According to the OSHA regulations, a confined space is anyplace that meets the following criteria: (1) Is large enough and so configured that an employee can bodily enter and perform assigned work; and (2) Has limited or restricted means for entry or exit; and (3) Is not designed for continuous employee occupancy.
On December 3, 1984, at a pesticide ingredient manufacturing facility owned by Union Carbide, a leak occurred in the Methyl Isocyanate (MIC) plant. Due to the toxic nature of the gases released and the plant's proximity to local residences, the death toll was in the thousands; both plant workers and nearby residents. The first recorded public meeting in response to this incident was on December 9th, in Institute, WV, the site of Union Carbide's only US MIC production unit. Full disclosure: my father was a research & development chemist for Union Carbide and Institute is about 10 miles down the Kanawha River from my hometown of Charleston, WV.
When thinking about the safe operation of boilers (and don't we all?), several systems can readily be named; flame control, fuel/air ratio; steam pressure control, levels in the vessel, etc. What about the water? It seems so passive, as long as there is enough for level control, what's the big deal? Well, it turns out, that as the steam produced by a boiler is used in the process, the condensate from that steam is returned to the boiler as feedwater. However, since 100% of the condensate is not returned, whatever solids had been in that water before it evaporated to form steam are left in the remaining water. Fresh feedwater is added to maintain levels, but even fresh water contains some dissolved solids. So over time, the water in the boiler system gets saturated with all sorts of dissolved minerals.