Mr. Kulkarrni,
If you look through the first attachment, 50% NAOH concentration and 82 Deg of design temperature would call for PWHT of the welds and pipe bends( following forming).This is an universal materials selection diagram which is illustrative in NACE- SP0403-2008 (formerly RP0403), Avoiding Caustic Stress Corrosion Cracking of Carbon Steel Refinery Equipment and Piping.
Generally maintaining a hardness lower that 200 HBW is recommended
Use of rubber lining is permitted subject to the fact that all only specific types of rubber are recommended of 50% NaOH caustic solution(see the 2nd attachment). The rubber lining has to be leak tight and there should be a good inspection technique to ensure the integrity of the lining is good after years of service. If the lining is not good and the C.S welds are not heat treated , there's every possibility that welds may get attacked.
Hence I would think PWHT or No PWHT should be a judgment call of the owner/client. After all the cost for PWHT will be lot cheaper than the cost of procuring the new equipment.
One argument could that operating temperature is 35 Deg C, i.e C.S (without PWHT) is still safe to use. However ultimately the design parameters are essential to adopt or waive PWHT for the welds and bends.
Thanks
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of manish kulkarni
Sent: Sunday, April 21, 2013 6:26 AM
To: materials-welding@googlegroups.com
Subject: [MW:17493] PWHT for CS weld in NaoH service
| Dear Experts, Any idea or ref. for using CS without PWHT of piping welds with NaoH service? The process fluid is almost 50% NaoH and design temp. of 82 Deg.C , operating temp. @35 Deg.C. Now the issue is by using rubber lining on ID side of piping, still PWHT is required for CS welds? Any literature available for NaoH service and CS materials? Regards, Manish Kulkarni , |
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