calculations for pressure vessel RT

Date 03-22-2010 17:00

Hi all,
Im fairly new to this side of code work, Ive welded on code work for many years and now im on the quality side of things with many questions. Im doing a mock up for a pressure vessel, i drew a small vessel (8" sch 80 sa 106B) with two 8" sch 80 sa 234 end caps for the ellipsoidal heads and a few fittings and had calculations done on it. This is what was sent back, for the end caps(ellipsoidal heads)
Radiography: Category A joints- seamless No RT
Head to shell seam- None UW-11(c) Type 2

UW-11(c) NO Radiography.Except as required in (a) above, or when the vessel or vessel part is designed for external pressure only, or when the joint design complies with UW-12(c).

(a) above- Category A and B welds connecting the vessel sections or heads shall be of Type No.(1) or Type No.(2) of Table UW-12
UW-12(c)- A value of E not greater than that given in column (c) of table UW-12 shall be used in the design calculations for welded joints that are neither fully radiographed nor spot radiograghed [ see UW-11(c) ].

Type No. (2) joint description is single-welded butt joint with backing strip other than those included under (1)

So if that vessel was calculated and that joint is category A - Type 2 does that mean that a backing strip has to used?

Any help would be appreciated
Thanks
Strat

By Rogers Karr Date 03-27-2010 04:07

Strat, let's see if I can help. I've heard Section VIII described as "the code written by engineers for lawyers". It is confusing and I'm still trying to understand it, too. Hope this is not to basic.

I think you got off track with "(a) above" in UW-11(c). The reference is to UW-11(a) Full Radiography. Your reference is to UW-11(a)(5)(a). UW-11(a) lists eight specific circumstances where welded joints are required to be radiographed in their entirety.
UW-11(a)(1) Vessels in lethal service
UW-11(a)(2) Joint thickness greater than 1 1/2 inches
UW-11(a)(3) Unfired steam boilers with design pressure greater than 50 psi.
etc

So, assuming none of the conditions in UW-11(a) apply, you are not REQUIRED to RT under Section VII.

Your pressure vessel engineer is telling you there are no Category A joints in the ellipsoidal heads as they are seamless. Seamless heads have a joint efficiency of 1.00 from the get go. If the heads were formed by welding, the longitudinal joints would be classified as Category A joints as defined in UW-3 WELDED JOINT CATEGORY. Similarly, since you are constructing the vessel out of 8" sch 80 (seamless) pipe there are no Category A joints in the vessel shell.

The head to shell seam is classified as a Category B joint. I suspect the engineer has mandated a type 2 joint but not requiring RT because he only needs a joint efficiency of 0.65 to meet the maximum allowable design pressure given the materials and thicknesses you have selected. Otherwise, he would mandate spot or full RT with a category 2 weld in order to increase the joint efficiency. He is not permitting a type 3 weld because the 0.55 efficiency rating would require thicker materials in either the shell or heads or both to meed the design pressure requirements. He hasn't mandated a type 1 joint because he doesn't feel you can weld from both sides of the joint and meet the requirements of a type 1 joint as defined in Table UW-12 or weld from one side and meet the requirements of UW-35. In short, if you a welding type 2 joints then by definition a backing strip designed to stay in place after welding is required or a lap joint as shown in Figure UW-13.1 sketch (i) can be used.

If you want to look at the role joint efficiency plays in the pressure/thickness part of vessel design, go look at UG-27 for shells under external pressure and UG-32 for heads with pressure on the concave side.

Hope this helped.

Rogers

By js55

Date 03-27-2010 15:27

Welcome to the forum. Additional knowledge of Section VIII is always an asset for us.

By strat

Date 03-29-2010 15:43

Rogers Karr,

Thank you very much for the reply, I think i see it a little better now after your reply. After reading UW-11(a) i understand what you are saying, what was throwing me of is that the engineer refered to UW-11(c) and category A joint, Rogers, this is what was sent back to me for the Ellipsoidal Head (8" Pipe Cap)

Radiography: Category A joints- Seamless No RT
Head to shell seam- None UW-11(c) Type 2

I tried for a week to get in touch with the engineer to ask these questions with no replys,
I think i have missed up for the moch up vessel is built with the heads welded with full-pen without backing, I told the guys to go ahead and weld them full-pen for i didnt know at the time, as i said this is just a moch vessel for a trial run, the AI Supervisor will be here in the morning. I remember going through this when i was at Alstom Powers and there were no backing on the joints, just full-pen on the shell when welded, I was the one welding, not the one checking calcs and so forth, plus we were building HRSG's which would not fall under section VIII.

Again Thanks,
Strat