Page 301 - Boiler_Operators_Handbook,_Second_Edition

286 Boiler Operator’s Handbook

that’s not counted as a pass. The gases turn up at the A special “puller” was required to seat the key caps so
back of the boiler and pass up through the superheater they wouldn’t leak as the boiler was filled.
and boiler tubes until they reach the top (first pass) then That old sectional header boiler provides a simple
drop down through the middle of the tubes (second look at the complex conditions surrounding circulation
pass) and finally up through the tubes at the front of in watertube boilers. Water separated from the steam
the boiler and out the stack. The baffles are made out of and boiler feedwater mixes in the steam drum (a com-
refractory and include tile laid on top of the screen tubes mon arrangement) then drops down the front headers
to form the bottom of the second and third passes. (which are exposed to the coolest flue gas) and rises up
The bottom two rows of tubes are called screen the sloped tubes going from the front of the boiler to
tubes because they form a screen that blocks the radi- the rear. In those tubes the water is heated to the point
ant energy from the superheater (more on superheaters of saturation and starts boiling, changing from water
later). They also protect the baffle. The sectional header to steam. The steam forms small bubbles in the water,
part of this boiler involved the forged square headers displacing the heavier water and reducing the density of
shown in the detail which were connected to the steam the steam and water mixture as it travels along the tube.
drum and bottom header by tube nipples (short lengths By the time the mixture reaches the rear headers
of tube) and contained handholes on the side to gain ac- it is significantly lighter than the water so the weight of
cess to the tube ends so they could be rolled. The headers the water in the front header is just like a piston pushing
were forged in a semi-square shape to provide a uniform down to force the water and steam mixture up the rear
surface for rolling the tubes. Drums are normally of suf- headers and back the return tubes to the steam drum.
ficient diameter that there is no problem rolling a tube in There’s only a little difference in pressure between the
them. water in the front header and the mixture at the rear
To gain access to the tube ends to roll them and header, perhaps half the height of the boiler (inches
for other parts the drums have manholes, usually a water column) but that’s enough to force the water and
12-inch by 16-inch oval opening. Handholes are simple steam to flow around with the flow rate of the steam and
openings in the drum or header that are closed by a water mixture through the top tubes at least five times
cast cover (Figure 10-16) which is inserted inside the the rate of the steam going out the nozzle, perhaps more.
boiler and bears on the inner surface of the shell, drum, In the case of this boiler all tubes are risers, the front
or header usually against a gasket so the internal pres- headers are downcomers.
sure of the boiler helps hold the cover in place. To keep Another form of straight tube boiler was the box
them in place when the boiler is not under pressure the header boiler which used fabricated boxes containing
bolt, nut and dog are applied. Key caps (Figure 10-16A) stud bolts (see discussion for firebox boilers) and hand-
are similar but tapered cast plugs that wedged into the holes opposite the tube ends in an arrangement very
header or drum openings to form a metal to metal fit. similar to the sectional header boiler. The straight tube

Figure 10-16. Handhole and cover Figure 10-16A. Key caps