Lap Riveted Boiler Seam - BAD!

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

Explanation for the residual "spring" in the defective lap joint.
When the plate was rolled to make the boiler, by virtue of the process, residual internal compressive and tensile stress were installed into the boiler shell plate. As shown, by the attached calculations, the internal fibers of the shell plate had to absorb 1.178" of compression while the external fibers had to stretch 1.179" in tension. Because this is a riveted boiler it is safe to assume that it was never subjected to a thermal stress relief treatment that a welded boiler would see. Therefore, throughout its service life, the internal fibers retained the residual compressive stresses and the outer fibers retained the residual tensile stresses.
Without trying to invoke any voodoo bending moments into the equation, look at the depth of the crack of the subject photo. Does it not appear to penetrate about half way through the boiler shell plate? If the plate was rolled, the opposite and equal tensile and compressive stresses that held the boiler shell plate circular still exist. If you grind away the inside half of the plate, would not the residual tensile stresses tend to pull the plate out in a straightening manner? Similarly, if you were to grind half way through the outside of the plate, would not the residual compressive stresses tend to pull the plate inward?
Can somebody with a shop run out and roll a 1" X 3/8" piece of flat bar into a 12" to 18" radius and slice it half way from both sides with a hacksaw to see which way it bends?

Kelly Anderson · **Joined:** Tue Sep 14, 2004 7:52 am **Posts:** 2477

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

Kelly,
This is a good time to introduce the Principle of Superposition in Strength of Materials calculations.
Please notice above the residual compression and tensile stresses are on the order of 1.0% due to rolling the plate. I uploaded the specification S-1 from the 1935 ASME Section II showing the expected elongation before failure to be greater than 25% for boiler plate.
The Principle of Superposition says that if a component is loaded in several different manners, each manner may be calculated independently and algebraically added to find the true resultant stresses.
If a beam is fixed to a wall with a point load on the end and also subjected to a uniform load for its entire length, then the calculation is generated for the point load condition and the uniform load condition, added together to get the actual stresses at any point and the true maximum stress in the subject beam.
The residual compressive stress due to rolling are insignificant in the lap joint stress equation. Please review the Strength of Materials section of a Machinery's Handbook for a more detailed explanation.

Ron Travis · **Joined:** Sun May 15, 2005 2:22 pm **Posts:** 1543

So, if I understand this correctly, both the inside and the outside of the rolled plate have rolled in stress. The outside of the boiler has tensile stress and the inside has compressive stress.

Then when placed in service, the boiler pressure places the lap joint in tension, which tries to “straighten” the joint, so to speak. When that happens, it places the inside of the outside plate and the outside of the inside plate into tension, which can cause cracks to form at those locations.

However, regarding the two possible crack locations, it would seem more likely for the tensile load from steam pressure to cause the outside of the inside plate to crack because that plate surface has residual tension, whereas the inside of the outside plate has residual compression.

jim templin · **Posted:** Mon Apr 12, 2010 9:49 pm

Until we can see the otherside of the affected plate, all manner of super duper super callifragilistic algebreic stress incantations are pointless. It would also be informative to know the maker,the age, the boiler's design, the mode of manufacture, and the quality of the steel used. Is this a British built unit, American, or a colonial/Austrailian locally made boiler?

I just do not think that a lot more theory will shed any light unless and until there is more information presented on the unit itself.

Mark Jordan · **Joined:** Fri Nov 07, 2008 11:21 am **Posts:** 490

Quote:

Until we can see the otherside of the affected plate, all manner of super duper super callifragilistic algebreic stress incantations are pointless

Ditto what Jim said.....

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

Ron Travis wrote:

Then when placed in service, the boiler pressure places the lap joint in tension, which tries to “straighten” the joint, so to speak. When that happens, it places the inside of the outside plate and the outside of the inside plate into tension, which can cause cracks to form at those locations.

However, regarding the two possible crack locations, it would seem more likely for the tensile load from steam pressure to cause the outside of the inside plate to crack because that plate surface has residual tension, whereas the inside of the outside plate has residual compression.

Seem, yes. This is where magnitude of the lap joint bending stress is the determining factor. Your first paragraph was correct. The residual stresses are overwhelmed by the bending moment stresses. Stress in a plate or beam is not simple or constant. Stress is constant in a wire suspending a brick. In a plate or a beam it can change from tension to compression and then back to tension, depending on the load and supports. If you have a Machinery's Handbook, read the chapter on Strength of Materials, three or four times. It has all the answers to this situation. Stay tuned.

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

Stan Ottaway wrote:

The American Society of Mechanical Engineers, Boiler and Pressure Vessel Code is an approximately 20,000 page document that provides rules and standards for the new construction of boilers and pressure vessels.

jim templin wrote:

I just do not think that a lot more theory will shed any light

Mark Jordan wrote:

Quote:

Until we can see the otherside of the affected plate, all manner of super duper super callifragilistic algebreic stress incantations are pointless

Ditto what Jim said.....

Soooo....... 20,000 pages of "of super duper super callifragilistic algebreic stress incantations are pointless."????????
Come on Guys!!!!! After almost 100 years the ASME Code is still only the minimum standards that say "If you do this, the boiler won't spontaneously or arbitrarily blow up and kill a lot of people." I had a standard routine when I was a State Inspector. When someone came into the office to argue a 'RED TAG", if they used the line, "common sense says", I would stop them cold, pull out the Section I and point to the 15+ pages of committee member names and say "Tell me that you are smarter than all these people AND all the engineers whose names appeared on these pages since 1914." Most people just shut up and walked away.
Many people died because of lap joint failures. You can still build a lap joint dome for a locomotive with restrictions and a factor of safety of 8. Columbia Boiler Co. in PA was still building lap riveted HRTs into the late 1960's. I have inspected a few. If you do all the math (super callifragilistic algebreic stress incantations) you will find they came out of the factory with a defacto factory of safety of 8. Written into most state rules (At least in Hawaii it still is) after 20 years the pressure is manditorily reduced to 50 psi. SF=16 now.
If you would, brush up on your algebra, I promise to keep 4 semesters of calculus out of the discussion, and before it is over you will understand why the first thing the ASME did in 1914 was restrict lap joint use on power boilers.

jim templin · **Posted:** Wed Apr 14, 2010 9:48 am

I am not a code jockey, I am a practicle boiler maker and have been so all my life, and now work as an inspector. I am also an ordained minister, and view calculations in the same way I view Scripture....The Bible is not God, but an attempt to explain some finite portion of an infinate God. The calculations are not physical reality, but are imperfect, mathamatical attempts to explain physical reality. Niether is a complete picture of the phenomenea they try to explain. And the Code, just like the Bible, is quite open to misinterpretation, and can be misused to the detriment of others.

And on a personal note, IMHO it is poor practice as a regulator to engage in veiled attacks on the integrity and intelligence of others...when in a dispute over an enforcement action, I can think of no more unprofitable way to pursue resolution of a situation by opening a code book and asking a man if he thinks he's smarter than the names therein. Knowledge and wisdom are two different things.

Still waiting on the photos of the otherside.

Mark Jordan · **Joined:** Fri Nov 07, 2008 11:21 am **Posts:** 490

Matt,

I, too, have all 20,000 pages +..........

BUT, before I look at page #1, I insist on more and better photos of the chunk of metal in question. I, and I think others, are skeptical that this is a boiler joint at all.

Please provide more and better photos before I'll even consider opening my book.

Thanks,
Mark

Dave · **Posted:** Wed Apr 14, 2010 1:36 pm

Equating science with mythology or constructing a conspiracy theory might be comforting if we are reluctant to challenge our mental and emotional status quo, but it diverts us from the understanding of the actual conditions shown and explained.

If we were to seperate the two bits of metal and look at them from the adjacent surfaces, we'd see the crack going across the plate just where it goes across the plate. I'm not more than an experienced practical mechanic myself, leaving the higher math and engineering to others more gifted in those fields, but from my practical perspective it seems to me that what we are being shown is what we are looking at, and it is acting like these things act, and we would do well to conceptually deal with it from a practical perspective when we run across them in the field, and perhaps take a more careful and intensive look as a result.

Certainly our understanding and ability to explain stresses and reactions using numbers and words is as imperfect as our ability to take in and process information and communicate it - but again, from a practical perspective it has pretty well allowed us to grow a safe and mature technology that we can rely on even with our imperfect abilities, and we avoid doing so at our own risk.

So, leaving all the static behind, what can we do to most reliably prove or disprove the integrity of lap seams in the field?

dave

jim templin · **Posted:** Wed Apr 14, 2010 2:33 pm

What conspiracy theories are you speaking of, Dave? All I am stating is the fact that calculations are not "science" but are imperfect attempts to explain the real science. One of the best determiners of lap seam intgrity is knowing its age, who made it, and a good visual supplemented by certain other nde prodedures. Look at Bro Jordans post about grizzely bears. That sums it up for me. (Letting the mythology knock pass)

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

jim templin wrote:

Still waiting on the photos of the otherside.

I too am patiently waiting, but since the guy with the part is the problem solver for most of the chemical and oil refineries online between New Zealand, Australia, Indonesia and India, I'm not going to pressure him. He said it was from a tractor boiler. Believe it!

Dave · **Posted:** Wed Apr 14, 2010 6:28 pm

I wasn't suggesting you in the conspiracy thing, Jim.

Bear in mind a lot of mythology was considered science at one time, such as the geocentric universe thing that got Galileo into such hot water when he dared disprove it. Mythology isn't an insult - I'm fond of it myself, enjoy it from a historical / political / philosophical POV, and would never suggest that whichever version you represent is any less valid than any other, so please do not take offense where none is intended.

I'm comforted that we now take a more rational, less supernatural view of interpreting things like mechanics, and frequently wish my English Major education was more Engineering intensive, but then I haven't pursued my planned career path for decades now.

dave

jim templin · **Posted:** Wed Apr 14, 2010 7:48 pm

I compredende'. Fyi I started out as an english major with a minor in social psych....we all see how that came out, don't we?