Tube rupture

[Chris 10]

Dear All,
I have read a couple of books on boiler, but could not find an answer to a question. I hope to find the answer in this forum.

All books describe that scale, corrosion and embrittlement cause weakening of boiler tubes. This could cause cracks, leaks & rupture, and boiler failure. However the explanations are not very detailed.

I am wondering when cracks leads to a boiler failure / explosion and when not?

I understand the flash steam" mechanism: pressure drops when there is a rupture in the boiler. Water changes to steam and re-pressurize the boiler (1 to 1700), and it causes explosion.

What is not clear to me is the following: when do crack in tube lead to this flash steam? and when not? when does it cause explosion? Apparently not every crack leads to an explosion.

Does it depend on the size of the crack? Does it depend on the location of the crack (wall furnace vs boiler tube)?

Many thanks in advance.

Chris

 

[metengr]

You mean leak before break of boiler tubes. This depends on several factors; the location of the tube in the boiler, extent of wastage (internal or external), type of cracks and orientation in the tube, service temperature and material.

 

[KoachCSR]

Cracks in boiler tubes will always lead to boiler failure, per se - if you cannot maintain water level in the steam drum, you cannot run the boiler. You'll need to shut down the boiler and repair the tube(s).

I have, thankfully, not witnessed a catastrophic tube rupture in a boiler; my understanding of one is that the expanding steam bubbles at and near the rupture will essentially throw large "slugs" of water into the boiler, potentially blowing out the tube sheet, collapsing the furnace, or even shift the entire boiler - depending on its operating saturation pressure at the time. It's these slugs that cause the damage, as the rapidly expanding steam itself has an exit path through the stack. Essentially, any crack is a bad day; the bigger the crack, the worse your day.

I'll also note that the more violent boiler failures I've heard about tend to revolve around fuel explosions - fuel-rich mixtures in the furnace and/or improper purge cycles.

Edited to add some clarification.

 

[EdStainless]

A large part of the point of modern boiler design (materials, temperatures, pressures, water control, and so on) is so that any tube failure in the boiler will result in a minor leak that can be detected and fixed before there is a catastrophic event.

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P.E. Metallurgy, Plymouth Tube

 

[blacksmith37]

This may refer to caustic stress corrosion cracking ; Where steam flashes from a very small leak ,eventually building up a caustic deposit on the crack/surface. Then caustic SCC causes a large failure.

 

[Compositepro]

Personally I think that "steam explosion" is a misnomer. From the context in which I have seen it used, it appears to refer to a situation where a large quantity of hot boiler water is released so quickly that when it flashes into steam it causes enough pressure to build on the outside of the leak that this pressure causes damage to equipment or building structures.