PWHT of repaired vessel 1

[jamesy]

I have a 25 year old plaincarbon steel pressure vessel 1" thick that was subject to PWHT & 100% RAD during original manufacture. The vessel was designed & constructed to BS1515, now superceded with PD5500. We no longer have the fabrication records so we do not know if the PWHT was applied to non pressure retaining welds.

We are installing new internals & have to weld a support bracket, 75mm angle with a 6mm fillet. Do we need to PWHT this weld? From PD5500 I dont think so but there are others who think I should because the original construction called for PWHT.

Thanks in advance
Jim

 

[metengr]

jamesy;
I don't have a copy of BS 1515 to provide you specific guidance on this issue. However, for most construction codes, the requirement for PWHT is normally driven by either base metal thickness for groove welds or nominal thickness of the weld deposit. There is one point to mention in that the owner of the vessel may still stipulate PWHT. The PWHT you mentioned during fabrication was probably required for groove welds for joining 25mm thick base material.

For P-No1 base material with no corrosion concerns related to stress corrosion cracking or hydrogen embrittlement in an as-welded condition, I would doubt PWHT is required for only a 6mm fillet weld. Your final call on this will be PD5500. You need to obtain a copy and review PWHT requirements for fillet welds, not groove welds.

 

[MikeMet]

PD5500 is very much like ASME B&PV Code, Div II. I don't know PD550, so take the following for what you paid. 1" thick CS typically does not required PWHT due to thickness. So PWHT may have been done for process reasons, such as lethal service or to prevent stress corrosion cracking. To disagree with metengr, carbon steel will stress crack in the as-welded condition in amines, CO, CO2, ammonia and caustic steams, which are all very common in the process industry and older versions of carbon steel are subject are also subject to SOHIC (stress oriented hydrogen induced cracking) which is a form of hydrogen cracking. That is why one can now buy HIC resistant carbon steel plate. So PHWT may be very important

 

[SJones]

There's no need to disagree with metengr:

For P-No1 base material with no corrosion concerns

Is the 6mm fillet weld being made on the pressure envelope? If not, there could be a case for omitting PWHT. If the weld is on the pressure envelope, and the vessel comes under COMAH, will you not have to make a safety, engineering and technical review? This review should then document the application or omission of PWHT.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[jamesy]

Thanks for the replies so far.

The fillet weld is on the pressure envelope however COMAH does not apply as the vessel is on an offshore platform. Another factor I ommitted in the original statement is the vessel is to be internally coated with Belzona 1341 for corrosion protection.

As MikeMet has pointed out the original PWHT was for improving the material to mitigate the potential for SCC in case the vessel ever saw sour service. The platform conditions are non sour service and never have been in 30 years of service.

 

[SJones]

Then refer to ISO 15156-2 and assess whether PWHT will be necessary to bring the failure (corrosion) risk assessment to ALARP.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[MJCronin]

This is an exellent discussion on a problem that seems to come up over and over.

Perhaps someone could develop this into a FAQ ??

-MJC

 

[SJones]

Good point MJC and, in which case, I shall elaborate a little further to provide a better FAQ:

1. Just because the conditions have been 'non-sour' for 30 years does not mean that requirements for H2S service can be ignored. Future field and/or operational changes can lead to increases in H2S. The approach to H2S should be documented in the safety case under the performance standards for safety critical elements (SCEs). The vessel should be classed as an SCE.

2. The main degradation mechanism that is under consideration is sulphide stress cracking (SSC). ISO 15156-2, A.2.1.5 states that linings are not acceptable for preventing SSC. So, the application of lining can be ignored.

3. Weld zone hardness will be a factor to consider in an assessment of the likelihood of SSC. The first way to review the hardness would be a check on the existing welding PQRs for the vessel looking for the hardness achieved and use of the actual material in the test weld as was used to make the vessel. These records would appear to be unavailable; so, what will happen? Ideally, the fillet welding procedure should be qualified using the exact same material as the vessel or, if not the same material, a fair equivalent. It should also have the same or higher carbon equivalent value (CEV) as the actual materials (not forgetting that a crack that initiates in the non-pressure retaining material can propagate through to the pressure retaining material once they are connected via a weld.

4. If a fair simulation indicates that hardness values for the fillet weld will not exceed the limits of ISO 15156-2 then it may be documented that PWHT is not considered necessary.



Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04