Page 284 - Boiler_Operators_Handbook,_Second_Edition

Strength of Materials 269

to boilers and pressure vessels the vessel wall is normal-
ly more flexible while remaining strong and can take the
stress; the shell simply changes its shape a little. Pumps,
blowers and turbines, on the other hand, go out of align-
ment when they are overstressed. A piping system that
has very little stress in the piping can still overstress a
pump or turbine to the degree that the impellers hit the
casing and shaft seals are rapidly worn and fail.
Anytime a customer has a problem with a pump
the first thing I look at is the connecting piping. I was
called to resolve the problem on one job where the
stress got so high that it broke the concrete pad under
the pump away from the floor and moved it almost
Figure 9-5. Packed type expansion joint
two inches. The maximum allowable forces on pump
or bellows joint on the anchors is readily determined by connections are described in API-610. When you look
multiplying the MAWP (maximum allowable working at those forces you’ll notice that some are so low that
pressure) of the fluid in the system by the cross sectional enough pipe to get from where it’s attached to the pump
area of the pipe (including the metal area) for packed to overhead will, along with the weight of fluid it holds,
joints like the one in Figure 9-5 or the bellows area of weigh enough to exceed the standard’s limits.
bellows joints. Perform the calculation where you have Over the years I’ve encountered many situations
these joints and I’m certain you will be amazed at the where the operators of a plant modified or had a con-
forces that are produced. It’s only one reason I do not tractor modify piping without careful analysis of the
recommend their use. flexibility; and they suffered the consequences. I’m not
I’ve also encountered many a valve that leaked talking about application of snubbers that are like shock
because the piping stresses applied to it were too high. absorbers and restrict the dynamic flexing of the piping
It happens frequently where large stiff piping is reduced (when it acts sort of like a tuning fork) they don’t restrict
at a control valve making the valve and its flanged joint the thermal growth. The wise operator realizes that the
the weakest point in the piping and the one that bends piping has to remain flexible and will not attach stuff to
or breaks. it or impair its movement to reduce its flexibility.
During the first trip on the last ship I sailed I had a I hope this little discourse in strength of boiler
piece of gasket blow out of a flanged joint and bean me plant materials gives you some guidance in operation.
on the head. It was a good thing I was more than three You should feel a little more comfortable with what
feet away because 90 psig steam followed the piece of you’re dealing with and, at the same time, gain some re-
gasket. After I replaced the gasket I cut off one of the spect for the pressures you’re operating at. Look at some
pipe supports that was obviously (at least to my engi- of the vessels you’re operating and calculate the force
neer’s mind) causing the stress at the gasket. by multiplying the area by the operating pressure then
The largest problem with stiff piping is its effect on dividing that result by the area of the metal holding that
pumps, blowers, and turbines. Where piping is attached force back.