Stress Engineers,
This is just a confirmation in relation to standard i read.
In case of short-time duration such as Steam out condition, does allowable stress range should be increased by multiplying a factor?.
As per my understanding, only allowable for occassional loading shall be considered.

Any comments, suggestions & explainations subject to this topic is highly appreciated.!

Does anyone in this forum can give idea to the topic I posted?
Please share your knowledge..!

Hi Nald

As per my experience, if we have to consider the steam out case temperature and pressure (which is more than Design condition) then we use the Steam out conditions Temperature and Pressure as the design condition with a confirmation from process department. And we will use the same allowable stresses without any factor for our analysis. This is a conservative approach.

Zahid

Nald,

You are correct. The increase in allowables for short term temperature/pressure excursions as described by B31.3 only apply to sustained stress evaluations. Displacement allowable stresses are not affected by these factors.

Zahid mech & Edward Klein,
Thank you very much for sharing your knowledge...

Hi Nald,

For short term condition as your described,it is state on B31.3 para.302.2.4 (1), our system is allow to exceed the allowable in certain of time.

[quote]
(1)Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than :

(a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr, or
(b) 20% for no more than 50 hr at any one time and no more than 500 hr/yr

Regards,
Windu

Windu,

Don't confuse stress evaluation with pressure-temperature design criteria. You are quoting the latter while the subject of this thread is the former.

Dave,


Actualy what I quote above is try to give additional information inline with zahid said, that we will use the same allowable without multiplying any factor when we want to consider steam out case. As long as the result is not more than what I quote above, our sistem is safe.

But, thanks to remind me.

windu.

windu_n,

Steamout is typically a low pressure event and not likely to require invoking the short increase in allowable stress that you quote from the code. Particularly since, steamout is typically less than 400°F and thus, for typical carbon steel applications, unlikely to impact wall thickness calculations as Sh does not drop off significantly up to 400°F.

Can you clarify how you are applying the code section you quoted above to the evaluation of steam out cases? I would like to be certain that we're all on the same page here.