Dear members, I would like to know what is the recommended time interval(no.of year or no.of hours in service) for inspecting the reformer (steam/Naptha))tube assuming that the temperature limits are within the design. What is the best tecnique to detect midwall fissure either by UT OR Electromagnetic Attenuation?.What is the recent trend in above inspection?Tube material is H39-WM(Paralloy). with advance thanks, gopalsankar

Dear sir, Reformer which i am mentioning is in operation for last 6 years.After commissioning and within 2 years we did UT attenuation survey and got 56-65 db at slected tubes(high temperature zone).Now,the subject reformer has completed about 30,000 hours in service continuously(in between 2- 3 short shutdown).Is it mandatory to carry out UT Attenuation survey again?Is it good practice to check the tubes with ET rather than UT for confirmation. regards, gopalsankar

Gopal 30,000 hr service duty for Paralloy WM material - i.e, on-set of creep fissures - you can expect but of not much relevance if the design conditions had been met. 3 shut downs in 30,000 hrs is good and nothing to worry from this account. Interval for Reformer tube scanning: Once in 2 years or during catalyst change time is ideal for the UT scan + RT of welds + confirmatory RT of tubes with mid-wall fissures + in-situ metallography. Every reformer operator should plan for this interval - immaterial - if the tubes had been operated within design or abused to keep track of progressive deterioration of mid-wall creep fissures and grade each tube for planned replacement in stages. Paralloy WM material can take technically take another 50-100 o C plus for operational temperature abuse. Nitridiing on the surface or surface oxidation is not possible in reformer tubes. If nitriding can be seen on the surface, then the tube would have seen direct flame impingement which would drastically reduce tube life . A direct flame impinged tube may fail in 15-20 minutes time also with run -away temperature increase . I had analaysed / seen reforemer tubes with close to entectoid phase morphology which failed in just 15 minuts of over-firing short-term rupture from outside. Such a thing is not possible by extrnal surface nitriding or oxidation. Nitriding, bore carburization and oxidation are generally assocaited with internal phenomenon due to steam-naphtha mixture interaction and not with external firing from burner deposits i.e., nitriding, bore carburzation and oxidation will be from inside of the tubes. If you expect Nitriding, ( if it had morphologically occured ), this means the tube had seen much higher than design temperature. You will observe an acicular nature nitriding - distinguishable by a special etchant - at the inner bore. So also bore carburization and oxidation if the reformer had been operated consistently at + 50-100o C than design temperature conditions. Nitrded tubes are hard and brittle with practically very little elongation. In fact, Nitriding effect comes at advanced stages of tertiary creep combined with iron-chrmium sigma phases . This means the tube will have severe mid-wall creep tertiary fissures and would have come to the stage of virtual retirement. This can happen if the tubes had been really hard fired at 950-1000 o C consistenly for production outages. In refinery service, this situation normally does not arise at 30,000 hours unles CPCL would have consistently fired the tubes at 930-960 o C + conditions (consistetnly). If nitriding is observed in the tube, it will not be isolated. Outlet Incoloy 800 H pigtail tube would be the first casualty and would also be equally affected. You could expect earlier failure in outlet hair pin or pigtail tubes (normally outlet hair pin tube is designed for 816 o C for Incoloy 800 H grade material). I would expect failures in pigtail tubes or in outlet headers or in sockolet welds than in reformer tubes if nitriding effect had been seen in reforerm tube. Paralloy material can stand the additional abuse of higher temperature. Nitriding effect will not be pronounced in this material. Paralloy,however, will have complications due to secondary precipitates from W & M. This has to be recognized while evaluation by in-witu metallography of both the welds and the parent Paralloy tube. Finally, reformer tube assessment should not be done only by an N.DT inspection angle - though it would give enough confirmation on mid-wall fissures. This should be done with experienced metallographyic analysis and also study to analyze the morphological transformation and the effect of this in predicting the remaining life of the tube. Most inspection companies who do a N.D.T survey and give some predictions based on the N.D.T results without understanding the significance of tertiary creep related effects and the complicated metallurgy in cast HK/HP/ HP modified / HP modified with micro-allloys. Such study can at best be useful in grading the effects of mid-wall fissures. This cannot give the real residual life for the tube. Residual life evaluation should combine the N.D.T and the metllography and the metallurgical transformation effects into consideration instead of only relying on N.D.T results. Do the inspection companies have this capability? Plant users have to apply their mind on such predictions. Trust this is of help to you. C.V.Srinivasan Nishi Engineers Pvt Ltd India Sept 14, 2005 E-mail: nishi@vsnl.com Trust this is of help to you >Dear Mr. Gopalashankar, >>Dear sir, >As your Reformer has completed around 30,000 hrs. it is >better to do the following : >(1) To evaluate creep void nucleation population by in-situ >metallography relication preferably at burner facing areas. >Record it for future comparison. >(2) If possible you can cut one bad tube and can have cross >sectional metallographic survey to assess creep void >population >(3) As you have already executed UT (automated? full body?) >scanning for mid wall cracking after commissioning now based >on the earlier result criticality you can decide for the >same on random basis order >(4) ET is principally applicable for non-ferromagnetic >material and as due to oxidation + Carburisation tube can >achieve definite degree of external + internal magnetism ET >is not really preferred. >(5) Give more stress on Visual , Effect of Nitriding on >external surface by observing high level of glossyness & >oxide layer shedding >(6) You can also do random RT on "welds" , "Bows" , >"dissimilar jts" . >(7) Guidelines of Mr. Srinivasan are of real help too. > >regards >D.Mitra >