Stainless steel NPT ball valve in Carbon Steel piping

[Gnoom]

Hi everybody,
I've been doing some searching in various threads of the forum, but I still remain with some doubts on this topic: I've seen various plants that use the typical NPT stainless steel ball valves in Carbon Steel piping, for example for vents and drains on cooling water piping, instrument connections compressed air and nitrogen piping, etc, but I'm not too sure if this is an often seen combination because of galvanic corrosion. Also, people seem to get vary scared by the idea of connecting a stainless steel flange to a carbon steel flange because of galvanic corrosion. There are some good commercial available solutions for this matter so this is an issue easy to solve. Here are my doubts:

First of all, does somebody have bad experiences with NPT SS valves in CS piping because of galvanic corrosion? If you basically only have good experiences, also let me know. Brass / Bronze valves should even be worse than SS valves with CS piping and even worse galvanized CS piping (a higher electrochemical voltage); if somebody has good experiences with those, also please let me know.

Second of all, is it correct to assume that when you connect a SS flange to a CS flange, you are talking about a lot of mass behind each material group and therefore it could cause way more galvanic corrosion, than what would be the case for instance for a tiny 1/2" SS ball valve on a big CS pipe? (assume for instance as a fluid / electrolyte cooling water in an open circuit with cooling water towers).
Is normally the internal corrosion critical for these cases, or can the external corrosion (outdoors, unsheltered) be just as important?
Thanks in advance for your help

 

[Gnoom]

If anybody has seen a good thread about this topic in eng-tips.com, also please let me know (the ones I found didn't quite clear my doubts)
Thanks1

 

[moltenmetal]

For any service where carbon steel is already a marginal selection from a corrosion resistance perspective, avoid the stainless steel valves.

For high ionic strength, well oxygenated untreated water, or where the water is hot and contains a lot of chloride, avoid using the stainless steel valves, and consider whether or not carbon steel pipe is the right choice too.

If the corrosion exposure is exterior only, or interior but mild (i.e. treated cooling water), I'd have no problem with the stainless valves- unless it was offshore or immersed etc. and the exterior conditions were severe such that you're relying entirely on the exterior coatings of the carbon steel for protection.

In treated water services with galvanized steel pipe and galvanized MI fittings, we prefer the stainless valves over the brass valves which are usually specified (I've seen many major oil/refinery specs which have brass/bronze valves in galv piping- I guess they tolerate the 1.5 volt corrosion battery they're generating). As you've noted, brass is more of a galvanic risk than stainless is in a galvanic couple with either zinc or steel.

As you've also noted, a small valve on a large line is OK from an anode/cathode area perspective. The same is true for a stainless flange on a carbon steel line. It's worse if the assembly is immersed. But carbon steel studs/nuts in a stainless flange have an unfavourable anode/cathode ratio if immersed.

 

[Gnoom]

thanks Moltenmetal. I did some more research on the internet as well and more or less came to a similar conclusion. For small SS valves on large lines it can be tolerated, but is not 100% recommendable. For SS to CS flange we normally apply an insulating gasket with insulating bushes and washers for the studbolts / nuts.
What still surprises me is that brass/bronze valves are commonly used with galvanized carbon steel pipe but there seems to be more fuss about SS with CS connections. It's probably mainly dry air on the inside of those galvanized pipes (for instance Instrument Air), but on the outside it's not.

 

[Duwe6]

Skip the insulating flange kits. Stainless valves work fine in c/s lines. By the time any galvanic mechanisms start to 'think about' causing problems, it will be time to repack/replace the ball due to normal wear and tear.

 

[moltenmetal]

You make a fuss over galvanic isolation at the flanges when you're joining a stainless line, pump or other large stainless component to a carbon steel line, again because of anode/cathode ratio.

 

[SJones]

You might make a fuss, but does an insulating kit actually work? Poor installation, bridging by instruments and earthing, bypassing through a conductive fluid, and so on.

Make your assessment of the corrosion risk and corrosion control requirements using resources such as API RP 571 and

http://www.nickelinstitute.org/Know...galvanic&pageIndex=0&sortBy=r&filter=keywords

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Gnoom]

ok, thank you all for your feedback, it has been helpful. Not an easy topic to resolve though for connecting for example a SS pipeline to a CS pipeline. Bridging by earthing is a factor that's hard to eliminate in plants; a lot of times eventually there will be some contact through supports, equipment or instrumentation to earth.