PWHT - Caustic service

I hope some may be knowing the company named Denora SpA which was once a world leader in Caustic plants and had developed new electrolyser technologies which is used to produce NaOH from high conc. saline water. As per thier experience, PP piping, PP valves and PP coated pump casings and GREpoxy resin based equipments and tanks were used for NaOH conc. is greater than 20% irrespective of temp. For conc. more than 90%, especially in caustic flaking plants, Complete system is made of proprietary Nickel alloys especially of Outokumpu of Finland.

Sriram of the group may contribute more who has extensive knowhow in this area of caustic service.

With regards,
Kannan

"Bathula Raghuram \(Mumbai - PIPING\)" <R.Bathula@ticb.com>

At temperatures above ambient, corrosion rates of carbon steel become greater and is accompanied by a risk of caustic stress corrosion cracking (CSCC). Low concentrations of caustic can be safely handled by carbon steel up to 180^oF/82^oC, where CSCC starts to become a risk factor, while the safe upper limit for a 50% solution is approximately 150^oF/65^oC, although cracking has occurred at temperatures as low as 120^oF/48^oC. The Caustic Soda Service Graph (attached pic) is a widely used guide for determining safe operating temperatures with respect CSCC of carbon steels at various concentrations of caustic.

Conclusion: PWHT generally recommended by process designer for CS used caustic service, mainly based on two parameters, concentration of the solution and operating temperature.

Darji Nilesh (Mumbai - Machinery)

In Process data sheet, Process has asked to perform PWHT for Centrifugal pump Material due to caustic service.
Pump casing material is A 216 Gr WCB.
Can any one clarify why It is required !! any specific reason !!

Regards,
Nilesh.