Page 341 - Boiler_Operators_Handbook,_Second_Edition

326 Boiler Operator’s Handbook

pumps are usually centrifugal. Low pressure feedwa- (a code requirement) they’re slightly oversized anyway
ter and small volume condensate pumps are usually because capacity picks up as the differential is lowered
turbine type pumps. Fuel oil is moved with positive to operating conditions.
displacement progressive cavity pumps of the screw Just because the pumps can be oversized don’t
and gear types. There are other options but their use ignore the possibility that an operator can compound
is not as common. Technological advances could alter the problem by making logical decisions. I was in one
one or more of these general rules in the future. If only plant with three boilers and four feed pumps. If two
someone could come up with something better than a boilers were on line the operator ran two pumps. During
centrifugal pump we could see dramatic reductions in the winter when there were three boilers on line… you
electric power consumption because many of the cen- got it, three pumps were running. It made no difference
trifugals run at efficiencies less than 50%. what the boiler load was, run a boiler and run a pump.
Pumps handle liquids, incompressible fluids, and A quick look at the instruction manual revealed
they’re an essential part of the boiler plant. Modern that any one pump could supply three boilers. Savings
pumps have become so reliable that operators tend to of electricity by only running one pump the year round
ignore them until something fails. I’ve been in many a was well over $50,000.00. I think it’s now obvious that
plant where the pumps have been there operating for so proper choices in the operation of pumps and monitor-
long that the manufacturer’s name that was formed in ing their performance as well as maintaining them can
the casting of the pump had corroded until you couldn’t make a significant difference in the cost of operating a
read it. When asked, the operators couldn’t produce an plant and can also justify a wise operator’s salary. Fair
instruction manual or anything else that would identify warning, however, simple numbers don’t always work.
the make and model of that pump. With rare exceptions pumps are powered by elec-
Now that’s confidence, it will last forever so we tric motors or steam turbines. We say the motor or tur-
don’t have to know where to get one to replace it! Dream bine ‘drives’ the pump so we call them ‘drivers.’ They
on. Pumps don’t last forever and their capacity and dif- all serve to rotate or extend and contract the shaft of
ferential capability declines as they age. Their efficiency the pump. The energy is transmitted through a metal
also declines with age and pumps that are so old you shaft that connects the driver to the pump. The rotating
can’t read the nameplate may be using twice as much parts of a pump can be mounted directly on the driver’s
electricity as they did when they were new or, more shaft or they can be mounted on their own shaft. When
likely, only pumping half of what they could originally. the pump has it own shaft it is also fitted with bearings
Monitoring the performance of your pumps is a wise to maintain alignment of the shaft in the casing of the
thing to do. pump. Regardless of operation, rotating or extending
Pumps are usually oversized too. I frequently and retracting the shaft moves and the design of the
discover that boiler feed pumps are selected so any pump must allow it to move without allowing the liquid
one of them can run the plant at full capacity (all boil- to leak out of the pump.
ers on) which we know doesn’t make sense when at When I was operating and maintaining pumps we
least one boiler is usually a spare. Then, to compound had to allow some of the liquid to leak. That’s because
stupidity, the engineer specified three or four pumps all we had to keep the liquid from leaking out of the
of the same size. It’s virtually impossible for an op- pump in large quantities was packing. (Also see pack-
erator to select a pump that matches the load when ing under maintenance. Packing seals the space along
they’re all too damn big! the shaft where it penetrates the casing to limit leakage.
In many instances replacing a boiler feed pump Some leakage through the packing is essential to lubri-
with one that will just barely handle a spring or fall load cate the packing to shaft joint. If the packing is tightened
will save enough electric power to pay for the pump in enough to stop or reduce the leakage too much then the
one summer. When you have more than two pumps the packing and shaft rub with deterioration of each.
capacity of each should be such that it takes all of them, As a matter of fact, it was so common for us to
less one, to carry the peak load. With three pumps they screw up a shaft with the packing that manufacturers
should each handle half the peak load. With four pumps started making rotating pumps with shaft sleeves to
they should each handle one third of the peak load. Five help with that problem. The sleeve was like a pipe or
pumps should be sized at one quarter the peak load, etc. tube that slipped over the shaft and was either clamped
Since boiler feed pumps have to be capable of delivering with other parts or threaded onto a matching thread on
water to the boiler when the safety valves are blowing the shaft so it was removable. That way, when we ran the