PWHT 1

[larryli2004]

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.

 

[gr2vessels]

Refer UW-2, the vessel in lethal service must be post weld heat treated. The Table UCS-56 is detailing the temperature and duration requirements, function of the weld thickness.
Cheers,
gr2vessels

 

[metengr]

Lethal service design requires PWHT.

 

[larryli2004]

But I think it still depends on wall thickness. As per UCS-56, even for lethel service, the PWHT is not required for wall thickness less than 1/2".

 

[metengr]

Here is some reading for your pleasure;

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.

 

[SNORGY]

Then the only question remaining is whether or not 2% H2S constitutes "lethal service".

If it does, then Alberta is in serious trouble...

Regards,

SNORGY.

 

[larryli2004]

well, as per lethal service's definition, even 100 ppm H2S belongs to this category.

 

[jte]

That is not entirely clear. Steam is lethal. So is Nitrogen. Yes, 100 ppm H2S is listed as an immediately dangerous to life and health level. But... when it leaks, its concentration is substantially reduced. Even percent level H2S vessels are commonly not considered to fall into the "lethal service" criteria for ASME VIII-1 purposes.

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

 

[larryli2004]

Jt,

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."

 

[SNORGY]

That definition is consistent - almost word for word - with the one found in CSA B51 (the Canadian "over-riding" Standard to ASME VIII Division 1). That said, I don't think I have ever worked on a project up here in Alberta where the "lethal service" definition was imposed for H2S service.

I think jte's post is correct.



Regards,

SNORGY.

 

[gr2vessels]

Interesting post, but leaves the question begging;- then what is a correct definition for lethal service?
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

 

[GenB]

I do not think this is lethal in sect VII-1, I would do 100RT and PWHT regardless plus do not forget the
vessel has to comply to NACE

 

[gr2vessels]

GenB;- what makes you think this is not lethal? C'mon guys, we're engineers...
gr2vessels

 

[stanweld]

Per U-2, the "The User or his designated agent shall establish the design requirements..."

"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.

 

[larryli2004]

Well, for high pressure vessel, it is normally not a problem because the wall thickness will be more than 38 mm anyway. Most time the argument is raise for low pressure vessel such as free water knockout (MAWP around 75-125 psig). Very often the wall thickness is around 1/2" for these vessels.

 

[SNORGY]

gr2vessels:

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.

 

[vesselfab]

I think most users would prefer to err on the side of caution. We do a lot of wet H2S fabrication that is not lethal. But it is entrained in the process fluid and not gaseous (my thoughts and not in spec). Requires HIC plate and 100% RT and PWHT. built the same and the lethal service would just require one hit of the hammer on name plate and 6 keystrokes on U-1A.

 

[Duwe6]

My general field defination of "Lethal Service" is: Will 2-3 breaths of the stuff incapacitate you [and since you can't leave on your own, then you die]? Phosgene and cyanide are No-Brainers. 2% Hydrogen Sulfide should be, too. You *will* hit the deck after the secdond breath.

 

[SNORGY]

You will indeed be knocked down, but you will also stop breathing and you will still have the chance of being rescued and making a full recovery if action is taken within a few minutes. Of course, past exposure history may alter that prognosis.


Regards,

SNORGY.