Page 125 - Water and wastewater engineering

3-22 WATER AND WASTEWATER ENGINEERING

TABLE 3-6
Definition of terms for Figures 3-13 and 3-14
Term Definition
Absolute pressure (h a ) Barometric pressure in vessels open to the atmosphere
Energy grade line (EGL) Total energy at any point in the pumping system
Fitting and valve losses (h fvs , h fvd ) Energy loss due to eddy formation and turbulence as the water passes through
a fitting or valve
Friction headloss (h fs , h fd ) Head of water that must be supplied to overcome friction losses in the pipe system
Hydraulic grade line (HGL) Locus of all pressure head values—always below the energy grade line by the amount
of the velocity head
Manometric suction head (h gs ) Suction gauge reading
Manometric discharge head (h gd ) Discharge gauge reading
Manometric head (H g ) Difference between the manometric suction head and the manometric discharge
head (h gd h gs )
NPSH A Net positive suction head available
NPSH R Net positive suction head required
Static suction head (h s ) Difference in elevation between the wet well water level and the reference datum
of the pump impeller
Static discharge head (h d ) Difference in elevation between the reference datum of the pump impeller and the
discharge water level
Total static head (H stat ) Difference in elevation between the water level in the wet well and the water level
at the discharge (h d h s )
2
Velocity head (v /2g) Kinetic energy in the water being pumped at any point where v velocity of water
2
and g is the acceleration due to gravity 9.81 m/s
Adapted from Cooper and Tchobanoglous, 2006.

Net Positive Suction Head (NPSH R ). The NPSH R is a function of the pump design and operating
conditions (capacity, speed, and discharge head). Each model pump has a different NPSH R . It is
provided by the manufacturer along with other data graphically on a head-discharge curve.
As part of the process of selecting an appropriate pump, the design engineer must evaluate the
available Net Positive Suction Head (NPSH A ) and, if appropriate, adjust the head (depth of water).
Two operating conditions are of interest: (1) source of water above the pump and (2) source of
water below the pump. Using Figure 3-15 to identify terms, the equations for these conditions are

For water above pump:

NPSH A h a h s h fs h va (3-7)
For water below pump:

NPSH A h a h s h fs h va (3-8)

where h va is the absolute vapor pressure and other terms are as described in Table 3-6 . The abso-
lute pressure of the standard atmosphere is a function of altitude above or below mean sea level
as shown in Table 3-7 . The vapor pressure is a function of the water temperature as shown in
Table 3-8 on page 3-24.