Re-rate of pre-1968 vessels 3

[MalcolmA]

Does anyone know the reason why vessels that were built to the 1968 or previous editions of the pressure vessel code cannot be re-rated using the current code?

 

[stanweld]

Rerating is not prohibited but you may not use the higher design allowable stresses of later (current) editions of Code. Substantive changes in materials manufacturing and inspection methods have occured since the 1968 edition and the Code's requirements have been amended to reflect them and incorporate low temperature testing requirements thus permitting the higher design allowables.

 

[LSThill]

MalcolmA (Mechanical)

what is the material specification for your static equipment built to the 1968?

L S THILL

 

[MalcolmA]

What I have is a vessel built to the 1952 edition of the code. The material is A-212-B F.Q.

 

[unclesyd]

Exactly what are you attempting to do in the rerate?

What environment will the material see?

 

[MalcolmA]

I am not actually doing a rerate of the vessel.

We have vessel fabrication drawings but no certified documentation for the vessel rating. There is not a U-1 and the nameplate is missing.

I am to perform the calculations to verify the rating of the vessel. I was under the impression that you had to use the code of construction if the vessel was built prior to the 1968 edition of the code. I now see that you just have to use the allowable stresses from that code.

The vessel is in pentane service.

 

[stanweld]

Without a nameplate or U-1 form, are you sure that the vessel was built in 1952 and not to an earlier edition of Code? The safety factor may have been greater than the 4:1 (SMTS:design allowable stress).

 

[MalcolmA]

The only documentation that I have for the code of construction is in the general notes on the fabrication drawings:


Design, construction and welding to be in accordance to the 1952 edition of the ASME code for unfired pressure vessels.

 

[jamesl]

Stanweld,

Can you elaborate what exactly happened in 1968 Edition ASME BPV Code to make NBIC to choose this edition as the cutoff year? I cannot find info about significant changes in 1968 Edition other than Div.2 was first published in that year. The new impact test rule was not implemented until 1987.

 

[unclesyd]

We have several vessels built in 1952 and 1953 from SA-212 GrB Fbx to the 1952 same code. Nearly all have had a change in service so most designs have been rechecked using current software and the 17500 allowable from the 1952 Code.
Our only deviation from the original design is that for most of the equipment we require PWHT for all thicknesses due to service.
The only piece of equipment that presented any problem as to reevaluation of the original design was a Horten Sphere used to store anhydrous NH3 for several years.

 

[metengr]

Not to answer on behalf of stanweld, the requirements for re-rating pressure retaining items and prohibiting the use of higher allowable stresses was based on guidance from API 510. This standard had the requirement in place for vessels manufactured prior to 1968.

Why? Because from my recollection, the Bessemer steel-making practice (which produced plate steels with higher phosphorus contents and lower notch toughness) was phased out around the late 1960's and the basic oxygen furnace (blowing with oxygen instead of air) and electric arc furnace melting became the choice of most fabricators.

 

[stanweld]

The limiting 1968 Edition is somewhat arbitrary and an overarching reason cannot be given. Having been involved in steel making and vessel fabrication from 1968, we seriously began to embrace toughness criteria in our vessel designs and steel making practices after about 1970. This is not to say that we did not previously do so, only that we addresed the issues to a much greater degree. Also refer to Metengr's response which is well made.

 

[PVDean]

Make sure you do a check for MDMT of that Firebox material. For thick 212b it can be quite high.

 

[jamesl]

1. I try to understand the rational to check MDMT per current code rule. Pre-1987 vessels were exempted from impact test to -20F. If you use current MDMT rule (I assume Curve A in UCS-66), MDMT will be so high that most likely you will not be able to operate the old vessel any more. There are plenty of such old vessels in operation right now.

2. In the old code, there is a 0.95 joint efficiency limit with full RT. When you rerate old vessels with latest software (with old allowable stresses), do you use E=0.95?

 

[metengr]

In the old code, there is a 0.95 joint efficiency limit with full RT. When you rerate old vessels with latest software (with old allowable stresses), do you use E=0.95?

Yes you should. In accordance with the requirements of the NBIC, you cannot take advantage of a higher joint efficiency for re-rating.