Page 485 - Chemical process engineering design and economics

462 Chapter 8

Net Positive Suction Head (NPSH)

To prevent cavitation in the pump (NPSH)A must be greater than (NPSH)R. The
design of the suction piping and not the discharge piping determines (NPSH).
A
The pump manufacturer specifies the (NPSH) R.
(NPSH) A = H zi + Hpi-Hps-HvPi

where the subscript i refers to the inlet of the pump.
Table 8.6 contains values for the first three terms. Next, calculate the vapor-
pressure head

32.2 (0.363)
g c p v
H Vp; = ——— = —————— 144 = 0.8393 ft (0.256 m)
g p 32.2 (62.3)

(NPSH) A = 7.00 + 34.0 - 0.290 - 0.8393 = 39.9 ft (12.2 m)

From Figure 8.4.2, (NPSH) = 22 ft (6.71 m) at 100 gal/min (0.379 m3/min). Be-
R
cause (NPSH) A > (NPSH) R the pump will not cavitate.

DISCHARGE SIDE OF THE PUMP
Pipe Sizing

Calculate a preliminary area.

Q gal 1 ft 3 1 min 1 s
2
2
A = — = 100 —— ————— ———— —— = 0.03713 ft (34.5 cm )
v min 7.481 gal 60 s 6ft
1/2
D = [ (4 / 3.142) (0.03713) ] = 0.2174 ft (2.609 in, 6.63 cm)
From Table 8.2A select a 3 in pipe. Therefore, D = 3.068 in (0.2557 ft, 7.79 cm)
2
2
and A = 0.05136 ft (47.7 cm ). Because the pipe size is greater than 2 in (5.08
cm), the discharge piping is also welded, requiring flanged connections to equip-
ment.
Now, the actual water velocity,
100 1 1
v = ——— —— ———— = 4.338 ft/sec (1.32 m/s)
7.481 60.00.05136

Copyright © 2003 by Taylor & Francis Group LLC