In both these cases the main air start compressor was started up with the isolating valve on the discharge line shut. In one case it was because the operator had not opened the valve after carrying out maintenance on the compressor, in the other it was because it had become practice to shut the discharge valve when the compressor was stopped because the non return valve was leaking by.
The relief valves which should have protected against over pressure did not lift because they were choked with burnt carbon. The photographs show what happened in one of the cases. Several tubes in the tube stack in the aftercooler failed due to the high pressure. This subjected the water jacket (part of the compressor casing) to the high pressure, which failed. The relief valve which should have protected the water casing did not operate because that was corroded shut.
In one of the cases, a member of the crew standing nearby was killed.
Relief valves on air compressors should be regularly overhauled and tested. The lift pressure should be set hydraulically using water. Do NOT test relief valves by shutting the compressor discharge valve! Relief valves on the water jacket should be tested regularly by lifting them with the operating handle.