De-rating a Vessel

[drials2]

This is a general question to satisfy my own curiosity:

When taking a vessel (sa-516) and derating the temperature
-vessel rated for 600F 300 psig,
-only operates at 50F 300 psig,
-so derate to 200F how is the new pressure calculated?

The allowable stresses dont change in section 2, part D but I know the modulus of elasticity is changing (increasing) so technically the material can "take" more stress. I am wondering how this is done via the asme code. Cant seem to work it out on my own.

Thanks,

David.

 

[SnTMan]

drials2, Sec VIII, Div 1 does not generally speaking use Mod of E in design. Carefully check the allowables for all vessel materials, some of them will start to decrease at 500 F or so. Those components may have a slightly higher MAWP. They may not govern the design though.

Essentially you will have a 300 psi / 200 F vessel with a Maximum Allowable Working Temperature of 600 F at 300 psi.

Regards,

Mike

 

[TGS4]

... but I know the modulus of elasticity is changing (increasing) so technically the material can "take" more stress.

No, it doesn't. The elastic strain for a given stress is different for a different value of Young's Modulus. That's all that the elastic modulus indicates. There is nothing there that relates to an allowable stress.

Focus solely on the allowable stresses and the rating of the components, as SnTMan said. Your B16.5 flanges pressure rating changes by quite a bit. For Group 1.1 materials, Class 300 flanges go from 570psi@600°F to 680psi@200°F.

 

[drials2]

The vessel will have a limiting component, agreed. But what I'm wondering is;

-If I have the same piece of material as listed above has MAWP conditions of 600F 300psig, if I drop my rated temperature to 200F, does my MAWP increase? That's a simpler way of asking what I am trying to get at.

 

[marty007]

The short answer, maybe.

If the limiting component is:
[ul]
[li]SA-516-70: yes, slightly due to allowable stress increase from 19.4ksi to 20ksi[/li]
[li]B16.5 flange: yes, as mentioned by TGS4 stated[/li]
[/ul]

The truth is, you're likely not going to buy yourself as much MAWP as you may have hoped. CS maintains the same allowable stress right up to 500°F before it beings to fall of a cliff.

 

[SnTMan]

drials2, if the allowble stresses are increased, yes, most likely, if not, no way.

Regards,

Mike

 

[drials2]

Again, this was just to satisfy my curiosity. My example was not the best but I think I understand;

The MAWP for a section 2, D material does not change in the lower temperature regions BC the allowable stress is not changing. Changing the allowable stress will change your MAWP and possibly thickness required if in the design stage of a vessel. Thanks for the replies.

 

[SnTMan]

Materials do not have an MAWP, components do.

 

[chicopee]

In the appendix section of the fired and unfired pressure vessel code, you'll have tables of temperatures vs. working stresses. If the working stress remains the same for the temperature range of 50-600 dF then then you have your answer, no increase in stress.