Stainless steel NPT ball valve in Carbon Steel piping
Stainless steel NPT ball valve in Carbon Steel piping
(OP)
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
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





RE: Stainless steel NPT ball valve in Carbon Steel piping
Thanks1
RE: Stainless steel NPT ball valve in Carbon Steel piping
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.
RE: Stainless steel NPT ball valve in Carbon Steel piping
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.
RE: Stainless steel NPT ball valve in Carbon Steel piping
RE: Stainless steel NPT ball valve in Carbon Steel piping
RE: Stainless steel NPT ball valve in Carbon Steel piping
Make your assessment of the corrosion risk and corrosion control requirements using resources such as API RP 571 and
http://www.nickelinstitute.org/KnowledgeBase/Techn...
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.
RE: Stainless steel NPT ball valve in Carbon Steel piping