Luis 2000 ppm of H2S is a high level of contamination. How long has this been noted ? Depending on this the attack on steel will vary on the weld, HAZ and the parent metal. If by chance the steel used is A 537 Cl 1 or Japanese HT 60 grade the chances of SSC (Type 1 or Type 2) Sulphide Stress Cracking -Type 1 or Type 2 one could expect. On line monitoring of SCC due to sulphide stress cracks will be a guessing game whether you use ultrasonic flaw detection or acoustic emission because you do not know which is the weld joint or HAZ area or parent metal is affected. You will not get any useful information by on-line monitoring by any of the techniques like UT Flaw Check or Acoustic Emission for such fine tight SCC cracks which will be distributed at randon depending on the indeterminate stresses present due to welds / HAZ, parent metal mis match stresses etc. You have to internally inspect this high contaminated Propylene Sphere by using conventional N.DT methods - First do 100% of all internal welds by Fluorescent MPT using hand held A.C/ D.C yokes to ASTM E 709 - which is the best suited for detecting fine SCC in LPG/Propane or Propylene or Butane Spheres / Bullets. ASTM E 709 is the standard to adopt. FLMPT is the best suited, fastest, reliable for identifying fine surface SCC in steels used in LPG / Propylen/ Propane or Butane Spheres. No other technique will give you better information. Second do 100% UT Flaw Check of all welds - Ultrasonic Flaw Detection internally.of all welds. ASTM A 577 is the standard to adopt. This will give accurate information on sub-surface cracks not detectable by FLMPT. Third do inaccessible areas by MPT by Dye Penetrant examination. ASTM E 165 is the standard to adopt. If you get indication of surface and sub=surface SCC (which i suspect could be SSC - Sulphide Stress Cracking Type 1 or Type 2) you have to do In-situ metallographic examiantion by replica techinique to ASTM E 112. This is to determine the morphology and type of SSC. Type 1 or Type 2 can be evaluated. ASTM E 112 is the standard to adopt. Conventional NDT also calls for Hardness check of welds / HaZ and the parent metal to ASTM E 110 - if you get surface cracks or sub -surface cracks. This can be done at random. Also do a corrosion profile Ultrasonic Thickness mapping to ASTM A 435. -98 profile pattern but more large spaced than ASTM A 435 grid profile HIC effect will not be prominent in this compared to SSC Type 1 or Type 2. Before all that a a throrough visual inspection of internal and external of the sphere to ASME Sec V article 28. I think you should insist on opening the vesel for a thorough NDT examnation coupled with in-situ metallography (replica metallgoraphy) instead of going for on-line monitroing by AE or UT Flaw. Trust this helps you to solve your problem. C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 may 30 E-mail: nishi@vsnl.com >We have a propylene sphere, which is feed from a FCC gas >concentration unit and has been highly contaminated with >lets say 2000 PPM of H2S, because of incapacity of >mercaptans removal in downstream Amine treating unit. We >think that there is a great possibility to have induced >Hydrogen. First- what kind of NDT would you recommend to see >if we have HIC? Ultrasonic Backscatter? Acoustic emission? >Metallography replicas? What kind of on stream inspection >could be done? Second-If by NDT we will find HIC, before >repairing, how do we get ride of induced hydrogen. Forum >empathy on this subject would be highly appreciated. > >Thank you very much in advance Best Regards > >Luis Marques

For the forum information on this topic, at last we have the opportunity to make an intrusive inspection to the propylene sphere, which was highly contaminated with, lets say 2000 PPM of H2S, in April/May this year. The inspection is still in course and at the moment we found through MP (Magnetic particles) lots of relevant crack indications in HAZ. We will keep going on with MP inspection because the half top part welds of the sphere haven’t yet been inspected. Regards Luis Marques