04 May Welding Flashback – Preventing Fires and Explosions

Welding Flashback – Preventing Fires and Explosions

Welding flashback can occur during oxy-fuel cutting, welding, brazing, or heating when a flame travels backward through the torch and into the hoses, regulators, or fuel-gas system. Proper equipment setup, inspection, operating pressures, and safe lighting procedures are essential to preventing fires, equipment damage, and serious injuries.

Understand the Hazards

Oxy-fuel equipment uses oxygen and a fuel gas, such as acetylene, to produce a high-temperature flame. Improper pressures, restricted gas flow, damaged equipment, or incorrect operating procedures can allow gases or flames to travel in the wrong direction.

Three conditions are especially important to understand:

Backfire

A backfire occurs when the flame momentarily burns back into the torch tip or mixing chamber and then goes out, often producing a sharp popping sound.

Backfires may result from:

• Holding the torch tip too close to the work.
• An overheated or damaged tip.
• A loose tip or connection.
• Dirt or debris in the tip.
• Incorrect gas pressures.
• Restricted gas flow.

Repeated backfires indicate a problem that should be corrected before work continues.

Sustained Backfire

A sustained backfire occurs when the flame continues burning inside the torch instead of extinguishing. It may produce a whistling, hissing, or squealing sound.

If a sustained backfire occurs, shut down the torch immediately according to the manufacturer’s instructions and allow the equipment to cool. Inspect the torch and correct the cause before attempting to relight it.

Flashback

A flashback occurs when the flame travels upstream from the torch into the hoses or other equipment. Unlike a simple backfire, a flashback can travel through the gas system and potentially cause hose failure, fire, equipment damage, or explosion.

Never continue operating equipment that has experienced a flashback until it has been properly inspected and determined safe for use.

Control Gas Pressure

Correct gas pressure is essential for safe operation.

When using acetylene:

• Never use acetylene at a pressure greater than 15 psig.
• Set regulators according to the torch manufacturer’s requirements.
• Never attempt to increase cutting or heating performance by exceeding approved operating pressures.
• Make sure regulators are designed for the specific gas being used.
• Do not interchange oxygen and fuel-gas regulators, hoses, or fittings.

Other fuel gases may have different operating-pressure requirements. Always follow the equipment and fuel-gas manufacturer’s instructions.

Prevent Reverse Gas Flow

Reverse flow can occur when one gas flows backward into the hose or system of another gas. This can create a dangerous mixture of oxygen and fuel gas within the equipment.

To reduce the hazard:

• Make sure oxygen and fuel-gas pressures are properly adjusted.
• Inspect hoses and connections before use.
• Keep torch passages and tips clean and unobstructed.
• Use appropriate reverse-flow check valves.
• Never use damaged or malfunctioning regulators, hoses, valves, or torches.

Remember that a check valve helps prevent reverse gas flow but is not, by itself, a flashback arrestor.

Flashback Arrestors and Check Valves

Where required by the equipment manufacturer or company procedures, properly rated flashback arrestors and reverse-flow check valves should be installed in the appropriate locations.

A flashback arrestor is designed to stop a flame from continuing upstream through the gas system. Combination devices may incorporate both flashback protection and reverse-flow protection.

Before beginning work:

• Verify that required devices are installed.
• Make sure they are installed in the correct direction and location.
• Inspect them for damage or contamination.
• Confirm that they are compatible with the gases and equipment being used.
• Replace or service them according to manufacturer requirements.

Never assume that a device is a flashback arrestor simply because a fitting or valve is installed between the hose and torch.

Inspect Equipment Before Use

Before lighting an oxy-fuel torch:

• Inspect the torch, regulators, hoses, fittings, and connections.
• Check hoses for burns, cuts, cracks, deterioration, or other damage.
• Make sure connections are secure and appropriate for the gas being used.
• Check the torch tip for damage, contamination, or obstruction.
• Verify that regulators are functioning correctly.
• Check for leaks using an approved leak-detection method.
• Confirm that required check valves and flashback arrestors are installed and serviceable.
• Remove defective equipment from service.

Never use oil or grease on oxygen equipment, valves, regulators, or fittings.

Lighting and Operating the Torch

Follow the torch manufacturer’s procedures for purging, lighting, adjusting, and shutting down the equipment.

• Purge oxygen and fuel-gas passages as required before lighting.
• Use an approved friction lighter or other manufacturer-approved ignition device.
• Never use matches, cigarette lighters, or other inappropriate ignition sources.
• Keep the torch tip the proper distance from the work.
• Do not allow the tip to become clogged or overheated.
• Never leave a lighted torch unattended.
• Do not use equipment that repeatedly pops, whistles, or backfires.

If equipment does not operate normally, stop work and determine the cause rather than attempting to work through the problem.

If a Flashback Occurs

If you hear a shrill whistle, experience repeated popping, or suspect that a flashback has occurred:

• Shut down the torch immediately in accordance with manufacturer instructions.
• Close the cylinder valves when it is safe to do so.
• Remove the equipment from service.
• Inspect the torch, hoses, regulators, check valves, and flashback arrestors before reuse.
• Replace any equipment damaged or affected by heat.
• Notify your supervisor as required.

Do not relight the torch until the cause has been identified and the equipment has been determined safe.

Remember

Welding flashback can rapidly turn ordinary oxy-fuel work into a serious fire or explosion. Correct pressures, properly maintained equipment, appropriate check valves and flashback arrestors, and manufacturer-approved operating procedures are essential.

Inspect before you ignite—if the torch backfires, whistles, leaks, or behaves abnormally, stop work and correct the problem.