Ventilation, purging and inerting

Ventilation is the main engineering control for confined space atmospheres. It removes contaminants, brings oxygen levels back to normal, keeps flammable gases well below their explosive limits and dilutes fumes and vapors produced by the work. Purging and cleaning remove hazardous contents before entry. Inerting does the opposite of ventilation: it deliberately removes oxygen to prevent fire, which makes the space unbreathable. This lesson explains how these methods work, how to estimate how long purging takes, and the common mistakes that make ventilation fail.

Cleaning and purging before entry

Before ventilation can do its job, the space should be emptied of as much of its hazardous contents as possible. Typical steps are:

• drain liquids and remove product, sludge and residues, from outside where possible;
• flush or wash the space, for example with water, steam or a cleaning solution, using spray nozzles from outside;
• purge the atmosphere by displacing it with air, water, steam or an inert gas, depending on the contents and the fire risk;
• test the atmosphere after purging and before entry.

Purging with steam or water can itself create hazards: steam heats the space, can generate pressure and leaves condensation that can trap gases; water may react with some residues. Purging a flammable atmosphere with air takes the mixture through its flammable range, so sources of ignition must be controlled and equipment must be suitable for flammable atmospheres. For this reason, highly flammable atmospheres are often purged first with an inert gas such as nitrogen, and only then with air, so the concentration never passes through the flammable range in the presence of oxygen.

Mechanical ventilation

Natural ventilation, relying on open hatches and wind, is rarely enough for a confined space, and most rules require or strongly favor mechanical ventilation. Portable blowers driven by electric motors, compressed air or (outside and away from the opening) engines push or pull air through ducting. There are two basic arrangements:

• supply (blowing) ventilation pushes fresh air into the space through a duct, diluting contaminants and pushing them out through the openings. It is the most common method and delivers clean air to the work area if the duct outlet is placed near the workers;
• exhaust (extraction) ventilation pulls contaminated air out through a duct, drawing fresh air in through the openings. It is effective at capturing contaminants at their source, such as welding fume, especially when the duct inlet is placed close to where the fume is produced.

Often both are used together, with supply air brought in at one end and exhaust taken from the other, creating a flow through the work area.

Getting ventilation right

Ventilation only works if the air actually reaches the places where people work and contaminants collect. Good practice includes:

• take supply air from a clean location, away from vehicle exhaust, engine-driven equipment, process vents and the exhaust outlet of the same system, so contaminated air is not recirculated;
• place the supply duct outlet near the bottom of the space or near the workers, and the exhaust inlet near the contaminant source or at the opposite end, so air sweeps the whole space without short-circuiting straight from inlet to outlet;
• consider gas density: heavy gases such as hydrogen sulfide and solvent vapors collect low and may need low-level exhaust, while light gases such as methane collect at the top;
• keep ducting short, straight and undamaged; every bend, kink and extra length reduces airflow, and manufacturers' charts show how much;
• use blowers rated for the atmosphere: in a potentially flammable atmosphere, the fan, motor and any electrical equipment must be suitable for that hazardous location, and the system must be bonded and grounded to prevent static sparks;
• never ventilate with pure oxygen;
• keep ventilating continuously throughout the entry when the hazard could return, and retest after ventilation starts, because ventilation can stir up sludge and release gas.

1910.146 requires the employer to provide ventilating equipment needed to obtain acceptable entry conditions, and the alternate procedures in paragraph (c)(5) allow continuous forced-air ventilation as the only control for spaces whose only hazard is atmospheric. Under those procedures the forced air must be directed to ventilate the immediate areas where employees are or will be present, and must continue until all employees have left the space, with periodic testing to confirm conditions.

How long does purging take?

If fresh air is supplied at a constant rate Q into a space of volume V, and mixing is perfect, the concentration of a contaminant falls exponentially:

Real spaces are never perfectly mixed: corners, baffles and poor duct placement leave pockets of contaminated air. Industrial hygienists account for this with a mixing factor K, typically between about 3 for a simple, open space with good duct placement and 10 for a complicated space with poor mixing. Solving for time gives:

A useful rule of thumb follows from this. Reducing a concentration to one thousandth of its starting value (99.9 percent reduction) needs \( \ln(1000) \approx 6.9 \), or about seven air changes with perfect mixing, multiplied by the mixing factor in practice.

Ventilation for hot work and coatings

Welding, cutting, grinding and coating in a confined space produce contaminants continuously, so ventilation must remove them as fast as they are generated. Local exhaust with the inlet close to the work captures fume before it reaches the welder's breathing zone; general supply ventilation alone may simply spread fume through the space. Coatings and solvents release heavy vapors that collect low, so exhaust is often taken from low points. Flammable vapor levels must be monitored continuously, and hot work must not start or continue unless flammables are at the very low limits set for hot work. Fire safety for hot work is covered in HSE-216.

Inerting

Inerting means filling a space or a piece of equipment with an inert gas, usually nitrogen, to reduce oxygen below the level at which fuel can burn. It is used when flammable materials cannot be removed, for example when work must be done inside a vessel containing catalyst or residues that could ignite, or when a vessel must be kept non-flammable while connected to a process. Inerted spaces typically contain only a few percent oxygen or less, which is immediately dangerous to life.

Entry into an inerted space is one of the most hazardous confined space activities. It requires:

• supplied-air respiratory protection with an escape supply, typically an airline respirator with an escape bottle or self-contained breathing apparatus, and backup systems;
• continuous monitoring of oxygen and of the inert gas supply;
• specialized training, procedures and supervision, often by specialist contractors;
• a dedicated rescue team at the space, equipped and ready to enter immediately, because an equipment failure leaves only seconds before unconsciousness;
• warning signs and controls to stop anyone else entering or standing at the opening, where inert gas may flow out and displace the air.

The area around an inerted vessel opening can also be dangerous. Nitrogen escaping from manholes, vents or open flanges has killed workers who were simply standing near the opening or leaned in to look.

When ventilation is not enough

Sometimes ventilation cannot bring the atmosphere to acceptable levels, or cannot keep it there reliably. Then the options are to postpone or redesign the work, to isolate and clean more thoroughly, or to use respiratory protection. Air-purifying respirators may be used only when oxygen is adequate, the contaminants and their concentrations are known and below the respirator's protection limits, and the atmosphere is not immediately dangerous. Supplied-air respirators are needed for oxygen-deficient, unknown or IDLH atmospheres, and their use in confined spaces brings extra requirements for training, fit testing, airline management, escape supplies and rescue. Respiratory protection is introduced in HSE-104; in confined spaces, it is always a last line of defense, never a replacement for testing, isolation and ventilation.

Finally, ventilation equipment itself needs care. Blowers and ducts should be inspected before use for damage, blocked inlets and torn duct sections, and electrical cords should be checked. Someone should be responsible for watching that the blower keeps running during the entry, because a stalled engine, a tripped breaker or a kinked duct can stop the airflow without anyone inside noticing.