PWHT in Caustic Service

[lcmechengr]

I have a caustic system that will typically operate at 90 deg F with a max of 20% caustic solution. The piping and equipment is constructed of carbon steel and has not been post weld heat treated. I have located several references that specifies that PWHT is only necessary for CS in 20% caustic solution at temps of 140 - 160 deg F. However, I do have a potential for upset temperatures in excess of 160 deg F that could last for a few hours. Are the mechanics of caustic corrsion so that any exposure to 20% caustic solution above the treshhold temperatures will compromise the integrity of the metal? Thanks for any input.

 

[metengr]

lcmechengr;
What you reported is indeed correct for exposure of CS to caustic solutions and resultant SCC (stress corrosion cracking) along weld regions. If you anticipate over temperature excursions, I would perform local PWHT of the girth welds for added resistance to SCC. The cost of PWHT is small in comparison to leaks and repairs in service.

 

[Shmulik]

Please refer to Fig. 7 for temp. / concentration range of metallurgical requirements:

http://www.dow.com/PublishedLiterat...soda/pdfs/noreg/102-00011.pdf&fromPage=GetDoc