For repairable hydrosystems, such as pipe networks, pump stations, and storm runoff drainage structures, failed components within the system can be repaired or replaced so that the system can be put back into service. The time required to have the failed system repaired is uncertain, and consequently, the total time required to restore the system from its failure state to an operational state is a random variable.
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TABLE 5.2 Reliability and Maintainability of Water Distribution Subsystems by Generic Group
|
Subsystem
|
MTBF* (x 106 hours)
|
MTTR* (hours)
|
|
Pumps
|
|
Centrifugal, open impeller
|
0.021660
|
7.825
|
|
Axial flow, propeller
|
0.074191
|
16.780
|
|
Power transmission
|
|
Concentric reducer
|
0.122640
|
2.000
|
|
Parallel shaft
|
0.710910
|
32.000
|
|
Right angle shaft
|
0.019480
|
1.400
|
|
Vertical shaft
|
0.031470
|
2.023
|
|
Variable speed, hydraulic
|
0.349500
|
—
|
|
Variable speed, other
|
0.014200
|
2.500
|
|
Gear box
|
0.045780
|
3.530
|
|
Chain drive
|
0.017850
|
8.000
|
|
Belt drive
|
0.091210
|
1.800
|
|
Motors
|
|
Multiphase
|
0.068000
|
6.853
|
|
Variable speed, ac
|
0.114820
|
8.000
|
|
Gas engine
|
0.023800
|
24.000
|
|
Valves
|
|
Gate
|
0.008930
|
3.636
|
|
Ball
|
0.011460
|
—
|
|
Butterfly
|
0.032590
|
1.000
|
|
Plug
|
0.028520
|
—
|
|
Controls
|
|
Electrical
|
0.100640
|
2.893
|
|
Mechanical
|
0.031230
|
8.000
|
|
Pressure (fluid)
|
0.035780
|
8.236
|
|
Pressure (air)
|
0.018690
|
3.556
|
*MTBF = mean time between failure; MTTR = mean time to repair; MTBF = MTTF + MTTR.
SOURCE : From Schultz and Parr (1981).
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