Boiler Seam Technology

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.