hi At tank Floating roof there is a drain to remove any rainwater at the top of the tank . I found pitting corrosion at some location in this drain. Any comments for the root cause and prevention. The drain spool is carbon steel. this drain move by the flex. joints with the floating roof. Thanks Mark

Pitting on your roof drain pot could be due to dirty accumulation such as sand dust, another reason would be initial small pin holes or dust during painting which grew up as pitting. Luis

Dear Mark, The drain you are mentioning about is the articulated roof drain, i.e, pipes connected with swivel joints. The possible reasons are depending on the location of the pitting, which u have not mentioned clearly. 1.If you have observed the pitting on the external surface of the pipe(the horizontal section),it is probably due to the water that generally settles at the bottom of the tank,i.e, below the hydrocarbon level. 2.It may be from the adherent salt deposits of the salt water by which the tank must have been hydrotested long back and not cleaned subsequently prior to commissioning. 3. If it is near the drain sump, it may be from the accumulated rain water, which got entrapped at some crevice. To prevent it, u may paint the external surface with primer which is compatible with the hydrocarbon. >hi > >At tank Floating roof there is a drain to remove any >rainwater at the top of the tank . I found pitting corrosion >at some location in this drain. Any comments for the root >cause and prevention. The drain spool is carbon steel. > >this drain move by the flex. joints with the floating roof. >Thanks >Mark