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 >Dear All, > >A convection heater runs in on/off mode (1hr on/4 hr shut). >Gas temperature at bridge wall is 1350F. Unfortunately, >during the runs the recirculation damper had broken and very >little flow passed thru (this is can be shown from the >chamber temprature profile) and at this time the temperature >controller did not alarm because it has not been set up for >K type thermocouple. (originally J type mode). Eventually, >chamber temparature got higher and damaged the first row P11 >finned pipe and the tubesheet. Tubes are connecting to a >header also. And there are 3 tubesheets, centered and end >tubesheets. Normally coil design for 1550F max. But with >above senario the tubes at first row snapped. The employees >blame the design and complains about tubesheets design and >expansion values wrong. Others think about this is happened >becuase of damper (data history shows it.) So did anyone >comeacross this kind of problem? Becuase of high temperature >right around (1203 thru 1582 max tmt and 1203 thru1780 fin >tip max metal temp.) cause to intergranular corrosion and >lets holes on the pipe and the nitrogen during the cool down >period gets in the chamber and thermally damaged the >tubes??? and SS 304 tubesheet good enaough for >thisconditions? >Thanks for input in advance.