Scotch Type Marine Boiler

[Maryak]

Neil,

By testing to 2 x WP you still have a safety factor of 4 on the initial test and 5 1/3 on all subsequent tests. Also hydrostatic testing is carried out cold.

The calcs are good to go working pressure calcs for temperatures up to 4500 F which is a bit over 400 psi but..................thinking on the side of safety is NEVER a bad thing.

Best Regards
Bob

 

[Diversion900]

;DThanks Bob,

now I get it, the 8 X safety factor IS built into the calculations already.

So if I set myself a max working pressure of 75psi I can safely test to 150psi on a cold hydro test..... :bow:


Cheers,Neil

 

[doubletop]

Thanks for this guys we now all know where we are on this. The internal tubes calc wasn't something I wasn't familiar with as the calcs I'd done previous were based on the Sandy'C example which my boiler tester had told me used the incorrect formula. He was going to give me the correct formula but I have it now.

Neil, I would expect your next challenge will be how to heat your boiler. I'm using standard 1" tube for the main flue with cross tubes which act like stays. You'll probably need something bigger but as you can see from Bobs calcs standard 1.5" tube isn't up to 92psi.

So next question Bob. Cross tubes act like stays so does their inclusion affect the safety margin value used in the formula?

Pete

 

[doubletop]

This weekends activities. The bronze didn't turn up so I did a few of the other bits and bobs. The main mounting brackets and the boiler access door.

Fly cutting the 1.5" radius for the mounting brackets with a home made fly cutter

The finished brackets with relief cut with a T slot cutter so they'll match the similar rails I did for the engine.

Rounding off the ends of the hinges on the rotary table

All the boiler door parts ready for fitting to the end plate

Loosely fitted together to give an idea how it will all look.

Just a bit of finishing required on all these parts. The current V3 drawings doesn't have the door details yet. I need to amend V4 as what I drew and what I built had some subtle differences. I may re-post V4 later in the week.

Pete

 

[Diversion900]

Lookin' good Pete

 

[doubletop]

Thanks Neil

Hopefully I can get the bushes made next weekend and then the soldering underway. That's when things went down hill a bit last time but I'm hoping for better results now that I have had more practice. The whole point being the more I do the better I should get. To quote Bogs strap line "If you don't try it, you will never know if you can do it."

Pete

 

[Maryak]

So next question Bob. Cross tubes act like stays so does their inclusion affect the safety margin value used in the formula?

Pete

Personally, I would use the shell calc for the cross tubes because the pressure is from the inside to the outside. I agree with you that they also act like stays for the main furnace/flue tube, so I don't believe that the safety factor is affected downwards. Using the standard formulae will, I believe, give adequate safety, acceptable to our codes here in Oz.

Hope this helps

Best Regards
Bob

 

[doubletop]

Bob

I understand your reply and re-read my question. I was suggesting that with the inclusion of cross tubes acting as stays to the larger burner tube it would allow a reduction of the safety margin used in the calculation (a value greater than 893) and hence it can be used at a higher pressure?

That then leads to another question. There seems to be no consideration to the length of the tube under pressure. I would have thought that 2 meters of tube under compression would be more susceptible to collapse than say 2cm. Or lets say a tube that has a length less than its diameter is less susceptible to collapse than a tube with a length of many multiples of its diameter?

On another subject bronze for the bushes arrived today. That took a week to come by NZ post from Auckland. A book I ordered from Amazon UK to 2 days to get to NZ.


Pete

 

[Diversion900]

;D :big:Sounds like your NZ "snail mail" is very similar to ours here in Oz Pete.


Cheers, Neil

 

[kvom]

An earlier post asked while a folded flue should be smaller on each fold.

As the flue gas travels it gives up heat. Thus there is less energy to transmit the further along it goes. A smaller tube will extract a proportionally small amount of the reduced available heat energy and thus be able to reach a higher temperature.

 

[Maryak]

I understand your reply and re-read my question. I was suggesting that with the inclusion of cross tubes acting as stays to the larger burner tube it would allow a reduction of the safety margin used in the calculation (a value greater than 893) and hence it can be used at a higher pressure?

To be honest I have not been asked this before and my pressure vessel certification to date has been for 12" : 1ft vessels. IMHO you are correct. This now comes down to an agreement between you and your local testing authority.

That then leads to another question. There seems to be no consideration to the length of the tube under pressure. I would have thought that 2 meters of tube under compression would be more susceptible to collapse than say 2cm. Or lets say a tube that has a length less than its diameter is less susceptible to collapse than a tube with a length of many multiples of its diameter?

To the best of my knowledge stress is not a function of length. The only reference, with respect to your question, I can find, is in the use of stays where the length and type of the stay has, in most instances, a length limit as a function of diameter, e.g. a 1/4" stay would have a maximum length of 7 1/2"

Finally a full size Marine return fire tube, (Scotch), boiler has a diameter greater than its' length of the order of 16:9, (Just like a new TV :).

I hope this helps.

Best Regards
Bob

 

[Diversion900]

So what you're saying Bob is that the proportions that Pete is using here, being 3"X5", and my plan to use 4"X6" is in fact A$$ about 'cos we are long and skinny instead of short 'n fat. :big:

Cheers, Neil

 

[Maryak]

So what you're saying Bob is that the proportions that Pete is using here, being 3"X5", and my plan to use 4"X6" is in fact A$$ about 'cos we are long and skinny instead of short 'n fat. :big:

Cheers, Neil

I guess I am but as long as it's safe what does it matter. ;D

About a year ago, I priced some 6" and 8" dia new copper pipe. Minimum length was 3 ft. they came out at $800 and $1000 each

so what is on hand is a great option yes ???

Best Regards
Bob

 

[Diversion900]

I can sympathize with that.......

I started out looking for 6" diam. but ran into similar brick walls (like prices and quantity). Trying to get someone to sell you a 6" length of 6 to 8" pipe is near impossible around my neck of the woods.

My local scrap yard came to the rescue with a 12" piece of 4" and I was stoked.....

Hence the project begins. :big:


Cheers, Neil

 

[doubletop]

About a year ago, I priced some 6" and 8" dia new copper pipe. Minimum length was 3 ft. they came out at $800 and $1000 each

Best Regards
Bob

Yes been there; for the 3" tube they wanted $280 for a meter. Scrap yard sold me some at $11/kilo (it varies). From the supplier catalogue 80mm = 3.12kg/mtr, 100mm= 4.69kg/mtr and 150mm = 7.86kg/mtr

So 3" tube it is and the aspect ratio has to go out the window. However, the KN Harris book has a picture of long and skinny version and that's what kicked me off. I can now build a long and skinny boat rather than a short fat one.

Neil so you are on your way then......???

Pete

 

[Diversion900]

Still have to get the PB, and some 1.5 and 1" tube.

The figures are pointing at these sizes, with the 1.5" tube requiring a reduction in max working pressure to 73psi, which is close enough for me.

I just have to put it all on paper to see how many small tubes I can fit on the third flu set.

This one is definately not a project to rush into

Cheers, Neil

 

[Blogwitch]

Hey Lads,

What you are forgetting is what you are going to be running with these small boilers.

You are not powering the Titanic, but a small steam engine, most probably for display or in say a model boat. From my experiences in model boats that are steam powered, you will most probably be running somewhere between 15 & 25 psi, so a boiler with a working pressure of 50 psi should be more than ample. Small engines you will find, don't like high pressures when running on steam, they run in a totally different way than when run on air.

By going down on your pressure requirements should relieve a lot of the worry over your material selection.

I found that staying on the lower pressure scales (I had mine with usually a max working pressure of 45 psi), I could use generally available plumbing sizes of copper tubing for making boilers for myself and friends, and the scrap yard got a fair amount of the 75mm & 100mm diameters in as offcuts, because most public buildings use copper instead of plastic piping for drainage because of the longevity of use required.

If you still want to go the higher pressure route, maybe a simplified yarrow boiler, with their smaller diameters of main boiler tubes (3 off) would put it within your price range. I have never built one, but I have seen model ones, and they can reach high pressures very quickly and still maintain that pressure easily.

http://en.wikipedia.org/wiki/User:Andy_Dingley/My_created_pages/Yarrow_boiler


Bogs

 

[doubletop]

John

I hope all is well?

I've had your sage like advice in mind during this build. This boiler is for the French marine engine I have just completed and 50psi has always been the max intended. As you'll recall from the video it runs really well on 20psi and even starts and reverses under no load at 5 psi (or maybe less). And I have the fixed pressure reducer already in place on the steam line of the engine.

The discussion with Bob and Neil was about establishing the suitability of Neils 4" pipe for a similar boiler.Somehow that discussion ended up at 92psi. but was interesting and informative.

I did think of doing a Yarrow at one point, as KN Harris has one in his book. However, I came across a small book in the archive of the public library circa 1949 'Flash Steam" its about flash boilers for model speed boats from around the 30's. It has some intriguing designs including one in particular made from a continuous length of 1/4" copper tube wrapped in multiple figure 8's much like a coil of rope wound on 2 pegs. They have the disadvantage of needing a continuous feed engine driven water pump but an interesting challenge. Maybe something for the future.

Regards

Pete

 

[Diversion900]

Thanks for the advice John, but my theory is that just because you have the capability of high pressure, doesn't mean you have to use it.

I feel a V8 ticking over at low revs doing the job easily is better than a 4 Cylindar running flat out to do the same job.

I have made a few small pot boilers before, and this one is going to be more a challenge and workhorse for maybe multiple engines at the same time.

I also have on my "to do" list, a larger working engine that may require this type of pressure.

I just liked he layout of this style of boiler and thought I would give it a go, its just coincidance that Pete and I seem to be on a similar build in the same timeframe.

Keep the advice coming guys, every little bit helps.

Cheers, Neil

 

[Blogwitch]

Pete and Neil,

I wasn't trying to say you were doing anything wrong at all, I was just giving a little insight that very heavy gauges in copper are not always required if you really want to make a small boiler. More so in the UK, where we have the 3bar/litre rule, where if your boiler is within it, very little rules and regulations apply to it's build and material thicknesses. More of a common sense build, rather than a 'done to the rules and regulations' one.

As I said, I have seen model 'Yarrow' boilers in operation, and I was astounded at the speed they got up to their high working pressure. From cold, just a couple of minutes.

Flash steam is having a bit of a resurrection here in the UK, and some are using electronics for the control of burners and water injection.

Have a Google search for 'Model Boat Mayhem' and when you open it up, search the site for 'flash steam'. There are people doing wonderful things with it on there.


John