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.
Safety
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 (103) |
Line | Cylinder | Cylinder | |
| Burning velocity in oxygen ft/sec (mm/sec) | 22.7 (6097) |
15.2 (4633) |
12.2 (3718) |
15.4 | |
| 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 (506) |
5.0 (600) |
|
| Ft3/lb gas at 70° F (m3/kg @ 21.1° C) | 14.7 (0.92) |
23.6 (1.4) |
8.6 (0.54) |
9.05 (0.57) |
|
| 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 (4378) |
— | 120 (757) |
133 (916) |
|
| Boiling range °F (°C) 1 atm | -84 | -259 | -44 BP | -54 BP | |
| Flame temperature in O2 °F (°C) | 5589 (3087) |
4600 (2538) |
4579 (2526) |
5193 (2867) |
|
| Latent heat of vaporization at 25° C, Btu/lb, (kJ/kg) | — | — | 184 (428) |
188 (437) |
|
| Total heating value (after vaporization) Btu/lb (kJ/kg) | 21,500 (50,000) |
23,900 (56,000) |
21,800 (51,000) |
21,100 (49,000) |
|
| Btu/ft3 (MJ/m3) | 2404 (90) |
1470 (55) |
900-1000 (34-37) |
2498 (93) |
2371 (88) |
Frequently Asked Questions
What is acetylene used for, and how hot does it burn?
Acetylene is a fuel gas used in oxy-fuel welding, cutting, brazing, and heating. Its flame temperature depends on how much oxygen it burns with. Air-acetylene reaches around 4,000 degrees F (2,200 degrees C), which is hot enough to solder, work glass, repair radiators, and braze plumbing fixtures, but not hot enough to weld steel. Burned in pure oxygen, acetylene can reach as high as 5,730 degrees F (3,166 degrees C), the hottest flame of any common fuel gas. By adjusting the oxygen-to-acetylene ratio, you can produce carburizing, reducing, neutral, or oxidizing flames for the job in front of you.
What grade of acetylene do I need?
Acetylene specifications are defined in Compressed Gas Association (CGA) Pamphlet G-1.1. Grade D at 98.0 percent purity is considered commercial acetylene. The usual grade supplied is about 98.8 percent, which is the standard acetylene welding grade and the right choice for most cutting, welding, and heating work. Purified acetylene at 99.6 percent (Grade 26) is also available and is prepared specifically for atomic absorption spectrophotometers and other analytical applications. If you are not sure which grade your process calls for, our team can help you match the grade to the application.
Why can acetylene never be used above 15 psig?
Acetylene is shock sensitive and decomposes explosively when piped above 15 psig and exposed to mechanical shock or an ignition source. That is why 15 psig is the maximum allowable regulator pressure and why acetylene is never supplied as a liquid. Instead, the gas is dissolved in acetone inside heavy-walled cylinders packed with a porous mass, which keeps it stable. Acetylene also has an unusually wide flammability range in air, from 2.5 percent to 81 percent by volume, and even a low-energy spark such as static electricity can ignite it. Transferring acetylene into another container is a filling-plant operation only, requiring proper manifolds, flash arrestors, and acetone-solvent cylinders.
How should acetylene cylinders be stored and handled?
Store acetylene cylinders upright, outdoors or in a well-ventilated area, away from hot surfaces, flammable materials, and any ignition source that can produce a flame or spark. Never drop cylinders or handle them in a way that could damage the filler. Use only equipment designated for acetylene, and make certain all hardware is steel or brass with copper content below 65 percent, since acetylene forms explosive compounds with copper, silver, and mercury. Electrical equipment must be explosion-proof and tools must be non-sparking. Correct any leaks immediately, and move cylinders that cannot be stopped outdoors for return and repair. Ventilation matters for a second reason as well: acetylene is nontoxic, but if it displaces oxygen below about 19.5 percent it can cause brain damage or death. Its distinctive odor is detectable at low concentrations, which gives early warning.