Page 342 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 327

pump with the packing dry and tore up the shaft sleeve that could damage the seal the flushing liquid is passed
all we had to do was replace it, not the entire shaft. I’ve through a strainer to remove those solids providing
been in a few plants where annual replacement of shafts clean flushing liquid and extending the life of the shaft
and shaft sleeves was common because the operators seal.
consistently tightened the packing too much. Sometimes the flushing liquid is improperly ap-
If you don’t know how much leakage is necessary, plied. If you open a pump to find erosion around the
try measuring the temperature of what leaks out and area where the flushing liquid is admitted check with
compare it with the temperature of the liquid inside the manufacturer. I’ve run into more than one pump
the pump, it shouldn’t rise more than 5 degrees. That that left the factory without a required orifice in the seal
doesn’t work for boiler feed pumps because the liquid flushing piping.
in the pump would flash. Usually we look for a tiny The seal materials have to be able to handle the tem-
stream flowing out of the packing as a rule. By tiny peratures under those vapor forming conditions. Some
stream I mean something no larger than a pencil lead. shaft seals require coolers to lower the temperature of
Over time you’ll learn how much you can squeeze down the liquid. HTHW circulating pumps, for example, have
on packing and learn not to go too far because you’ll strainers and coolers on the flushing liquid. Maintaining
end up rebuilding the pump. The pump will usually tell the coolers and strainers is an important factor in keep-
you when it’s too tight because it will wear the sleeve or ing those pumps in operation. Care is required to keep
shaft until it gets enough flow. In that case, listen to what the temperature of the liquid within an acceptable range
the pump is telling you and allow that leakage. because it can also get too cool producing thermal shock
A pump construction ensures the packing, or where it mixes with the fluid in the pump.
seals described below, are not subjected to discharge
pressures unnecessarily. By designing pumps with the Alignment
packing or shaft seal at the lowest possible pressure When the pump and its driver are riding on
point in the pump wear and tear on them is reduced. separate bearings the two shafts are connected with a
Some pumps still have a seal or packing exposed to the coupling. Rotating shafts are equipped with flexible
highest pressure and construction is modified to reduce couplings which allow the two shafts to be centered in
the effect of the high pressure. Packed pumps will have their own bearings. Shafts that extend and retract can be
lantern rings (Figure 5-8) which permit bleeding off of connected with rigid clamped couplings or a coupling
the high pressure leakage to the suction of the pump containing a bearing that allows one shaft to swing like
with the rest of the packing exposed only to the suction that for a chemical feed pump.
pressure. Proper alignment of couplings is essential for long
In the case of condensate pumps and others that pump life. If the alignment is poor the coupling will ap-
can operate with pressures below atmospheric, pressure ply alternating forces to the shaft, constantly bending it
on the suction side supplied by a connection from the back and forth until it finally breaks if the bearings or
discharge provides fluid to seal and cool the packing. packing don’t fail first. I will not go into the alignment
Modern rotating pumps are commonly supplied of a reciprocating shaft pump because you should not
with a shaft ‘seal.’ It’s a special construction with very have to do it. If you do have to work on a straight recip
hard materials consisting of rotating and stationary make it a point to follow the directions in the instruction
parts that provide the liquid seal. Those materials are manual carefully. The following discussion on aligning
machined to very close tolerances so there are only a few rotating pumps should give you all the clues you need
hundred thousandths of an inch separate them when to know about what has to be done; you will still need
operating but the two materials do not touch because a the manual to see how to do it right.
minute amount of liquid separates them. In many cases The process of aligning a pump and driver begins
the liquid forms a vapor between the two wearing sur- with determining the differences between operating and
faces and the vapor becomes the lubricant with no leak- cold conditions. A boiler feed pump, for example, will
age evident at the seal. heat up when the pump is in operation so its shaft can be
Most of them require some flushing of the seal to higher when it’s operating. A pump and turbine combi-
keep it cool enough to operate properly, using a small nation can have different changes in shaft position. You
line from the pump discharge to the seal to provide normally do not need to correct for operating tempera-
flushing liquid. When there’s an opportunity for the ture on most pump and turbine combinations because
liquid to contain small particles of rust or other solids both will be centerline supported. That means the pump