Re-Rating for Lower Design Conditions
Re-Rating for Lower Design Conditions
(OP)
A CS pressure vessel that was designed & constructed in 1975 to ASME Section VIII. The vessel was designed for 1700 psig @ 700F.
Can we use the same vessel for same service at lower design conditions (The new requirements are 521 psig @389F) Without re-rating it?
A New Name plate will be required
Can we use the same vessel for same service at lower design conditions (The new requirements are 521 psig @389F) Without re-rating it?
A New Name plate will be required





RE: Re-Rating for Lower Design Conditions
RE: Re-Rating for Lower Design Conditions
RE: Re-Rating for Lower Design Conditions
Typically the design pressure and temperature have some margin against the operating conditions. Sounds as though you are simply increasing the margin. Good for you. As long as you are not changing other things like the corrosion allowance it seems to me as though you are proposing to operate the vessel (well) within its design envelope.
I'm a bit perplexed as to why you would require a new nameplate and why a rerate/derate is being considered here. Typically a derate would be an effort to gain corrosion allowance and in that case it makes sense to alter the vessel properly with appropriate calc's and paperwork. So are you also changing the CA or something else which would impact the design? Is this a Div 2 vessel in which case the User's Design Spec may need to be modified?
jt
RE: Re-Rating for Lower Design Conditions
If it's the latter, you shouldn't have to do anything. The (original) design conditions should be thought of as limits NOT to be exceeded. You have every right run the thing below those limits. Just make sure the vessel hasn't degraded to the point of not being good for its original (current) rating.
If you're talking an official de-rate due to thinning or some other mechanical reason then metengr is correct in directing you to your local jurisdiction or governing code, API-510 or NBIC for example.
RE: Re-Rating for Lower Design Conditions
There may be some advantage from a "process and instrument diagram" and other official documentation point of view though, as it helps keep all the documentary ducks in a row, removing any confusion when corporate memory dissipates years down the track.