125psi Copper Boilers (was 7" Pipe)

[10 Wheeler Rob]

Bill gave the answers you really want to know, the first was thT copper prperties drop off significantly with higher pressures and the tempatures asociated with and you need to know your local laws governing operation to make sure your operating within the legal limits for liability.

Others answered that from an operational point of view most model engines in 1 1/2" scale gain very little additional preformance over 100 psi pressure.

I have two 3/4" scale locomotives, safeties set atound 85 to 90 psi range. They both run quite well between 50 psi and the safety set points.

Rob

[Andrew Pugh]

James,

Nothing "hokie" about it at all, strictly good mechanical engineering.

As Harry stated, the problem is the rapid drop off of the mechanical properties of pure copper at temperatures associated with steam temps above 100 PSI. Do some research, and find out that this is true. Thicker copper does not solve the problem, nor does adding more stays.

Think of it like a steel boiler with no water on the crown sheet. When the crown sheet gets too hot, no amount of extra staybolts will keep it from collapsing from the loss of the mechanical properties due to the higher temperature. With copper, it is just a much lower temperature the the strength drops off.

Your mention of full sized locos with copper fireboxes is not a valid comparison, as the full sized engines do not use pure copper, but a copper alloy that has better mechanical properties at the higher temperatures.

Note that heat exchangers in higher temp service are not pure copper, but of a material with higher strength at higher temps.

Regards,

Doug

EDIT: Doug points out in a later post that creep is important to consider, as it is another physical property that varies (becomes worse) with increasing temperature. I did not consider creep below, and do not wish to imply any sort of safe working pressure for a copper boiler.

Hello Doug, your post comes off like you are saying adding thickness and stays doesn't help anything. I don't think that was your intent, but I wanted to clarify some things, so the following is not directed at you, rather information for all who are interested.

Of course you can't change the physical properties of copper or any material by changing the boiler design, but you can certainly work around the high-temperature physical properties by adding stays and material thickness. Adding thickness and stays will reduce the stresses on the material.

Good mechanical engineering would be designing the boiler with the physical property relationships in mind and spec'ing material thickness and stay spacing etc. to result in an acceptable safety factor for the expected operating conditions.

http://www-ferp.ucsd.edu/LIB/PROPS/PANOS/cu.html -physical properties of pure copper
http://www.engineeringtoolbox.com/satur ... d_101.html -properties of saturated steam

Between these two sources, it appears the substantial drop in yield strength for pure copper starts at ~475K, or ~200C, or a boiler gauge pressure of ~215psi. The temperature of the fire side of the boiler sheets and tubes will of course be higher than the water side, and consideration would have to be paid to this in the design. Using copper super-heater tubes would be ill-advised.

EDIT: Doug points out in a later post that creep is important to consider, as it is another physical property that varies (becomes worse) with increasing temperature. I did not consider creep above, and do not wish to imply any sort of safe working pressure for a copper boiler.

What copper alloy is/was used in full size steam locomotive fireboxes?

[alanstepney]

All I remember about this one is that it was made from "high purity oxygen free copper".
(And the copper sheet cost a fortune!)



(It is a full size boiler having a new firebox fitted on the Severn Valley Railway. From memory, it was for a GWR Manor class loco. If so, it has a working pressure of 225psi.)

[Bill Shields]

We have pictures of the same firebox. I probably have a picture of you sticking your head in the firebox.

I too remember the man telling me 'PURE COPPER' no alloy - and worth more than my house.

[Johnny O]

Hello
I have started building a 4" o.d. scotch type marine boiler using only type K copper tubing. I do not see pressing it up over 80 psi but wanted the comfort of not blowing it up. I also fitted 4 phosphor bronze stays in across tube sheets. I might put in galloway tubes and still need a method of firing it. My safety is a pmr 75 psi if I remember correctly.I'll post a pic if I can figure out the resize problem.
Thanks
Johnny O

[gwrdriver]

Johnny,
This will not take into consideration what you will add to or take away from the basic tube, but plugged into model boiler formula (from Martin Evans, 1976) using .137" as the wall thickness, 0.80 as the temperature allowance, 8 as the safety factor, and 25Kpsi as the UTS value for copper, the Type K tube will have a maximum capacity of about 165psi.

WP = [.137" x 25000 x 0.8 x 2] ÷ [4.125 x 8] = ±165psi

If we add in an allowance of 0.80 for silver soldering the rated capacity falls to about 130psi.

WP = [.137" x 25000 x 0.8 x 0.8 x 2] ÷ [4.125 x 8] = ±130psi

Some people will say that a more true representation of the UTS for annealed copper is 14Kpsi, in which case the rating for a 4" tube would only be 75psi. Sounds way too low doesn't it (and I think it is) but that's what the formula gives.

WP = [.137" x 14000 x 0.8 x 0.8 x 2] ÷ [4.125 x 8] = ±75psi

[David Powell]

With reference to the use of copper in small boilers i have read that some movement or settling may occur while under the initial hydro test to twice working pressure, and that the movement or settling in fact hardens the copper and returns the UTS to a higher level than that of the copper made soft to allow it to be flanged or otherwise shaped, Is this true? Certainly in all the tested and used copper boilers I have ever repaired the plates have seemed to be in a fairly hard condition as far as I can tell. Regards David Powell.

[gwrdriver]

David,
I have no science to back this up but when in discussions about such things I maintain that work hardening occurs during use, for a couple of reasons. First as you say, the movement during testing, but there is also movement during each subsequent pressurization and a hardened condition increases with each pressurization. Then there is age, and over time copper will re-acquire hardness. So there we have two known effects working toward hardening.

There is then the question of whether annealing takes place during steaming. I would allow that at any elevated temperature there may possibly be a very small amount of annealing taking place in some areas, a firebox sheet for instance. However considering the threshold temperature for copper is around 760°F, steaming would result in a VERY small amount of annealing, not enough IMHO to keep pace or overtake the hardening which is going on during normal operations. The end result, after a time is a hardened state.

[David Powell]

We own and run a fine 3 1/2" gauge Britannia. It has a copper boiler professionally built some 30 yrs ago. We have been warned that, though the design makes for a safe boiler ,we must not hydraulically test it to more than 1 1/2 times Wp lest we create some permanent unwanted distortion in the combustion chamber/ firebox tubeplate area. Does anyone else have knowledge/ experience of this potential problem, or any problems or failures related to the design. Regards David Powell.

[gwrdriver]

I don't know anything specific about the 3.5"ga Brit boiler design that warrants special handling, although there may be, depending upon which design it is, AND if it was built to the full specification of the design. Beyond that, 1.5X is the (almost) universal target pressure for subsequent hydro tests on model boilers.

[Marty_Knox]

On full size steam locomotive boilers in the United States it is 1 1/4 (125%) x working pressure.

[Doug_Edwards]

James,

Nothing "hokie" about it at all, strictly good mechanical engineering.

As Harry stated, the problem is the rapid drop off of the mechanical properties of pure copper at temperatures associated with steam temps above 100 PSI. Do some research, and find out that this is true. Thicker copper does not solve the problem, nor does adding more stays.

Think of it like a steel boiler with no water on the crown sheet. When the crown sheet gets too hot, no amount of extra staybolts will keep it from collapsing from the loss of the mechanical properties due to the higher temperature. With copper, it is just a much lower temperature the the strength drops off.

Your mention of full sized locos with copper fireboxes is not a valid comparison, as the full sized engines do not use pure copper, but a copper alloy that has better mechanical properties at the higher temperatures.

Note that heat exchangers in higher temp service are not pure copper, but of a material with higher strength at higher temps.

Regards,

Doug

Hello Doug, your post comes off like you are saying adding thickness and stays doesn't help anything. I don't think that was your intent, but I wanted to clarify some things, so the following is not directed at you, rather information for all who are interested.

Andrew,

You make some good comments, but I meant exactly what I said: "Thicker copper does not solve the problem, nor does adding more stays." If it were just a tensile issue, you would be correct, but one of the mechanical properties not take in to consideration with copper is creep of the material. I am short on time currently, so have cut and pasted the following I posted from this same discussion 3.5 years ago. Do a search on creep of copper under pressure based on temperature, and I think you recognize the problem with copper boilers at temps above 100 PSI.

Regards,

Doug

"The copper alloy used in the full sized copper fire boxes had more resistance to creep at elevated temperatures above those we use at 100 Psi steam. At 100 psi with pure copper we are already on the downhill slope as far as the creep rate increasing, and ending up with a failure due to creep rupture. The increase of temps above 100 psi increases the rate of creep. Creep rupture in copper is a sudden failure at stress below the yield point after a small percent of elongation.

If you are basing your comments that we can run copper boilers above 125 Psi based on the fact that the full sized ones did, you are doing the readers a disservice. That information is incorrect. The copper we use is pure copper, and not the alloy used in the full sized ones.

The copper alloy is still being used on full sized Uk loco restorations; I did a quick check and found it mentioned on two current websites of restorations. The alloy was discovered about 1906, and was in common use in the UK by some time in the 20's. The only change I can remember hearing about was that some time in the 40's they started going to an oxygen free copper to allow renewing of fireboxes by welding. I think it was LNER that was experiencing some sort of operating condition that was giving them a short life of about 10 years. They were working on renewing/extending the life of the fireboxes by welding the wasted areas.

I took the time to write this not so much for you, but for the others who might read this and accept what you have said without knowing the difference.

The increase in the rate of creep above temps over 100 psi is why the AMBSC and the ASME restrict model copper boilers to 100 Psi."