Acetylene (C2H2)

Acetylene is a colorless and tasteless gas with a garlic-like odor. It is flammable and can be an asphyxiant.

It is one of the fuel gases used in oxy-fuel gas welding, which is any welding procedure that combines a fuel gas with oxygen to produce a flame.

The heat and temperature produced by an acetylene flame depend upon the amount of oxygen used to burn it. Air-acetylene produces a flame temperature of around 4000° F (2200° C). This is hot enough to solder aluminum work glass, repair radiators and braze plumbing fixtures. It is not hot enough to weld steel.

When acetylene is burned in pure oxygen, the flame temperature may be as high as 5730° F (3166° C). However, the flame temperature and the amount of heat generated (measured as BTUs or kilogram-calories) depend upon the ratio of oxygen to acetylene used. Acetylene can produce carburizing, reducing, neutral and oxidizing flames.

The specifications for acetylene are found in the Compressed Gas Association (CGA) Pamphlet G-1.1. Grade D (98.0 percent) is considered “commercial” acetylene. The usual grade is about 98.8 percent acetylene. This is the standard acetylene welding grade. Purified acetylene (99.6 percent) is also available.

Caution: Never use acetylene at a regulator pressure higher than 15 psig. This fuel gas is sensitive to shock and may explode at higher regulator pressures. Acetylene is not supplied as a liquid for similar safety reasons. The gas is dissolved in acetone and supplied in heavy-walled cylinders filled with porous mass packing material.

Purified acetylene (Grade 26) is prepared for use in Atomic Absorption Spectrophotometers.


Acetylene mixed with air or oxygen in a confined space will explode when ignited. Acetylene decomposes explosively if piped at pressures above 15 psig and exposed to mechanical shock or ignition source. Acetylene forms explosive compounds with copper, silver and mercury. Use steel pipe and fittings and pressure gauges with steel or stainless steel bourdon tubes. Copper alloys if used must contain less than 65 percent copper. Acetylene has a very wide flammability range in air of 2.5 percent to 81 percent by volume. Also very low energy sparks such as static electricity can cause ignition and explosion.

Store acetylene cylinder outdoors or in well-ventilated areas away from hot surfaces or flammable materials and ignition sources such as flames or any equipment that can generate a spark. Cylinders must be stored in an upright position. Acetylene cylinders should not be dropped or handled in such a manner as to damage the filter. Use only cylinders and equipment especially designated for acetylene. Never attempt to put acetylene in any other container, equipment or pipeline at pressures above 15 psig. This can be done only at filling plants with proper manifolds, flash arrestors and cylinders with acetone solvent. Make certain all hardware is steel or brass with copper content below 65 percent. Also no silver or mercury can be present where acetylene can react with it.

Correct any leak situations. Leaking cylinders that cannot be stopped should be placed outdoors and returned for repair.

All electrical equipment must be explosion-proof. Tools used around acetylene must be non-sparking (brass or aluminum bronze is required). Articles of clothing that develop static charges should not be worn where acetylene is handled in volume and leakage may be present.

Work in well-ventilated areas. Acetylene is nontoxic but if it displaces oxygen in the air to levels below about 19.5 percent it can cause brain damage and even death. However, acetylene has a distinctive odor which is readily detected in low concentrations so that warning is given of the possible hazard.

Fuel Gas Properties
Safety Acetylene Natural Gas Propane Propylene
Shock Sensitivity Unstable Unstable Stable Stable
Explosive limits in oxygen, % 3.0-93 5.0-59 2.4-57 2.3-55
Explosive limits in air, % 2.5-80 5-15 2.2-9.5 1.9-11
Maximum allowable regulator pressure, psi (kPa) 15
Line Cylinder Cylinder
Burning velocity in oxygen ft/sec (mm/sec) 22.7
Tendency to backfire Slight Considerable Slight Slight
Toxicity Low Low Low Low
Reactions with common materials Avoid alloys
with more than
67% Cu
Few restrictions Few restrictions Few restrictions
Specific gravity of liquid, (60/60° F) 0.507 0.5220
Lb/gal liquid at 60° F(kg/m3 @ 15.6° C) 4.22
Ft3/lb gas at 70° F (m3/kg @ 21.1° C) 14.7
Physical Properites
Specific gravity (air = 1) @ 60° F (15.6° C) 0.906 0.554 1.55 1.476
Vapor pressure at 70° F psig (20° C, kPa) 635
Boiling range °F (°C) 1 atm -84 -259 -44 BP -54 BP
Flame temperature in O2 °F (°C) 5589
Latent heat of vaporization at 25° C, Btu/lb, (kJ/kg) 184
Total heating value (after vaporization) Btu/lb (kJ/kg) 21,500
Btu/ft3 (MJ/m3) 2404