Copper boiler Bushing thread

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bobs7-62steamair

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Currently working on a 4" diameter copper boiler using the water tube design from Cli Shay book by Robert Maynard. Question is what thread should I use for outlet Bushings to connect PR valve, pressure gage, whistle, steam supply etc. The book indicates 3/8-24 thread for most outlets. One connection on top of boiler indicates 1/8NPT thread.
Is a tapered pipe thread better for sealing or is the straight thread with a suitable sealer better? Last boiler I built about 18 years ago was a steel jacket and firebox with copper flues that were expanded into the tube sheets which worked well for the locomotive it powered. That boiler was strictly NPT type threaded fittings. Application for this copper boiler is to power bench top stationary steam engines. Any wisdom/advise from experienced copper boiler builders appreciated.
Regards.
Bob
 

Entropy455

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https://www.surpluscenter.com/images/techhelp/SAE.pdf

Straight threads (aka SAE O-ring) require a positive seal (see above link). These types of fittings are widely used for hydraulics and air, but I suppose with the correct temperature-rated O-ring, it would hold back low-energy steam also. Note that the end of the tapped hole contains a chamfer which contains/crushes the O-ring. Said another way, the fitting is supposed to be tightened until the flange on the male threads bottoms out - which means the O-ring needs a cavity to reside. If that chamfer is missing or too big, the O-ring will not seal properly. When everything is set up properly, these joints will hold back 6000 psi oil without a drop.

Pipe threads are tapered (self-engaging), which means the threads themselves will seal the joint (normally). It is best to use some sort of thread sealant when making up pipe thread joints. Pipe thread joints are good if you make them up once and leave them alone. For joints that require repeated disassembly, or are subjected to high vibration, it's best to eliminate threaded joints completely (i.e. welded/brazed joints, with flanges).

The other thing to remember about pipe threads - cutting threads into a pipe reduces its safe working pressure by 2/3. This is why steam power plants predominantly use welded joints throughout. For a "low-energy" hobby model however, pipe threads will probably be ok. Just be sure to take into account a 2/3 reduction in safe working pressure of the pipe/tube.
 

Entropy455

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Also, make sure the pipe/tube is thick enough for cutting threads into it (normally schedule 40 or larger).

It is a bad idea to try and cut threads into schedule 5 or schedule 10 pipe (the pipe is left paper-thin at the minor-diameter of the threads).
 

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