[Steam-engine] pressure calculations

[Jeff Detwiler]

Hello Andy and all,
First let me say I was impressed with the pictures of Wauseon on Ben's site.  I didn't know there were so many familiar folks there, all 800 miles from Pawnee... We'll have to make it next year Beth.

Second, let me answer your Working Pressure questions. You've got your formula correct, and this will calculate the safe pressure to the pound. The problem lies in the joint efficiency numbers and safety factors.

>From the 2004 Pawnee Steam School at Rushville, and Brian Vaughn of B&B Steam Restoration:

A double riveted butt and strap joint is 82% efficient.
A triple riveted butt and strap joint is 87% efficient.

A Double riveted lap garners a 70% efficiency rating, while 
A Triple riveted lap(75Case) offers 75% efficiency.

Now for safety factor.  The NIBC book on boilers as well as the FRA guide allows safety factors of as little as four, but most states have established 5 as the safety factor, no matter what type of joint you have. Some states(TN) have even enacted rules that limit all lap seam boilers to 100 pounds, even if they are like new thickness and can support a higher pressure with the calculations!

With your original thickness of .312 and factor of 5, your boiler would carry 126.6 lbs safely, thus your original book number is correct.  Return flue engines presented somewhat of a conundrum in that they were not stayed surfaces, but rather a firebox in a tube. The forces were exerted on the fireflue in the same way they they exerted on the shell, but it is a different dynamic than simply a barrel full of water and steam.

If your ultrasonic testing shows good averages throughout, you should be able to use a pressure between 100-125 safely. Just do the math, but don't use a safety factor higher than 5, or the Max drops off in a hurry.

Hope this helps, Andy.  Now a question for the masses.  Does anyone know of a good 65 Case looking for a new home?  My Keck Gonnerman afflicted friends are probably going to disown me but I need the perfect complement to a great side geared engine, and I have run several 65's and have fallen in hopeless love with a 65 with a 6 bottom plow in tow. Amy ideas?

Thanks,
Jeff Detwiler
Oklahoma Steam Threshers


--- Andy glines <pioneersop96 at yahoo.com> wrote:
at the Pawnee steam school Brian Vaughn shared a
formula used to calculate the safe working pressure of
a boiler

P= (tensile strength)(thickness)(joint
efficiency)/(shell radius)(safety factor)

tensile = 55000
thickness = .3125 (new thickness)
Joint efficiency = 82% (lap seam)
radius = 19
safety factor = 8 (lap seam) 

using this formula I get 92.7 psi.  This is on the
boiler in new condition.  The boiler on my Huber
originally carried 125 with a 135 pop.  It seams like
I need a different formula or to adjust this one. 
What do you think?

=====
Andy Glines
Evansville, IN


		
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