PWHT 2

[farzam]

Hi All,

My question is related to our Deaerator vessel(ID=3800mm,TL-TL=15500mm, thk=12mm, Temp.=119c, P=1.9barg)
Based on our process data sheet,PWHT shall be applied for vessel, but our subcontractor does not want to consider PWHT and explains that if they perform PWHT the vessel will be defomed because its thin shell plate.please let me know your idea about this matter.its possible to perform PWHT for this vessel?

Thank you

 

[metengr]

Most of the deaerator vessels I have seen are constructed of carbon steel, and I would assume your vessel is as well, based on your stated design conditions. I also believe your PWHT requirement is an excellent idea even though the wall thickness for PWHT of carbon steel falls below most construction code requirements. PWHT will reduce susceptibility to either corrosion fatigue or SCC in service.

Yes, PWHT of the vessel can be performed in sections or stages of the vessel with resistance heating elements. The entire vessel would need internal bracing as temporary support if you were using a large furnace.

 

[farzam]

Thank you metengr
I'm fully satisfied by your explanation

 

[codeeng]

I'm not aware of any process reason to PWHT a deaerator. We typically do not apply PWHT when its not required for process or Code reasons.

 

[unclesyd]

Code or no Code I think a little CMA is in order as pertaining to the Best Available Engineering Practice.

I don’t know the status of the recommendations that came out in the 80's concerning the construction, inspection, and repair of deareators after several fatal incidents involving same. I do know that when we replaced a unit in the mid 90's it did have a PWHT. There must have been some pretty sound reasoning as I recall there was no screaming and hollering about carrying out the PWHT.

There was quite a lot of information published about the failures.

http://www.nationalboard.org/NationalBoard/Articles/Classics/classic6.aspx

Here is a little information from the web.

http://www.altairequipment.com/technical_files/DA_hei_vs_altair.pdf

 

[NickelMet]

There is a NACE document Recommended Practice 0590 (Recommended Practice for Prevention, Detection,
and Correction of Deaerator Cracking) that may be of use to you. The deaerators in my plant are all PWHT.

~NiM

 

[metengr]

One additional comment on this subject, weld repairs that are required for in-service deaerators (aka DC heaters) should be subjected to local PWHT. Again, current construction codes do not require PWHT because the shell thickness is well below the requirement for PWHT.

We have included this recommendation in the NBIC Part RB because of reported cases of environmentally assisted cracking (EAC) after weld repairs to these vessels located at or below the waterline.