PWHT for Process Reasons.

[Chemitofreak]

Hi,

When does a process engineer specify PWHT (Post Weld Heat Treatment) for process reasons?

Does anyone have any idea about it.

Thanks in advance.

Dinesh S SHELATKAR.
Process Engineer

 

[EmmanuelTop]

The requirement for PWHT arises from a potential hydrogen cracking problem which exists due to:

(1) sensitive microstructure of the metal,
(2) high levels of hydrogen, and/or
(3) high stresses.

Construction codes that have to be followed define PWHT with regards to materials, wall thicknesses, strengths, alloy content, etc.

Dejan IVANOVIC
Process Engineer, MSChE

 

[TD2K]

I've commonly seen Refineries call out PWHT for piping in specific services:

- caustic
- amine
- HC containing sufficient H2S to qualify as sour under NACE MR0175. Some for sour HC check weld hardness and only PWHT if the hardness exceeds a certain level.

I have a feeling I'm missing one other common fluid but I'll need to check that at work.

(edit) sour water is another stream that is commonly called out for PWHT in my experience. Some companies do, some companies don't.

 

[SJones]

It shouldn't be called out by the process engineer, it should be called out by the materials and corrosion engineer. You can look through API RP 571 and NACE SP0472 to get an idea of where PWHT might be used to minimise the likelihood of any pertinent damage mechanisms occurring (assuming that downstream/upstream oil & gas is your interest).

Steve Jones
Corrosion Management Consultant

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

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

 

[marty007]

Another chemical that I see this in is chlorine production. I can't remember, but it may come out of the chlorine institute pamphlets.

 

[Chemitofreak]

Thanks to one and all.

@ TD2K,

Can you please specify the criteria for selecting PWHT for caustic and Amine serivces, or is it a case that all system containing caustic and amine are to be considered for PWHT.

To All,

I have came across a FEED process datasheet in which the requirement of PWHT due to process reasons needs to be specified and that is why I have raised this issue.

Dinesh S SHELATKAR.
Process Engineer

 

[EmmanuelTop]

As Steve Jones said, going through API RP 571 is a good start. There is a chapter related to Caustic and Amine service corrosion. Also look at

http://www.amine-gas-treatment.com/dbimg/1142527275.pdf

Dejan IVANOVIC
Process Engineer, MSChE