We have an air cooler condenser in a Vacuum Hydrobon process unit. Operating temperature is 260%C2ºC. Operating pressure is 55bar G. Hydrogen partial pressure is 670 PSI. Susceptible corrosion mechanisms are HIC, SSC, SCC, HCL and HTHA. The project specified for the air cooler tubes & headers, 1 %C2%BD Cr %C2%BD Mo steel with NACE requirements according MR 0175 & MR 0103 standards. Because of difficulties in performing PWHT requirements for strength welds of 1 %C2%BD Cr %C2%BD Mo steel tubes into the tube sheets headers, we are suspicious about the materials project choice. What proven steel materials in the above conditions would be an alternative to 1 %C2%BD Cr %C2%BD Mo steel? An answer from the forum would be appreciated. Best Regards Luis Marques

Luis In my opinion, your Project department is correct to choose NACE MR 0175 guideline for material selection of !.25 Cr/0.5Mo usage. PWHT T11 weld joints (1.25 Cr / 0.5 Mo) will offer better resistance for this operating temp (260 deg cent) and for this Hydrogen Partial Pressure 670 PSIG . PWHT of strength weld should not be a problem to do and it is needed to reduce chances of HIC / SSC in this service usage after frist two years of operation. From my view, plain carbon steel and non-stress relieved T 11 tubes will develop SSC or HIC at this service duty after the first two years. If attaining higher temp of PWHT (720 + or minus 10 deg cent) is a problem to achieve in the field, in tube sheet to tube sheet welds - due to heat dissipation problem, you could think of lower temperature of 600-640 deg cent and hold for longer time. Any way PWHT is needed for T 11 to P 11 tube sheet joint to tube sheet plate. I am sure you would recognize the code practice : For every 50 deg cent drop other than 720 deg cent PWHT for T 11 material, you have to increase holding time in PWHT correspondingly as per ASME/TEMA code practices. Carbon steel may give problem later at this temp either due to HIC or SSC. Hydrogen Partial pressure given by you (670 PSIG) is a border line case for Carbon Steel for this temperature from HIC ir SSC. Recognizing this long term problem, i would suggest you to go by recommendation of your Project department and NACE MR 0175 guidelines for this service duty. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 13, 2006 E-mail: nishi@vsnl.com > >We have an air cooler condenser in a Vacuum Hydrobon process >unit. Operating temperature is 260%C2ºC. Operating pressure is >55bar G. Hydrogen partial pressure is 670 PSI. Susceptible >corrosion mechanisms are HIC, SSC, SCC, HCL and HTHA. The >project specified for the air cooler tubes & headers, 1 %C2%BD >Cr %C2%BD Mo steel with NACE requirements according MR 0175 & MR >0103 standards. > >Because of difficulties in performing PWHT requirements for >strength welds of 1 %C2%BD Cr %C2%BD Mo steel tubes into the tube >sheets headers, we are suspicious about the materials >project choice. > >What proven steel materials in the above conditions would be >an alternative to 1 %C2%BD Cr %C2%BD Mo steel? > >An answer from the forum would be appreciated. > >Best Regards > >Luis Marques

Luis API 941 Nelson Curve limit is a border line in your case - for both carbon or carbon -moly steel.. When in border line limit of either temperature or partial pressure consideration, you have to automatically choose only the next higher alternative low alloy material to tackle likely HIC problem after a couple of years. Your Project department had done this. It is not a conservative upgrading ( it appears to me). Also, it is not an imponderable task to do PWHT of an air cooler tube sheet to tube strength weld joints even if the tube sheet thickness is higher. Your operating pressure is not that high (around 55 KSIC) to call for a heavy wall thickness tube sheet. You should be able to do good PWHT at a lower temperature with the Project department recommended material of T 11 for tubes and P 11 for tube sheet. To my mind, there is no need to think of a different material other than T 11 and P 11. You do not have to think of an exotic material and the related problem associated with exotic material grades from high fabrication and procurement cost and the associated problem of dissimilar weld problem with use of exotic material. Site strength weld between T 11 and P 11 tube to tubesheet strength weld and some notional fear, from your side, for a successful PWHT of the same should not deter you to think of an exotic material for this medium pressure and medium temperature use in hydrogen partial pressure service. I am sure that in your country there should be experienced fabricators who could do successful site strength welding with T 11 and P11 material and also successful site PWHT of the same to code needs or at a lower temperature if heat dissipation is a problem in doing the same. For this purpose, because of notional fear of carrying out a PWHT for low alloy steel, it appears to me, change of material to an exotic grade is not warranted. P 11 is not as dangerous compared to P 22 or P 5 or P9 grades for this site PWHT. Only in P5/P9 grades some possibility of some residual left over stresses is likely if heat distribution is not uniform. There is no technical contradiction - in my opinion, for your Project department to suggest T 11 and P11 for this service. Successful PWHT can be done even for heavy wall thickness tube sheet of over 200 mm . For your service pressure and temperature, maximum wall thickness of tubesheet may come around 75 mm which is not difficult to PWHT in my opinion. Trust this is of help to you to go ahead with T11 and P11 choices C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 14,2006 E-mail: nishi@vsnl.com >Thank you both Mjcronin/C.V.Srinivasan. > >From the operating 260%C2°C (500%C2°F) and 671-psi Hydrogen >partial pressure, the most current issue of API 941 Nelson >Curves Figure 1 (6th edition, 2004) we are very close but >still below the carbon steel curve. The point is within >20%C2°F (7%C2°C) of the bottom of the curve. Note that C-%C2%BDMo is >not included in that curve. API 941 specifically calls >C-%C2%BDMo steels out in an appendix to the main document. And >in that document states "...most companies no longer specify >C-%C2%BDMo steel for new or replacement equipment used for >operation above the carbon steel curve in Figure 1 because >of the uncertainties regarding its performance after >prolonged use%E2%80%9D. So the project is a little bit >conservative but correct when specifying 1 %C2%BD Cr %C2%BD Mo for >our service. Our problem is with technical difficulties in >performing PWHT requirements for strength welds of tubes >into the tube sheet headers, to meet MR0175 AND MR0103 >requirements > >To PWHT (720 + or minus 10 deg cent) is a problem to achieve >in tube sheet to tube sheet welds - due to heat dissipation, >As C.V Srinivasan says PWHT at lower temperature of 600-640 >deg cent and hold for longer time can be the option. >Ideally for our purposes we would like to have a material, >which doesn%E2%80™t need to be PWHT and at the same time meets >MR0175 and MR0103 requirements, I suppose we are begging the >earth and the even, I don%E2%80™t know if this technical >contradiction is possible. > >Once again thank you very much for your empathy > >Best regards > >Luis Marques