PWHT at the existing piping and reactor for further preventing some HTHA 1

[lovescroll]

Hello
Our plant has a debottleneck project, they decided to increase the unit's operating temperature from 140 to be 200 degC at the partial pressure of hydrogen around 400 psia
after reviewing all of condition with safety margin added, all of piping including reactor shall be done a PWHT, while existing condition is not.

So my question is
can we do PWHT to increase the resistance for HTHA at the existing unit or not ? if we still afraid of hydrogen trapping inside , can we do hydrogen bake out before doing PWHT?
* base on our existing condition, i can make sure that no susceptible to HTHA for this time,

 

[Dhurjati Sen]

lovescroll
Please refer to API Recommended Practice 571, Figure 5-56 – Recommended pressure and H2 partial pressure limits per API 941.

It seems you are operating well below the limits. Do cross-check once again.

 

[Chumpes]

Hi
curves from actual API RP571 are no more up-to-date because there is a new revision of API RP941 (2016) dedicated to HTHA damage.

at Top=200°C and ppH2=400psi = 27.6 bar (remember that partial pressures are always absolute pressures and not gage pressures, same as pressure drop !) i can read on the Nelson curve from API RP941 latest revision that you are more than 50F/50psi below the Nelson curve for carbon steel without PWHT, and carbon steel without PWHT may be suitable for the service (you are just out of hot H2 service.
It is usual for users to apply additionnal margin such as specification of PWHT in addition to API minimal requirements, in case of new equipement, so you still have to comply to your company's philosophy for material selection in hot H2 service, that may be different for new and old units.
In case of old equipement, because your service is not very severe (below the Nelson curve for CS without-PWHT with significant margin), I think it is possible to reuse the vessel provided regular inspection for incipient HTHA attack is performed (old equipment, no-PWHT...).
As previously said, I would not perform PWHT of an old equipment.
As you would know reading reference documents, hydrogen that diffuses into the bulk metal is trapped in the form of CH4 molecules after HTHA damage mechanism, and these CH4 molecules cannot diffuse backwards and remain trapped. I dont think it exist heat treatment that are able to recover the original resistance to HTHA of carbon steel materials.

regads

 

[blacksmith37]

A stress relief does not improve resistance to HTHA .You got a reasonable "bake out" during cool down from operation , assuming it took several hours.

 

[DBreyer]

I have to disagree with blacksmith37.
The latest revision of API 941 offers a curve for non-PWHT carbon steel that is is approximately 50°F and 50 psia partial pressure H2 below the existing carbon steel curve.
As such there is a clear advantage in PWHT of carbon steel.
For new equipment, however, I would still be careful when predicted operating conditions (excluding design margins) are close to the CS curves as experience has shown that the exact mechanism of HTHA is still not very well understood.
I recently worked in a facility where HTHA damage (2 leaks within 2 years) of non-PWHT CS occurred even below the new curve after very prolonged service (> 25 years)
Following the Tesoro incident the CSB even proposed inherently safe design for new equipment, operating above 400°F, which would be at least 1.25% Cr steel. This was not adapted by API.

For existing equipment if you are going to be well below the curves (actual operation) continued use of non PWHT CS might still be

Daniel Breyer
Inspection Engineer

http://lnkd.in/dFcQA-w

 

[lovescroll]

Thanks for clear discussion
I actually agree with you all to not to take some pwht at the existing equipment and reduce some margin and increase some confidence rating instead.

By the way we found that the new procedure of regeneration process will take a system to be 260 degree C for 48 hours. And incubation curve at page 10 @api941 only support at non weld or pwht condition only. So i think the way we go to pwht might be suitable. What do you think ?

But i do not have any idea to take pwht at the existing reactor (diameter 2 meter with 10 meter height) please give me some idea.

 

[Chumpes]

@blacksmith37 : maintaining temperature for retro diffusion of hydrogen out of metal during cool down will not be beneficial regarding HTHA damage which includes decarburization and methane molecules. you may have mistaken HTHA with hydrogen embrittlement...
PWHT is demonstrated to have a positive effect on resistance of CS to HTHA according to reference document.

@DBreyer : what was rougthly the conditions (Top / ppH2op) at which HTHA of non-PWHT carbon steel was experienced? our practice is to ask for PWHT of CS while above the CSB curve for carbon steel (PWHT or not, one single CSB curve for CS), rougthly above 200°C (400F) at ppH2 < 55bar. In my opinion, PWHT should be applied to CS in hot H2 service (above 200°C at ppH2 < 55bar), which is more conservative than API that allows CS without PWHT for less severe conditions.
I would not follow CSB recommandation asking for low alloy steel above 200°C because numerous equipement in CS worked just fine in moderate service for long time.

@lovescroll : so you re going for PWHT the old vessel ?

 

[lovescroll]

Actually i won't prefer. At least i prefer a new one fabrication. But cost might be effective as well.

So. I need to confirm that with the regeneration conditon like this (260 deg C with 350 psia of H2PP and non pwht carbon steel. Is it necessary to do pwht ??

Are there any curve like incubation curve in page 10 api 941 - 2016 (for using with carbon pwht) to let me ensure that no HTHA even short period of regeneration for carbon steel w/o pwht

 

[metengr]

PWHT of an existing vessel can be tricky and probably would be performed as a alteration. First, the tricky part is PWHT of an entire vessel in service with existing nozzle and pip loads and dead weight requires analysis to avoid creep deformation under exposure to PWHT.

The reason I mentioned alteration is because original fabrication of the vessel was done with qualified WPS that most likely were not qualified with PWHT. This could alter the toughness/strength characteristics of the existing vessel material (weld and base material). This is also considered as an alteration in the 2017 NBIC.

 

[Chumpes]

So. I need to confirm that with the regeneration conditon like this (260 deg C with 350 psia of H2PP and non pwht carbon steel. Is it necessary to do pwht ??

It is recommended by API to have PWHT of new carbon steel vessels in such service, even transient, refer to API RP941 for more information. transient operation is less harmfull than continuous.
in case your actual vessel is not PWHT and your new process service is similar to former one, i would advise regular ispection for incipient HTHA attack.

Are there any curve like incubation curve in page 10 api 941 - 2016 (for using with carbon pwht) to let me ensure that no HTHA even short period of regeneration for carbon steel w/o pwht

no, API RP941 incubation time curve is no more applicable to CS witout PWHT but only to CS+PWHT. The only other incubation time curve is for C-0.5Mo and should not be trusted. no other curve exists to my knowledge

 

[Lee SiHyoung]

I think we have to make an approach from metallurgical aspect.
I do agree with that the stress relief from PWHT definitely makes steels less susceptible to crack. However, please note that the mechanism from HTHA is based on the diffusion of H2 and the reaction between Fe3C and H2 rather than directly connected with stress relief. (I think the reason why PWHT is applied is not exactly due to HTHA but other mechanisms such as Creep which is normally accompanied with HTHA in hydrogen service.)
The decomposition from Fe3C will induce reduction of strength of steel and be led to failure in case it is serious. Referring to many global references, HTHA is related to not only weldment but base metal(See section 5.1.3.1.7 a) from API 571) while PWHT is only with weldment. (Post Weld Heat Treatment)

Best Regards,

Lee SiHyoung
Engineer – Material & Corrosion Specialist
Tel : (+968) 22306214
Mob : (+968) 98891587
Email : lee.sihyoung@worleyparsons.com