Page 282 - Boiler_Operators_Handbook,_Second_Edition

Strength of Materials 267

You aren’t expected to design the boiler but I think you it has no trouble returning to its original shape and you
should have an understanding of how the design is de- can’t rupture it unless you’re a real blow hard or used
termined and that’s why I’ve subjected you to this math compressed air. If you suck on it the results are much
business. different. By removing the air you expose it to external
Calculating the stresses and required thickness of pressure from the atmosphere and it collapses and, it’s
a pipe or boiler shell is rather simple. Complications usually permanently deformed.
enter the equations when you have openings in the pipe The stresses that are applied to anything exposed to
or shell; for example, all the holes in a water-tube boiler external pressure produce both compressive and bend-
drum. In those cases allowance for the holes is based on ing stresses and usually the bending stresses produce
the required thickness of a cylinder without holes and the failure. Cylinders and other parts exposed to external
how much metal has to be added where the cylinder pressure and flat parts of vessels exposed to internal pres-
is complete to make up for the holes in other locations. sure are thicker than they would have to be for the same
A steam drum where the tube holes are two inches in pressure applied internally or they are made with stiffen-
diameter and installed on 4-inch centers has to be about ing rings, bars, etc., to help them resist the bending forces.
twice as thick as one without the holes. The corrugated steel furnace of a firetube boiler (called a
Normally the code doesn’t require any special Morrison tube after the man that determined it would be
consideration for an occasional opening for a connec- stronger) handles external pressure better than a simple
tion smaller than two inch nominal pipe size. Larger cylindrical furnace of the same thickness and diameter
openings may have enough extra material in the cylin- because the corrugations stiffen the cylinder.
der (because the standard steel plate thickness, greater Calculating stresses becomes a lot more complex
than what was required by the code formulas, provided when you’re making a valve, flange or other pressure
it). It may be included in the structure of the opening retaining structure. Many standard arrangements have
(like manhole rings) or a doubler (additional steel plate been developed and, in most cases, tested to failure to
surrounding the opening) that’s added to provide the determine their strength. That was a much easier propo-
required material. If you would like to know any more sition in the days before computers and all their capabil-
about boiler design and construction requirements I ities. We have standards based on a maximum operating
would suggest you take one of the courses provided by pressure. The ones you’ll normally encounter are 125,
the National Board. 150, 250, 300, 400, 600, 900, 1,500, 2,000 and 3,000.
Cylinders under internal pressure are easy to un- An important thing to know about those standards
derstand and the calculations are rather simple once you is they have secondary ratings. Perhaps you’ve seen a
get the gist of them. We have other situations that are valve with “500 WOG” cast onto it and wondered what
complex, cylinders under external pressure is one. All it’s about because the other side has “250” and you un-
the tubes of a firetube boiler and its furnace are cylinders derstand it to be a 250 psig valve, what’s that other stuff
with the pressure on the outside of the tube. I’m sure about? The 500 WOG means the valve is also rated for
you know there’s a difference in the amount of pressure operation at a maximum allowable pressure of 500 psig
a cylinder can withstand depending on whether it’s on if it’s used for water, oil, or gas at normal atmospheric
the inside or the outside. temperatures. 250 psig steam is at 400°F and the valve’s
strength is less at that temperature.
An operator argued violently with me once about
CYLINDERS UNDER EXTERNAL PRESSURE this, he was absolutely certain that I was endangering
his life by allowing a boiler feed pump to operate at a
Even as a child we knew our soda straw would 270 psig discharge pressure when it was fitted with 250
collapse if it got plugged with some ice cream in our psig valves. A copy of the valve manufacturer’s table
shake and we continued to suck on it. To clear the plug of secondary ratings (that’s what they’re called) for
we would blow on it. Sorry, for those of you that don’t or the valve didn’t convince him. You shouldn’t question
can’t remember, shakes used to be made with real milk secondary ratings (he had taken a position and didn’t
and ice cream mixed by something like a blender. want to back down from it) because any manufacturer’s
You can get the same evaluation by blowing into chart is based on standard tests and they’re all alike, the
or sucking on the top of a plastic soda or water bottle. secondary ratings are an American National Standard.
The bottle could change shape while you are blowing on The secondary ratings allow for differences in
it if it isn’t a cylinder (it becomes more cylindrical) but the maximum temperature of the system and permit