chilled water coil: steel nipple brazed to copper coil header

[NEplantengineer]

Recently came across a situation on a hvac chilled water cooling coil where the coil connections were comprised of steel pipe nipples brazed (2 - one supply and one return) to the coil's copper vertical header pipe. Does anyone know if this is common? Seems like this would invite a galvanic corrosion problem.

Posted also in the corrosion engineer's section.

 

[Drazen]

It should not, brazing is way to joint different materials, and in HVAC installations that kind of things happen rather often.

It is allowed even in plumbing, though it is forbidden to have steel downstream of copper, but that is related to potable water pipework, not chilled water.

I would recommend that you make serious warm testing, as different expansions of copper vs. steel can make trouble if brazing is not done properly.

 

[willard3]

It's bad practice to join dissimilar metals without an electrical interruption between them.

Disconnect what you have and put in unions or fittings with dielectric isolation.

 

[Drazen]

Sorry, but it is not bad practice at all, it is common practice which is applied very often. Turning from stell pipe of larger diameters to copper pipe of smaller diameters is very common.

Dieletric uninons serves completely different purpose - to enable mandatory potential equalisation and are used for both, same-metal or different-metal pipework.

 

[ChrisConley]

What is mandatory potential equalization and how is that different from disconnecting dissimilar metals to prevent galvanic corrosion?

My understanding is that dielectric unions are used for the sole purpose of preventing galvanic corrosion.

We do allow other methods (grooved couplings are an effective way to change material type as they keep the metals apart) as it is common to change from steel to copper, but I would argue uncommon (and bad practice) to connect steel piping directly to copper piping.

 

[Drazen]

I am bit surprised to see rather large difference in opinions, so I made a little check.

Google shows, at first glance, "dielectric union" used for home installations "to prevent galvanic corrosion".

And it is first time I see it, with over 15 years of work. Galvanic corrosion is allways mentioned, but more as urban legend, while in practice I simply see that mix all the time.

Not only that reference copper manual, like the one from Yorkshire company does not make any mention of it, but copper system manufacturers offer variety of copper-to-steel fittings, and even clearly declare that copper can be used in mixed installations without limits (except in cases I mentioned in previous post).

Canadian fitter's handbook I use in some international projects shows such fitting with direct fit of metal surfaces, without non-metalic seal.

Non-native use of english terms can make some trouble though it is adopted by manufacturers, and even official standards.

Dieletric isolation fitting in my (very wide) surrounding is exclusively used to isolate indoor metallic pipework from outdoor utility lines (gas or water or public hot water), to enable potential equalisation within buildings according to electrical codes.

In industrial and commercial buildings, electric continuation of every piece of pipework is mandatory for the same purpose (as potential equalisation is applied at limited number of spots), and each flange joint with non-conductive seal has to have additional bypass to ensure contact, which is apparently in direct contradiction with this "Googled" dielectric union...

Therefore, I could hardly call bad practice something what is every day practice for decades.

 

[quark]

I am with Drazen on this one. For us, it is a standard practice and I have AHUs of 10 years old running without above mentioned problems. Earlier practice was to have stub end flanges to the coil.

 

[Drazen]

Looking at very opposite comments from some members, whose experience and knowledge I respect for long, I made some more Internet search, and it is like dicovery to me that this small detail have strongly opposing practice between many European countries and North America.

While it is almost completely ignored in Europe, there are many US and Canadian recommendations to use that dielectric piece.

It was even more interesting to find out that many maintenance and facility managers in North America clearly state their opinion against use of such joints though they seem to be officially specified often.

It is possible that there is some gap between standard announced once upon a time, and actual practice that gives very tangible feedback.

 

[ChrisConley]

I'm finding this debate highly interesting as well, because I did not know that the debate even existed.

I find myself reexamining my understanding of the use of the dielectric connection and will continue to research.

 

[willard3]

Whether difference in potential is driven by using the piping system for electrical ground (stray currents) or galvanic corrosion is irrelevant. Dissimilar metals should be electrically isolated from each other to prevent corrosion.

How well dielectric fittings work well is another question.

 

[TXiceman]

This is pretty common practice for water coils in the HVAC industry.

If a customer insist, tey can generally be drovided with a dielectric flange set.

Ken

Ken
KE5DFR