Dear Members, I am an old member of this forum. 'Need some help. Recently we faced a corrosion problem which need to be explained. The Vacuum Slop line of Vac Distillation column was found corroded. The line is of P5 (5 Cr -1/2 Mo) and handles Vac Slop at approx 360 C. The crude oil in CDU keeps on changing (no high TAN crude till date) and so not much data about the crude can be given. The flow rate thru' the line is quite less. Corrosion is mainly deep, distinct pit / circular groove of about 15mm dia with 4-5 mm depth. Generally, corrosion in P5 material is very unlikely - although I have seen failure of a dia 4" line of P5 metallurgy in similar service (Slop / RCO) once - some 15 yrs back. 'Hope you could share your experience and guide me to probable cause of failure. Regards Chakra

Dear Debashis, Aqueous condensation or air ingress is ruled out. Regards Chakra >P5 material selected for high temperature operating >performance basically . At 360C no free water could be there >and primarily Slop can form sulphidic corrosion. >Pitting corrosion resistance depends on PREN and for P5 >material it is less. >But here the situation is marked Pitting. Most probably it >could be due to low flow/stagnancy where aq. condensation >and/or air ingress can be there lead to pitting propagation. >regards

Dear Krish, Thanks for your response. We have already done a Failure Analysis after getting the cut section of the affected pipe. Our view is similar to yours. Let me share our observation : 1) We are processing low TAN crude sometimes (0.5 to 0.8) 2) The corrosion is predominantly on bottom part of Horizontal section (6 O'clock) by deep grooving / channeling and few deep groove on top section, formed by coalescence of deep pits. Few isolated grooving in vertical section of pipe too. 3) The service is Vac Slop, (just below HVGO section) and may remain slow moving (flow is less). The original vac slop generation was 70 M3 / hr which reduced recently to 10 M3 after revamp. So, the line may remain partially filled sometimes. 4) Unlike CDU where corrosion is more associated with high velocity / turbulence and boiling of Nap Acid, corrosion in VDU takes place by re-condensation of Nap Acid and preferred in slow moving stream. 5) Nap Acid is generally gets concentrated in HVGO preferentailly (almost 1.5 times more than original conc. in crude) Our understanding is - Nap Acid got condensed in the Vac Slop line. Nap acid is having a density of 0.95 to 0.98 - which is higher than the vac slop and settled in the bottom section. The isolated deep grooving is due to condensation on surfaces - some of which might have been un-wetted as well. The available literature indicates that -(CH2)n group in Nap Acid plays an important role and with n=3 or more, alloy steel get easily corroded. A similar Failure Analysis was presented in API Subcommittee presentation in 2003 Spring which is also available in internet. 'Hope the learned members of this forum may comment further on this. Regards Chakra