Page 347 - Boiler_Operators_Handbook,_Second_Edition

332 Boiler Operator’s Handbook

group. How much liquid can be pumped under vary- is not as dense as ice water, it only weighs about 59.4
ing differential pressures is the most important line on a pounds per cubic foot and while pumping that lighter
pump curve. Any pump curve will normally have several feedwater the pump will only produce 95.3% of the
of those depending on different construction and operat- discharge pressure that’s produced when pumping ice
ing conditions. As stated above, the NPSH (NPSHR un- water, enough to be significant when operating at high
derstood) will be shown when it’s important. The pump boiler pressures. It’s also important to note that cen-
will also have horsepower lines or efficiency lines or trifugal pumps are volumetric machines, they pump so
both. Either the horsepower or the efficiency will permit many gallons, not so many pounds so the 95.3% should
calculation of the other value because there’s a standard also be applied to any calculation that converts the gal-
formula for hydraulic horsepower. lons per minute to pounds per hour.
The flow-differential curve is the first one to look The horsepower or efficiency lines are primarily
for. In many cases they will be the darkest lines on the used by engineers in selecting pumps, trying to buy the
paper. The normal form of a curve lists the differential one with the lowest operating cost. At least that’s what it
on the left side of the curve and the flow on the bottom. should be. You, on the other hand, can use those curves
Differential is typically listed in feet, meaning head, to get an idea of the best mix of pumps for a given opera-
and you have to convert that value to psi to see how tion or to provide answers to problems with the pump.
much pressure boost you can get out of the pump. Some You may have a choice of running one or two pumps
curves will show psi because the pump isn’t affected and decide that running one should be more efficient.
much by density. The rate of flow is normally listed in While that’s a logical decision it isn’t always the case.
gallons per minute but don’t be surprised to see gallons Running one large pump far out on its curve could be
per hour or hundreds of gallons per minute. If there’s no less efficient than running two smaller pumps because
label you should be able to safely assume gpm. they’re operating at a better efficiency.
Now I know that you’re going to find most curves I always tell this story to make an important point
will have several lines. There are several lines because the regarding pump efficiency. I was asked to look at a
pump can pump more than one type of liquid and some problem with boiler feed pumps at a major laundry in
have variations in construction. The typical centrifugal Washington, DC. The owner complained that he was re-
pump, where the curves are almost always for cold wa- placing the pumps every six months. They didn’t sound
ter, will have different lines for the choices of impeller too bad but it was obvious that they were cavitating
diameters. Normally the curve is marked with the design during normal operation. A look at the pump curves and
point so you can see what diameter impeller was installed installation revealed inadequate suction head was the
in your pump, otherwise you’ll have to look elsewhere in problem. I searched catalogs for alternates and submit-
the manual to find out what size impeller you have. ted a recommendation for purchasing different pumps
Once you’ve identified the line you can tell what for two reasons. One was the NPSHR of the recommend-
the differential pressure will be for a given pumping ed pumps was less than what was available. The other
volume. Sometimes it’s valuable for determining how reason was the new pumps did the job at 3.5 horsepower
much you’re pumping based on the difference in pres- and the existing pumps took 7.2 horsepower. Yes, there
sure. Other curves will address characteristics of the is that big a variation in pump efficiencies. The savings
liquid. Fuel oil pumps, for example, will have a number in motor horsepower was worth $1,480.00 per year.
of lines on the curve for different viscosities of the oil. The owner balked at my recommendation because the
Unless specifically stated to the contrary a pump new pumps cost twice as much apiece, $2,500.00 more
curve is supplied to show the flow and differential char- than the ones he had. To this day I don’t know what
acteristics pumping cold water at 32°F and a density of happened because I was never called back to the site.
62.4 pounds per cubic foot. That provides a basis for de- If he had installed those expensive pumps he would
termining the differential pressure at other fluid densi- be avoiding the $6,480.00 he had been spending for
ties. Since we seldom pump ice water you have to adjust horsepower and replacement pumps every year. If you
the head characteristic of a pump curve to determine the have any opportunity to choose a pump be conscious of
actual differential pressure which will always be lower power requirements in addition to NPSH.
than what the curve indicates. This gains some impor- Another point to consider in using pump curves is
tance with water at high temperatures and is important the occasional use of a pump for a purpose other than
for things like boiler feed pumps. originally intended. You can use the curves to see if the
Boiler feedwater at 227°F (a common temperature) pump will work and make certain you don’t overload