Page 279 - Water Engineering Hydraulics, Distribution and Treatment

EXAMPLE 8.11 LEAKAGE LOSSES VERSUS SPECIFIC SPEED FOR DOUBLE-SUCTION PUMPS
Specific speed S is a dimensionless quantity used to characterize turbomachinery, and is of significant importance in selecting a pump
for a specific application, since there is a relationship and internal leakage within the pump as shown in Table 8.3 for double-suction
split case centrifugal pumps.
Table 8.3
for double-suction pumps in Example 8.11
Leakage losses in %
Specific
speed S
of power input (%)
500
9.5
4.3
1,000 Specific speed versus leakage losses 8.7 Dual Water Supply Systems 257
1,500 2.6
2,000 1.8
2,500 1.4
3,000 1.2
3,500 1.0
Determine and discuss the specific speeds and the expected inherent losses in % of power input for two pumps:
3
1. Pump A: it has a 356-mm-(14-in.)-diameter impeller with its optimum efficiency at 0.2145 m /s (3,400 gpm) and 54.9 m
(180 ft.) head when operating at 1,900 rpm.
3
2. Pump B: it has a 356-mm-(14-in.)-diameter impeller with its optimum efficiency at 0.0126 m /s (200 gpm.) and 79.2 m (260
ft.) head when operating at 3600 rpm.
Solution 1 (US Customary System):
For pump A:
0.5
0.5
S = NQ ∕H 0.75 = 1,900 × (3,400) ∕180 0.75 = 2,255.
sv
Pump A would have an inherent loss of about 1.6%.
For pump B:
0.5
S = 3,600 × (200) ∕260 0.75 = 786.
Pump B would have an inherent loss of about 6%.
Solution 2 (SI System):
For pump A:
0.5
0.5
S = 51.7 NQ ∕H 0.75 = 51.7 × 1,900 × (0.2145) ∕54.9 0.75 = 2,255.
sv
Pump A’s expected inherent loss would be about 1.6%.
For pump B:
0.5
S = 51.7 × 3600 × (0.0126) ∕79.2 0.75 = 786.
Pump B’s expected inherent loss would be about 6%.

8.7 DUAL WATER SUPPLY SYSTEMS 1 for potable use; the other system is for lower quality water
(seawater or untreated raw freshwater, or treated/reclaimed
The term dual water supply systems refers to two water sup-
wastewater) for firefighting and toilet flushing purposes. In
ply distribution systems in a city. One is a freshwater system
Hong Kong and Saudi Arabia, dual water supply systems
have been in use for more than 40 years.
1 Section 8.7 is based on a timely article written by Dr. Daniel A. Okun a few Water supply networks are sized so that they can pro-
months before he passed away in 2007. The article appeared in the February vide the large flows needed for firefighting, but this creates
2007 issue of Water21, a magazine of the International Water Association,
pages 47–49; reproduced courtesy of IWA Publishing. potable water quality problems due to the long time water