NACE SP0472 Amine Service PWHT holding time and temperature 2

[Chumpes]

Dear all,
i could not find the answer to my question in other threads in the forum, that's why i have to ask you the following question...

ASME
PWHT of CS (P-No 1) with thickness above 125mm as per ASME VIII div. 1 (table UCS-56-1) asks for 2 hr plus 15 min for each additional inch(25 mm) over 2 in. (50 mm)

Same as ASME VIII div.2 (table 6.8): For t_n > 125 mm: 595°C, 2 h plus 0.6 min for each additional millimeter over 50 mm

for my 262mm thickness pressure vessel to be operated in amine service, this gives PWHT holding time = 4h07min

NACE SP0472
3.4 For amine and caustic cracking services, an effective PWHT procedure shall consist of heating weldments to 635 ± 15°C (1,175 ± 25 °F) for a hold time of one hour for each 25 mm (1.0 in), or a fraction thereof, of metal thickness, with a minimum hold time of one hour.

for my 262mm thickness pressure vessel to be operated in amine service, this gives PWHT holding time = 10h30min


The problem is that we have to apply NACE SP0472 vs. amine service, but the mechanical properties cannot be achieved for such a long holding time (10h30 @635°C), and NACE states that lower temperature and longer holding time is not acceptable for stress relief for amine service.

Can I go for holding time as per ASME (much shorter) keeping temperature as per NACE (635°C) ?

Why PWHT holding times as per ASME are so much shorter that holding times as per NACE ?

thank you in advance

 

[SJones]

Because ASME is not considering the environmental damage mechanisms associated with the contained fluid.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Dhurjati Sen]

Start with the WPS. How many passes would be there to finish the welding?

You can try doing PWHT multiple times as you go on welding, abiding SP0472 every time. This way you could maintain the mechanical properties, i.e. the material does not get too soft.

Tough job though.

DHURJATI SEN
Kolkata, India

 

[Chumpes]

Thanks for your answers

@Sjones
Well yes, ASME does not consider corrosion damage, but NACE does not consider much mechanical resistance either, excepted that(extract from NACE): "PWHT procedures as described in 3.4 and 3.5 can negatively affect the tensile strength and impact toughness of the base metal. Each situation should be evaluated with the steel supplier to determine whether this specified thermal stress relief is adequate for all other considerations in addition to ASCC prevention"
The answer here is "no, PWHT as per NACE for amine service is not adequate vs. mechanical properties"...

@Dhurjati Sen
I do not know the figure but for a 262mm thick vessel yes, there will be multipass welding.
What do you mean by "doing PWHT multiple times as you go on welding, abiding SP0472 every time" ?
Do you mean short time PWHTs as per NACE but considering the actual thin thickness of the latest pass only ?

The problem with this NACE PWHT (duration 10h30) is that we need to design for one additionnal PWHT cycle for future repair, so mechanical properties must be maintained for 21h PWHT, which cannot be achieved with the actual PWHT methodology... mechanical properties are maintained only for PWHT duration shorter than 17h @635°C.

 

[0707]

Yes Dhurjati Sen gives a possible solution, weld 25mm and PWHT, then weld another 25mm and PWHT and so on til finish the weld.

good luck

luis

 

[DBreyer]

I would suggest talking to your customer before you do the stepwise PWHT.
Maybe suggest a weld overlay, so your only have to do PWHT of the base metal acc. to ASME code

 

[Dhurjati Sen]

The root pass, hot pass and maybe few more passes that would make 25 mm thickness can be initially Post Weld Heat Treated. After all, the process fluid (rich amine) would have its effect over this zone mainly. You can extend the first PWHT zone even upto 100 to 150 mm (eg Steam and Mud drums of boilers where this is routinely done). Surely you would get desired mechanical properties after the first PWHT as per NACE SP 0472 in this zone, say a thickness of 100 mm.

The next part would be tricky. What do you do with the remaining 162 mm?

That's why I mentioned the WPS at the beginning of my earlier post.

There has been a case where the vendor did not finish the welding and sent the vessel at site, for the very same reason maybe.

Of course, this should not be your solution.

DHURJATI SEN
Kolkata, India

 

[Chumpes]

Dear all,

@0707 (Luis): thanks for your comprehensive explanation
@Dhurjati Sen : thanks for the suggestion of multiple PWHT

>> I am discussing the possibility of performing multiple short PWHT layer by bayer for better mechanical properties of the weld, i will let you know

Still, multiple short PWHTs layer by bayer will not change anything regarding the loss of mechanical properties of the base material (cumulated PWHT time for base metal = NACE's PWHT time)... but base material composition (CE etc.) may perhaps be adapted to sustain the longer cumulated time PWHT required for amine service as per NACE... ongoing discussions...


@DBreyer : thanks for your answer... i understand that you suggest SS WOL of the welded zone in order to avoid amine SCC of the weld and to avoid requirement for NACE's PWHT, but in that case you will still have to perform PWHT as per ASME (lower temperature and shorter time): I think you cannot PWHT only the base material and avoiding the PWHT of the weld. I will discuss some solution based on your suggestion (SS WOL vs NACE's PWHT).

thanks again, see you soon

 

[Chumpes]

Dear all,
In order to inform you about the conclusion on this topic :

A new repair procedure that respects NACE SP 0472 for repair's PWHT was proposed: defects identified are localized in the outer or in the inner half thickness section, and repairs from the outside or from the inside will allow repair operation to be limited to half of the thickness only, which will require PWHT time for the half thickness only and will eventually allow achieving suitable mechanical properties after repair.

Regards