Gas burner for pot boiler

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Re: Gas burner for pot boiler

Post by GTB » Mon Aug 01, 2022 8:10 am

jefran wrote:
Sun Jul 31, 2022 4:21 pm
it is so quiet that the ambient noise outside made it hard to tell whether it was burning, and in the first few minutes it went out a few times, but it then ran nicely for about 20 minutes, with the loco running on blocks. I had to put some more water in a couple of times but the initial tests seem promising.
Glad to hear that it worked first time. Burner design is very much a 'suck it and see' exercise, as there is no published design data around for garden scale butane burners. What little info that exists is empirical and about building burners for internally fired boilers.

There are two main reasons I use the Roundhouse type of burner. One - they run quietly, especially when turned down after working pressure is reached, so much so that I have trouble hearing them outside. Two - they are more efficient than the older slotted design.

If you don't have machine tools and skills to make burners, you can use mesh to quieten down the slotted type, but it also restricts the gas flow, which reduces heat output.

To be honest, I'd seriously consider ditching the liquid butane feed arrangement for the much simpler gas feed normally used in commercial models. Some of your lighting and running issues may be due to variation in the liquid feed rate.

Drilling all the holes in poker burners is a problem as you found. My bench drill has enough slop in the quill that it is useless for precision work. I use my small mill for most of my drilling work, as it allows for co-ordinate drilling. Next best choice of machine, in the absence of a mill, would be to use a vertical slide in a lathe.

I now use a spotting drill to put each row of hole centres in the metal, as it's hard to centre punch a long straight row of hole centres by hand. Before a friend put me on to using a spotting drill, I used to use a centre drill (slocombe drill) to mark the centres.

Drilling holes in a tube isn't easy, but by holding the tube in a milling vice, carefully setting up and using spotting drills, I've drilled the holes in seven scratchbuilt burners so far and I'm still using the same 1.4 mm drill.

Regards,
Graeme

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Re: Gas burner for pot boiler

Post by jefran » Sat Aug 06, 2022 5:03 pm

After today's experience I am certainly going to switch the gas tank arrangements; the easiest thing to do is put another bush in the top of the tank and take the gas from there. Today the burner wouldn't stay alight for more than a minute at a time and there were surges which brought orange flames above the boiler for a few seconds. It may have been a little more breezy, but the only other thing that is different from last time is that the loco had its full bodywork on, as this was supposed to be the day she pulled a train for the first time in 20 years. I can't see that the cab and tanks would affect air flow to the burner, and they don't reduce the gap that the hot gas comes out of. What is more puzzling is that although she works nicely on air and on blocks at 20-30 psi, and takes some effort to stall with a finger, she would not even move herself on the same pressure of steam. I brought her straight back inside, put her back on the blocks, connected the air and, apart from a blip of full forward gear to get the condensate out, she ran just as before. The fire certainly needs some work, but there was definite and sustained steam pressure in the boiler, so the gas supply isn't the only problem.

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Re: Gas burner for pot boiler

Post by GTB » Sun Aug 07, 2022 3:19 am

jefran wrote:
Sat Aug 06, 2022 5:03 pm
What is more puzzling is that although she works nicely on air and on blocks at 20-30 psi, and takes some effort to stall with a finger, she would not even move herself on the same pressure of steam.
Unfortunately garden scale live steamers can do that. It depends on how big the compressor is.

I use an old airbrush compressor for testing chassis on air. It is rated at 1cfm, so puts out about 28 litres/min of air. A standard Roundhouse 2" x 6" boiler with one of their burners can only generate about 12 litres/min of steam with the burner flat strap and roaring it's head off. At the normal running setting, it will be generating about 6 litres/min of steam and that's all R/H cylinders need to pull a train, if they are working properly.

Get the burner running on the bench to your satisfaction first. I aim for a steaming rate of around 5 ml/min of water being boiled at working pressure and gas usage of about 1 ml/min. Measuring the water usage is easy. If you have a digital balance (the kitchen variety usually measure to the nearest gram) you can weigh the loco before and after filling the boiler, then at the end of the run to find the water usage. Gas can be a bit trickier to weigh in the loco. I usually weigh the gas can before and after filling the tank and use all the gas during the run. Some gas is lost by venting during the fill, but the weights will be near enough in the greater scheme of things. The density of water is 1 g/ml and for liquid butane is 0.6 g/ml, so the volumes are easily calculated from the weight.

If the boiler/burner performance is up to scratch, then look at the cylinders and valve gear. If the loco has run satisfactorily in the past, then the likely cause after 20 years of storage is sticky valves, or deteriorated piston rings, allowing internal steam leaks.

I use Roundhouse burners and cylinders as the design yardstick for my scratchbuilt locos. The burner will use 1 ml of butane to boil 5 ml of water per min and that will steam a pair of 9/16" dia x 5/8" stroke cylinders. That can be scaled down for smaller cylinders and within reason, scaled up for slightly larger ones.


Regards,
Graeme

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Re: Gas burner for pot boiler

Post by LNR » Sun Aug 07, 2022 9:30 am

jefran wrote:
Sat Aug 06, 2022 5:03 pm
she works nicely on air and on blocks
I might be completely off target here, but as you have mentioned that it runs OK on blocks( presuming wheels hanging free) it couldn't be that with weight on the wheels, bearing condition or loco attitude on the rails, is not causing a drag or bind somewhere? Just a thought.
Grant.

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Re: Gas burner for pot boiler

Post by jefran » Mon Aug 08, 2022 6:06 pm

I am going to try and eliminate the weight/angle question by taking my airbrush compressor outside and seeing how she runs with a long air hose. I will time how long the compressor is on load for vs time off and try and work out how much air the loco is drawing - once I dig out the spec for the compressor. Those air/steam consumption figures are most useful, and the results I get will give some idea of how efficient my loco is vs one from Roundhouse. Meantime, I have machined a 1/4" x 40 bronze bush and silver soldered it into the tank: I pressure tested the tank when I made it, but I took it up to 160psig on water anyway. I will leave it to dry out overnight and reassemble everything in the morning to try again. The original union was silver soldered directly into the tank, so I will have to plug it: is there a better way than soldering a piece of 1/8" rod (rather than copper pipe) into a nipple and tightening it in with a union nut?

As the loco ran on blocks on steam last week, I am hoping that the problem is the intermittent state of the fire, but I will have a better idea tomorrow.

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Re: Gas burner for pot boiler

Post by GTB » Tue Aug 09, 2022 3:12 am

jefran wrote:
Mon Aug 08, 2022 6:06 pm
The original union was silver soldered directly into the tank, so I will have to plug it: is there a better way than soldering a piece of 1/8" rod (rather than copper pipe) into a nipple and tightening it in with a union nut?
I make my own pipe fittings and just turn up a blank cone when I want to block off an unused line. Soldering an offcut of brass or copper rod into the cone, or olive, might be a bit quicker I suppose.

Grant is likely thinking of wear in the chassis bearings and after 20 yrs in storage, oil could also dry out and turn gummy. It would probably be easier to support the loco upside down on the bench and run it on air, than fiddling around with an airline outside on the track.

As you did, it is always good practice to do a hydraulic test on a pressure vessel after any repairs, or modifications. It wouldn't be the first time a silver soldered joint opened up a pinhole when reheated.

My comment about compressors when testing on air was just putting numbers on why a chassis can sometimes run better on air than on steam. A workshop compressor is an unlimited supply of air for a small loco chassis, while model boilers can usually only supply just enough steam to successfully run the loco. when everything is working properly. I'm not sure if measuring air usage would tell you much, but it would be easier to do on the bench than on the track.

Graeme

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Re: Gas burner for pot boiler

Post by jefran » Tue Aug 09, 2022 6:02 pm

Progress has been affected by a family bereavement, so please don't think that I am ignoring you if I don't give much in the way of response for a few days.

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