Page 283 - Boiler_Operators_Handbook,_Second_Edition

268 Boiler Operator’s Handbook

using less expensive, but perfectly fine, materials for simulate the effect of the pipe expanding. You could also
some processes. You will always find 600 psi rated steel clamp it in two vises properly positioned and heat it up
valves and flanges on feedwater piping for boilers with but that’s a little more complicated.
a maximum operating pressure of 600 psig even when Keep in mind that pipes get stiffer as they get
the feedwater pressure is as high as 800 psig because the larger, note the sag in different sizes of pipe when you
secondary ratings of 600 psi standard valves and flanges pick them up in the middle; bigger is stiffer, smaller is
is 900 psig with 250°F feedwater. An abbreviated copy of more flexible. You can also relate to the fact that valves
secondary ratings is in the appendix. and other devices in the piping make it stiffer. When stiff
piping is heated it tends to grow in length and diameter.
Getting a little larger in diameter isn’t much of a prob-
PIPING FLEXIBILITY lem to handle but the added length is.
Sometimes the pipe can do the same thing that
Tension, compression, and bending stresses are all railroad tracks do, just spring sideways a little to con-
involved in determining the flexibility of boiler plant vert the straight line of pipe to a shallow S. That doesn’t
piping. I should explain that what we’re talking about cover much of a change in length and we’re just lucky
when we mention the words “piping flexibility” is the that railroad tracks don’t get that hot. Other examples
stresses in the piping and the stress and forces applied to include roads. I can remember one hot summer when a
boilers, pumps, turbines, building structures and other lane of the Baltimore Beltway got so hot that the pave-
things the piping is connected to where those forces and ment buckled up at a joint producing the equivalent of a
stresses are produced by the thermal expansion or con- two foot high speed bump. Luckily I was driving in the
traction of the piping. other direction but I saw two cars hit it and they didn’t
I can still recall being asked to look at a problem fare well. The compression stress gets so high that any
in the warehouse section of a plant where a wall had little change in cross section (the roadway joint) permit
been damaged. The wall was at the south end of a large translation of some of that compression stress to bend-
warehouse, it was made out of concrete block and it had ing stress and, in that event, the roadway bends.
a very large hole in it right around a piece of insulated I can also remember looking at two 16-inch HTHW
pipe. In the shipping area opposite the wall was a pile lines in an underground tunnel where they made a
of broken concrete block. You could see the remnants 45-degree bend. The adjacent support for the piping had
of a thin steel plate that was welded around the pipe moved, shearing off its anchor bolts. The piping move-
to seal the opening in the wall (required for a fire wall ment drove it back so far that some conduit behind it
construction) still hanging on the pipe. According to was overstressed in tension and split like an old paper
the drawings a similar plate was in the north wall of soda straw to produce a gap over an inch wide exposing
the warehouse. In between those two plates was 84 feet the wires inside. You have to respect the forces associ-
of four inch steam piping; a straight 84 feet of pipe. ated with thermal expansion.
Operating pressure was 150 psig and the pipe was in- Back to piping flexibility. Usually you don’t notice
stalled when the outdoor temperature was about 70°F. any problems with it in a boiler plant because the de-
Using the tables and procedures in the Appendix you signers are aware of it. That doesn’t mean the designer
can determine that the pipe would lengthen by about 2 did it right. There are times when the installing contrac-
inches when heated. Since the pipe had no place to go tor changes the piping arrangement and it produces
but straight south it broke the wall. Later a ‘U’ bend was excessive stresses. If you fail to maintain joints in the
installed in the pipe inside the warehouse and the wall piping or the piping supports you may have some prob-
repaired. Unlike the stiff straight piece of pipe the pipe lems with overstress.
with the U bend was sufficiently flexible that the pipe I’ve seen welded steel elbows buckled because an
bent slightly and the seal plates and walls were able to adjacent packed type expansion joint (Figure 9-5) froze
withstand the forces applied to them. up. This form of expansion joint allows the pipe to ex-
If the concept of piping flexibility doesn’t gel in pand into the space between the flanged connection and
your mind I suggest you do what I have done in the past a bare end. They have to have anchors somewhere else
to picture it, make a model of the piping out of a piece of to take the axial pressure forces of the pipe or the darn
coat hanger wire then grasp it at two points where it will thing will come apart. If you have some of these joints be
be anchored (attached to something that doesn’t give) very certain that the anchors aren’t corroded.
then try to move your two hands toward each other to The forces produced by a packed expansion joint