Page 122 - Water and wastewater engineering

INTAKE STRUCTURES 3-19

The AFD allows changes in the flow rate by changing the frequency of the alternating current (AC)
electrical supply. The selection of the number of pumps and drive arrangement is based on an eco-
nomic evaluation of the alternatives that includes the capital investment for the pump(s) and the
drive(s) and the required operating floor area as well as the cost of power and maintenance. The AFD
has a higher capital cost than constant speed drives but is often more efficient to operate because of
lower power costs that result from being able to match pumping rate to the demand for water.
The power input of the drive is estimated from the following equation:

QH t
P (3-3)
E p

where P brake power, kW

3
specific weight of fluid, kN/m
3

9.807 kN/m for water at 5 C
3
Q flow rate, m /s
H t total dynamic head, m
E p efficiency of pump

Pump Capacity. The flow rate of the pump is called the capacity or discharge ( Q ). The capac-
3
ity is usually expressed in cubic meters per second (m /s) for large pumps and cubic meters per
3
hour (m /h) for small pumps.
Although the wet well structure is designed for anticipated demands at a design life of 50
to 75 years, common practice is to provide initial pumping capacity for a 20-year life. As the
demand increases over time, additional capacity may be added as required by the addition or
replacement of pumping units (Honeycutt and Clopton, 1976).

Head. The term head is the elevation of a free surface of water above or below a reference datum.
The reference datum for a vertical flow centrifugal pump is the inlet to the impeller. Two cases are
illustrated in Figures 3-13 and 3-14 . The definition of terms used in the figures is given in Table 3-6
on page 3-22.
The total energy required to deliver the water from the wet well to the discharge point at the
water treatment plant is commonly calculated in terms of equivalent elevation of a water column.
This is termed the total head or total dynamic head (H t or TDH). It is expressed in meters (m). It
is calculated as

2
v d
TDH H stat h ent h fs h fd h f vs h f vd (3-4)

2 g
where, with the exception of h ent (which is the entrance loss), the terms are described in Table 3-6 .
The friction losses may be calculated from a revised form of the Hazen-Williams equation:

.
⎛ Q ⎞ 185 ⎛ L ⎞
h fs or h fd 10 7 ⎝ C ⎠ ⎜ ⎝ 487 ⎟ ⎠ (3-5)
.
.
D
where h fs or h fd head loss in m, L equivalent length of pipe in m, and the other terms are as
defined previously.