Dear Luis, Thanks for the information. we have been observing the hydrogen blistering in the Overhead accumulator since long time and the equipment is manufactured quite old, when the stringent control on the impurities were not adopted. We have taken care of the high quality steel as we have recomemnded for killed carbon steel with the control on P(0.005%) and S(0.005%). In addition to that we are recommending Monel lining as we have experience of successful application of monel in the similar services. On another application Condensate bottom distillate - Kerosene Hydrotreater feed drum, we have observed similar blistering, but unfortunetely the steel quality is poor and we have observed lamination by UT scanning. The process analysis shows, S-1000 ppm, Mercaptan - 300 ppm, moiture content - 100 ppm. Under these conditions the Monel lining apperas to be suitable lining. Please let me have your valuable feed back. Shabbar Hussain Corrosion Engineer