[shahrulizwan abdul hamid]

Hi All,

The project that i am working on currently is applying hot dip galvanized pipe and fittings for 2″ and below for Instrument Air service, whereby size 3″ and above is applying normal Carbon Steel for such service.

Based on the Branch Table in the Piping Material Specs for this line class, the type of branch for 2″ (galvanized) , branching from 3″ (CS) main line is reinforce type.
FYI, the 3″ bare CS pipe will not be galvanized.
The Material Engineer has been requested to look into the Branch Table once again due to this issue.

Based on your experience, is it normal practice to weld a galvanized pipe to CS pipe? I think it can be hazardous due to the zinc.
If it has been done before (welding galvanized pipe to CS pipe), what are the conditions or precautions to be met?

regards,
AjakPiper

[Anton Dooley]

The fitter may need to grind the zinc coating off the galvanized 2″ pipe prior to welding, and touch up with galvanized paint after.

Also, welding to galvanized pipe can produce toxic fumes – this should be recognized as a safety concern.

Could a flanged connection of the main header be considered?

[Mick McCafferty]

I find it strange that 3″ and above is not galvanized as my understanding that galvanization is for cleanliness in IA Service. Typically 2″ and below would be galvanized threaded pipe and fittings and above above that welded with the welded portions made into flanged spools then hot dip galvanize the spool.
Grinding the zinc coating back far enough to allow safely welding would only work for the outside surface and it is the inside which is intent for galvanizing.

[Mark Layport]

There are a number of issues with this topic. First as Anton has point out welding on galvanized pipe DOES produce toxic flumes and will make the welder very ill for about a 24 hr.s period …this doesn’t happen with one whiff but after 15 to 30 minutes of continuous welding, especially in enclosed areas. IF you ever suffer this poisoning drink alot of milk!
Secondly “normal” instrument air system that uses a galvanized piping spec call out 2″ and below because they are a threaded connection. Now you may be able to find 3″ or even 4″ galvanized threaded fitting, but they are considered “odd ball”, and when ANY piping component has that label you can bet that there cost with be ever high, and lead time to get them will be extended too, so the delay will add to the cost …(and I’m not even getting into the fact that at those odd ball sizes they probably don’t meet piping spec being used).
Lastly, and probably most important, connecting galvanized steel to bear carbon steel (by any means …threaded or welded) leads to galvanic action or “bimetallic corrosion”, this is where an electro-chemical process in which one metal corrodes preferentially when it is in electrical contact with another. I have seen where galvanized pipe is thread to a carbon steel valve and within 4 to 5 months corrosive pitting at the connection had caused leaks.
NOW if this increase in pipe size or where connection galvanized piping is absolutely necessary there is a way to do this, and that is with the use of bronze coupling or a dietectric union (http://www.jomarvalve.com/products/categories/dielectricunions.html) …basically this is where bronze is positioned between the galvanized spec piping and the bear carbon steel piping, the bronze stops the electro-chemical process.
This type of info is necessary to the good designer to layout their piping and to remind the Jr engineers that think they know what they are doing! *G*

[Author]

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