Page 233 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 203

Most standard pump curves illustrate the effect of When the performance of a pump handling water is
changing impeller diameters on characteristic perfor- known, the following relations are used to determine the
mance (Figure 3-36A). Note change as reflected in the performance with viscous liquids [17]:
different impeller diameters. However, the slight change
in efficiency is not recorded over the allowable range of (3-32)
impeller change.
Recognizing the flexibility of the affinity laws, it is bet- (3-33)
ter to select an original pump impeller diameter that is
somewhat larger than required for the range of anticipat- (3-34)
ed performance, and then cut this diameter down after
in-service tests to a slightly smaller diameter. This new per- BHP,i, = (Qvi,) (l-Ivis) (SpGr)/3960(£,i,) (3-35)
formance can be predicted in advance. Once the impeller
diameter is too small, it cannot be enlarged. The only Determine the correction factors from Figure 3-56 and
solution is to order the required large impeller from the Figure 3-57, which are based on water performance
manufacturer. because this is the basis of most manufacturer's perfor-
mance curves (except, note that the "standard" manufac-
Example 3-15: Reducing Impeller Diameter at Fixed turer's performance curves of head vs GPM reflect the
RPM head of any fluid, water, or other non-viscous). Do not
extrapolate these curves!
If you have a non-cavitating (sufficient NPSH) operat- Referring to Figure 3-56 [ 17]:
ing 9-inch impeller producing 125 GPM at 85 feet total
head pumping kerosene of SpGr = 0.8 at 1750 rpm using 1. The values are averaged from tests of conventional
6.2 BHP (not motor nameplate), what diameter impeller single-stage pumps, 2-inch to 8-inch, with capacity at
should be used to make a permanent change to 85 GPM best efficiency point of less than 100 GPM on water
at 60 feet head, at the same speed? performance.
2. Tests use petroleum oils.
Q2 = Q1 (cl2/d1) (3-23) 3. The values are not exact for any specific pump.
85 = 125(cl2/9)
d2 = 6.1 in. diameter (new) Referring to Figure 3-5 7 [ 17]:

The expected head would be 1. Tests were on smaller pumps, l-inch and below.
2. The values are not exact for any specific pump.
1)
H2 = Ht(d2/d 2 (3-24)
H2 = 85(6.1/9)2 The charts are to be used on Newtonian liquids, but
= 39.0 ft (must check system new total head Lo deter- not for gels, slurries, paperstock, or any other non-uni-
mine if it will satisfy this condition.) form liquids [17].
Figure 3-56 and 3-57 are used to correct the perfor-
The expected brake horsepower would be mance to a basis consistent with the conditions of the
usual pump curves. In order to use the curves, the fol-
2
1
BHP2 = BHP (d /d 1)3 (3-25) lowing conversions are handy:
BHP2 = 6.2(6.1/9) 3
= 1.93 BHP (use a 2- or 3-hp motor) Centistokes = centipoise/SpGr
t]fects of Viscosity SSU = Saybolt Seconds Universal
= (Centistokes) ( 4.620) at 100°F
When viscous liquids are handled in centrifugal pumps, = (Centistokes) ( 4.629) at 130°F
the brake horsepower is increased, the head is reduced, = (Cernistokes) (4.652) at 210°F
and the capacity is reduced as compared to the perfor-
mance with water. The corrections may be negligible for Example 3-16: Pump Performance Correction For
viscosities in the same order of magnitude as water, but Viscous Liquid
become significant above 10 centistokes ( l O centipoise for
SpGr = 1.0) for heavy materials. While the calculation When the required capacity and head are specified for
methods are acceptably good, for exact performance a viscous liquid, the equivalent capacity when pumping
charts test must be run using the pump in the service. (text continued on page 206)