Page 350 - Boiler_Operators_Handbook,_Second_Edition

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obviously running backwards he became very belliger- If a centrifugal pump was installed in connecting
ent, telling me I didn’t know what I was talking about, piping with no valves and stopped the liquid would
that he checked the rotation himself, and that couldn’t flow right back through the pump because there are
possibly be the problem; besides, how could I know be- no suction or discharge valves to block that flow. Some
cause I hadn’t seen the pump running. operators have a problem understanding how the pump
If you’re now a wise operator I’m sure you already even works. Centrifugal pumps simply grab the liquid
know how I knew. He proceeded to show me how he and throw it. The impeller flings the liquid into the
had determined the rotation, using a logic appropriate volute of the pump (Figure 10-71) where the velocity
to a centrifugal pump, which was wrong for the crescent pressure is partially converted to static pressure and
gear pump we were looking at. I suggested several times delivered to the discharge. To get an idea of its opera-
in the discussion that he would discover I was right tion go to the kitchen, fill a pot or bowl half full of water,
if he looked at the instruction manual. Finally, show- and start stirring it with a spoon. Stir the water in one
ing signs of rage, he stepped over to the center pump, direction ( a pump only runs in one direction) and you’ll
yanked away the envelope that was still wired to the notice that the level of the water in the bowl will vary
motor lifting eye (as at all three pumps), extracted the from low in the middle to highest at the outside of the
instructions, flipped through the pages until he found bowl. That difference in level is the head of your bowl
a graphic, and pointing at the graphic approached pump at shutoff.
me saying “see, right here it shows…” He suddenly Stir faster and the head goes higher and when you
stopped, turned to look at the pump again, shrugged spin it fast enough the water starts coming out of the
and said “I’ll get it changed tomorrow” then walked off. bowl. Setting it under the spigot to add water and stir-
There’s many a lesson in this story but making sure the ring it fast enough so the water spills out at the same
rotation is correct on your pump is the one you should rate you’re adding water and you have a simple version
get right now. of a centrifugal pump. Note what happens when you do
various things with the spoon and you’ll have a pretty
Centrifugal Pumps good understanding of how a centrifugal pump oper-
Our most common pump, the centrifugal, acquired ates.
that position for reasons other than efficiency and en- A centrifugal pump does not move a fixed volume
ergy costs. In my experience it is also misapplied more of liquid like a reciprocating pump. The amount of
than any other pump. The range of efficiency of pumps liquid moved varies with the differential. The flow of
in service, again in my experience, runs from 30% to water pumped from a tank will vary with changes in
70%. Now that has to be one serious variation, a pump the height of water in the tank or the discharge pressure
at 30% efficiency will use 2.33 times as much energy as at the outlet of the pump. If you open the spigot on the
a pump operating at 70% efficiency. I told the story in sink up so more water flows in and don’t change the
the section on NPSH that reflects the differences that can rate you’re stirring it you will see more water flowing
exist in pump performance and it’s primarily with cen- even though you aren’t doing any more work. It might
trifugal pumps. Why is there such a variation? Because
engineers, contractors, and or owners all either ignore
those significant differences or are so intent on getting
the lowest first cost that they’ll choose a pump that will
chew up all the difference in first cost in comparison to
an efficient pump in less than a year or two.
I’m an energy engineer and I’ve evaluated pumps
for power costs for years but I also seem to be a voice
crying in the wilderness because I keep finding them
and continually encounter people that will purchase
that cheaper pump anyway because the power cost isn’t
their problem. I’ve covered the matter in the previous
paragraphs and I hope you learn to apply this knowl-
edge of pump power requirements to operate your plant
wisely even though, in normal situations, you have been
given inefficient junk to operate. Figure 10-71. Centrifugal pump impeller and volute