Page 184 - Water and wastewater engineering

WELLS 4-39

The open area of the screen is

2
.
A (0 097 m /m )(31 m ) 3 0 . m 2

The velocity is
⎛ 1 ⎞
3
(32m /h ) ⎜ ⎝ 3 600 s/h ⎠ ⎟
v , 2 0003 m/s
.
.
3 0m
Thus, the entrance velocity criteria are met with the longer screen. The decision on
whether or not to shorten the screen (or reduce its diameter) is based on economic con-
siderations: well efficiency, the cost of the screen, and design life.
k. Pump selection
The required head and discharge are used to select an appropriate pump from manufac-
turer’s data. The total dynamic head (TDH) is calculated as
TDH static lift to ground level static lift to the top of the reservoir

h v minor losses
o
h f
The drawdown was estimated as 6.8 m in step d(3). The 6.8 m is measured from the
static piezometric surface that is 5.2 m below grade. The drawdown piezometric surface
will be a total of 5.2 6.8 12.0 m below the ground surface.
The total dynamic head required for the pump is
h
.
TDH 12 0 m 50 m to the top of the reservoir h f h v minor losses

From the problem statement h f h v minor losses 0.15 m. Therefore,
.
.
.
TDH 12 0 m 50 m 0 15 m 62 15 m
From manufacturer’s catalogs, find a pump characteristic curve with maximum effi-
3
ciency at 32 m /h and 62.15 m TDH. For this example, the pump in Figure 4-15 on
3
page 40 was selected. At a discharge of 32 m /h, a head per stage of 3.2 m is shown. The
number of stages to achieve the TDH is
TDH 62 .15 m
No stages 19 4.or 20 stages
.
head/stage . 3 2 m/stage

From the box at the left of the head-discharge curve, the maximum number of stages
is 35. Therefore this pump will work.
l. Elevation of pump intake
The pump intake elevation must meet both the requirements of the regulatory agency (in
this instance MSDWA rules) and the NPSH.

The MSDWA requires that the pump be set a minimum of 1.5 m above the top of the
well screen. The required NPSH is that at run-out. For the pump in Figure 4-15 , it is
about 1.5 m. Using Tables 3-7 and 3-8 for the water temperature and altitude given