Rerate of a Vessel by PWHT

[weldtek]

I'm wondering if any members have had any experience with re rating a vessel by using UCS 68(c
In this case the customer would like to re rate their 70's vintage vessel, fabricated from SA285 GrC, from 44 F to 14 F, by way of PWHT.

 

[stanweld]

Why would they want to do that? It was good to -20F when designed to the 4:1 Safety Factor. Or do they want to increase the pressure by using the 3.5:1 Safety Factor?

 

[vesselfab]

if it was thicker than 1" maybe the curves limited to 44*F and PWHT would lower it 30*F

Talk with the AI

If I remember correctly 285 is fairly fine grained and should be ok, depending on thickness.

I do not know why in 70's anyone would use 285 for vessel fabrication

 

[weldtek]

Stanweld,
No pressure increase that I'm aware of. I've asked the same question, - why?

Vesselfab,
Material is < 1".

Thanks,

 

[DVWE]

What about the weld procedures used on the vessel?

They would have to be requalified, no?

 

[vesselfab]

i am assuming it's 5/8" thick curve A is about +44*f

LOOK AT UCS-68(c)

It says an additional 30*F reduction in MDMT if vessel is PWHT for no other reason by code if P-1

so material is sa--285-c it's p-1

an existing vessel is not pwht so adding pwht for no other reason

oughta be good....but check with the local authority

 

[stanweld]

The impact testing "curves" didn't exist in ASME VIII during the '70s and the present Code does not apply to current operations as long as the original design conditions apply. It would appear that the customer is unaware of that fact unless its insurance agency or the Jurisdiction has initiated further reviews.

vesselfab,
SA-285C was often used to fabricate vessels in the '70s. It is coarse grain manufactured. Unless ordered at that time as Firebox Quality, it was not fully killed which was considerably cheaper than SA-515 at the time. My company made thousands of tons of SA-285 during the early '70s.

 

[weldtek]

DVWE,
You raise a good point and one I've raised with the customer. The chance that a 70's era fabricator is still around is slim enough, and then consider the chances his WPS' were qualified with and without PWHT.

But, the bigger question is, as was brought out by Stanweld, why not fall back on the the Code rules in effect at the time of construction, allowing this material to be used down to -20 F. By the way, I've since received a drawing and U1A, and, it was FBX material.

 

[vesselfab]

Well I wonder how the vessel would be stamped with a MDMT if the curves were not around then

I know Exxon had there own set of curves in the 70's

hmm..i was building vessels in the early 70's and we normally used 515-70. Guess it all depends on who the user was.

 

[DVWE]

why not fall back on the the Code rules in effect at the time of construction, allowing this material to be used down to -20 F.

Absolutely. As stan has pointed out, your customer is probably unaware of this.