Page 219 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 191

ple 3-5: NPSH Available in Open Vessel Not at Sea Then: NPSH available = S + (Pa - PT) (2.31/SpGr) - hSL
Level, Use Figure 3-39 = -8 + (60 - 44) (2.31/0.58) - 12 = f43.8 feet (3-10)

Conditions: vessel is at altitude 1500 ft, where atmos- This presents no pumping problem.
pheric pressure is 13.92 psia = pa,
Example 3-8: Closed System Stem Surface Condenser
Liquid: water at 15O"F, vapor pressure P, = 3.718 psia NPSH Requirements, Use Figure 344
SpCr = 0.982
Assume vessel liquid level is 12 ft below centerline of This is a closed steam surface condenser system with
pump, SL = -12. condensate being pumped out to retreatment facilities.
Friction losses: assume calculated to be 1.1 ft of liquid. From the conditions noted on the diagram,

Friction loss in suction line side = 2.92 ft
Then: NPSHA available = S + (Pa - PLvp) (2.31/SpGr) - hs~
= -12 + (13.92 - 3.718)(2.31/0.982) - 1.1 Absolute pressure in condenser = p' = 1.5 in. Hg Abs
= +10.88ft (3-10) = l.s(l.13 ft/in. Hg)
= 1.71 ft water
Water from steam tables at saturation = 1.5 in. Hg Abs
The worst condition case should be calculated using @ 91.72"F
SIL, since this represents the maximum lift.
Vapor pressure, P'~, at 1.5 in. Hg Abs = 1.5(1.13)
= 1.71 ft water
Example 3-6: NPSH Available in Vacuum System, Use NPSHAavailable = +10 + (1.71 - 1.71) - 2.92
Figure 3-41A = +7.88 ft

Conditions: vessel is liquid collector at 28 in. Hg Vacu- The suction head or lift for the pump (separate calcu-
urn (referred to a 30 in. barometer). This is 30 - 28 = 2 lation from NPSHA) is:
in. Hg abs, or Pa = [ (14.7/30)] (2) = 0.98 psia. The 28.42 in. vacuum Hg (gauge) is equivalent to 1.5
Liquid: water at 101.2"F, vapor pressure = 0.98 psia. in. Hg Abs
Assume vessel liquid level is 5 feet above centerline of
pump, S = + 5', worst case, S, = 2' 28.42 in. vacuum (1.137) = 32.31 ftwater
Static submergence = 10.0 (see figure)
Friction losses: assume to be 0.3 foot of liquid
Friction/entrance losses = 2.92 ft
Net static submergence = 7.08 7.88 ft
Then: NPSHA available = S + (Pa - F??) (2.31/SpGr) - hSL Equivalent suction lift = 25.23 ft [Note: 32.31
= + 5 + (0.98 -- 5.98)(2.31/0.994) - 0.3 - 7.081
= + 4.7 ft (3-1 0)
(= vacuum effect less net submergence)
Worst case = 1.7 (not practical design)
The pump selected for this application (water boiling CONDENSER
at 0.98 psia) must have a required NPSH less than 4.7 ft,
preferably about 3 to 3.5 ft. This is a difficult condition. If
possible the vessel should be elevated to make more head
(S) available, which .Nil1 raise the available NPSH.

SHA Available in Pressure System, Use
Figure 3-41(b)

Conditions: vessel contains butane at 90°F and 60 pia
system pressure. Pa = 616
4
slicane vapor pressure, P,, at 90°F = 44 psia, SpGr
= 8.58.
Assume liquid level is 8 feet below pump centerline, Figure 3-44. Surface condenser condensate removal. Closed sys-
S = -8. tem steam surface condenser NPSH requirements. (By permission,
Cameron Hydraulic Data, 16th ed. Ingersoll-Rand Co., 1979, p.
Friction losses: assume to be 12 ft of liquid. 1-1 2.)