Shashank: If your metal temperatures are 850C I would think that you might be coking on the tube side. Also, at 850C, the strength of P5 and P9 will be low. However, at 850C, either oxidation or sulfidation could be the damage mechanism, depending on if you are burning with excess air or not. Perhaps if you were to analyze some OD corrosion products. Hope this helps! David Hendrix The Hendrix Group Inc.

Shashank For your design temperature : 435 and 498 o C for Inlet and Outlet temperatures - both P 5 and P 9 are OK. P 5 and P 9 are a known material usage for this type of heater service in refineries in many parts of the world ( > half a century). Davind Henderix and Luis have probably given their views - based on your hypotheitcal guess on 850 o C - for the 1.5 mm thinning seen in the heater tubes. How did you guess this temperature can be seen in P 5 or P 9 material for this service usage? This is a presumptuous view as P 5 or P 9 are not designed for operation at temperature of 850 o C. At the theoretical temperature, given by you, both P 5 and P9 will fail by creep or stress rupture at short time operational run. Heaters in such service fail by thinnning - ie.. horizontal flat tubes - at the convection zone- by condensation and flow pattern change. If the convection zone tubes are finned chances of external sulphur deposit during shut down accelerates the corrosion in the fins and also on the outer of the tubes - both straight ones and in elbows / bends. If the thinning is at the radiant zone, i.e, at the first few rows from bottom burner, please look at possibility of burner nozzle and flame impingement to reduce wall thickness. You will observe bulging of tubes. If you suspect thinning on the outer ends at the bottom row or middle row tubes at the radiant zone, then take a caliper measuremnt on all directions to measure OD and see if the likely impingement is restricted to one side of the coil. Sulphur vapor condensation in fired heaters and external corrosion is a known cause or limitation to P 5 or P 9 in this service for tube wall thinning from external pitting and corrosion. For internal, please also look at possibility of steam injection and start and shut down is causing tube thinning in all rows of tubes. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 12, 2005 E-mail: nishi@vsnl.com >Shashank >At a first sight, without knowing details of your unit, it >seems that you are having sulphurization corrosion caused by >oxidizing reactions trough S02 from your burners. You have >to be careful because if the thinning of your outlet end >tubes is great they can fail by fluency and suddenly you can >have a fire or even an explosion. As David Hendrix said if >your metal temperatures are 850C the strength of P9 material >is working in the creep range. Another possibility for your >thinning is carburisation because at that temperature you >might have coke inside your tubes, your material is loosing >carbon which is migrating to internal and external surfaces >of your tube section and is becoming thinner and thinner. >How is your scale? Is it brittle carbon? If so the >likelihood for carburisation is great. > >Regards > >Luis Marques