RTJ Grooves corroding in wet CO2 environment

[GL431]

The service is wet CO2 containing gas on oil field. Because of the pressure class, RTJs are chosen ahead of the compressor scrubbers as per piping code. Piping is SS and scrubber itself is CS. The ring gasket is SS. (Sequence: piping SS, Gasket ring SS, spacer, SS, gasket ring SS, vessel flange CS) Corrosion has been found in the troughs of the RTJ on the vessel flange causing the joints to leak. Two postulates are out there: wet CO2 condensing in the crevises and causing the corrosion in the CS troughs or galvanic corrosion between the SS piping and gasket and the CS steel causing accelerated corrosion in the trough on the vessel flange.

Has anybody else experienced this problem with RTJs on oil fields? Would this be a reason to override the B31.3 requirement for RTJ for the pressure class and specify flat gaskets instead? This would eliminate the crevice corrosion possibility and would also insulate the joint against galvanic corrosion.

On another vessel, we have found that placing a CS spacer between the SS piping and the CS vessel (sequence: piping SS, gasket ring SS, spacer CS, gasket ring SS, vessel flange CS) causes the CS spacer to preferentially corrode instead of the vessel flange. This is cheaper (obviates the reworking of the vessel flange) and is somehow predictable (one looks out for leaks at the piping side of the spectacle blind). Is this a good solution?

It has also been suggested to use CS gasket rings instead of SS ones. I do not favour this alternative, because I fear that the CS gasket could fail catastrophically because of both postulated corrosion mechanisms.

I would appreciate your views.

 

[unclesyd]

You can get RTJ gaskets plated but in your case I don't think it will help much.

You might want to look at this gasket as it seals inside the RTJ groove. Pikotek has applied for full approval from API for this gasket.

http://www.pikotek.com/Products/Brochures/VCS Flange Gasket Brochure.htm

Pikotek now has backup from Enpro, Garlock gaskets, so the engineering assistance should be there in spades.

 

[GL431]

unclesyd, do you know what "applied for full approval from API" entails? Does it mean it will still take some time before it can be used on the oil fields as a standard feature? When the above-mentioned problem arose first, the Pikotek gasket was discussed as a possible solution. I forgot about it when I wrote. (Thanks for reminding me.)

I think this should be a common problem and I wonder whether there are also other solutions other than a different, non-standard gasket technology? On the plants I know, this would mean a large scale gasket change-out and long trial periods.

 

[unclesyd]

My information is a a little dated in respect to the API approval as I was made aware of it early last year. I haven't mined the Pikotek site but from the amount of gaskets being used by different companies I would think that specific application testing has been completed.

I would contact Pikotek directly with any concerns. There have probably been some progress that I'm not aware of as Enpro saw fit purchase the company.

 

[SJones]

Why is there stainless steel piping and a carbon steel vessel? Did somebody get the materials selection wrong? Weld overlay of the flanges and the inlet nozzles might not be a bad thing.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[EdStainless]

I like the use of a sacrificial spool, but weld overlay on that flange would help improve the life and lower the risk of failure.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[GL431]

Thank you all for your input. I have not posed the question why SS piping feeds into the CS scrubber to the design staff yet. It could be that there was a supply limit (scrubber supplied by compressor manufacturer and rest of plant by EPC) and that somebody overlooked the dissimilar metal apsect. But there are many spec breaks like this in a plant, no? I see a lot of this: e.g. transition from wet gas section (SS) of the plant to the dry gas section (CS); transition from feedgas (CS) to wet gas (SS); Glycol contactor is a CS shell (with SS insert) and all the surrounding material is SS. The transition is always at a vessel flange. Perhaps there are other conditions at those points and we have not seen the problem surfacing there yet.

Ed, at one of the vessels a CS spectacle blind is in fact the sacrificial equipment (at the glycol contacter I have mentioned above). This blind has leaked through the groove before and it is being watched. If an SS weld overlay is put on at the flange, will the problem not be re-located to another point in the scrubber? Or can dissimiliar metal corrosion be solved that easily?

 

[bmoorthy]

You may overcome this by overlaying on all the flanges.