Page 216 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

188 Applied Process Design for Chemical and Petrochemical Plants

in feet through which water would have to fall to acquire then the true total head = (45.5 + 4.5) - (8.6 + 1.4) =
the same velocity, expressed as foot-pounds per pound of 40 ft, and the difference in gauge readings would be 45.5
liquid. - 8.6 = 36.9 ft, or an error of ’7.8%.
Most designers ignore the effects of velocity head, but
h, = v2/2 g, feet of fluid 3-7 the above brief examples emphasize that the effect varies
depending on the situation and the degree of accuracy
where h, = velocity head, ft desired for the head determinations.
v = liquid velocity, ft/sec
g = acceleration of gravity, ft/sec-sec Friction

As a component of both suction and discharge heads, The friction losses for fluid flow in pipe valves and fit-
velocity head is determined at the pump suction or dis- tings are determined as presented in Chapter 2. Entrance
charge flanges respectively, and added to the gauge read- and exit losses must be considered in these determina-
ing. The actual pressure head at any point is the sum of tions, but are not to be determined for the pump
the gauge reading plus the velocity head, the latter not entrance or discharge connections into the casing.
being read on the gauge since it is a kinetic energy func-
tion as contrasted to the measured potential energy. The NPSH and Pump Suction
values are usually (but not always) negligible. Present
practice is for these velocity head effects at the pump suc- Net positive suction head (in feet of liquid absolute)
tion and discharge connections to be included in the above the vapor pressure of the liquid at the pumping tem-
pump performance curve and pump design, and need perature is the absolute pressure available at the pump
not be actually added to the heads calculated external to suction flange, and is a very important consideration in
the pump itself [SI, selecting a pump which might handle liquids at or near
It is important to verify the effects of velocity head on their boiling points, or liquids of high vapor pressures.
the suction and discharge calculations for pump selec- Do not confuse NPSH with suction head, as suction
tion. In general, velocity head (kinetic energy) is smaller head refers to pressure above atmospheric [1’7]. If this
for high head pumps than for low head units. Sometimes consideration of NPSH is ignored the pump may well be
the accuracy of all the other system calculations does not inoperative in the system, or it may be on the border-line
warrant concern, but for detailed or close calculations and become troublesome or cavitating. The significance
velocity head should be recognized. The actual suction or of NPSH is to ensure sufficient head of liquid at the
discharge head of a pump is the sum of the gauge reading entrance of the pump impeller to overcome the internal
from a pressure gauge at the suction or discharge and the flow losses of the pump. This allows the pump impeller to
velocity heads calculated at the respective points of gauge operate with a full “bite” of liquid essentially free of flash-
measurement. ing bubbles of vapor due to boiling action of the fluid.
Regardless of their density, all liquid particles moving The pressure at any point in the suction line must
at the same velocity in a pipe have the same velocity head never be reduced to the vapor pressure of the liquid (see
[ 1 I]. The velocity head may vary across a medium to large Equation 3-6). Both the suction head and the vapor pres-
diameter pipe. However, the average velocity of flow, that sure must be expressed in feet of the liquid, and must both
is, dividing the total flow as cu ft/sec by the cross-section- be expressed as gauge pressure or absolute pressure. Cen-
al area of the pipe is usually accurate enough for most trifugal pumps cannot pump any quantity of vapor, except
design purposes. possibly some vapor entrained or absorbed in the liquid,
Using the example of Reference [25], for a pump han- but do not count on it. The liquid or its gases must not
dling 1500 GPM, having a 6-inch discharge connection vaporize in the eye/entrance of the impeller. (This is the
and 8-inch suction connection, the discharge velocity lowest pressure location in the impeller.)
head is 4.5 ft and the suction is 1.4 ft, calculated as shown For low available NPSH (less than 10 feet) the pump
above. If the suction gauge showed 8.6 ft, the true head suction connection and impeller eye may be considerably
would be 8.6 + 1.4 = 10.0. If the discharge head showed oversized when compared to a pump not required to han-
105.5 ft head, the true total head would be 105.5 + 4.5 = dle fluid under these conditions. Poor suction condition
110.0 ft, less (8.6 + 1.4) or 100 ft. The net true total head due to inadequate available NPSH is one major contribu-
would be 110 ft - 10 ft = 100.0 ft. Looking only at the tion to cavitation in pump impellers, and this is a condi-
gauge readings, the difference would be 105.5 - 8.6 = tion at which the pump cannot operate for very long with-
96.9 ft, giving an error of 3.1% of the total head. As an out physical erosion damage to the impeller. See
alternate example, if the discharge head were 45.5 ft, References [111 and [261.