×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Tank floor scan for SS 316 Chemical storage tank

Tank floor scan for SS 316 Chemical storage tank

Tank floor scan for SS 316 Chemical storage tank

(OP)
Hi anyone,

Tank has built to API 650 and will be inspecting in accordance with API 653. Tank material is Stainless steel 316. Tank has installed at Jetty in marine environment.

Now, i looking for tank floor scan to detect underside pitting corrosion, Anyone experienced with the inspection technique for SS tank floor, please advise me.


Thank for any help.

RE: Tank floor scan for SS 316 Chemical storage tank

Scanning with a Phased Array transducer, set up to sweep from +15° to -15° from vertical with a small sector of that same 'ducer making a straight-beam scan to record the nominal thickness surrounding the pits and other defects located by the sweeping beam. This will be slow and expensive.

Scanning using a recording UT 'box' and a 'paintbrush' transducer. Paintbrush = 'ducer with a large number of tiny elements arreinged in a row, to give several inches of coverage in each swipe.

Why do you suspect pitting of 316 s/s? The normal failure mode is Chloride Stress-Corrosion Cracking - SCC.

RE: Tank floor scan for SS 316 Chemical storage tank

(OP)
Actually both pitting and CSCC are suspected.
Pitting corrosion -We found some SS 316 piping in marine environment had noted pitting corrosion approx 1-2 deep beneath surface accumulation.
How about CSCC? Can you recommend the appropriate NDE or Advance NDE to detect this problem?

As found on 316 piping
http://files.engineering.com/getfile.aspx?folder=b...


When removed to bare metal surface

http://files.engineering.com/getfile.aspx?folder=c...

RE: Tank floor scan for SS 316 Chemical storage tank

CSCC is a 'standard' problem for the Nuke Power industry - they call it IGSCC [intergranular stress corrosion cracking]. The techniques developed are essentially a) 45° small UT transducer* with a highly trained and experienced operator
b) WSY 'ducer that generates 'creeping waves' that follow the ID [far side] of the material for 1-2" and find breaks in the surface, i.e. surface-breaking cracks; requires a specially trained operator.
c) Phased Array using an angled sweep that covers 45° to 70°.

My preference would be the PAUT*, and I'd start at the weld seams - CSCC starts where the stress is highest, typically at weld seams, and typically causes transverse cracking.

*at 45° the tip of the crack gives a unique signal "crack tip diffraction", and that can be used to give an accurate depth [size] of that crack. The "Cracks are Bad, we gotta fix them" philosophy is not a good choice if the cracks are inaccessible. Sometimes 'watchful waiting' is the best method. You only have a problem when a minute crack achieves some significant, >20% depth. On the other hand, if that surface is accessible, using Dye Penetrant - PT, locate all these tiny cracks and buff them out with a sanding disk, verifying the full removal by reapplying the PT developer.

RE: Tank floor scan for SS 316 Chemical storage tank

(OP)
Thank you so much Duwe6 for the huge information.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources