Page 299 - Boiler_Operators_Handbook,_Second_Edition

284 Boiler Operator’s Handbook

so it is difficult, if not impossible, to install it by rolling. tube boiler which includes simplicity in design. They’re
Also, I wouldn’t want to be the guy that has to pick up relatively easy to clean completely on the fire side, once
that tube roller. you get those heavy doors off. They can be packaged in
Sometimes furnace tubes are called Morrison most of the sizes, they contain minimal refractory. Tube
tubes, and it’s done without distinction. Some furnace replacement is less expensive because all the tubes are
tubes are not Morrison tubes; they’re the ones that are straight. They also hold a larger volume of water com-
basically a simple cylinder. Morrison is the guy that re- pared to a watertube boiler so they absorb load swings a
alized the furnace tube could be made thinner and still little better.
withstand the external pressure without collapsing if
it was corrugated (Figure 10-14). If you look closely at
WATERTUBE BOILERS
Figure 10-11 you can see that boiler has a Morrison tube.
Now you know the difference, if it’s corrugated it’s a
Whether tubes are straight or bent is probably the
Morrison tube and if it’s not it’s just a furnace tube.
first distinguishing characteristic for a multitude of de-
The section through a firetube boiler in Figure 10-
signs of watertube boilers. I started operating straight
14 also reveals another important element of their con-
tube boilers and learned later that there was such a thing
struction, staybolts. The tube sheet isn’t supported by
as a bent tube boiler. Actually the last boiler I operated
the boiler tubes in the top of the boiler (what we call the
while in the merchant marine was a straight tube boiler
steam space) so staybolts are required to keep that por-
and in the process of rebuilding and retrofitting boilers
tion of the tube sheet from buckling out. Part of a boiler
with Power and Combustion in the 1980’s we designed
internal inspection is checking the fillet welds attach-
a new burner installation and furnace modifications for
ing the staybolts to the top of the boiler shell, and the
a straight tube boiler that had a riveted drum.
staybolts themselves, for corrosion. The staybolts nor-
You may never see a riveted boiler outside of a
mally penetrate the tube sheet and their welds should be
museum because they are no longer built and many
checked on the outside as well as the inside.
have failed, never to fire again. Most state laws require
There’s another classification of firetube boiler that
replacement of any riveted boiler that has a failure after
you may encounter. They’re called “oil field boilers”
a certain age and those laws have effectively eliminated
and they’re designed for that application. Boilers used
riveted boilers. When I mention a riveted boiler the
in oil fields get little care, normally run on raw water
normal response is a question, “how did they keep them
with little condensate return and don’t get the quality
from leaking?” The answer is caulking, not the goo in a
treatment provided by a wise boiler operator so they’re
tube type you’re thinking of. To caulk a joint in a riveted
designed for the abuse. They have thicker shells, thicker
boiler you used a special chisel and a good heavy ham-
tubes, and lower heat transfer rates.
mer to deform the metal at the joint working the two
There are many advantages to a scotch marine fire-
together.
Blacksmiths still weld metal by heating the mate-
rial until it’s soft then beating two pieces together to
form one piece. Most of the time we managed to seal
the joints in a boiler by caulking them cold. The real
problem with riveted boilers wasn’t leaks, it was cracks
forming between the rivets. The crack formation was
eventually identified as a byproduct of tiny leaks that
left water concentrated in the metal to metal joint and
caustic corrosion cracking (see water treatment). A lack
of skilled riveters and caulkers and the development of
gas and electric arc welding, which formed a stronger
and cheaper joint, produced the change from riveted
boiler construction to welded construction.
Just like firetube boilers need a shell to contain the
water and steam most watertube boilers require drums or
headers to close off the ends of the tubes, provide a path
for the water and steam to flow into and out of the tubes,
Figure 10-14. Morrison tube