The 400C design is good and conservative. Usually these tubes are not advisable above 800F ( 427C) due to a specific gradual microstructural change management . You have been observing 500C - 550C from 2003 - till date i.e. around 26,000 hours. A Discussion : - On an oversimplification, the term Elevated Temperature (ET) of metals & alloys is usually defined to be about one half the absolute melting temperature (Tm deg. K) at and above which temperature the strength of metal is limited mainly by creep . For Carbon and Low Alloy steels ET begins approximately at 0.36 Tm deg. K ( approx. 370 deg. C) and prolonged overheating above this temperature leads to gradual creep from primary to secondary to tertiary creep and ultimately leading to ruptures.. In carbon and low alloy steels that are subjected to moderate temperature exposure for longer periods of time are often suffered from a microstructural changes called %E2%80%9CGraphitisation%E2%80%9D and when subjected to more higher temperatures for times suffered from another type of changes called %E2%80%9C Spheroidal Carbidation%E2%80%9D.. It results from instability functions and concurrent gradual decomposition of pearlite colonies into ferrite and free carbon (graphite) or spheroidal carbides respectively and embrittle steel parts. Generally, Graphitisation is the usual mode of pearlite decomposition for carbon and low alloy steel at temperature just below about 550 deg. C under prolonged exposure time of around 9,000/ 10,000 hours . Spheroidal Carbidation is the mode of pearlite decomposition at and above 580 deg. C under comparatively short term exposures . Inferences : In your case ( based on your factual inputs ) you are running in Graphitisation mode w.r.t. run lenght of 26000 hrs. Now Graphitisation as itself is not much harmful so far as on-line or shut-down faliures are concerned but if the graphitisation chain forms an eye-brow pattern under close chain links then it has got significant impact on embrittlement and failures. Also it promotes creep void as your system is operating in crep zone. Now as you have not experinced any failure till date either you continue with reduced temperature loading or in case any opportunity you take a sample and do metallography and just see the nature of graphitisation. If it is very isolated and scattered no need to worry much but if it is eye-brow linking fashion you should worry and if your Process owner can allow operating failure you RUN otherwise start inventory craetion and replace . regards and others pl. comment. Debasis Mitra Debasis.Mitra@hpl.co.in

Alani If you have been experiencing coking and your tube skin temperature has been 500-550 %C2ºC (932-1022F) from 2003- current. It is advisable to shut down the heater for decoking and check your tube condition. (Cut a section of your tube for evaluation of remaining life). A106 Gr B is not recommended to work above 400%C2ºC. If you keep on working in actual conditions you are highly reducing your tube heater remaining life, you are risking a tube rupture. For estimation of heater tubes remaining life documents such as API530, and API579 are good references. Regards Luis