Page 124 - Water and wastewater engineering

INTAKE STRUCTURES 3-21

2
v /2g
d
h h fvd
fd
EGL HGL
2
v d /2g

h gd
H TDH
h d
t
H g H stat

Pump
Gauge Gauge
Datum

h gs h s

h
h h ent
fs fvs
2
v d /2g
H baronetric
NPSH A

Key
H
vapor
Butterfly valve
Absolute zero pressure
Check valve
FIGURE 3-14
Terminology for a pump with a negative suction head.

Net Positive Suction Head. Liquids at temperatures above their freezing point have a corre-
sponding vapor pressure. If the pressure in a pump suction pipe is reduced below the vapor pres-
sure, the liquid will flash, that is, it will form a vapor. Because no water pump of ordinary design
can pump only vapor, flow to the pump decreases and the pump is said to be “vapor-bound.” A
more serious consequence occurs when the vapor and water mixture move toward the pump dis-
charge. Here the pressure is increased, and the vapor bubbles collapse from bubble size to particle
size. This implosion is violent and destructive. The implosion blasts small particles of metal from
the impeller. This process is called cavitation. Ultimately it destroys the impeller. The most com-
mon method to avoid cavitation is to provide enough head on the pump suction so that the pressure
in the suction pipe is always greater than the vapor pressure of the liquid. This is called the required