Boilers

Good afternoon gentlemen,

I'm currently having a culling session in my garage and have come across a simple steam oscillating engine as a kit of parts and also a piece of copper tube roughly 50 mm dia and i'm wondering if somebody can tell me if there is a minimum thickness for the tube thickness for making a simple boiler to produce 75 psi max.

Thank you,

Martin P

Edited By martin perman on 22/04/2021 17:57:54

Martin, do you need 75PSI for a simple oscillator?

Chris Gunn

Martin, just had a look at the Polly vertical, 1 3/4" OD copper is used, they do not give a thickness, the ends are 16 or 18G, so I would suggest the tube should be the same for a low pressure boiler, i have mad a few from 2" x 16G in the Polly style, as said 30psi will be plenty to run a smal engine.

Chris Gunn

Gentlemen,

Thank you, I got the pressure from a book called Ellie the steam tram which uses an oscillating engine and the safety valve is set to blow at 75 psi so I used that as my starting point, I know mamod stuff runs on approx 30 psi.

Martin P

According to the magazines LBSC's Tich boiler is 18g with 16g tubeplates and 13g backhead

75 psi is about 60 psi more than you will need, assuming its not doing much work. At high psi the steam pressure will cause the cylinder to lift off its pad thus defeating the point of using a high psi. I dont use 18g on any of our boilers as its not strong enough to use on an end plate which has a water sight gauge on it - the pressure will distort the plate and the gauge will break. 16g will do for your size of boiler. You could use 18g safely on the main tube but its so easily dented that its not worth it for us. Hope this helps.

The book mentions 75 psi because that is what the WeeBee safeties we buy from the US are set for. They are excellent. If you build the Ellie boiler with solid 3 mm end plates and the single central stay, you can't go wrong. The 2" tube I used in my G1 boilers is quite thin walled, maybe even 20 G. Interestingly, once my Ellie gets pressure up, as soon as it starts to run on the track, the gauge shows close to zero. It seems to use the steam as it is made. As has been stated above, a freely running oscillator needs very little pressure to run.

The boiler design in the book has a 1/4" stay passing through both ends and the end plates are 2mm thick, it doesnt give the gauge of the tube though.

Martin P

Edited By martin perman on 22/04/2021 22:28:54

Page 34 in the Notes: 18 gauge or 1.2 mm.

Burst pressure of even the least spec 2 " copper pipe is well over 1000psi , the ends will come off well before

the pipe bursts.

+1 This table at Engineering Toolbox gives working and burst pressures for various diameters and types of copper pipe.

Although the safe pressures reduce with increasing temperature, for 2" pipe it suggests:

Annealed working pressure 309psi.

Drawn Working     607psi

Annealed burst  pressure   1910psi,

Drawn burst     3365psi

The working pressure of the weakest 2" copper pipe is 4 times greater than 75psi, and the same pipe has a safety factor of about 25. Drawn copper pipe can take twice the pressure. The pipe is unlikely to fail!

Dave

Edited By SillyOldDuffer on 23/04/2021 12:23:22

Gentlemen,

just picked up this thread after another session in my garage/workshop, thank you for your time, from what I read my piece of copper tube is adequate for the job.

Martin P