Acceptable formulaes for boiler design

[ejparrott]

I'm deep in to designing the steel boiler for my Manning Wardle now, in conversation with suppliers of barrel tube and flat platework. I have a copy of the Australian code here and studying it lots.

I have spec'd an Ø8.5" tube from ASTM A106 with 5/16" wall thickness - thats that out of the way. I'm looking at the flat platework now.

I'm designing the boiler for a WP of 120 PSI - I'll do the initial steam test to 100 and see how she performs, raising to 120 if I thing there's any benefit safe in the knowledge that the boiler can take it and has been hydraulic tested to account for it.

My problem, the AMBSC table for plate thickness versus stay pitch doesn't account for this pressure. It equates to 828kPa, and the code only goes to 700kPa in the predetermined stay table. I used the formula at 3.7.2.1(1) to find the square pitch and got 111.some number...lets call it 110 to be safe, based on a firebox plate thickness of 8mm, and a corrosion allowance of .75mm, and the code utilises Ø19mm stays.

Am I over spec'ing the plate thickness? If I were to use 6mm plate instead, it would bring the stay pitch down to 81, say 80.

I've not had to design a steel boiler before, so I havent the gut instinct that I would usually rely on.

Cheers

[alanstepney]

If you are using 5/16" wall tube, (and I would prefer to see it thicker), then I would make the plates 5/16" or more.

A very rough-and-ready calculation gives 2.5" stay spacing using 3/4" diam stays, and that would cope with a WP of 130 psi. and far more on the stays. (But as they waste away, that is no bad thing.)

I must emphasise that this was quick back-of-envelope, and should not be relied upon to be accurate.

(Anyone else wants to check and tell me I am wrong, please do so!)

[lennard1200]

I think the Oz. code allows for 100 1lb max. pressure & the code specs would reflect that. You may have to rethink the specs if using a higher pressure.

L

[ejparrott]

I should have added some more info too, little that it is.

The intention is to use 12mm plates for the front tubeplate and for the firebox tubeplate as per the ABMSC suggestions. On this design the barrel is welded into the throatplate unlike a Romulus boiler, and I had no intention on using anything less than the same thickness plate as the wall of the tube to aid welding. The ABMSC code doesn't seem to show to well, I was thinking of putting a 45degree weld prep on the water side and welding, and then welding from the outside too. Would this be overkill/unnecessary and should I just spec the outside weld only?

Very much appreciate your replies, and will enquire about 3/8" thick wall tube.

[3405jimmy]

If you go to this page on my website and click the email address for Andrew I am sure he will be able to offer some good advice. He designed my boiler for a similar pressure

northernsteam.com/sneak_peek.html

Jim

[alanstepney]

I've only built 1 1/2 steel boilers (the half was because a pal bought it before I had completed it), and it is many years since I did the calculations for boiler design, so please take that into account.

I would weld both sides, using a double-vee or double-bevel weld.
I would make the stays 1/2", and for that, either 2" or 2.5" spacing. (I'd need to work out which is best.)

The first steel boiler I designed and made was 10" diam, and used 1/2" wall tube and 1/2" plate.
Back then, we had to do all the test pieces for the insurance company and they inspected it at all stages.
Both my design and welding was approved, and, more importantly, the boiler and engine worked well.
But, as I said, that is long ago and it is worth getting other opinions.

[ejparrott]

Hi Alan, thanks for your input - noted your remarks about the time etc.

I've just been having a chat with a fellow boiler inspector, 7¼" Sweet William has the same size barrel as I'm proposing albeit shorter - 8½" nominal diameter (actually 219.1mm) and 5/16" wall (actually 8.18mm). On that basis I'm happy that its thick enough, but will still run pressure calculations on it later to be sure. They have a tame coded welder, so I may enquire as to whether he fancies do the welding - I will not!

The use of internal and external welds does appeal to me for fitting the barrel to throatplate, I'm, glad to have a second opinion the same.

Thanks

[ejparrott]

I've checked the maximum allowable working pressure for the tube using the formula P = s x t x c / r from here:http://www.classicsteamengineering.com/index.php?action=printpage%3btopic=250.0

I get P= (48000/4) x .322 x 1 / 4.3125

P = 3864/4.3125

P = 896 PSI

With a wastage of .75mm allowable under the ABMSC, and no mention made at all in the blue book, I'm happy that the tube is sufficient for my needs. I've never yet had a boiler come to me with barrel problems, its only ever been firebox failures.

[cwr]

I remember a link to a website in a thread on this forum that had a lot of formula for boiler design (though possibly copper boilers). Anyone recall what website site it was?

[fostergp6nhp]

www.alanstepney.info/page17.html

[ejparrott]

Having spoken to RSA tonight, I've had a tip off. Use Google to find IBR 1950 - Indian Boiler Regulations 1950, last updated 2000. Very comprehensive document, listing all that is needed, although it does take a bit of reading.