The Steamboating Forum

Interesting read for the would-be home boiler maker...

https://www.nationalboard.org/index.asp ... 164&ID=441

https://en.wikipedia.org/wiki/Fatigue_(material) also has a nice treatment of fatigue failures, including some notable examples.

When I was in school studying mechanical engineering ('78-'84), it was felt that a fatigue limit stress in mild steel (below which no fatigue failures are observed at 1 million load cycles ) was typically order one half the ultimate tension strength. More recent research indicate that this limit may not actually exist; given enough cycles (10 million to 10 billion), all metallic components may fail.

Our hobby steamboat boilers are typically very conservatively engineered since with our small sizes and low pressures the primary way these boilers become unserviceable is leaks caused by corrosion. Thus such boilers typically have factors of safety (ratio of maximum stress in service vs strength limit of material) of well over 2, and sometimes over 10. In addition, the only significant cyclical stresses seen are due to raising and lowering boiler pressure, something that happens far less often than many of us would like to manage.

Hydro-test your boilers yearly to 1.5 x safety valve setting... install that safety w/ a union to make it easy.

- Bart

When I went to look at my boat the first time and the seller rolled it out of the storage unit, the first thing that caught my eye was a shutoff valve in line with the safety. Installed, I was informed, to make it convenient to do a hydro test. Lunatic out for a picnic!

Unscrewed and thrown in the car before I even towed the beast home.

Remarkable.