Page 239 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 209

where eM = minimum safe flowing efficiency, overall fluid handled and the calculated NPSHA condition. For
pump, fraction NPSHR refer to corrections discussed earlier.
H, 0 = head at no flow or shutoff, ft
cp = specific heat of liquid, BTU /lb/°F Centrifugal Pump Specifications
Kf R = temperature rise in liquid, °F
Figure 3-61 presents specifications for a centrifugal
e. Read minimum safe now in GPM from pump per- pump. Although the process engineer cannot or should
formance curve at value of minimum efficiency cal- not specify each item indicated, he must give the perti-
culated in (d). nent data to allow the pump manufacturer to select a
pump and then identify its features. Pumps are selected
Example 3-18: Maximum Temperature Rise Using Boiler for performance from the specific characteristic curves
Feed Water covering the casing size and impeller style and diameter.
Often the process fluids are not well known to the pump
Using the example of Reference [6], assume a pump manufacturer, therefore the materials of construction, or
with characteristic curve and added temperature rise data at least any limitations as to composition, must be speci-
as shown on Figure 3-59 is to handle boiler feed water at fied by the engineer.
220°F, with a system available NPSHA = 18.8 feet. The
vapor pressure of water at 220°F is 17.19 psia from steam Example 3-19: Pump Specifications, Figure 3-61
tables and the SpGr = 0.957. Correcting the 18.8 feet
NPSHA: psia = 18.8 (1/ [2.31/0.957)] = 7.79 psia at The pump specified identifies the design data, key por-
220°F. tions of the construction materials and driver data as
The vapor pressure to which the water may rise before required information for the pump manufacturer. If the
it flashes is 17.19 psia + 7.79 psia = 24.98 psia. pump is to be inquiried to several manufacturers this is all
From steam tables (or fluid vapor pressure tables), that is necessary. The individual manufacturers will iden-
read at 24.98 psia (for waler of this example), tempera- tify their particular pump selection and details of con-
ture = 240°F. struction materials and driver data. From this information
Therefore, allowable temperature rise of the water a pump can be selected with performance, materials of
(this example) = 240° - 220°F = 20°F. construction, and driver requirements specified.
A plotted curve as shown on Figure 3-59 [33] shows In the example the manufacturer has been specified
that at point A a rise of 20°F on the temperature rise from available performance curves, and the details of
curve corresponds Lo a flow of 47 GPM minimum safe for construction must be obtained. The pump is selected to
the pump handling 220°F, with NPSHA of 18.8 feet. operate at 22 GPM and 196 to 200 feet head of fluid, and
An alternate estimate for minimum flow [ 11]: must also perform at good efficiency at 18 GPM and a
Minimum flow (for water) through pump, head which has not been calculated, but which will be
close to 196 to 200 feet, say about 185 feet. Ordinarily, the
QM = 0.3 ?, 0, GPM (3-41) pump is rated as shown on the specification sheet. This
insures adequate capacity and head at conditions some-
where P, = shutoff horsepower what in excess of normal. In this case the design GPM was
0
determined by adding 10 percent to the capacity and
For cold liquids, general service can often handle ti Tr of allowing for operation at 90 percent of the rated efficien-
up to 100°F, a rule with approximately 20% factor of safety: cy. Often this latter condition is not considered, although
factors of safety of 20 percent are not unusual. However,
Q, 1 = 6 P, 0 /,1.T" GPM (3-42) the efficiency must be noted and the increase in horse-
power recognized as factors which are mounted onto nor-
Li.Tr= permissible temperature rise, °F. mal operating conditions.
Sometimes the speed of the pump is specified by the
The NPSHR required at the higher temperature may purchaser. However, this should not be done unless there
become the controlling factor if cavitation is not Lo occur. is experience to indicate the value of this, such as packing
The minimum flow simply means that this flow must cir- life, corrosion/erosion at high speeds, and suspended
culate through the pump casing (not recirculate with no particles; as the limitation on speed may prevent the man-
cooling) back to at least the initial temperature of the feed, ufacturer from selecting a smaller pu!.Tip. In some cases it
if excessive temperatures are not to develop. The best prac- must be recognized that high heads cannot be reached at
tice is to request the manufacturer to state this value for the low speeds in single stage pumps. Table 3-7 presents sug-