Page 235 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 207

/oo 200
90 180
80 160
7'0 140
*
L ILL
Q) 2 50 .- r100
af
20 :E 40 'z 80
15 E30 $60
af IO 20 40
.a? I 20
0
b
in
0
0 100 200 300 400 500
Capacity , Gpm.
Figure 3-58. Typical curves showing the effect on a pump designed for water when pumping viscous fluids. (By permission, Pic-a-Pump,
4 959, Allis-Chalmers Mfg. Co.)

Temperature Rise md Minimum Flow or, alternate procedure [33,6]. For low capacity:

When a pump operates near shut-off (low flow) capac-
ity alnd head, or is Iiandling a hol material at suction, it (3-37)
may become overheated and create serious suction as well
as mechanical problems. TO avoid overheating due to low
flow, a minimum rate (GPM) should be recognized as where H,, = total head of pump at no flow or shutoff or at any
necessary for proper heat dissipation. However, it is not flow rate with corresponding efficiency from
necessarily impossible to operate at near shutoff condi- pump curve, ft
tions, provided (1) it does not operate long under these
conditions, as temperature rises per minute vary from less e = pump efficiency at the flow capacity involved (low
than 1 "F to 30-4OoF, or (2) a by-pass is routed or recycled flow), decimal
from the discharge through a cooling arrangement and
back to suction to artificially keep a minimum safe flow Another alternate procedure [ 101
through the pump while actually withdrawing a quantity
below the minimum, yet keeping the flowing temperature AT, = (GPM) (Hso) (SpGr)/3960 (3-38)
down [SI].
See Figure 3-59 and Figure 3-60 for a graphical solution
to the equation above for temperature rise. Figure 3-59
1. Temperature rise in average pump during operation illustrates the characteristics of a boiler feed water pump
[GI. set to handle 500 GPM water at 220°F for a total of 2600
feet head. The temperature rise curve has been superim-
posed on the performance chart for the pump, and values
42.4 Ps,
ATr = ~ ,')F/min [25] (3 - 36) of ATr are calculated for each flow-head relationship. Note
w, cp how rapidly the temperature rises at the lower flows. This
heating of the fluid at low flow or no flow (discharge valve
shut, no liquid flowing through the pump) can be quite
where [25] rapid and can cause major mechanical problems in the
pump's mechanical components. The maximum temper-
ATr = temperature rise, "F/min
ature rise recommended for any fluid is 15°F (can be a bit
P,, = brake horsepower at shutoff or no flow higher at times for the average process condition) except
when handling cold fluids or using a special pump
Mil = weight of liquid in pump, Ibs
designed to handle hot fluid, such as a boiler feed water
cp = specific heal of liquid in pump pump of several manufacturers.