Page 131 - Water and wastewater engineering

3-28 WATER AND WASTEWATER ENGINEERING

Standards Institute (ANSI) has provided the following guidance for the Preferred Operating
Range (POR) for vertical pumps (ANSI/HI 9.6.3-1997):
For specific speeds less than 87, the bounded region is between 70 and 120 percent of the
BEP flow rate.
For higher specific speeds, the bounded region is between 80 and 115 percent of the BEP
flow rate.

Specific speed is defined as
.
nQ 0 5
n s 075 (3-9)
.
H t
where n revolutions per minute
3
Q flow rate, m /s
H t total dynamic head, m
ANSI also established a wider region than the POR called the Acceptable (or Allowable)
Operating Region (AOR). It “is that range of flow recommended by the pump manufacturer over
which the service life of a pump is not seriously compromised. Minimum bearing life will be
reduced and noise, vibration, and component stresses will be increased when a pump is operated
outside its POR. As a result, service life within the AOR may be lower than within the POR.” The
pump may be operated in this region for short, infrequent periods without significant equipment
deterioration (Cooper and Tchobanoglous, 2006).

Example 3-6. Using the pump head-discharge curve shown in Figure 3-17 for the system head
curve shown in Figure 3-16 , and data from Example 3-5, specify the following at the average
flow rate: (1) BEP, (2) number of stages, (3) efficiency at BEP, (4) the operating range of flow
rate, (5) motor power, (6) the depth of the wet well, and (7) the location of the pump intake.

Solution:
a. From Figure 3-16 , the maximum total system head is 18.4 m at the average flow rate
3
of 6,000 m /d. On Figure 3-17, the BEP is found at a flow rate of
3
6 000 m /d 3
,
250 m /h
24 h/d
and a head of 3 m/stage.
b. The number of stages required is
Total system head 18 4 . m
No of stages 613. or 7
.
Head per stagge 3
This is less than the maximum number of stages allowed of 35 noted in the box in
Figure 3-17 .
3
c. From Figure 3-17 at 250 m /h and 3 m head per stage, the efficiency is 78 percent for
impeller A.
d. From Figure 3-17 , the rpm 1,750. The flow rate is

3
250 m /h 3
0 0694 m /s
.
,
3 600 s/h