HMEM Lifetime Supporter
- Dec 29, 2012
- Reaction score
- Indianapolis, Indiana
I run this boiler at our club each month (Sydney society of Model Engineers) I will miss this month as I have family things to attend. We have a stationary steam display with about 20 models which are supplied steam from a boiler we have as a permeant thing in our shed and is piped to a stand with the models on display for the public to watch. I set my models up with the others.
One thing I just remembered the friend who gave me the drawings did say that the original design was a 5 inch boiler which he resized to 6 inch as he had that size material and I do not know if any recalculating was ever done would this change things much. I will check with him to see if he has to original drawing before he changed the dimensions. Also I just checked my fire tube dimensions they are .373" OD with .031" (.8mm) wall thickness.
I agree with your comment regards keeping the brain active I am 71 now and still enjoy going into the workshop each day.
Did you ask the mesh diameter of the burner i am using, it measures about 3.75" diameter.
There are two major things you need to check when a boiler is returned to operation after an inspection or put in service for the first time. Did the engineer or designer do a good job in the design calculations and was the boiler properly built or was something missed in the repair. Each boiler is certified for a design pressure and operated at something less then that. (usually 75% of design). There are several tools used to inspect boilers but the ultimate test is the hydro. Always with water never air. The hydro test will identify leaks and inadequate design as the parts will fail under load. The test pressure is usually established by code and might be as high as 150% of the design number. But at the very least it should be tested a pressure slightly above your intended operation pressure. Model builders do not normally have access to the tools necessary but in general their boilers are often operated at low pressures. A hydro is a good simple method of establishing a first criteria that the boiler is able to hold pressure at ambient temperature. Things change when heat is applied and its possible to have a good hydro only to have the boiler fail due to damage from thermal expansion. So often in these types of conversations the testing of the safety valves is ignored. On large equipment they are required to be pulled and inspected separately. I also know that models will be built and operated by many different skill levels. Not so with larger units. They not only need a boiler certification but a authorized certification to do the work. So about the only advice we can offer to a model builder is make sure your design is well thought out. Test it if you can and if you are going to operate it around others you might need a certificate from a jurisdictional authority to do so.Hi John. Good to hear about you club's activity. Please post a Web address?
My club have 2 shows per year - outside, and always planned well in advance for cold wet days in June and September. Amazing planning required! I sit in a small gazebo tent with 3 tables of club models. I run about 4 members' models on air around 20psi, and one steam engine on a small boiler at around 20 psi. A guy with infernal combustion models usually demonstrates 3 or 4. The club track runs steam and electric rides every weekend in summer. A good crowd of lads!
I'll explain my concerns about firebox wall thicknesses.
I first met this issue when I had sized tubes and made a boiler for use at 40 psi NWP. Then decided with recycling the calculations that I could reset it for 80psi NWP. But at around 130 to 140 psi during an hydraulic test (target 160psi) a tube collapsed. Although the tube had been sized to all the boiler calculations (hoop stress etc.) there were NO standard calcs for tubes with external pressure.... and flue tubes and firebox tubes are tubes stressed with external pressure.
People who do these calculations have some vicious factors for de-valuing the compressive strength of tubes dues to any shape deformations, or stress raisers, that distort the uniformity of stresses on the tube and cause zones that instigate collapse.
Your firebox tube - 5", 10 gauge - has a stress raiser where the fire door connection is soldered. The extra stiffness here creates stress raisers that de-rate the overall pressure of collapse. But I can't find any calculations for that. So all I can assume is that it is a "perfect" tube.
Notwithstanding, the "perfect tube" - with a stress limit of 3000 psi (ASME Limit) - seems good for 53psi NWP boiler pressure....
Now, I know the calculations use 3000psi stress limit because of the degradation in strength of copper at elevated temperature, and allowing an adequate safety factor to account for any imperfections. These imperfections can be as little as 1mm ovality of the tube when in the working condition. I.e. something small enough you won't necessarily notice! The fire hole could do this. I don't know, to be able to advise.
What I do know, but based on my imperfect calculations, is that
1. There are lots of designs that use the same or slightly thinner walled firebox tubes as you have.
2. The factors of safety are there to accommodate variables that may or may not affect an individual boiler.
3. A modern boiler design - for a locomotive - commercially designed, so all the calculations will be professionally certified - has a firebox of 4 inch diameter and wall thickness around 6mm.... !!
So in conclusion (and having been a professional Engineer) if I identify something that suggests an increased risk, I must inform the user of reasonable and sensible action to take.
Hence I feel responsible in suggesting the boiler should have "a professional" decide if the firebox tube is adequately safe for the Regulations, for the NWP you have. In most clubs I think there is a professional charged with boiler certification, so I suggest you discuss it with him? Possibly the club insurance covers you if you steam the boiler at your displays? Mine does, providing the club's Engineer has issued or witnessed the current boiler certificate.
The only "moral" issue I have - please forgive me for fussing about this - is that there is a huge difference of risk between a factor of safety of 8 (design requirement) and the proven factor of safety of 1.5 or 2 - that is the proof test by hydraulic test.
The hydraulic test is not intended to supercede the design FOS.
Recently I saw a project where someone proved their design for a 100 psi NWP boiler with an hydraulic test over 800psi. But Regulations only need the boiler made to that design to be hydraulically tested to 200psi...
Now a simple solution to stiffen and strengthen tubes subjected to external pressure, is a series of strengthening rings fitted internally. I would need to do some calculations before I could suggest sizes.
Meanwhile, to answer your question about a design scaled-up from 4" to 5" for the firebox tube: I'll have to check, but it may be that the 5" tube can only take 16/25th of the pressure of the 4" tube before collapse. That is 64% of the pressure. So a 4" firebox tube for 95 psi NWP would only be good for about 60psi NWP at 5" diameter.
Just think of a plastic Coke bottle. Or a beer tin. Or aerosol can... It will take 90 psi internal pressure... but hardly any external pressure without collapsing. Put some boiling water into one, fit the top and watch a few psi of atmospheric pressure collapse the bottle when the steam condenses! Have you flown? And seen a half-full water bottle collapse when the cabin pressure drops just 3 or 4 psi?
I hope this helps? - and doesn't worry you!
Incidentally, if there are Engineers on the thread that can correct anything I have stated, I would appreciate your help? I have made mistakes before, and don't want to wrongly advise anyone!