Page 346 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 331

liquid. The pump parts either spin in the vapor produc- is specified by the pump manufacturer for the design
ing no pressure or the vapor will constantly compress operating condition and is usually shown on the pump
and expand. The net result is the pump stopped pump- curves. It’s the required NPSH for that pump at the
ing. In some cases this will cause a surge of discharged rate of flow. Some of the requirement is a function of
liquid back into the pump which then gets pumped out how much the liquid has to accelerate at the inlet of the
again and that liquid surging back and forth damages pump impeller because some of the static pressure of the
the pump. suction head has to be converted to velocity pressure to
Sudden formation of vapor in a pump driven by a get the liquid into the impeller. The NPSHA is what’s
steam turbine will result in rapid over-speeding of the available, the actual NPSH at the inlet of the pump. That
pump and turbine. Occasionally that happens so fast value always has to be higher than the NPSHR.
that the turbine over-speed trip can’t respond before the Operating a pump when the level in a tank it’s tak-
turbine blades start flying out of the casing! ing suction on is too low can result in serious damage to
When the bubbles start forming they will collapse the pump. Allowing a pump to continue operating when
later when the pump increases the pressure in the liquid. the suction head is inadequate doesn’t make sense. If the
In centrifugal and turbine pumps the result is bubbles tank is almost dry there’s nothing there for the pump to
forming then collapsing and the liquid rushing in, to move anyway, shut the pump down to prevent it being
fill the voids as bubbles collapse, hammer away on the damaged. Remember priority number three?
parts of the pump. We call that ‘cavitation’ and it’s evi- I’m not talking about short term conditions here
dent by a small to fair amount of noise that you can hear. because I know we occasionally run a pump to the point
It’s also evident when you dismantle the pump. You will of losing liquid. Stripping a fuel oil tank before cleaning
see heavy wear consisting of lots of tiny indentations is one example. In that case you should be prepared to
where the bubbles collapsed. To prevent pump damage stop the pump the instant it loses suction so you limit
you have to be sure you have adequate NPSH. the potential for damage. I’ve cleaned all the metal shav-
The NPSHA is the difference between the suction ings out of many a fuel oil strainer after somebody let a
head and the vapor pressure of the liquid. To get away pump run for several minutes after the tank went dry.
from the math let’s assume a pump submerged to its Then I helped rebuild the pump.
centerline in a tank of boiling water at sea level. Since You’ll notice on the pump curves that the NPSHR
the water level is right at the inlet the suction head is increases as the flow through the pump increases. Throt-
zero gage, 15 psia. Since the water is boiling the vapor tling the discharge of a pump to reduce the flow will
pressure is 15 psia and the NPSHA is zero. By submerg- also reduce the NPSH required and can stop a pump
ing the pump in the tank so its centerline is four feet be- cavitating. Although this is occasionally required under
low the surface and there’s no suction piping to produce unique operating conditions it shouldn’t be the normal
friction, we increase the NPSHA to four feet. case. If you have to operate the pump at the lower suc-
I should also explain what happens when the wa- tion head then you would do well to have the impeller
ter is colder. Lets assume our pump is in a tank of con- turned down to reduce its capacity and horsepower
densate at 162°F. The vapor pressure at that temperature requirement.
(check the steam tables, is 5 psia. Subtract from 15 psia Cut it down! What’s that about? It’s a way of mak-
to get an additional 10 psi of pressure that the suction ing a pump fit its application better. It can’t always
can drop before the water boils. Checking the head ta- be done. However, in many cases it’s something that
bles we find that the 10 psi converts to about 23 feet and should have been done and wasn’t. If someone simply
we can add that to the four feet the pump is submerged orders a new impeller giving the manufacturer nothing
to get an NPSHA of 27 feet. but the pump model number they could very easily get
To help explain how a centrifugal pump can lift a full size impeller, not one that was trimmed for the
water out of a lake once it’s flooded, the NPSH of water application. You can tell what your impeller diameter
at 60 °F is minus 14.5 psig equal to 33.5 feet. A pump should be by the pump curve that came with the original
can lift 60° water that far before it will start boiling. Of instructions.
course the pump can’t pump the air out so you’ll have
to install a foot valve in the lake and fill the piping and Pump Curves
pump casing with water to get it started. Once it’s start- Pump curves provide answers to a lot of questions
ed it will pump the water. about our pumps. If you feel compelled to throw out a lot
Now, back to the two additional labels. The NPSHR of unnecessary paper never include pump curves in that