Confined space is easy to recognize when it involves entry into a storage tank or pressure vessel. People would apply a systematic procedure to avoid incident. The tank or vesssel would be isolated by disconnecting or blinding the connecting lines. It then be cleaned, test the atmosphere and use special air mask if necessary.
However, some confined spaces are less obvious. The following incidents are quoted from the book written by Trevor Kletz, "Still Going Wrong!" page 22.
Use liquid nitrogen in a trench.
Two men used liquid nitrogen to freeze water lines in a trench, as part of a cut and weld job. Ther was too little ventilation to disperse the nitrogen as it evaporated and they wer overcome. No safety harnesses were worn and no oxygen meter was used.
Plastic sheet wrapped at one end of an open pipe makes it a confined space.
During a plant shutdown, a piece of equipment was removed from a 1.2 m diameter pipe. No one entered the pipe but the inside was inspected by shining a light into it. Bright sunshine made it difficult to see anything so a black plastic sheet was wrapped over the end of the pipe. There was a strong breeze so to hold the sheet in place two men sat on one edge of the sheet and two others held it over them. The two sitting men then inspected one of the open ends.
They then tried to do the same at the other open end of the pipe. Unfortunately, there was a flow of nitrogen coming out of the end of the pipe and the two men were overcome. One died and the other recovered after five days in hospital.
Both the man who died and his coworker were men of great experience. The day before, one of them had asked for nitrogen to be injected in order to protect the catalyst. The injection point was 50 m and several floors away and he may not have realized that the nitrogen would exit through the 1.2 m diameter pipe. He certainly did not realize that a plastic sheet held loosely over the end of the pipe turned it into a confined space.
The company's entry procedure did not draw attention to the hazards of temporary enclosures. Obviously it should have, but even if it had, would the men have remembered this fact? Instructions are no substitute for knowledge and understanding, that is, knowledge that confined spaces can easily be formed. Knowledge that nitrogen in quite small amounts can reduce oxygen level to a dangerous extent and knowledge that waht goes in must come out and that whenever we put anything into a plant we should ask where it or something else will exit. The root cause of the accident was the failure of the company to give their employees this understanding of the hazards.