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Harris 43-2 Acetylene

Harris 43-2 Acetylene FAQs

Answers to common questions about the nozzles, mixers, heating heads and cutting attachments used with a modular oxy-acetylene torch.

What pressure should I run acetylene at?icon-minus-redicon-plus-red

For most welding and cutting jobs the HSE says acetylene pressure should not exceed 0.62 bar (9 psi), unless the equipment has been specifically designed for a higher pressure. Decomposition of acetylene can be triggered more easily and proceed more rapidly if the gas in the hoses is above the pressure recommended by the supplier, so do not simply wind the regulator up to get more heat.

The right working setting for a particular nozzle or mixer is on the manufacturer's chart or data plate. Use only a regulator, hoses and torch parts made for acetylene and marked to the correct standards, and follow your own risk assessment.

Do I need flashback arrestors and non-return valves, and what is the difference?icon-minus-redicon-plus-red

A non-return (check) valve fitted on the torch stops gas back-feeding into the hoses, but the HSE notes it will not stop a flashback once one has occurred. A flashback arrestor is a different device that stops a flame travelling back towards the cylinder.

HSE guidance is to fit arrestors on the regulators of both the acetylene and oxygen cylinders and, for long hoses, on the torch end as well. Inspect these devices regularly and replace damaged ones. They are not a substitute for safe working practice, so keep to the manufacturer's replacement and testing advice.

How do I purge and light an oxy-acetylene torch safely?icon-minus-redicon-plus-red

Before lighting, purge the hoses one at a time by opening the gas supply to each hose for a few seconds, then close the torch valve. This flushes out any flammable mixture of gases in the hose. Work in a well-ventilated area and light the gas with a spark igniter, not a butane lighter or a match held close to your hand.

Use the correct gas pressures and nozzle size for the job. The order in which to close the torch valves at shut-down differs between published instructions, so follow the sequence in your torch manufacturer's manual. Only use the equipment if you have been trained.

What causes backfires and flashbacks, and how do I reduce them?icon-minus-redicon-plus-red

The HSE says poorly maintained nozzles cause turbulent gas flow, which increases the risk of flashback. Inspect nozzles regularly, make sure they are not blocked by dirt or spatter, and replace damaged ones. Holding the nozzle too close to the workpiece can overheat it and cause a flashback.

A flashback can also happen if a flammable mixture of fuel gas and oxygen is left in the hoses when the torch is lit, which is why purging matters. Tips and mixers that are loose or not properly seated are another common problem, so tighten them by hand or as the maker instructs.

How should acetylene cylinders be handled and stored?icon-minus-redicon-plus-red

An acetylene cylinder contains a porous mass soaked in acetone, so it is built differently from most gas cylinders. HSE guidance says not to drop or jar it and not to roll it across the floor. Keep it upright, and if it has been laid down and returned to upright, leave it for an hour to settle before use.

Check the identification and look for damage before use, and turn off all valves after use. In the UK, acetylene cylinders are normally painted maroon, but the label is what identifies the gas. If a cylinder looks damaged, the label is unclear, or it has been involved in a fire or flashback, do not use it and speak to your gas supplier.

How do I choose the right cutting nozzle?icon-minus-redicon-plus-red

The cutting oxygen bore is chosen by the thickness of material, so larger bores are used on heavier plate. Nozzles are made as one-piece or two-piece designs, and the choice depends on the fuel gas, whether the torch is hand or machine operated, and the maker's design. Standard nozzles have a central oxygen bore surrounded by preheat ports.

Always match the nozzle to the fuel gas and to the torch or cutting attachment it was made for. The maker's chart gives the recommended gas pressures for each size, so check it rather than guessing from a similar tip.