Query on boiler design - Thick or thin wrappers?

Hi,

I have to build a boiler for my 3 ½” 2-6-4T Jubilee. It’s a Martin Evans design. I have the boiler plan, but live in Australia and so have to modify it so that it’s compliant with the Australia boiler code. I have no issue doing this, but have a few options and so was after peoples thoughts on teh best way to go. I am designing it for 90psi operating pressure.

The inner and out wrappers are 3/32” on teh original design. Now I can go with either 2.5mm or 2mm, but the stay spacing will alter.

Here’s my choices:

2.5mm wrappers

24.5mm stay spacing, so less stays

Stays need to be 5mm diameter

2mm wrappers

19.5mm stay spacing (so more stays)

Stays can be thinner at 4mm diameter

Now with the thinner wrappers I get 2mm more width in the firebox. It doesn't sound much, but this increases the grate area by 5%.

I was thinking although there’s a few more stays with the thinner 2mm wrappers the extra grate area is a good thing. So I was thinking being 3 ½" and already having a narrow firebox it was worth going with the thinner wrappers.

So is a little bit more work and few extra stay worth the effort to get the slightly wider firebox?

Superheaters

The original design also has three superheater tubes that are ¼” OD. Are three tubes really needed on a loco this size? Two tubes would make it simpler in the smokebox.

What do others think?

Greg

Things that are never included in the design and should be , in my opinion.

1. A heat sink , to bolt the superheater ends to on the top of the firebox wrapper ( fire side).

2. A pad to take the firebox hinge , silver-soldered to the backhead.

3. Be very sure of your regulator design so that you can incorporate the sealing best suited to the regulator

in the end tubeplate.

4. Double check the position of the gauge glass bushes ( for obvious reasons).

5. Double check the position of those Jubilee boiler blow down valves ! NOT easy to get to !

Apologies if teaching granny, etc.

Thanks.

Got most of that covered.

1. The superheater tubes won't extend into the fire box. If they stop at the end of the tubes I can use copper tube with a bronzed end. If I extend into the firebox it'd have to be stainless steel which is a bit more involved for me.

2. Yes will do this. I have had a loco with things screwed to the backhead and apart from not liking it the fittings also leaked.

3. Still thinking about the smokebox end of the reg fittings. The code requires a bushing in the smokebox plate and being a bepaire the regulator has a dog leg and so can't be just screwed in from the firebox end.

4. I've repositioned the gauge glass mounts to ensure the bottom of the glass is 10% above the crown.

5. Blow downs - Still thinking about these. They won't on the side as per the original design. The code requires the bushings to have a minimum seat thickness of 2.5mm. So if I had a bush on each side it would reduce the firebox by 5mm. No something that I want to do. So the blowdown will be squeezed in the front between the axle or at the back. Haven't yet decided. It won't be ideal which ever way I go.

Edited By Greg H on 06/09/2016 12:41:49

Hi Greg,

Your queries are very specific to the Australian Boiler Code.

I suggest you show the Jubilee drawings to your club boiler inspector and follow whatever advice he provides.

If it were me, I would use 2.5mm for the outer wrapper and inner wrapper to the firebox. I would use a much closer stay pitch than the Aussie code, which will of course necessitate quite a few more stays.

I really need to dig out the drawings for the boiler then PM you. I would probably re-arrange the tube layout, and set the smokebox tubeplate further back, and would definitely fit silver solvered phos bronze bushes for all fittings and attachments

It is quite an early Martin Evans design.

I am surprised you havent posted same on the modeleng.proboards.com forum.

Cheers,

Julian

Edited By julian atkins on 06/09/2016 22:36:17

Hi Greg,

I assume you are building a copper boiler. What minimum thickness does the Aust. code specify. (I only have the code for steel boilers). Best to check with the boiler inspector who is going to certify your boiler.

3/32" = 2.38mm which is closer to 2.5mm than 2.0mm, therefore I would go for the slightly thicker material. The life of the boiler will be greater if there is a bit more material to corrode away.

Paul

Hi Julian,

My questions are not specific to the boiler code. It's a general query on design principles for good performance, not compliance with a standard. What I was hoping for is some thoughts on whether a bit more effort to get a slightly wider firebox is worth the effort. I realise most in the UK probably just copy the original design and so not a lot of thought may go into making changes to improve a design.

I have no issue designing a boiler to the code. A club boiler inspector has signed off on my design. This design was with 2.5mm wrappers, but something has got me thinking about the size of the firebox and whether a slightly wider one was worth the effort..

Your thoughts on the tube layout and possible improvements would be appreciated. I have just replicated Martin's tube arrangement for the boiler with three super heaters tubes, not the later version with the siphon and one tube.

The bushes will be phos. bronze. So your thoughts are the thicker 2.5mm wrappers.

Hi Paul,

The code allows copper wrappers down to a thickness of 1.6mm. Obviously the stay spacing is closer. It would be 15.5mm. I thought 1.6mm was getting a bit thin and the closer spacing makes it a bit hard. So I thought this was a step too far.

So two for a thicker wrapper. I thought a wider fire box may have been favoured.

It is such a minimal difference I doubt you would see a change in performance. It is more when people want to make a steel boiler for an originally copper design that the often double thickness of material will eat into water space and/or grate area.

I have noticed that the Australian code does allow for what would seem less stays than the traditional UK designs certainly some of the steel traction engine ones have very few stays.

I made a mistake. The inner wrappers on the original design were only 1/16", not 3/32". So if I go with the 2.5mm wrappers my firebox will be 2mm thinner than the original design.

Martin Evans did a revised design. The only real difference was the inclusion of a siphon. There was also only one super heater tube instead of three. I suspect this design probably done soon after the original design in 1959 was more about putting a siphon in a 3 1/2" loco than a need to redesign the boiler.

Edited By Greg H on 07/09/2016 10:00:26