valves: Types and Characteristics
To prevent serious accidents, it is essential to install safety devices between the gas source and the torch. The main safety valves used in oxy-fuel cutting are:
1. Gas Non-Return Valve (Check Valve)
- Function: Prevents gases from flowing back toward the supply source.
- Characteristic: Allows flow in only one direction.
- Installation: Typically placed at the torch inlets and at the outlets of the pressure regulators.
Practical example: If oxygen tries to flow back toward the fuel gas cylinder, the valve automatically shuts off.
2. Flashback arrestor
- Function: Stops the flame from propagating in the event of a flashback.
- Typical Components:
- Built-in non-return valve.
- Cooling element (usually a metal mesh) to extinguish the flame.
- Thermal shut-off valve: automatically cuts off gas flow if overheating is detected.
- Standards: Commonly comply with standards such as EN 730-1 or ISO 5175.
Practical example: If a flame travels back into the hose, the flashback arrestor extinguishes it and blocks the gas flow, protecting the gas supply source.
3. Thermal Safety Valve
- Function: Shuts off gas flow when abnormally high temperatures are detected.
- Characteristic: Contains a fusible element that melts and activates a shut-off mechanism.
- Application: Especially useful in environments with a risk of prolonged heating.
Practical example: If a hose is accidentally exposed to a heat source, the thermal safety valve automatically stops the gas supply before a flashback can occur.
What to Do in Case of a Flashback?
- Close the oxygen valve: This will stop the combustion. Then close the Chemtane 2 gas valve.
- Allow the equipment to cool down.
- Check all connections and components.
- Take the equipment to a maintenance workshop for inspection.
Final Considerations
- Always use high-quality, certified safety valves on both the gas regulator and the torch, and for each type of gas (oxygen and Chemtane 2).
- Check gas pressures: Ensure that oxygen and Chemtane 2 pressures are correct for the cutting task.
- Perform regular maintenance on the equipment: This includes cleaning the torch and checking all connections.
- Inspect the torch and hoses: Replace any damaged or leaking components.
A proper understanding and selection of safety valves can make the difference between a safe operation and a serious accident in the oxy-fuel cutting process.
