[Steam-engine] pressure calculations

[Andy glines]

Thanks for the answers.  You folks filled in a lot of
the blanks as I knew you would.  I was trying to take
careful notes but obviously missed a few pionts. 
Maybe a couple of these formulas could be published in
the steam school book (hint hint).  So that you folks
know what I'm dealing with ... I have a 1905 Huber 16.
 Double riveted lap seam, I am assumming 55000psi
steel.  Boiler has 2 stays and 14 3" tubes running the
full length.  New thickness = 5/16"  Initial thickness
readings show full thickness until my transducer broke
(need a replacement).  I know there are some thinner
spots as there is some significant pitting on the OD
resulting from the jacket.

--- Jeff Detwiler 
<jdetwiler at detwilerandassociates.com> wrote:

> 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.  

=====
Andy Glines
Evansville, IN


		
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