Page 230 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

202 Applied Process Design for Chemical and Petrochemical Plants

(3-20)

(3-21)

For a fixed speed [25] :

(3-23)

(3-24)

(3-25)

For geometrically similar impellers operating at the
same specific speed, the affinity laws are [25,11]:

Capacity , Gpm.

Figure 3-55. Relation of speed change to pump characteristics.
(3- 28)
2. For changes (cut-down) in impeller diameter (not
design) at fixed efficiency: [ 111
where: condition of subscript (2) represents the new non-
cavitating or desired condition, and condition of sub-
script (1) represents the condition for which a set of con- (3 - 29)
ditions are known.
These relations do not hold exactly if the ratio of speed
change is greater than 1.5 to 2.0, nor do they hold if suc-
tion conditions become limiting, such as NPSH. (3- 30)
Figure 3-55 illustrates the application of these perfor-
:]
mance laws to the 1750 rpm curves (capacity, brake horse- BHP, = BHP, [ [ 21 3
3
power, and efficiency) of a particular pump to arrive at (3- 31)
the 1450 rpm and 1150 rpm curves. Note that the key
value is the constant efficiency of points (1) and (2).
When the speed drops to 1450 rpm, capacity drops: where dl is the original impeller diameter in inches, and
subscript (2) designates the new or desired conditions cor-
Q2 = 204(1450/1750) 169 GPM responding to the new impeller diameter, d2. All perfor-
mance changes occur simultaneously when converting from
The head also drops: condition (1) to condition (2), no single condition can be
true unless related to its corresponding other conditions.
An impeller can be cut from one size down to another
H, = 64(1450/1750)2 = 44 ft
on a lathe, and provided the change in diameter is not
greater than 20 percent, the conditions of new operation
and: can be described by the type of calculations above. A cut to
reach 75-80 percent of the original diameter may adverse-
(BHP), = 6.75(1450/1750)3 = 3.84 BHP ly affect performance by greatly lowering the efficiency [4].