PWHT
PWHT
(OP)
Can someone calrify for me regarding PWHT on sour service vessel? The vessel will be used for natural gas with 2% H2S. In ASME Section VIII Div 1, I believe it is called lethel service. The vessel material is A516 Gr. 70N. Per table UCS-56, my understanding is that if the vessel wall thickness is over 1/2", then the PWHT will have to be done becuase it belongs to lethel service. However, someone argues that PWHT is only required when the wall thickness is over 38 mm.





RE: PWHT
Cheers,
gr2vessels
RE: PWHT
RE: PWHT
RE: PWHT
UW 2(a) When vessels are to contain lethal1 substances, either liquid or gaseous, all butt welded joints shall be fully radiographed, except under the provisions of UW-2(a)(2)and UW-2(a)(3) below, and UW-11(a)(4). ERW pipe or
tube is not permitted to be used as a shell or nozzle in
lethal service applications. When fabricated of carbon or
low alloy steel, such vessels shall be postweld heat treated.
The only reference to wall thickness for PWHT exemption is here
UW-2, 4
(d) Pressure vessels or parts subject to direct firing [see
U-1(h)] may be constructed in accordance with all applicable
rules of this Division and shall meet the following
requirements:
(1) All welded joints in Category A (see UW-3) shall
be in accordance with Type No. (1) of Table UW-12, and
all welded joints in Category B, when the thickness exceeds
5⁄8 in. (16 mm), shall be in accordance with Type No. (1)
or No. (2) of Table UW-12. No welded joints of Type No.
(3) of Table UW-12 are permitted for either Category A
or B joints in any thickness.
RE: PWHT
If it does, then Alberta is in serious trouble...
Regards,
SNORGY.
RE: PWHT
RE: PWHT
Now, good practice would be to have the vessel PWHT'd. But that is good practice, not a Section VIII-1 requirement. It's important to keep the distinction.
jt
RE: PWHT
I agree the concentration will be reduced. But as per the quote I copied from ASME Section VIII, I think it does belongs to lethal service.
"By "lethal substances" are meant poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled. For purposes of this Division, this class includes substances of this nature which are stored under pressure or may generate a pressure if stored in a closed
vessel."
RE: PWHT
I think jte's post is correct.
Regards,
SNORGY.
RE: PWHT
This debate without consensus leaves the issue open to unscrupulous users and manufacturers, allowing them to abuse the code for their peculiar benefit.
Also, we are a forum with some essence and we should be able to address issues of this kind with the code writers, members of this forum. Obviously, using the formal means offered by the code.
gr2vessels
RE: PWHT
vessel has to comply to NACE
RE: PWHT
gr2vessels
RE: PWHT
"Such consideration shall include...
(2) the definition of lethal services."
Many Users (Owners) have often not considered vessels containing fluids described by larryli2004 as being in lethal service (cost driven). I know of one or two major users who have. Without doubt, if a major leak were to occur in that system, (and it didn't ignite/explode) it would be lethal but a "very small" release would not be. The definition of "very small" is somewhat nebulous.
RE: PWHT
RE: PWHT
Yep. Pretty much sums it up.
Under the current Code definitions, and with the understanding of what the effects of H2S can do to people - and steel, for that matter - engineers should have no choice but to consider it lethal.
My personal interpretation of "lethal" would be "remote or no possibility of revival immediately following exposure". HF acid, phosgene, and other chemicals might fit such a definition, but technically, H2S would not.
Regards,
SNORGY.
RE: PWHT
RE: PWHT
RE: PWHT
Regards,
SNORGY.