Inadequate PWHT holding time for Carbon Steel piping in caustic service (NACE SP-0403)

[kondori]

Dear all,

We have a situation in our project (petrochemical plant construction) where in our caustic service piping (mostly 4-6mm wall thickness and small OD), all the weldments have been post-weld heat-treated, but it has been done acc. to ASME B31.3 (not as per NACE SP-0403 requirements). The PWHT holding temperature has been around 600- 620C, and the holding time is 30 minutes in almost all cases. As you know, NACE SP-0403 asks for a minimum holding time of one hour and a temperature range of 635+_14 C.

At this stage of the work, going back and repeating PWHT ACC. to NACE SP-0403 will be a nightmare. Considering the ASME PWHT has been done, we believe that these weldments still have a chance and should not be rejected without further evaluation, and that's why I am posting this thread.

What is the correct way forward here...?
I presume we have to cut a sample (or samples) of a welded joint (or prepare a PQR test coupon) and send it for corrosion testing. I can go through the available references myself and find something, but I do not pretend to be a corrosion engineer, and I do not want to make mistakes here.
Please note that all the field hardness test results have been satisfactory.

Thank you for your time and assistance.

 

[r6155]

In B31.3 2022 Table 331.1.1 the holding time is 15 min min. for P-1 material.

 

[kondori]

In B31.3 2022 Table 331.1.1 the holding time is 15 min min. for P-1 material.

Our PWHT meets the requirements of ASME B31.3.
That's not the issue.
The issue is that we did not meet the NACE SP-0403 requirements.

 

[EdStainless]

Should I tell you how many thousands of hardness reading it would take make me feel any comfort about this.
How are you checking hardness on the ID of the welds and HAZ?
Look at your actual chemistries, where are they in terms of CE?
Look at the actual data from the PWHT.
Were the times and temps to the low side or the high side?
If the CE is low and the the times and temps are high then you are not off by much and this might work.
Is re-PWHT more of a nightmare than in service failures?
Are you ready to put it into the record that you signed off on these deviations?

 

[kondori]

Should I tell you how many thousands of hardness reading it would take make me feel any comfort about this.
How are you checking hardness on the ID of the welds and HAZ?
Look at your actual chemistries, where are they in terms of CE?
Look at the actual data from the PWHT.
Were the times and temps to the low side or the high side?
If the CE is low and the the times and temps are high then you are not off by much and this might work.
Is re-PWHT more of a nightmare than in service failures?
Are you ready to put it into the record that you signed off on these deviations?

Thank you, EdStainless, for your reply.

Update:
Today, I found out that originally, they have been following the specification and performing PWHT acc. to NACE SP-0403. The reason why they have changed things and reduced the holding time & temperature has been due to excess distortion and oxidation after PWHT. The EPC contractor has submitted a TQ, to reduce the holding time to 30 minutes (as a solution), and they have received approval for it (this has fixed the excessive distortion & oxidation issue, though).

Answers to your questions:
- The field hardness tests have been obtained only from testing the outside surfaces (due to lack of access to the inside of our small-OD caustic piping system), but we did lab hardness tests from inside during qualification tests (PQR), and they were mostly around 120 HV (never above 140 HV).
- Our material is NACE MR0175 certified, and the CE numbers are low.
- We have always had good control over our PWHT operations, and we repeat the PWHT anytime we detect any deviations in time/temp parameters. Please note that our PWHT holding temperature has been around 600- 620C, and the holding time is 30 minutes in almost all cases.

Considering the above information and the fact that residual stress levels should be considerably lower after PWHT, I also do not think this piping system would cause huge problems in operation. Yet, I am looking for a way (especially corrosion testing) to make sure that's the case. I would rather have some favourable corrosion test results to rely on when signing off on such deviations.

Thank you again,

 

[hacksaw]

your customers standards for their process govern.

 

[r6155]

1) If preheat was performed before welding, I can tell you that for 4mm to 6mm wall thickness (small pipe diameter), 30 minutes PWHT is adequate. 2) Also refer to Figure 1 in NACE SP-0403 "carbon steel no stress relief necessary"

 

[Dhurjati Sen]

If you go for various lab tests like metallography, hardness, tensile strength etc., you won't get much difference between samples heat treated as per NACE or ASME.
I have faced such situation and the non-conformity that I raised was closed based on such tests.
I was a quality assurance / control engineer at that time and the project manager could not understand the real consequence of such poor fabrication practice as a service failure might take place say after 5 years of commissioning and no failure analysis would detect the real cause of failure which would be poor fabrication / quality control practice at the project stage.
But the pertinent question here is who you are and what sort of organization are you working for.

 

[r6155]

More
1) Did you perform hardness tests before and after PWHT?
2) Did you control the heating and cooling rate during PWHT?

 

[DekDee]

Who signed off the TQ approving the reduction in time ?
Our role in Quality is to identify issues and communicate these to the relevant engineers.
It is the Engineering department who ultimately decide if the request is acceptable.
IMHO this should not have been a TQ but a Deviation and therefore Client sign off is imperative.

 

[DekDee]

Does your caustic concentration and metal temperature show the welds to be in Area A, B or C of Figure 1 of NACE SP-0403 ?
As stated - Engineering are responsible for providing this information, not Quality.