Page 354 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 339

systems because you can use the recirculating line of an in heavy starting current for too long and the starter
idle pump to bleed some liquid back through it and keep tripping.
it hot so it’s ready to operate the moment it’s started. When the pump is operating under system startup
On the other hand, that’s no small amount of conditions you may have to leave the discharge valve
water! If the engineer didn’t include that flow with the throttled (I know they’re normally gate valves and
design capacity of the pump you might find yourself shouldn’t be throttled) until system pressure builds.
short of pump capacity at high loads. However I’ve only Not all pumps are furnished with non-overloading mo-
encountered that problem once because the pumps are tors. If a boiler feed pump is running when the boiler
normally oversized. Engineers usually oversize pumps, pressure is way below normal and the feedwater valve
including the recirculating flow before applying a safety runs wide open it’s possible for the motor to overload.
factor. What that flow represents is a lot of electrical en- Look at that curve in Figure 10-76, you’ll notice that the
ergy to replace steam energy. The power used to pump horsepower at the design operating flow (indicated by
that liquid heats it up but electric power to do that costs the little triangle) is less than the maximum. Draw a ver-
a lot more than the fuel. tical line at the design flow (point of the triangle) and a
This is one place where an operator can reduce horizontal line from where it intersects the horsepower
power costs. As long as the loads are such that the curve to the right to read the pump horsepower at that
feedwater valves should always be open, shut off the design condition. It’s always possible to pick a motor
recirculating line. The pump will back up on the curve, smaller than the maximum horsepower of the pump.
producing a little higher feedwater pressure, and the Even though you pump to a higher pressure with curve
horsepower consumption will decrease. Open the valve A you can’t pump as much volume as you can with
when loads are low and periods of shutoff are possible. curve B so horsepower is less.
You won’t save electricity then, but you will save on If, however, the pressure in the boiler drops so the
demand because forced draft fans and other equipment system curve is ‘C’ then the flow can increase consid-
are at lower loads when you reinstate the recirculating erably and the motor horsepower requirement for the
feedwater pumping load. pump at that point so much greater it could overload the
I should mention that there are feedwater systems motor. If you have a pump with a limited horsepower
that recirculate large quantities of water to maintain motor you have to take action to prevent it running out
a constant feedwater pressure or constant differential on the curve when the boiler pressure is low. Normal
between feedwater and steam pressure. Sometimes practice is throttling a valve down. Don’t count on the
it’s nothing more than an engineer’s concept of what throttling of the feedwater valve, it could suddenly go
should be done because the feedwater pressure gets wide open.
too high as flow is reduced when the pumps have very There are hundreds of variations in pump con-
steep curves. On the other hand the pressure regulation struction because of the many different applications.
is there to reduce pressure drop across the feedwater The shape of the vanes in the impellers can vary from
control valves because they either can’t shut off at the highly efficient backward curved to radial depending
higher differentials or they throttle so much that the on desired efficiency weighed against the solids in the
valves wear dramatically. I say change the damn feed liquid they pump. They can be rubber lined for such
valves and save electricity but not everyone agrees with purposes as pumping a slurry of limestone or ash. They
me. Another solution is installing variable speed drives can be “canned” where the rotor of the motor is sealed
but the economics aren’t always there. in an enclosure with the pump to prevent the leakage
When starting a centrifugal pump it’s common of hazardous liquids. The most common arrangement
practice to open the suction valve, start the pump, then is the horizontal split case pump (Figure 10-77) but the
open the discharge valve. The reason is the pump can’t ANSI pump (so called because the National Standard
draw any more horsepower than what’s used at shutoff establishes fixed mounting dimensions so all manufac-
during startup, reducing the load on the motor. turer’s pumps are interchangeable) is gaining popular-
I recall one time when a discharge check valve had ity. They’re end suction pumps that require the piping
failed to close on a pump but we needed the pump in be disconnected to get to the pump for maintenance.
operation. When the pump driver stopped the fluid sim-
ply flowed backwards through the pump and tended to Turbine Pumps
rotate it backwards. The additional motor load required When I say turbine pumps some people get the
to reverse the rotation before starting to pump resulted impression of a centrifugal pump powered by a steam