Full Vacuum vs Half Vacuum 5

[sshep]

My Friends,

I know that this subject has come up in forums several times, but my research of the site does not give a satisfactory explanation so I am posting the question again.

Is there any API, ASME, or other documentation which rationalizes why a half vacuum rating would be sufficient for vessels subject to steam out?

We are designing several new plants and there is a push to design vessels for steam out using a half vacuum condition rather than full vacuum. The saturation pressure of steam at half vacuum is in excess of 180F, so it is a hard thing for the process engineer to swallow to rationalize how half vacuum rating is sufficient protection for all the possible ways to collapse steam during a steam out activity. Early in my career I saw a vessel collapsed during cooldown after steam out, and myself and most of my peers have always specified full vacuum ratings. Now we are being asked to accept a half vacuum rating on the basis of "thats the way alot of companies do it", which inspires no confidence.

best wishes,
sshep

 

[moltenmetal]

If you're relying on procedures alone to protect against vacuum collapse, then I'd give my head a shake on that. You need either reliable vacuum resistance or vacuum relief to have a hope of sufficient idiot resistance in my opinion.

As to jte's suggestion that half vacuum design is sufficient to resist the near full vacuum you'd encounter in a steam-out totally gone wrong with no vacuum relief, I suspect jte is correct. That the vessel is overstressed to some degree during an uncontrolled external pressure event like this is not really something I'd worry myself about, other than trying to design the case out by use of relief.