3 Controlling Confined Space Hazards

Hierarchy of Control

The Hierarchy of Control describes the preferred methods of controlling hazards.

Isolation and Lock-out

Isolation means the process by which a confined space is removed from service and completely protected against the release of energy and material into the space by such means as:

• blanking or blinding
• misaligning or removing sections of lines, pipes, or ducts
• a double block and bleed system
• lock-out of all sources of energy
• blocking or disconnecting all mechanical linkages

Blanking or blinding means the absolute closure of a pipe, line, or duct by inserting a solid plate that completely covers the bore and that is capable of withstanding the maximum pressure of the pipe, line, or duct with no leakage beyond the plate.

Double block and bleed means the closure of a line, duct, or pipe by closing and locking two in-line valves and by opening and locking a vent valve in the line between the two closed valves.

Line breaking means the intentional disconnection of a pipe, line, or duct that is or has been carrying flammable, corrosive, or toxic material, an inert gas, or any fluid at a volume, pressure, or temperature capable of causing injury.

Ventilation

Ventilation is used to control the atmospheric conditions of a confined space by replacing unsafe air with clean, breathable air. For most confined spaces, ventilation must be provided mechanically, with fans.

There are two main methods of mechanical ventilation:

• General Dilution (Supply) ventilation blows fresh air in.
• Local Exhaust ventilation draws contaminated air out of an area.

It is important to remember that ventilation does not reduce the amount of contaminants released, so there are limits on when it can be used:

• For general ventilation to work well, workers should not be too close to the contaminant source.
• The contaminants must not be highly toxic.

Ventilation is required when:

• Testing indicates a hazardous atmosphere out of acceptable range.
• There’s a potential for atmospheric conditions to move out of acceptable range.
• The work can cause a hazardous atmosphere, such as welding, cutting, using sealants, etc….

Supply ventilation (pushing clean air in) usually works best for diluting contaminants that are widely dispersed. Supply ventilation also increases the comfort level for workers.

Local Exhaust Ventilation

• Captures contaminants at their point of origin and removes them.
• Local exhaust ventilation is the best way to control flammable and toxic materials produced at a single point.
• Whenever possible, use local exhaust ventilation during hot work and cleaning with solvents.
• Keep the exhaust intake close to your work.

When selecting ventilation equipment, the size of the blower or ejector and duct work is determined by the volume of air required to ventilate the space. The air volume is measured in C.F.M.’s (cubic feet per minute).

Be especially cautious if you use mechanical ventilation during certain hazardous work, such as hot work or applying coatings that are very toxic! Re-test the air often. Better yet, monitor continuously. You may also require respiratory protection.

Other considerations when ventilating confined spaces:

• Begin ventilating far enough in advance to ensure that the air will be safe for entry.
• Have an alarm system to indicate failure.
• If using a portable generator to power the fan, ensure exhaust from generator is going downwind.
• Never take the fan into a potentially explosive atmosphere. Most are not explosion proof!
• Place the outlet where air currents will disperse the exhaust quickly, without endangering nearby people.
• The exhaust may have to be filtered, to prevent pollution.
• If the exhaust could be flammable, remove all ignition sources from area.

Recommended air changes prior to entry:           

Recommended air changes per hour during entry:

Atmospheric Testing

Detector Tubes

• Sealed glass tubes.
• Tubes are specific for the chemical (or class of chemicals).
• Tube ends are snapped-off, one end inserted in pump, and a prescribed amount of air is pumped through tube.
• Chemical reaction within the tube results in color change, which is read on a scale.
• May experience a high error rate (up to 25-30%).

Multi-gas Monitors

• Sensors measure concentrations of gases and vapours.
• Most are 4-gas monitors, and measure O2, %LEL, CO and H2S.
• Results are read on a meter or display; audible & other alarms.
• Units are portable.
• Calibration and functional testing is critical.
• Gas monitors test only for specific hazards. This is a special concern where other toxic atmospheres may be present!

• On start-up, zero the gas monitor’s sensors in fresh, clean air.
• Perform a bump test to demonstrate that your monitor is functioning properly. Verify that each sensor is responding to the gas.

Procedures for Testing

• If possible, the preliminary test should be from outside. Use a hydrophobic filter.
• Test entire space. Test top, middle and bottom. Beware of stratification.
• Test before ventilating to establish baseline.
• Test while ventilating to ensure contaminants have been removed and system itself is not causing a hazard.
• Test again before entry.
• If the source of a contaminant cannot be determined, entry shall be prohibited until the source is identified and appropriate controls are implemented.

Sampling

Lighter than air gases & vapours

Heavier than air gases & vapours

When testing vertical spaces, take samples from at least three levels (top, middle, and bottom). Most monitors utilize a pump for this purpose.

Contaminants have different specific gravities and will develop layers in a confined space. This is called stratification.

Retest whenever there’s a change in work or conditions. At a minimum, retest:

• At the start of each permit period.
• The start of each work shift.
• Whenever the space and immediate area have been left unattended
• When there is a change in workers.

Continuous monitoring is required whenever a hazardous atmosphere may exist:

• Work being done may affect air quality
• Exhaust ventilation is being used to control a hazardous atmosphere
• Air quality could change based on nature of confined space (e.g.., sewer)

Continuous monitoring is always recommended!

Accurate continuous monitoring requires sampling at the proper location (normally this is near a worker’s breathing zone). Another appropriate sampling location may be where external contaminants could leak into a worker’s occupied space.

Sampling information should be recorded, usually on the permit, based on the frequency determined by the risk assessment. These records should include the name of the person carrying out the test, time of the test, and test results.