PWHT for caustic soda pipe 11

[flinana]

Hi,

I have the following problem. We have a 2" carbon steel pipe for caustic soda (50%)for which our customer(Refinery)requires a PWHT. The question is whether or not it is techically necessary to carry out this test since it is not required by ASME B31.3. We believe that the pipe will not suffer from corrosion if the PWHT is not carried out.

Thanks,

 

[BigInch]

Apparently this is on a pipe that has not been taken out of service for welding or reconstruction of some kind. I doubt corrosion is the issue either. How long has the line been in service? It may just be a fatigue strength check after many years of service in order to insure integrity. With the number of refineries that have had catastrophic failures during the last few years, it might not be such a bad idea to just say "yes" and get out of the way. Its a 2" line, shouldn't matter much in the way of cost and could make a lot of difference in safety and peace of mind. Is it really that big a deal?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[pennpiper]

flinana
You wrote "The question is whether or not it is techically necessary to carry out this test "
The PWHT is not a test. PWHT is Post Weld Heat Treat.
As in put it in an oven and heat it up to (a high temperature) and let it cool at a controlled rate.

You may want to check with someone with NACE (National Association of Corrosion Engineers) for criteria for welding on Caustic lines.

 

[Zapster]

flinana,I believe that you are incorrect in your assumption about corrosion. Caustic stress corrosion cracking is a problem encountered at many refineries and that resulted in your client’s requirements for PWHT. Proper PWHT reduces the stress adjacent to the welds and thus will reduce the potential for caustic stress corrosion cracking.

 

[MJCronin]

What is the service temperature of the 50% caustic ?

If it below 120F, you should have no problem.

The additional cost requirement of the PWHT would be better spent on a thicker pipe... (Sched 80 ?)

-MJC

 

[jte]

Codes provide minimum requirements. One requirement is that the user of a code read the introduction and scope sections and have competent engineers and designers to implement it. ...the Code is not a design handbook; it does not do away with the need for the designer or for competent engineering judgement.

If your client (that would be the folks who are operating the refinery and on the receiving end of the explosion / caustic spray) states that the PWHT is required for process / materials reasons then it would be extremely poor engineering judgement to take the position that "the code doesn't require PWHT so we won't spend the money doing it." Engineers need to produce reliable designs, not just follow the minimum requirements of codes.

Caustic cracking is real. It causes real failures. Failures cost more than doing it right the first time.

jt

 

[unclesyd]

I heartily second the post by jte.

We have an extensive NaOH distribution system encompassing 50% to 1% and our requirement is that All piping will undergo PWHT with no exceptions. Even with the PWHT were are careful about the routing of these lines for people protection.

Part of our process produces Amines and Imines where again all the piping has to have PWHT though not required by code.

 

[flinana]

Thanks to all, I meant Treat not Test sorry. Problem is of course money as always. The story is that our client is not directly refinery. It is a Biodiesel turnkey project situated next to a refinery. The thing is that under contract we need to apply refinery specs, however if we can technically prove that some refinery specifications are not needed according to standards, we can obviate it.

As unclesyd mentions, refinery applies this to all pipes regardless of sch, but ASME specifies this treatment only for certain schedules.

 

[SJones]

API 571 is a refinery recommended practice. Clause 4.5.3 on caustic cracking will tell you all that you need to know (and will also point out that PWHT makes good sense irrespective of whether multiple specifications can be played off against each other to cherry pick the path of least resistance).

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[MJCronin]

unclesyd and Steve Jones...

I thought that I had some good (although old) references ( including the ancient PPG guidelines)regarding system design of 50% caustic soda systems.

Caustic cracking is a concern at 50% solution, but according to my recollection, its only when the operating temperature is >120F. PPG also recommends SCH 80 piping

While I do not doubt your experience, I am looking for an industry guideline/ white paper that endorses the PWHT of ALL PIPING JOINTS....

Is there something out there more authoritative than the API 571 note ?

What about CS welded tank construction ?

Is PWHT required there also ?

-MJC

 

[unclesyd]

SJones,

Right On!

 

[DanStewart]

There are some things that "deminimus" engineering just should never apply to. That includes systems that require some sort of intrinsic safety. Caustic soda is nasty, nasty stuff. Caustic cracking happens more often that you think. At my paper mill, we use caustic for everything. Although there is no specific engineering "standard" that specifies the design requirements of a caustic handling system in a paper mill, we go the extra mile to ensure the containment of this extremely hazardous chemical is maintained. That means we, too, PWHT on all applicable piping, etc.

And, on a side note, if the customer said do it.....and they're paying for it....(as long as it was in the contract, else send them a change order request).

 

[itdepends]

Is the line heat or steam traced in any way? (to prevent crystallisation at low temps). If so- even if the normal operating temperature is below the level at which CSCC can occur- you would be well advised to apply PWHT

 

[stanweld]

flinana,
Please note that under 300 (b)(1)of B31.3 the Owner has overall resposibility for the design, construction, testing, etc. of the piping system. The Owner is responsible for the corrosion design requirements including provisions to mitigate stress corrosion cracking in service.

 

[flinana]

stanweld, what do you mean by 300 (b)(1)of B31.3

 

[stanweld]

ASME B31.3, Chapter 1, Section 300 (b) (1). "The Owner of a piping installation shall have overall responsibility for compliance with this Code and for establishing the requirements for design, construction, examination and testing which will govern the entire fluid handling or process installation of which the piping is a part."