J. A. CABLE
FOR VISCOUS FLUIDS To performnce data given f o r water,
appb correctionfactors, and derive performance of the pump when handling a more viscous liquid
to estimate how a centrifugal I pump ' oftenwillnecessary alter in performance with a change in the t IS
'
viscosity of the fluid pumped. The estimation procedure is simple. Charts available from the Hydraulic Institute provide a rapid method, accurate enough for all but borderline cases. However, the correction factors are so often misapplied that it seems worthwhile to go through the correct procedure. When the properties of the fluid pumped are changed: -No density correction is required, if discharge pressure is measured in height of fluid pumped -Viscosity change affects pump performance and requires correction. A sketch of the chart from the Hydraulic Institute Standards, giving the corrections, appears on the right -Net positive suction head is affected by viscosity. A more viscous fluid requires a higher suction head -Both fluid density and fluid viscosity affect driver requirements The engineer must know how to apply the correction factors to derive value from them. It is important to rewgniee that these factors apply to the pump and not to the rating required. Once this is realized, the application of thecorrections fallsintoa workable pattern. The correction factors are applicable to any pump. They are based on its water performance and are given in terms of its point of maximum efficiency. If it were always possible to use a pump at the capacity at which it gives maximum efficiency, all corrections would be alike. However, by nature of their application, centrifugal pumps are used over a fairly broad range. An example will show how to correct performance data for any point in this range.
F
z
CAPACIW IN 100 CPM VOL 5 5
NO. 1 J A N U A R Y 1 9 6 3
43
EXAMPLE An oil is to be pumped at a rate of 750 gallons per minute; a head of 100 feet is required. Viscosity of the oil is 1000 S.S.U. Three possible situations can arise, depending on pump capacity Case 1. Pump used at its point of maximum efficiency. For this pump, we will call capacity at maximum efficiency QN,~. Using the chart given in the 10th edition (1958) of the Revised Hydraulic Institute Standards (see example page 43) enter the chart at 750 gallons per minute. Proceed vertically to the point representing 100 feet, then horizontally to the line for 1000 S.S.U., and then again vertically to obtain:
If selection is all that is wanted, stop here. If it is desired to draw a full curve for the viscous performance, proceed to determine the following: From Water Performance Curve Data Point
1 .OQ N J 1.~QN,I ~ . ~ Q N , I
Efficiency correction Quantity correction Head corrections At 1.2QN At 1.OQN At 0.8QN At 0.6QN Haodcontctim C, at 7.0QN
C,
CQ
0.64 0.95
CH
Hw
Ew
790 950 632 475 0
109 93 116 121 130
82 78 80 72 0
Resultant Oil Curve
0.89 0.92 0.94 0.96 oMlitd to the hed ot the cafim'ty where CH at 7 . 2 Q ~is @plied to the head
maximum ejkiency is obmin'md. T h at 7ZO% of thiscapairy a d s o forth.
To get 750 gallons per minute of 1000 S.S.U. oil, the waterratingofthepumpmust be 750/0.95 = 790g.p.m., and oil efficiency will be 0.64 X water efficiency. To get 100 feet of head (assuming in this case that the pump being selected is most efficient at about 790 gallons per minuteofwater), weuse the CHfactorat 1 . 0 Q ~ and get 100/0.92 = 109 ft. as the water rating necessary. A small error enters the calculations here. The Hydraulics Institute chart should be entered at the point of maximum efficiencywhile pumping water (that is, 790 gallons per minute) and the first line should be drawn to the corresponding had-109 gallons per minute. When corrections are large, trial-and-error will give a more accurate answer. But for the case given here, the difference can be disregarded.
J . A . Cable i s a Supervisory Engineer in the fluid dynamics hpmhdnt of AUis-Chalmers Manufacturing Co.
AUTHOR
44
~.~QN,I 0.0
Qw
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Data Poimi
Q = 0.95Qw
l.OQN,I 1.2QN,1 O.8QN,, 0.6QN,1 0.0
750 903 600 452 0
H = Cx X Hw
109 X 93 X 116 X 121 X
0.92 = 0.89 = 0.94 = 0.96 = 130
100 82.7 109 116
E = 0.64 E a
52.5 50 51.2 46 0
Note that head at shutoff is not affected by viscosity, and efficiency is zero, of course. Brake horsepower would be somewhat higher for the oil than for water.
Cane II. Pump will perform at less than its best e 5 ciency range. Let's assume that for some reason, such as high suction lift or an increased future rating, we have to select a larger pump. We must then seek a new set of correction factors based on this pump's best efficiency range. As an example, assume that our pump in this case is most efficient at 1250 gallons per minute. This point we will call QN,z. The chart is entered, as before, at the capacity at which the pump is most efficient-now 1250 gallons per minute. The head correction factors also refer to QN,z so that for our rating we should use the Cn factor at 0.6Q~,because 0.6 X 1250 = 800 g.p.m.
What difference does pump size make?. If the chart had been entered at 790 gallons per minute, and the head corrections applied to 790 gallons per minute, the factors given in the “incorrect” column would be read :
CE CQ
cn At 1 . 2 4 ~ At 1 .OQN At 0 . 8 4 ~ At 0.6QN C, chosen for 750 g.p.m.
COWCCt
Inconart
0.68 0.96
0.64 0.95
0.91 0.94 0.96 0.97 0.97
0.89 0.92 0.94 0.96 0.92
As you can see on the graph opposite, use of the wrong factors on an oversize pump results in a selection that .is over safe. Case III. Pump will operate beyond its most e 5 a e n t point. On the other side of the fence, we might have chosen a pump smaller than average for some reason such as favorable flooded suction conditions. Suppose our best efficiency is at 625 gallons per minute which then rightly becomes 1.0QN,3. Our required 790 gallons per minute of water capacity is then at about the 1.2QN,3point. The table compares the right and wrong correction factors in this case: Concct cE
CQ C” At 1 .2QN At 1 .OQN At 0 . 8 Q ~ At 0.6QN C, chosen for 750 g.p.m.
I~W~CCI
0.62 0.94
0.64 0.95
0.89 0.92 0.94 0.96 0.89
0.89 0.92 0.94 0.96 0.92
This means that if we select the pump on the basis of 790 gallons per minute and 109 feet head, we are again w r o n g b u t this time on the unsafe side. Olher Factors in Pump Selection
An undersized pump can invite trouble if suctioi. conditions are at all marginal. The suction condition factor, called net positive suction head, is affected by viscosity in the same way as the head correction applying to the rating. -This means that when the fluid is more viscous than water, the pump requires more suction head than does a water pump. Of course, all these factors also affect the sizing of th motor that will drive the pump. You have to use th corrected viscous efficiency, and at the same time yo1 must consider the specific gravity of the liquid pumped I n summary, then, to handle a more viscous fluid, pump must be selected on an equivalently higher water rating basis. Proper use of the correction factors will then give you a better chance to get a pump that is just right, not too big or too small, and one with a properly sized driver. I
VOL 5 5
NO. 1
JANUARY I963
45
