Re-Rating for Lower Design Conditions

[TanEng]

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

 

[metengr]

Check with your local Jurisdiction or agency that regulates pressure vessels. They may provided specific rules you need to follow for a de-rating, and stamping of the namplate.

 

[dcasto]

There is no reason to rerate the vessel. The only thing you want to do is put the vessel under your Mechanical inspection program and monitor it as normal. If the vessel was not under your company's control since 1975, then it should be inspected and tested to your company's policies and proceedures which may include an API based Inspection/Fitness for Use Recommended Practice.

 

[jte]

hunnan-

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

 

[dtn6770]

By "lower design conditions" do you mean to literally reestablish the official vessel rating (de-rate) or are you just going to operate it at the lower pressure and temperature?

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.

 

[robsalv]

IFF the vessel is still fit for service/suitable for the original design conditions, there shouldn't be any reason [from the vessel's point of view] to down rate it unless you officially want to capture its new design conditions and take credit for the extra corrosion allowance this gives you.

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.