A Quick Guide to Atmospheric Testing in Confined Spaces

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Introduction

A 'confined space' is defined as a space which is enclosed by its nature and possesses the risk of death or serious injury from hazardous substances or dangerous conditions. By this definition a wide net is cast, with many obvious and some not so obvious examples e.g.

  • storage tanks

  • reaction vessels

  • enclosed drains

  • open-topped chambers

  • combustion chambers in furnaces

  • or even, poorly ventilated rooms

Some places may become confined when work is carried out or during fabrication, hence, vigilance is always necessary and dynamic assessment of risk always to be carried out.

In all circumstances you must carry out a suitable and sufficient assessment of the risks and decide on measures for safe working in compliance with the Management of Health and Safety at Work Regulations 1999, regulation. Most risk assessments will require the assessment and consideration of:

  • task

  • working environment

  • working materials and tools

  • suitability of those carrying out the task

  • arrangements for emergency rescue

If the assessment highlights potential for serious injury from work in confined spaces the Confined Spaces Regulations 1997 apply. Under these regulation key duties are imposed but not limited to:

  • avoid entry to confined spaces

  • if entry to a confined space is unavoidable, follow a safe system of work

  • put in place adequate emergency arrangements before the work starts

These are detailed extensively in Confined Spaces Regulations 1997

Atmospheric Hazards in Confined Spaces

Atmospheric hazards in a confined space are those that expose entrants to a risk such as death, entrapment, injury, or acute illness from one or more of the following causes:

Oxygen

An atmospheric oxygen concentration below 19.5% (oxygen deficiency), or above 23.5% (oxygen enrichment).

Combustible Gases

A flammable gas or vapor in excess of 10% of its lower explosive limit (LEL) yet still remaining below the upper explosive limit UEL). Lower Explosive Limit (LEL) Vs. Upper Explosive Limit (UEL)

  • The lowest concentration (air-fuel mixture) at which a gas can ignite is called lower explosive limit (LEL). Concentrations below this limit are too lean to burn.

  • The highest concentration that can be ignited is its upper explosive limit (UEL). Above that concentration, the mixture is too rich to burn.

RAE Systems’ one- to five-gas monitors read out in both % LEL and % by volume. For example the LEL of methane is 5% by volume, and the UEL is 15% by volume. When a confined space reaches 2.5% methane by volume this would be equal to 50% LEL. (5% methane by volume would be 100% LEL.) Between 5 to 15% by volume, a spark could cause an explosion. Different gases have different % by volume concentrations to reach 100% LEL. Some examples are: Propane’s LEL is 2.1% by volume; Pentane’s LEL is 1.5% by volume; Hexane’s LEL is 1.1% by volume and gasoline’s LEL is 1.3% by volume.

Toxic Gases

An atmospheric concentration of any toxic compound above the permissible exposure limit established by OSHA, NIOSH, ACGIH or HSE EH40. Common toxic compounds include, VOCs, carbon monoxide, ammonia, chlorine, hydrogen sulfide, sulphur dioxide, nitric oxide etc.

Monitoring a Confined Space Atmosphere

Monitoring the air inside a confined space is required prior to entering. Testing a confined space for atmospheric hazards should be done remotely before entering, and should be done in this order:

  • Oxygen. Ensure that proper oxygen levels are present.

  • Combustible gases. Ensure that combustible gases are not present.

  • Toxic Gases. Ensure that toxic gases are below the OSHA, HIOSH, ACGIH or EH40 permissible exposure limit. Common toxic gases in a confined space could be hydrogen sulfide (H2S) and carbon monoxide (CO), but other toxic compounds could be present. RAE Systems offers monitors to evaluate all these gases either individually or simultaneously.

  • VOC. VOC exposure is one of the most overlooked hazards in confined space entry. Volatile organic compounds (VOCs) are organic compounds evaporate easily at room temperature. Some familiar substances containing VOCs are vapours associated with fuels such as petrol, diesel, heating oil, kerosene, jet fuel, benzene, butadiene, hexane, toluene, xylene, and many others. All of these are potentially very harmful, and if present should be monitored to ensure that they are below safe limits.