britannia boiler air test

A passing thought regarding trying to test my locomotive (three and a half inch Britannia) with compressed air through the boiler. Then it occured to me that my compressor may not be up to it. I opened the tap and qt least a 100 pounds per square inch with no wheel movement. My compressor has a tank and although not very large thought it is not small. I have to admit there were some leaksfrom the backhead, any thoughts welcome as I dont want to waste time with something that is not going to work?

Even if the output of the compressor can't keep up with a running engine you should at least have got some movement until the pressure had dropped off significantly.

Does it have any drain cocks? opening those will show if air is at least getting to the cylinders. If so then close the cocks and rotate wheels by hand, you should at least get a bit of a kick or kick back at some point in which case it's timing that needs adjusting

Is there air coming out of teh chimney at a high rate, that may indicate the vales are not seating and air is going straight from valve chest to exhaust

I'm assuming that is the loco's gauge that is showing 100psi in the boiler

Edited By JasonB on 31/12/2022 16:58:01

Is your boiler certificated ? Air testing even at 3 1/2" could be quite dangerous and certainly not recommended. As for the motion work, 100 psi is ample to turn the wheels. What is the boiler working pressure as my 5" runs on a max of 90psi. If this is a new build, then there could be all sorts of problems including very stiff pistons / cylinders. Slight leaks from fittings would not stop the wheels from turning. Do be aware that if you assist the wheels to turn with your fingers, should they flick over with your finger in the way it will do some damage - to your fingers. OUCH !

Regarding the compressor, you will need quite a lot of air, however the wheels (once run in) should turn even at 20 psi.

Things to look at;-

Tightness of piston/bores

Water feed pump and eccentrics too tight

Regulator and passage of air to the valve chest/ piston valves

Timing of valve events

Good luck

Bob

You are quite right about fingers being damaged in loco motion when trying to turn by hand, it took me ages to clean the blood off mine.

Happy New Year

Brian B

Thanks to you both for the informative answers, My boiler is a pro build from Stuart models so should be OK, also the motion has been in action separate to the boiler and runs quite well. I did not build the chassis, but have worked on the details and boiler fittings since February, man thanks again

Where to start with this...

Has the boiler been hydraulically tested? There are good reasons for not testing it with air, and if it hasn't been tested you should not be putting 100psi into it.

100 psi is much too high a pressure for testing a small engine with no load. You will damage it. If it is a new engine and very stiff you might need 50 psi to shift it initially, but 20-30 psi should be plenty. If you have a pressure regulator on the compressor, start at the lowest pressure and increase gradually.

If it won't run at 100psi, then there is something wrong with the engine. Even if the compressor is too small to run the engine continuously, it should start and run until the pressure in the tank drops off.

Bear in mind also that the engine will need lubrication, especially the cylinders and valves. 

Edited By Redsetter on 31/12/2022 17:07:06

If you have a Stuart built boiler it will have paperwork to tell you the hydraulic test pressure, the running pressure is half that of the hydraulic pressure.

Assuming you have opened the regulator and knowing the chassis has run on air before it may just be you steam piping is blocked with solder or your backhead fittings are leaking worse than you think.

As others have said using compresed air to run an engine is not a good idea unless it has it's paperwork. Leaks on the backhead - from fittings, or the boiler ? You opened the regulator - did you move the reverser into gear ? A Stuart model boiler, I was not aware that they made loco boilers at least not recently ? Noel.

Thanks for the many replies, just to alay any fears, my boiler has been tested when first manufactured and looks and feels a very well made thing. I suspect I need to clear any leaks from the backhead before the chassis will work, which it has sans boiler

Oh dear, test with water - leak, test with air - BANG

Regards Ian.

Given the motion ran before the boiler was fitted, I think it more likely that the link between boiler and cylinders is blocked than a large quantity of air is escaping before reaching the pistons. If the boiler is holding 100psi, it's unlikely to be leaking seriously, and I'd expect a big air leak to be obviously noisy.

I'd make sure everything is open between boiler and valve-gear. Input to regulator; regulator opening and closing correctly; pipe between regulator and valve-gear clear. Superheater tubes if fitted. From my armchair I suspect a solder blob, lodged dirt/swarf/flux, or kinked pipework etc.

Recently fixed daughter's shower in her new flat. After a year, enough plumber debris had caught in the filter to derange the shower's thermostat. Cleaning the filter fixed it.

Dave

Edited By SillyOldDuffer on 01/01/2023 12:03:53