Why can you not use High pressure compression fittings with steam

Been doing some research for components for use with steam and it seems that you are unable to use high pressure compression fittings with steam as they are not rated for use above 65 deg C but can take 200-300 PSI.
The only reason that I can think of is metal creep.

Would be great if some one could confirm reason or given reason why not.

cheers

Peter

 

Hello Peter:

High pressure fittings are the same as domestic fittings, too rigid.

A couple of hundred yards down the four foot and they will break up leaving steam leaks all over the place.

It is as simple as that really.

Regards

 

I'd suggest that the main reason you can't use them is that nobody makes them! Most modern steam pipework is in steel and, except for the smaller bores, has welded joints. From my experience, it is rare to find copper being used for such purposes outside our world. Copper tends to be used for hot and cold water systems. Steam pressures above 100psi aren't too common in industry, either, but that is not to say that they don't exist. I've come across Ermeto and similar compression fittings being used successfully on small bore steam pipework but I don't think that the manufacturers recommend it and I certainly wouldn't specify it!

 

Dezincification as below

http://www.corrosionsource.com/technicallibrary/corrdoctors/Modules/Forms/dezinc.htm

 

good info there. However, that, in itself, is not a reason for not using compression fittings, as the original enquiry was about.

 

Dezincification eventualy causes the fitting to become weak and more to the point, porous.
Porous steam fitting not a good idea.

 

Why do compression fittings have to be of brass? Dezincification has nothing to do with the fact that compression fittings aren't used, as per the original question.

 

As you seem to know the answer to the question I have removed all mine.

 

.

 

There is a tendency for some compression fittings to cut into the pipe coupled, with the vibration found under the engines operating conditions you can have pipe failures. The olive in a compression fitting needs to be harder than the tube that it is fitted to so as to let the fitting bite onto the pipe. Compression fittings are designed to fit onto tube where most steam engines use pipe there is a difference between the two. Most railways are conservative over pipe fittings especially if going mainline where things have to be done to a certain standard and style. If you are preserving an engine you are also preserving the way things were made and departures from this are also not always welcome.
Keith