Confined Space

Confined spaces in the workplace include any chamber, tank, pit, pipe, flue or enclosed space in which either or both of the following situations are possible:

• Dangerous fumes are liable to be present to such an extent as to involve risk of fire or explosion, or persons being overcome by fumes.
• The supply of air is inadequate, or is likely to be reduced to be inadequate for sustaining life.

Hazards in confined spaces can be broadly separated into the following two categories:

• Atmospheric hazards, e.g. oxygen deficiency and presence of toxic or flammable gases or vapours
• Physical hazards, e.g. slips and falls, moving machines, exposed electrical components,
  engulfment and drowning

Legal Requirements on Confined Space Work
Safety measures for confined space work are stipulated in Regulation 25 of the Workplace Safety and Health (General Provisions) Regulations. These include:

• Removing any sludge or deposit liable to give off dangerous fumes before confined space entry.
• Preventing entry of dangerous fumes into the confined space.
• Adequately ventilating the space to sustain life before entry and during work.
• Testing of the space for oxygen and any flammable or toxic gases and vapours.
• Wearing suitable breathing apparatus if any space cannot be made safe for entry.
• Wearing a safety harness and lifeline where practicable, and having a standby person keeping watch from outside the space.
• Ensuring there is a sufficient supply of emergency response equipment such as breathing and reviving apparatus, belts and ropes.
• Having a sufficient number of employees trained and practised in the use of emergency response equipment, and in CPR.

Under the Workplace Safety and Health (Risk Management) Regulations, employer and contractor must conduct a risk assessment in relation to the safety and health risks posed to any person who may be affected by his undertaking, and take all reasonably practicable steps to eliminate any foreseeable risk.
In addition, safe work procedures (SWP) must be implemented to control the risks. The SWP must include the safety precautions to be taken in the event of an emergency.

Employees or any other persons at the workplace who may be exposed to a risk to their safety and health must be informed of:

• The nature of the risk involved
• The measures implemented to control the risk; and
• Applicable safe work procedures.

Oxygen Deficient Atmospheres


The level of oxygen inside a confined space can be reduced to a dangerous level due to:

• Chemical reactions (rusting, decomposition and fermentation)
• Absorption by porous materials (e.g. activated carbon)
• Displacement by inert gases (e.g. nitrogen and carbon dioxide)

Oxygen is vital for sustaining life. Many physiological effects emerge when the oxygen content is below the minimum safe level. The symptoms range from mild headache to permanent brain damage or death at a highly deficient level.

A confined space may be entered only if it contains at least 19.5% but not exceeding 23.5% oxygen by volume

Toxic Atmospheres

Air contamination inside a confined space occurs when hazardous substances inside the space become airborne. Depending on the type of contaminants, the effects can be irritation, asphyxiation, or systemic poisoning, even at low concentrations.

Common toxic/poisonous gases:

• Solvent vapours (e.g. acetone, toluene, trichloroethylene, xylene)
• Carbon monoxide
• Hydrogen sulphide
• Petroleum vapours (e.g. naphtha)

SolventsSolvents are petroleum derivatives and are commonly found in products such as paints, cleaning agents and adhesives. Due to their highly volatile nature, solvents can rapidly accumulate at dangerous levels in unventilated confined spaces. Acute exposure usually results in narcosis as many of the vapours depress brain function and the central nervous system. Chronic exposure can cause systemic poisoning and damage the organs.

Carbon Monoxide (CO)
Carbon monoxide is a colourless and odourless gas that is produced from incomplete combustion. The gas is a chemical asphyxiant. It binds strongly to red blood cells, preventing the flow of oxygen to the brain. In the absence of oxygen, the brain cells die, leading to unconsciousness and even death.
The Permissible Exposure Level (Long Term) for carbon monoxide is 25 ppm.

Hydrogen Sulphide (H2S)Hydrogen sulphide is a rapidly acting systemic poison which, at high concentrations, paralyses the respiratory function and causes asphyxiation. When breathed in for prolonged periods at low concentrations, it dulls the sense of its characteristic rotten-egg odour.

At high concentrations, the sense of smell is readily deadened, so odour cannot be used as an early warning sign.

The PEL for hydrogen sulphide is 15 ppm (Short Term), and 10 ppm (Long Term).

Flammable Atmospheres

Flammable substances in a confined space can cause fire and explosions in the presence of an ignition source e.g. open flame and sparks. Flammable sources include:

• Residual gases or vapours e.g. petroleum vapours
• Leaks from gas cylinders or pipelines e.g. acetylene, liquefied petroleum or natural gas
• Underground marsh gas (methane)
• Vapours evaporated from solvents e.g. toluene, xylene

The concentration of any flammable gas or vapour in a confined space must not be more than 10% LEL. If hot works is to be carried out in the space, the space should be free from any flammable substance.
Other Hazardous Gases
Acid FumesSkin reddens and blisters when exposed to acid fumes. When inhaled, a sore throat and shortness of breath result. Severe exposure can cause pulmonary oedema with a potentially fatal result when fluid accumulates in the lungs.

Ensure that the concentrations of toxic gases or vapours in the confined space are below the Permissible Exposure Levels (PEL).

Useful points to note when assessing the conditions of a confined space:

• Contents - Previous contents in the space
• Reactions - Possible reactions that can happen inside
• Operations - Nature and type of operations to be carried out inside, including the type of materials to be used
• Potential hazards - Inadvertent introduction of contaminants from outside environment

Control and Preventive Measures

The following measures should be taken where appropriate to prevent deaths and injuries from
confined space work.

Risk Assessment
Prior to commencement of work, a risk assessment must be carried out to identify the hazards associated with the work, assess the risk of accident that may occur, implement SWP and take appropriate measures to eliminate the hazards, or to reduce the risk.

Isolation
Before entry, valves and pumps to all pipes leading to confined spaces must be locked and tagged to prevent the entry of hazardous materials.

Gas Check
A competent person must test and certify the atmosphere of confined spaces for oxygen, flammable and toxic gases or vapours prior to entry. Confined spaces can only be certified safe for entry if:

• The oxygen level is within 19.5% to 23.5%;
• The level of flammable gas is less than 10% of the LEL; and
• The concentration of toxic gases or vapours is below the PEL.

While a person is inside the space, continuous or regular gas testing should be conducted to
ensure that the space remains safe.

Calibration of Gas Meters
Gas monitoring devices must be regularly maintained and their accuracy verified with functional (bump) tests and full calibrations. A functional test is a brief exposure of the gas monitor to a known gas for the purpose of verifying sensor and alarm operation. If the instrument fails to operate properly following any functional test, a full instrument calibration should be performed.

A full calibration involves the use of (certified standard) calibration gases, and requires verification that the gas concentrations listed on the cylinder label match the concentration setting for calibration in the instrument. Calibrations must be carried out by trained personnel, and records kept.


Entry Permit


Before entering a confined space, a permit must be issued by a competent person certifying that all hazards have been assessed and precautionary measures have been taken to ensure the safety of entrants.



Ventilation
When a confined space is occupied, suitable and adequate ventilation should be provided at all times to provide fresh air and/or to dilute and remove any contaminants to a safe level. It is recommended that a combination of forced or supplied and exhaust ventilation be used to ensure adequate ventilation of the space.

Forced or supplied ventilation introduces fresh air into the space typically through the use of a fan or blower while exhaust ventilation removes contaminants from the space by drawing air out using an extractor.

Standby PersonA standby person should be stationed outside the confined space to keep a look out and render help in the event of an emergency.

Safety Appliances
When entering a confined space, a safety harness and lifeline should always be worn. This will facilitate retrieval during an emergency. Suitable respirators should be worn where toxic gases or vapours are known to be present. Air supplied respirators must be used if the space is likely to be deficient in oxygen or contain unknown or high concentrations of air contaminants.

Rescue Plan and Equipment
A written rescue operation plan should be established and on-site equipment such as retrieval devices and breathing and reviving apparatus should be readily available for emergency use.

TrainingTraining should be provided to all persons involved in confined space work, including worker (entrant), standby person, supervisor and gas tester. The training should cover the following
areas:

• general hazards associated with confined space
• safety and health precautions with respect to entry into confined space
• entry permit system and safe work procedures
• emergency response

Other Safety Measures

Diesel-driven and petrol-driven engines such as pumps and compressors should never be placed inside a confined space.

Host employers should ensure that contractors are competent for work involving a confined space. They also need to brief their contractors on any precautions or procedures to be implemented.

Procedures for emergency response must be established and communicated to all personnel on-site.

For more safety requirements on entry into and working in confined spaces, please refer to:

• The Workplace Safety and Health (General Provisions) Regulations
• Singapore Standard - CP84: Code of Practice for Entry Into and Safe Working in Confined Spaces.
  

(Reference: Technical Advisory for Confined Spaces by WSH Council)