Guide
Inert gas confined-space entry: the permit the gas test, continuous monitoring, the standby person and a rescue that does not add victims
Entry into any vessel, pit or tank that has held nitrogen, argon, helium or CO₂ needs a written permit, positive isolation, an air purge, a gas test showing 19.5 to 23.5 % oxygen at top, middle and bottom, continuous monitoring, a standby person outside and a planned non-entry rescue.
- Entry atmosphere
- Oxygen 19.5 to 23.5 % by volume; flammables below 10 % of the lower explosive limit; toxics below their exposure limit
- Three-step test before generator entry
- Acetylene in nitrogen
- Purge end-point for acetylene pipework
- Outgoing nitrogen to contain not more than 2.5 % oxygen before acetylene is admitted
- Purge gas
- Nitrogen only for acetylene generators; CO₂ prohibited because of static electricity
- Rescuer deaths
- In NIOSH investigations of confined-space fatalities
Confined spaces filled with inert gas are among the deadliest places in industry, and a large share of the people who die in them were not the first person to go in. The space looks empty, the air smells normal, the first entrant collapses without a sound, and a colleague climbs in to help and collapses too. Nothing about this is unpredictable, and every step of the procedure below exists because one of those deaths showed it was needed. This guide covers entry into any vessel, tank, pit or enclosure that has held nitrogen, argon, helium or carbon dioxide, including the acetylene generators, gasholders and purifiers that are purged before maintenance.
Rules that matter
The physiology is set out under nitrogen safety and asphyxiation: the body's breathing drive responds to carbon dioxide, not to a lack of oxygen, so an oxygen-free atmosphere produces no sensation before unconsciousness. Below about 10 % oxygen a person can collapse in a few breaths; below 6 % death follows in minutes (CGA SB-2). Three features of confined spaces make this worse:
Why inert-gas spaces kill
- The inert gas is often there deliberately, from a purge, a leak test or a blanket, and people forget it has not gone away when the supply is turned off.
- Gases stratify. Argon and CO₂ pool at the bottom, helium and hydrogen at the top, and a reading at the manway says little about the floor of a tank.
- The exit is narrow. A person who feels unwell in an open shop walks out; a person at the bottom of a vessel has to climb.
The rescuer pattern recurs throughout the investigated record. NIOSH's investigations of confined-space fatalities found that around 60 % of those who died were would-be rescuers. The procedure therefore treats rescue as part of the entry, planned before anyone goes in.
The permit
Every entry is authorised on a written permit under a work-permit system of the kind EIGA Doc 40 describes, and EIGA Doc 123, the acetylene code of practice, requires permits for generator entry and cleaning. The permit records, at minimum:
| Element | What it covers |
|---|---|
| The space and the job | Which vessel, what work, how long |
| Hazards | Previous contents, purge gas used, residues, energy sources |
| Isolation | Every line blanked or disconnected; drives locked off; listed by tag |
| Purge and ventilation | Purge medium, air purge, ventilation method during entry |
| Gas test results | Oxygen, flammables, toxics, at top, middle and bottom, with time and tester |
| Monitoring | Personal and fixed monitors, alarm settings, calibration dates |
| Personnel | Entrants, standby person, supervisor, rescue team |
| Equipment | Harnesses, tripod and winch, breathing apparatus, communication, lighting suitable for the zone |
| Rescue plan | How a collapsed entrant is removed without anyone else entering unprotected |
| Sign-off and hand-back | Authorisation, time limits, closure when the space is reinstated |
A permit is valid for the stated job, the stated time and the stated people. Any change, including a shift change, a break in the work or a change in the job, means re-testing and re-authorising.
Rules that matter
Positive isolation means physically separating the space from every source of gas, liquid or energy: removing a spool piece or fitting a rated blank or spade, not trusting a closed valve. Inert-gas supply lines are the commonest omission, because nitrogen is thought of as harmless; a passing valve on a nitrogen header fills a vessel while people are inside. Agitators, screw feeds, pumps and heaters are locked off and tagged at the isolator.
Purging usually comes in two stages. Flammable gas is first displaced with inert gas, which makes the space safe from fire and lethal to breathe; then the inert gas is displaced with air. Three Indian and European rules apply to acetylene equipment:
Isolation and purging
- EIGA Doc 237 cl 6.1 permits only nitrogen for purging acetylene generators; carbon dioxide is prohibited because fast-flowing CO₂ generates static electricity, a hazard also recorded in the C.P.I. 1968 acetylene code.
- IS 15190 (Part 1):2002 cl 12.6.3 sets the nitrogen purge end-point for acetylene pipework at not more than 2.5 % oxygen in the outgoing gas before acetylene is admitted.
- Oxygen is never used to purge or ventilate anything; see oxygen enrichment and deficiency in the workplace.
The sequence for a plant is covered in detail under acetylene plant start-up, shutdown and purging.
The gas test
The atmosphere is tested from outside, before entry, with a calibrated multi-gas instrument and a sampling line long enough to reach every level.
| Parameter | Acceptance for entry | Source |
|---|---|---|
| Oxygen | 19.5 to 23.5 % by volume | OSHA 29 CFR 1910.146 |
| Flammable gas | Below 10 % of the lower explosive limit | OSHA 29 CFR 1910.146 |
| Toxic gases (CO₂, H₂S, phosphine, CO) | Below the occupational exposure limit, for example CO₂ below 0.5 % | ACGIH; OSHA |
| Sampling points | Top, middle and bottom, and any pocket or baffle | Common practice |
For acetylene generators EIGA Doc 237 cl 8.3 sets the order of tests explicitly: first acetylene in nitrogen, to confirm the flammable gas has gone; then acetylene in air below the LEL, once air has replaced the nitrogen; and finally oxygen in nitrogen before personnel exposure, the asphyxiation check. A purifier or a sludge-laden vessel can release acetylene, phosphine or hydrogen sulphide after the first clear reading, which is why the test is repeated, not filed.
Continuous monitoring and ventilation
A clear test at the start is not a clear space for the whole job. Gas desorbs from sludge, carbon beds and porous linings; ventilation fans stop; an adjacent line passes. Each entrant wears a personal monitor alarmed at 19.5 % and 23.5 % oxygen and, where relevant, at a flammable and a toxic limit. For heavy gases the monitor is worn low; for light gases, high. Forced ventilation with air runs throughout the entry, supplying fresh air to the work position and extracting from the point where the gas collects. Any alarm means every entrant leaves at once, and the space is re-ventilated and re-tested before re-entry under the permit.
Work inside the space can create its own hazard. TIG or MIG welding releases argon or argon-CO₂ mixtures that collect at the bottom of a vessel, as the argon safety guide explains, and hot work inside an acetylene plant vessel brings the controls in hot work and maintenance in an acetylene plant into play as well.
Rules that matter
The standby person stays at the opening for the whole entry with no other duty. They keep every entrant in sight or in voice contact, watch the ventilation and the monitors, control access, and at the first sign of trouble raise the alarm and begin rescue from outside. They do not enter, whatever they see, unless wearing breathing apparatus as part of a planned rescue.
Rescue is planned on the permit before entry:
The standby person and the rescue
- Non-entry rescue first: each entrant wears a full-body harness on a line attached to a tripod and winch at the opening, so the standby person can lift a collapsed entrant out without going in.
- Where internal geometry makes a straight lift impossible, a trained rescue team with self-contained breathing apparatus stands by at the opening for the duration.
- Escape breathing sets are carried by entrants where a sudden release is credible.
- The site emergency plan and, where needed, the local fire service are told before the job starts.
- First aid for an asphyxiated casualty is removal to fresh air, oxygen therapy by a trained person if available, and resuscitation; there is no antidote.
Rules that matter
Rules to apply
- Presume every space that has held an inert gas is unbreathable until tested.
- Isolate positively; a closed valve is not isolation.
- Purge flammables with nitrogen, then purge the nitrogen with air; never use oxygen, and never use CO₂ on acetylene plant.
- Test at top, middle and bottom before entry, and monitor continuously during it.
- Keep a standby person at the opening at all times, and never let anyone enter to rescue without breathing apparatus.
- Close the permit only when the space is reinstated and everyone is out.
KIGL's industrial nitrogen is the purge gas for most of these jobs; the full data sheet at /sds/nitrogen should be at the permit point.
Inert gas confined-space entry — frequently asked questions
What is a confined space?
A space large enough to enter, with limited means of entry or exit, not designed for continuous occupancy, and where a hazardous atmosphere can arise. Tanks, vessels, acetylene generators, gasholders, purifiers, pits, sumps, trenches, ducts, cold rooms and the load spaces of gas vehicles all qualify. A space that has ever held an inert gas is presumed to contain one until it has been purged with air and tested.
Why do so many confined-space deaths involve rescuers?
Because the first casualty collapses without warning and the natural reaction is to go in after them. In an inert atmosphere the rescuer collapses within the same few breaths. NIOSH found that around 60 % of confined-space fatalities it investigated were would-be rescuers. The rule is absolute: the standby person never enters without breathing apparatus, and rescue is planned before entry, not improvised after a collapse.
How is the atmosphere tested before entry?
With a calibrated multi-gas instrument, from outside, through the opening, at the top, middle and bottom of the space, because light gases such as helium collect high and heavy ones such as argon and CO₂ collect low. Oxygen must read 19.5 to 23.5 %, flammables below 10 % of the LEL and toxics below their limits. For acetylene equipment EIGA Doc 237 requires acetylene in nitrogen, acetylene in air, then oxygen, in that order.
Is a single gas test enough?
No. Conditions change: residual gas desorbs from sludge, porous materials and carbon beds; ventilation can fail; a neighbouring line can pass. Monitoring must be continuous throughout the entry, with a personal monitor on each entrant and, for longer jobs, a fixed sampler at the work position. Any alarm means everyone leaves immediately and the space is re-tested before re-entry.
What does positive isolation mean?
Physically disconnecting every line that could admit gas, liquid or energy: removing a spool piece or inserting a rated blank or spade, not relying on a closed valve. Inert-gas supply lines are the ones most often left on a single closed valve, and a passing valve fills the space while people are in it. Electrical and mechanical drives such as agitators are locked off and tagged.
Can nitrogen or CO₂ be used to purge before entry?
Nitrogen is used to remove flammable gas, but it makes the space unbreathable, so a second purge with air must follow before any entry. For acetylene plant, EIGA Doc 237 permits only nitrogen, never CO₂, because CO₂ generates static electricity. IS 15190 Part 1 sets a 2.5 % oxygen end-point for nitrogen purges of acetylene pipework. Never purge with oxygen.
What does the standby person do?
Stays at the entrance for the whole entry, keeps the entrants in sight or in voice contact, watches the monitor and the ventilation, controls who enters, and raises the alarm and starts non-entry rescue at the first sign of trouble. The standby person has no other task and does not leave until relieved or until all entrants are out.
What does a non-entry rescue look like?
Each entrant wears a harness attached to a line that the standby person can use with a tripod and winch to lift them out without entering. Where the geometry makes that impossible, a trained rescue team with self-contained breathing apparatus must be on standby at the opening, and the emergency services are told before the job starts, not after a collapse.
References
- Bureau of Indian Standards. (2002). Acetylene pipelines — Code of practice (IS 15190 (Part 1):2002).
- Compressed Gas Association. (n.d.). — Oxygen-deficient atmospheres (CGA SB-2).
- European Industrial Gases Association. (n.d.). — Hazards of inert gases and oxygen depletion (EIGA Doc 44).
- European Industrial Gases Association. (n.d.). — Safe operation of acetylene generator systems (EIGA Doc 237).
- European Industrial Gases Association. (n.d.). — Work permit systems (EIGA Doc 40).
- NIOSH — Worker deaths in confined spaces (Alert and case studies)
- Occupational Safety and Health Administration. (n.d.). Permit-required confined spaces (29 CFR 1910.146).
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