Steam Supply Pipe corrosion

anniemqu

Installed a new Crown 125,000 BTU residential steam boiler (2 years ago). At the end of this heating system it developed a leak at a 2" union on the steam supply near boiler piping (carbon steel). Took apart union found threads on 2 year old piping corroding away. Added 8 ounces of dry measure ComStar Steam Clean each year. All suggestions for cause and correction are welcome!

Jamie Hall

The piping is carbon steel, not iron? What is the union material? First guess would be an electrolytic corrosion problem -- particularly if it is all the way around the threads, and not just on the bottom. Unprotected steel can corrode remarkably fast, unlike black iron...

EBEBRATT-Ed

If it's a black union it's made of malleable iron and the pipe would be back steel although wrought iron used to be used I don't think it is available any longer. (hopefully they didn't mix black and galvanized)

The steam side seldom corrodes and usually lasts "forever".

I suspect the union connection leaked from the start, it was probably a small leak that didn't show water and evaporated as fast as it leaked.

Just repair the connection & if you can use a 300# union they are better than the standard 150# malleable union. A "Dart" union is even better but more expensive.

bob_46

Ed, What's Dart ? We always used railroad unions (ARR) .

EBEBRATT-Ed

@bob , Dart unions have a beveled seat with no step. Also are heavier construction. You have probably seen them. There claim to fame is that "supposidly" they can be put together crooked and not leak.

So Bob, what's a railroad union?? never herd that one

http://www.anvilintl.com/products/anvil-pipe-fittings/malleable-iron-fittings/mi-unions-slass-300/832-figure-dart-union-brz-to-brz-seat-union

bob_46

Got my 50 year pin and I never heard of a dart union, go figure. A place I worked at for a number of years used refinery specs. . On low press steam we used 300 Lb malleable fittings. The unions were marked AAR for Association of American Railroads I am not sure what was different about them. Anything high press we switched to 3000Lb. forged steel fittings. Black pipe to me means steel, I never saw wrought iron pipe. I think that was pre war. I ran some ductile iron pipe building **** plants.

EBEBRATT-Ed

@bob, I think I have seen fittings marked AAR, never knew what it meant. Yes, the 300# unions are a lot better.

when I started (73') I worked for an oil company and we did some underground oil tanks. This was well before all the EPA stuff.

Some engineers used to specify "wrought iron" pipe for use underground because it was supposed to be more corrosion resistant than regular steel pipe.

I remember we had some on the pipe rack and I believe is came marked with red paint. I was told not to used it unless on a job where it was specified because it was more difficult to thread. Don't even know if it's available any more.

EBEBRATT-Ed

@bob some information on wrought iron pipe.

http://machineryequipmentonline.com/hvac-machinery/pipes-pipe-fittings-and-piping-detailswrought-iron-pipe/