lcq3365ck If the UT attenuation study had graded the tubes - to 60 dB - then you do not have to unduly worry about the performance of 35Ni25CrNb tubes after 13 years. Hydrogen reformer tubes in a refinery generally operates at a much lower temperature of 50 degree cent less compared to a fertilizer plant steam-naphtha reformer furnace or in a methanol plant tubes. Second 35 Ni25Cr Nb can stand a higher temperature of operation at close to 950 deg cent. UT attenuation pattern - if the study done by the inspection company is correct - and they have proerply graded / assessed the tubes to Grade A / B with < 60 DB- this assessment clearly shows that your tubes had been operated well within design conditions for the last 13 years. OD increase is not a determining factor for retiring tubes. In OD measurement there could be errors unless you have monitored the same location for the last 13 years to make a total comparison over the years. This OD measurement practice was in vogue, in some older design plants - when UT and other NDT tests were not regularly practised. UT attenuation study and RT confirmatory and in-situ metallography will give both qualitative and quantitative accurate assessment than OD measurements. Technically, you can get, if you wish , to consider to increase the frequency of UT attenuation more frequent when opportunites arise to grade the more severely affected tubes for future planned and selective replacement. This will assure or give more confidence to you and the inspection agency for recommending selective replacement. Your purpose of doing - NDT assessment viz., UT attenuation is to evaluate the critical and selective tubes which would have seen higher temperature of operation (in other words higher creep fissures in 10-13 years) and then plan for selective replacement. Unless you are planning fora major furnace modification in the near future with other associated capital expenditure, from energy savings and increase in throughput in refinery, considering that the tubes had not exhibited severe grading of creep fissures (even after 13 years in operation), i would guess complete replacement of all tubes may not arise. Hope this helps you for future monitoring and future replacement planning. C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 23, 2007 E-mail: nishi@vsnl.com >In our Refienry we have hydrogen reformer which is about 13 >years old. Even after13 years operation when we recently did >ultrasonic attanuation test, the tubes have been graded in A >and B grade only. The attenuation in all the tubes is less >than 60 dB. >These are 35Ni25Cr niobium. >The OD increase of tubes is maximum upto 1.5 % >My question is with this data which shows that tubes are at >present in satisfactory condition. should we plan for >replacement as already 13 years have passed or should we >wait for some negative indication in NDT, before we plan for >repalcement. > And what is the repalcement criteria for OD increase, >beyond which the tubes need to be replaced. Literature is >varying from 2 to 4% increase in dia. >I understand some fertiliser plants are repalcing the tubes >straight away after 10 to 12 years without waiting for any >NDT. >thanking You