Guide
Fusible plugs on acetylene cylinders: the 98–104 °C alloy what it protects against and what it cannot do
A fusible plug on a dissolved acetylene cylinder is a brass or stainless plug filled with an alloy that yields at 100 °C +4/−2 °C under IS 13497:2017. In a fire it melts and vents the cylinder before heat drives it to rupture. It does not protect against overfilling or internal decomposition.
- Yield temperature, India
- 100 °C +4/−2 °C
- International ranges
- ISO 3807:2013 98
- Classic alloy
- Bismuth 39.4 %
- Plug body copper
- Brass body not over 65 % copper
- Production tests
- Every plug pneumatically tested at 60 kgf/cm²; each batch yield-tested; extrusion test 24 h at 55 °C under 34 kgf/cm²
The fusible plug is the one component of a dissolved acetylene cylinder designed to fail. It is a small threaded plug in the cylinder's top fitting, filled with an alloy that melts a little above the boiling point of water. In a fire it yields and lets the cylinder vent before heat can do what pressure alone never would. Indian law fixes its yield temperature more tightly than either ISO or the American standard, and the reasons behind that window explain both what the plug does and what it cannot do.
What the Indian standard requires
IS 13497:2017, the specification for fusible plugs for welded DA cylinders, sets out the plug in detail:
| Requirement | Figure | Clause |
|---|---|---|
| Device type | FP-2 fusible plug to IS 5903 | cl 5 |
| Yield temperature | 100 °C +4/−2 °C (98–104 °C) | cl 5; Annex B certificate |
| Body material | Extruded brass, copper not over 65 %, or non-magnetic stainless steel | cl 3.1 |
| Thread | 1/8 inch NGT external taper, gauged to IS 15894; 12 mm wrenching size | cl 4.1, 4.3 |
| Fitting | Must not protrude above the bung top face when tight | cl 4.2 |
| Flow capacity (type test) | Qa = 1.148 Wc m³/h of free air at 686 kPa absolute; Wc not taken below 10 litres | cl 6.2 |
| Yield test | Every production batch, by IS 5903 method; two retests on failure | cl 7 |
| Extrusion resistance | 55 °C for 24 h with at least 3.35 MPa (34 kgf/cm²) air on the cylinder face | cl 8.3 |
| Leak test | Every plug, pneumatically at 5.90 MPa (60 kgf/cm²) | cl 9 |
| Fixing | PTFE tape wound clockwise from below; hand start; 20–25 N·m torque | cl 10 |
The same 98–104 °C window appears in IS 3224:2021 cl 7.1.5 for acetylene valves and in Rule 37(viii) of the Gas Cylinders Rules 2016, which requires a relief device, if fitted, to operate at 53 kgf/cm² or at 100 °C +4/−2 °C. IS 7312 cl 10.2.2 places the plugs on the bung on both sides of the valve shank opening, so in Indian practice they sit in the top fitting, under the valve guard, not in the shell wall. Seamless cylinders under IS 7312 cl 6.2.3 carry no plug in the body.
Note the copper limit. The plug body is held to 65 % copper, stricter than the 70 % that Rule 37(v) allows for the valve. Acetylide-forming alloys are kept to the conservative figure on the component that sits wettest in acetone and acetylene.
Three regimes, one floor
| Source | Yield range |
|---|---|
| IS 13497:2017 / GCR 2016 Rule 37(viii) | 98–104 °C |
| CGA C-13:2000 §6.4 | 208–224 °F = 97.8–106.7 °C |
| ISO 3807:2013 cl 4.6 | 98–110 °C |
| ICC 8AL practice, as compiled by S. A. Miller (1965) | 99–101 °C; Bi 39.4 / Sn 30.8 / Pb 29.8 % |
All three current standards share a floor of 98 °C and differ on the ceiling; India's 104 °C is the tightest. The floor is set well above any temperature a cylinder meets in storage, even in direct Indian summer sun, to avoid nuisance venting. AIGA 022/26's narrative remark that plugs typically melt above 107 °C is the outlier and is not a specification.
What the plug protects against
The fire hazard for a DA cylinder is heat, and heat acts in three ways at once. Solubility of acetylene in acetone falls steeply as temperature rises, so gas comes out of solution and pressure climbs. The loaded solvent expands at about 0.23 % per °C (as compiled by Miller), shrinking the gas space towards the point where the cylinder becomes liquid-full and develops hydraulic pressure. And a shell wall heated locally can start decomposition in the adjacent mass. The plug converts a sealed cylinder into a vented one before these combine.
The type tests show the design intent. IS 7312:1993 cl 20.6 burns a fully charged cylinder lying on a rack in a fire of dry pine timber and 2 litres of kerosene; it passes with no appreciable bulging, no extensive decomposition or breakup of the filling, a pressure rise of at least 170 kPa before the plug releases, and release within 10 minutes. ISO 3807:2013 Annex F uses a chimney fire with flue gas at about 650 °C, passing if one or more plugs release and the cylinder does not burst. The US ICC bonfire test that Miller described required at least 25 psi rise before any device functioned, functioning within ten minutes, and no explosion.
Miller's own verdict, from experience of that test, is the reason the plug matters: either the safety devices function as intended, or there is eventually a violent explosion. There is no benign middle outcome in a cylinder fire. He also judged that safety devices had proved their value in practice as an automatic provision of free venting.
Rules that matter
What the plug cannot do
- It does not protect against slow heating below 98 °C A cylinder warmed to 60–80 °C, for instance against a furnace or in a closed vehicle in the sun, has lost much of its gas space but its plug has not moved. Overfilling and heat in that range are controlled by the filling ratio and the 65 °C type test, not by the plug.
- It is not proved to act before hydraulic filling in every case Miller's worked examples for a 79 %-porosity cylinder reached the liquid-full condition at 88 °C and 109 °C depending on gas purity. For a 90–92 % mass at the IS 7312 acetone charge the temperature has to be computed; the plug is the governing device only if that figure lies above 104 °C.
- It does not stop an internal decomposition A flashback into the cylinder is stopped by the porous mass. The plug only vents the heat and gas that follow.
- It does not respond to pressure It is a thermal device. A cylinder overpressured at ambient temperature will not vent through its plugs.
Why some countries do not use plugs
As compiled by S. A. Miller (1965), British solid-drawn cylinders were hydraulically tested at 1 000 psi (68 atm) and fitted with two bursting discs, in the valve and a base plug, set at 85–100 atm; continental Pilger-process cylinders, tested at 60 kg/cm², generally had neither discs nor plugs, and in some European countries plugs were actually prohibited. The 1965 compilation does not give the reason for those prohibitions. One known drawback is that gas released by a melted plug can burn and heat neighbouring cylinders; in at least one investigated US fire, flames from damaged cylinders melted the plugs on others and caused further releases. Indian and American practice both fit plugs, and GCR 2016 Rule 37(vii) allows a cylinder fitted with a fusible plug to be tested at 53 rather than 60 kgf/cm². See bursting discs for the other device family.
Inspection and replacement
Plugs are checked at every filling under IS 16271 cl 3.2.2 and cl 3.2.4 and leak-tested after filling under cl 3.5.1. At periodic inspection IS 8433:2017 cl 6.5 requires damaged relief devices to be replaced and checked for gas tightness. CGA C-13 §8.4.8 lists the defects (extruded metal, corrosion, bending and peening) and states bluntly that defective fusible metal devices shall not be peened, altered or repaired. Extruded alloy is a warning in itself: it means the cylinder has been hot enough to soften the alloy. A plug that has operated means the cylinder has been in a fire, and Rule 24(2) of the Gas Cylinders Rules 2016 condemns it. The periodic inspection page covers the full examination.
Rules that matter
Rules to apply
- Never paint over, seal, tape or plug a fusible plug, and never fit a cap that blocks it; IS 7312's valve cap is perforated for this reason.
- Do not store DA cylinders near furnaces, ovens, radiators or in closed vehicles; the plug will not save a cylinder at 80 °C.
- Report any cylinder with a weeping, extruded or damaged plug; do not use it.
- In a fire, cool from a protected position and keep people away from the plug side; the cylinder fire page sets out the response.
KIGL dissolved acetylene cylinders carry plugs on the valve pad as IS 7312 provides; the cylinder standards are listed on acetylene cylinder standards.
Fusible plugs on acetylene cylinders — frequently asked questions
At what temperature does an acetylene cylinder fusible plug melt?
In India the alloy must yield at 100 °C +4/−2 °C, that is between 98 °C and 104 °C, under IS 13497:2017 cl 5; IS 3224:2021 and Rule 37(viii) of the Gas Cylinders Rules 2016 give the same window. ISO 3807:2013 allows 98–110 °C and CGA C-13 97.8–106.7 °C, so the Indian range is the tightest. The CNG plug value of 125 ± 10 °C in IS 3224 is unrelated.
What is a fusible plug made of?
The body is extruded brass with not more than 65 % copper, or non-magnetic stainless steel, threaded 1/8 inch NGT and fitted into the bung beside the valve. The core is a fusible alloy of bismuth, tin and lead; the composition used on US ICC 8AL cylinders was Bi 39.4 %, Sn 30.8 %, Pb 29.8 % with a melting point of 99–101 °C.
Why does an acetylene cylinder need a fusible plug when it has a strong shell?
Because in a fire the danger is heat, not ordinary pressure. Rising temperature drives acetylene out of the acetone and expands the loaded solvent until the cylinder can become liquid-full, and a hot wall can start decomposition in the mass. The plug vents the cylinder before the shell is overwhelmed. Miller's experience of bonfire tests was that the safety devices either function or the cylinder eventually explodes violently.
Do all acetylene cylinders have fusible plugs?
No. Indian welded cylinders to IS 7312 carry them on the bung on both sides of the valve, and the shell test pressure may be reduced from 60 to 53 kgf/cm² when they are fitted. Seamless cylinders carry no plug in the body. British solid-drawn cylinders used bursting discs, and many continental cylinders had neither; the 1965 literature compiled by S. A. Miller records that plugs were prohibited in some European countries.
Can a fusible plug be repaired or re-used?
No. CGA C-13 states that defective fusible metal relief devices shall not be peened, altered or repaired and must be replaced. IS 8433:2017 cl 6.5 requires a damaged plug to be replaced and checked for gas tightness. A plug showing extruded metal, corrosion, bending or peening is replaced; one that has operated means the cylinder has been in a fire and is condemned under Rule 24(2).
What should I do if a fusible plug starts to vent?
Treat it as a cylinder fire emergency. Clear the area, raise the alarm and call the fire brigade; cool the cylinder with water spray from a protected position, continued until it is cold, as the Gas Cylinders Rules 2016 Form E conditions require for hot cylinders. Do not try to plug the vent. The cylinder is scrap afterwards.
How tight should a fusible plug be fitted?
IS 13497:2017 cl 10 requires the stem to be wrapped clockwise with PTFE tape, the plug started by hand, and a fixing torque of 20 to 25 N·m. The plug must not protrude above the top face of the bung when fully tightened, so that the valve guard protects it.
References
- Bureau of Indian Standards. (2017). Fusible Plug for Dissolved Acetylene Gas Cylinder — Specification (IS 13497:2017, First Revision).
- Compressed Gas Association. (n.d.). Guidelines for Periodic Visual Inspection and Requalification of Acetylene Cylinders (CGA C-13).
- Government of India. (2016). Gas Cylinders Rules, 2016 (G.S.R. 1081(E)). Petroleum and Explosives Safety Organisation.
- International Organization for Standardization. (2013). Acetylene cylinders — Basic requirements and type testing (CFR incorporation-by-reference copy) (ISO 3807:2013).
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