Confined space entry — tanks, manholes and silos: gas testing, isolation, standby person and rescue

Short answer: the most dangerous ones cannot be seen or smelt. Oxygen deficiency arises from rusting steel, fermentation or displacement by nitrogen or carbon dioxide. Hydrogen sulphide (H₂S) from decaying sewage quickly paralyses the sense of smell. Carbon monoxide (CO) comes from engines and combustion. Methane and solvent vapours create explosive atmospheres.

Physical hazards add to this: engulfment in bulk material in silos, agitators or pumps starting up, liquid or steam entering from pipelines, heat, falls on descent and difficult evacuation through a narrow hatch. Each must be covered by a pre-task risk assessment, not just the general job card.

Short answer: oxygen → flammables → toxics. Oxygen comes first because catalytic flammable-gas sensors under-read in oxygen-deficient air. Test from outside with a pumped probe before fully opening the hatch and before each entry, then continuously with a detector worn by the entrant.

Gases stratify, so sample at several levels: near the top (methane is lighter than air), the middle (CO is close to air density) and the bottom (H₂S, CO₂ and solvent vapours are heavier). Allow the probe time to draw the sample — usually a few seconds per metre of hose per the manufacturer's instructions.

Polish rules set no entry thresholds; the site procedure does. Common values are oxygen 19.5–23.5% vol. (OSHA-based practice; some Polish procedures use 19–23%), flammables below 10% LEL and toxics below occupational exposure limits — for H₂S 7 mg/m³ (about 5 ppm) and for CO 23 mg/m³ (about 20 ppm), plus short-term limits. Check current values in the regulation. The detector needs valid calibration and a bump test before use.

Short answer: physically, not just by closing a valve. Lines carrying liquids, gases or steam are isolated with a blind flange (spade) or by removing a spool piece; a closed valve can leak or be opened by mistake. Agitators, pumps and conveyors are switched off and locked out under the LOTO procedure — each entrant applies their own padlock.

The tank is then emptied, flushed or purged and ventilated mechanically, usually by forced supply air. Never 'improve' the atmosphere with pure oxygen — enrichment sharply increases flammability of clothing and materials. In potentially explosive atmospheres fans and lights need a suitable protection level and power tools reduced voltage.

Short answer: the standby person stays outside at the hatch at all times, keeps visual, voice or radio contact, watches detector readings, keeps others away and orders evacuation when needed. They do no other tasks meanwhile.

The standby person must not enter to rescue without raising the alarm and wearing independent breathing apparatus. NIOSH analyses show that about 60% of confined-space fatalities were would-be rescuers. Rescue starts from outside: winching the casualty out by the line clipped to the harness, calling emergency services and ventilating.

The entrant therefore wears a full body harness with a line led outside, and a tripod or other anchor with a rescue device stands over the hatch (EN 795, EN 1496). A rescue plan with named people and equipment must be ready before entry — fire brigade response usually takes longer than survival time without oxygen.

Short answer: no, if oxygen may be lacking or the concentration is unknown or high. Filtering devices clean ambient air but do not supply oxygen. Confined spaces call for isolating equipment: self-contained or airline breathing apparatus with a clean air source.

Filters also have limited service life and are gas-specific. Respiratory protection should follow the risk assessment and measurements; see the guide on PPE and workwear for records and inspections.

Short answer: a written permit to work (PTW) or written instruction for particularly hazardous work stating location, scope, people (issuer, supervisor, entrants, standby), isolation, test results, PPE, rescue equipment, communication and validity. Add a job briefing, current medical clearance and training of participants.

The permit is closed after the work — once everyone is out, tools are removed and locks are taken off in the agreed order. Record test results with time and detector number; they are key evidence in any incident analysis.

The permit to work module guides the isolation checklist, test results, assignment of entrants and standby and signatures at start and close-out. Last minute risk assessment (LMRA) lets the team confirm conditions right before entry.

Equipment registers track detector calibration and inspections of tripods, harnesses and breathing apparatus. The Lone Worker module can add communication but does not replace a physical standby person at the hatch. The diagram below illustrates the process and is not a screenshot.