From my point of view for the intermittent working condition of your heater, P11 is not a good material which doesn%E2%80™t withstand sudden forced rapid cooling. P11 when subjected to rapid cooling tends to form martensitic structures which are very brittle. Also at 843%C2ºC your temperatures SS 304 tube sheets and tubes are intermittently subjected and stressed to very different thermal expansion. SS 304 in your working conditions tends to form Sigma phase during the heating and cooling. Sigma phase is a non-magnetic intermetallic phase composed mainly of iron and chromium which forms in ferritic and austenitic stainless steels during exposure at 560%C2º-980%C2ºC (1,050%C2º-1,800%C2ºF). It causes loss of ductility, toughness and is generally strain intolerant at temperatures under 120%C2º-150%C2ºC (250%C2º-300%C2ºF) which in your case is your cyclic cooling and heating conditions. It is possible that your damper failure has also contributed to the damaged. My advice is that you should discuss with the designer of your heater in the sense of reviewing tube and tube sheet materials for on/off mode runs (1hr/4hr shut). Good luck Luis Marques

Paul / Luis Luis's observations on P 11 behaviour when subjected to rapid cyclic heating and cooling may be relevant to some extent. P 11 has an inherent characteristics of "temper embrittlement" in cyclic heating and cooling. If that is taken care, by careful slow heating after a shut down cracking problem may not persist . Besides this, P 11 is not meant for a continuous service duty of > 450 deg cent. From the description of what was explained in Paul's question, the coil was subjected to higher temperature excursions - which may be detrimental to P11's performance. SS 304 was also subjected to a sudden cyclic temperature excursion of 800+ deg centigrade. Even SS 304 tube sheet cannot stand this high temperature excursion / variations even for a short duration. To me, it appears the basic problem lies in damper failure. With damper failure chances of some run away temperature excursion in both P 11 tube, P 11 header and in SS 304 tube sheet are always possible. For damper to fail, Paul's operation personnel should look into any operational miss for the heater coil failure. For possible run away heating and cooling, like what was described, Paul may have to look from design improvements with material improvement combined - if needed. Problem is : compounded by operational failure combined with material failure. Both needs improvement - from my view of this problem Trust this will help to solve C.V.Srinivasan Nishi Engineers Pvt Ltd India July 20,2007 E-mail: nishi@vsnl.com >From my point of view for the intermittent working condition >of your heater, P11 is not a good material which doesn%E2%80™t >withstand sudden forced rapid cooling. P11 when subjected to >rapid cooling tends to form martensitic structures which are >very brittle. Also at 843%C2ºC your temperatures SS 304 tube >sheets and tubes are intermittently subjected and stressed >to very different thermal expansion. SS 304 in your working >conditions tends to form Sigma phase during the heating and >cooling. Sigma phase is a non-magnetic intermetallic phase >composed mainly of iron and chromium which forms in ferritic >and austenitic stainless steels during exposure at >560%C2º-980%C2ºC (1,050%C2º-1,800%C2ºF). It causes loss of >ductility, toughness and is generally strain intolerant at >temperatures under 120%C2º-150%C2ºC (250%C2º-300%C2ºF) which in your >case is your cyclic cooling and heating conditions. It is >possible that your damper failure has also contributed to >the damaged. My advice is that you should discuss with the >designer of your heater in the sense of reviewing tube and >tube sheet materials for on/off mode runs (1hr/4hr shut). > >Good luck > >Luis Marques