We are often asked the question: is it a legal requirement to fit flashback arrestors?
If you are using acetylene in England, Scotland or Wales, the simple answer is yes. The Acetylene Safety (England and Wales and Scotland) Regulations 2014 require a flame arrestor to be fitted to acetylene systems.
What about oxygen and propane? The British Compressed Gases Association Code of Practice 7 requires flashback arrestors to be fitted to all oxy fuel gas systems as part of a safe system of work.
A safe system of work is defined as a method of working that puts in place control measures from a risk assessment, in order to eliminate hazards where possible and complete the job with minimum risk.
So, you need to fit flashback arrestors.
What is a backfire?
When cutting, welding, gouging or bevelling steel, oxygen mixes with a fuel gas to produce a flame. The flame condition required for welding and cutting is achieved when the quantity and velocity of gas fed through the system (gas exit velocity) matches what the nozzle needs (combustion velocity).
If the volume of gas supplied to the nozzle is too low, or if something is restricting the flow, the combustion velocity becomes greater than the gas exit velocity and the flame burns back into the nozzle. A sharp crack is normally heard, and the flame may either extinguish or reignite at the tip.
This is known as a backfire. Backfires may be caused by:
- Incorrect regulator pressure settings
- Kinked or crushed hoses
- Low cylinder contents
- A blocked nozzle where spatter is restricting gas flow
- Selecting too large a nozzle for the flame required
Sustained backfires
A backfire usually travels back only to the point where gases mix. If the explosion is severe, enough heat can be generated to ignite the forward flowing gases. These gases can then ignite, backfire and ignite again very rapidly. This is often heard as a sharp bang followed by a high pitched scream or whistle. Black smoke may come from the nozzle.
This is called a sustained backfire and it is a very serious situation. The operator must stop it immediately by starving the flame of oxygen.
To stop a sustained backfire safely:
- Close the oxygen valve on the torch
- Close the fuel gas valve
- Close both cylinders
- Plunge the torch into water
- Inspect all equipment carefully for damage
What is a flashback?
A flashback is a backfire where so much energy has been generated that the explosion is forced back along one of the hoses until it either bursts or reaches the flame arresting element in the flashback arrestor.
If flashback arrestors are not fitted, there is a real risk of the explosion travelling through the regulator and into a gas cylinder, with extremely serious consequences.
The most common cause of a flashback is mixed gases in the hose at the time of lighting the torch. Operators must follow correct lighting procedures and purge hoses before light up.
Selecting flashback arrestors
There are many types of flashback arrestors available. Check that any arrestor you buy complies with a recognised standard such as BS EN ISO 5175-1 – Gas Welding Equipment for Safety Devices incorporating a flame (flashback) arrestor.
Flashback arrestors should be replaced every five years, or as per the manufacturer’s recommendation.
If you have experienced a flashback and need training on safe oxy fuel gas set up and use, please contact us on 01270 758890 or admin@gassafeconsultants.co.uk
