Local exhaust ventilation in large confined spaces.

In North America, there is an ongoing effort to repair the effects of corrosion in large boilers and pressure vessels. In recent years this work has been done from scaffolds erected in the boiler or vessel. The largest pressure vessels in British Columbia are known as Kamyr Digesters, and are found in Kraft pulp mills. The boilers are found in pulp mills across Canada and the USA, and in thermal power plants in east of the Rocky Mountains in Canada and across the USA. The scaffold structures are made up from 50 mm diameter steel columns and struts that are assembled with steel wedges. The length of these components and their fitting dimensions are standardized across North America. All the scaffold components can be moved into the space through a 500 mm diameter manhole, and assembled without mechanical aids. The processes involved in corrosion repair work include abrasive blasting, air-carbon arc gouging, grinding, welding and metal spraying. All these processes generate substantial quantities of toxic air pollution. Typically workers have been protected from the air contaminants by air supplied respirators and general ventilation.