Exhaust ventilation system removes airborne contaminants from bus service pit
A large public transport organisation services and maintains a fleet of buses. The buses operate on LPG, petrol and diesel fuels. Vapours from the fuels and exhaust gasses were a hazard to employees especially employees who worked in the service pit where vapours and gasses have a tendency to settle. A special cause for concern was when large (60 gallon) fuel tanks were removed and repaired. Attempts were made to remove all the fuel from the tanks. However, in this process, fuel sometimes spilled on the service pit floor releasing vapour, or vapour escaped from the empty tank and contaminated the pit and workshop area. Welding was occasionally carried out in the pit and the inability to disperse the welding fumes created another hazard.
Environmental conditions may mean concentrations of the vapours and/or gasses may build up to dangerous levels. Potential hazards for employees working in or around the service pits included:
- Explosion or fire should the flammable gasses or vapours contact an ignition source
- Asphyxiation of employees working in the service pit
As the workshop was undergoing redevelopment, management and staff saw an opportunity to design and install a ventilation system for the pit. A local exhaust ventilation (LEV) system was installed with exhaust ports in the side of the pit. Vapour and fumes were ducted from the pit and exhausted to the atmosphere above the roof of the garage. Fitted to the exhaust pipe of every vehicle idling in the pit was a connecting pipe that led, under a plate in the pit floor, to the ventilation system. Fitting a connecting pipe through the ceiling (the usual method) was seen as a potential hazard.
The design had to ensure that any contaminated air in the pit would be evacuated prior to employees entering it. To achieve this goal a time delay switch was incorporated between the ventilation and lighting systems. The time delay mechanism means the exhaust ventilation system operates for two minutes before lights in the service pit are illuminated. All lighting in the pit was built intrinsically safe. During this period approximately 1.5 times the air volume of the pit is evacuated. This engineering control measure is supported by a safe work procedure which requires that no employee may enter service pits without fitted lighting being illuminated. This procedure is regularly monitored by supervisors for the area. It should be noted that the servicing of vehicles powered by LPG requires additional precautions.
Hierarchy of Controls:
Administration – Procedures
- The risk level to employee health and safety associated with flammable and toxic gasses in motor service pits have been greatly reduced.
- A combination of engineering and administration controls have reduced the levels of dangerous vapours and gasses in the service pit.
- The risk of employees unknowingly entering service pits which may contain harmful substances has also been reduced.