There is several reasons of crack on pipes, such as unexperienced welder, design error, metal fatigue, poor management, material failure etc. One cause can make a crack or sometimes it happens by combined reasons or chain reaction.
The picturs below, an engineer posted and asked questions to other engineers the reason of pipe failure. Size of the pipe attached to main pipe is 3″ STD. He didn’t mention the length and I assume that it’s about 3meter by height of truck in the oter picture. Then weight would be 35~38kg. The pipe is attached to main pipe by 6mm plate at below section and elbow at upper side. There is only two points holding the pipe to main pipe. And one is not even support.
We need more specific data to understand the reason caused the crack however, still we can list some factors.
Firstly, design must be improved. 3″ pipe is not firmly attached but it’s hanging. The pipe is forced vertical direction without proper clamping support. The permanent force influences with the pipe which means the pipe has loaded. The area of the crack is proving that the first location of pipe was high. And it was being forced vertical direction becuase of hot temperature. The weather looks summer and steel products expands by heat. And it shrinks at night due to temperature drop. It cause metal fatigue especially on welding point. In a day, steel expands and the elbow attached to upper area also could be expanded to below direction. Secondly, fluid makes continueous vibration. Also fluid makes pressure, means that pipes expand to the direction of out diameter. Then pipes are stressed and can be broken at HAZ area if welding was wrong or PWHT was’t proper.
To prevent this kind of problem, clamp must be used and suppots on the ground must be installed.
Original pictures from http://www.eng-tips.com/viewthread.cfm?qid=385717
The Original Posted by Jethro