R. R. #4, Red Deer, Alberta, Canada
Lap Seam Versus Butt and Double Strap Seam I have read quite a few comments in the last year or so on the two types of boiler seams used on the barrel part of the boiler. I would like to try to throw a little light on the subject if I may.
Lap Seam This seam is made up where the barrel is rolled and lapped over itself to make the complete circle of the barrel, then it is riveted together with one, two, and three rows of rivets.
Most all professional engineers will tell you that by lapping the plates that you in fact do not make a true circle, and thus create a point where when under pressure the barrel of the boiler is trying to become a true circle, thus where the seam joins, it's supposed to be the working point, where possible cracking can occur, 'whether this is true or not, I do not know.' I never found a seam that was cracked.
Also, one other reason for engineers pointing out the weakness, if there is any in the lap seam is where two plates are just lapped over and riveted, you have a single shear force on the rivet and this is where your safety factor, catches heck from the engineers. If you do not understand single shear, and double shear, in a riveted joint, it almost explains itself.
We will come back to the rivets in the two types of seam further on in this article.
Butte and Double Strap Seam This type of barrel is rolled in a perfect circle and the edges butt against each other. Then a plate is placed on the inside of the seam, so it extends equally on both sides, and the full length of the barrel another plate is placed in the same manner on the outside of the barrel and then usually 3 rows of rivets are put through all the plates on each side of the barrel seam edges. Thus you have 6 rows of rivets in your butte strap boiler. Now we come to the double shear on rivets as follows:
Now we get to one of the reasons which make the butt strap seam superior, is in the double shear plus the two strap plates which also add adhesion and strength to the seam. But most important and the least thought of, is the fact that more material in the lap seam boiler is used up by rivet holes per area than in the butt and strap seam. So this will increase the factor of safety in a butt and strap seam way above the lap seam. This can be proved on paper mathematically by figuring material lost in rivet holes to actual plate strength.
Also, on most all butt & strap boilers all pipe fitting holes are reinforced with extra plate.
The thickness of a butt & strap boiler barrel, may be less than a lap seam, and this is usually because of the extra thickness required to make the lap seam as safe as possible because the lap seam plate is weakened more by the material wasted in rivet holes. This may sound like a repeat but read it carefully and it does make sense, more material is lost per area in lap seam rivet holes than in butt & strap seam.
When the engineers and designers draw up the blueprints for traction boilers, they could have used a 1/8' barrel and probably it would hold 100 PSI steam pressure. However, through experience and calculating material strength, they had to design a boiler that could stand high pressure internally, and then support a motor and gearing; the barrel also had to be strong enough to be supported by the front wheels, and also stand the stress of movement, plus the strain of pulling plows, etc. So with all this in mind, they designed boilers which stood up quite well after some field trials and some changes to improve design. Enough said.
Here in Alberta where I believe the toughest boiler code exists, all lap seam boilers will soon be X-rayed on the lap seam and if found okay will be allowed 100 PSI.
All butt & strap boilers in good shape can still operate on 175 PSI and believe me, most inspectors are tough up here.
I have spent many years repairing and building boilers. I have never seen evidence of cracking etc. on a boiler seam, however, this does not mean it could not have been there.
But I have seen Firebox failure due to human error or stupidity, and this is why I always end my letter with a caution.
To young engineers please ask questions if you don't understand. It is no crime if you do not know.
And to all you older experienced engineers if you do not make your knowledge available to these young engineers, you are robbing these young men and women of the greatest gift you can give-Experience.