Refinery Vacuum Transfer Line Corrosion

[NARL]

While steaming out equipment during a maintenance shut down, we recently discovered a leak in our crude unit vacuum section transfer line. The line, which runs from the vacuum heater to the vacuum tower, is a 36” clad pipe. Materials for the base material is 1/2” thick ASTM A515 Grade 55 carbon steel, and the cladding is 1/8” thick ASTM A240 TP405 stainless steel.

The leak occurred at a mitred elbows, slightly downstream of the mitre weld. Upon thorough inspection it was found that there was only one leak in the line, at the location of the mitre weld, but there was also severe corrosion/erosion of the cladding at the transition where the pipe connects to the tower (this transition consists of an enlargement which goes from 36” to 48” prior to connecting to the 48” tower nozzle). There was also some minor thinning of the cladding at other points downstream of welds in straight runs of pipe.

Has anyone had similar experiences, if so, has the cause been confirmed and what are the inspection techniques being used to find this type of localized corrosion/erosion? Any thoughts or discussion would be appreciated.

 

[mcguire]

NARL
I see that there are no responses to your question, so I guess no one else has direct experience in this either. I have one comment, however. Why would one go to the expenses of cladding the 1/2 inch carbon steel with stainless and then using the least corrosion resistant grade of stainless available? 405, at 11% chrome, is barely stainless. You could get an order of magnitude improvement for very little more money.

 

[Knorris]

Narl,
I have seen this type of corrosion at the weld seams and other localized corrosion before. If you are going to clad again, use a nickel base alloy, not a 400 series SS. If you need to repair, and wish to use a different method and material. Please contact me. We have a field service that can overlay your problem areas with Hastelloy, Inconel, Monel just to name afew.

Ken Norris
National Thermospray Inc.
281-373-0609
knorris@hvof.com

http://www.hvof.com

 

[telme]

NARL
Refer ANSI/ASME B31.3 Refinery Piping code,para 306.3-Limitations of Mitre Bends, 306.3.2 "A mitre bend which makes a change in direction at a single joint angle greater 45 degree shall be used only for Category D fluid service." Category D fluid service is basically nonflammable,nontoxic or noncritical fluid.So firstly use of mitre in critical service like heater transferline is not advisable.secondly use of 405 cladding is again a mistake. I don't know what is the sulphur content in the stream or is there is any presence of Napthanic acid but in any case 405 does not give much money's worth.Selection of Austenitic Stainless steel would have been better.(316type)Also if you are repairing, try to eliminate the mitres and put smooth long radius bends as that will reduce the velocity and vaporisation problems- the main contributor to your present problems.

 

[NARL]

Thanks for the comments, I agree with the point regarding the use of the 405SS, it is a relatively low grade of stainless, however the line has a good service record. It was originally installed in the early 1970s, the plant was shut down for a number of years soon after, but has been in service for about 18 years now. Aside from this leak the line is still in good condition, thus I think for our application we have received a reasonable service from this material.

The average TAN for the crude we run is 0.10 and the Sulphur content 2.4 Wght %, not a particularly nasty material. Some conversations I have had lean toward localized cavitation at the point of flow disturbance (i.e. the welds which were not ground flush) any similar experiences?

 

[rustbuster]

Narl,

To inspect for this phenomena I suggest pulse echo UT (temperature dependent) and TOFD (Time of flight diffraction) UT for the weldments. Both methods should work well at >300F

 

[rustbuster]

Narl,

Sorry, got cut off on the previous note. Temperature <300 F.

Contact RTD

http://www.rtdquality.com

for information on the techniques