The design period is not the same as the life expectancy. The design period is the length of time it is estimated that the facility will be able to meet the demand, that is, the design capacity. The life expectancy of a facility or piece of equipment is determined by wear and tear.
The complete water supply project includes huge and costly constructions such as dams, reservoirs, treatment works and network of distribution pipelines. These all works cannot be replaced easily or capacities increased conveniently for future expansions.
While designing and constructing these works, they should have sufficient capacity to meet future demand of the town for number of years. The number of years for which the designs of the water works have been done is known as design period. Mostly water works are designed for design period of 20-30 years, which is fairly good period.
New water supply projects are generally made large enough to meet the demand for the future. The number of years selected for the design period is based on the following:
- Regulatory constraints.
- Population growth rate
- Interest rate on Capital.
- The useful life of the structures and equipment.
- The ease or difficulty of expansion.
- Performance in early years of life under minimum hydraulic load.
Although other indicators may lead to the conclusion that a long design period is favored, serious consideration must be given to the impact of low flow rates in the early years of the project. In addition to the behavior and efficiency of the unit operations, the impact
on the energy efficiency of the equipment should be evaluated. A successful alternative is the use of modular units and construction of hardened facilities without installation of mechanical equipment until the units are needed.
Design periods that are commonly employed in practice and commonly experienced life expectancies are shown in Table below;
|Type of facility||Characteristics||Design period||Life expectancy|
|Large dams and pipelines||Difficult and expensive to enlarge||40–60||100|
|Wells||Easy to refurbish & replace||15–25||25|
|Treatment plants||Fixed facilities Difficult and expensive to enlarge or replace||20–25||50|
|Equipment||Easy to refurbish or replace||10–15||10–20|
|Distribution systems Mains 60 cm||Replacement is expensive and difficult||20–25||60|
|Distribution systems Laterals and mains 30 cm||Easy to refurbish or replace||To full development||40–50|
As per CPHEEO Manual, the recommendations for design period of various facilities is as below;
The water supply infrastructural is normally designed for a period of thirty years after their completion. The time lag between design & completion of a project should also be taken into account. Necessary land for future expansion of facilities should be acquired in the beginning itself. Expensive tunnels and large aqueducts should be designed for ultimate project demand. Where failure of components is forseeable, such components shall be provided in duplicate.
Some major components of Water Supply Project may be designed for a design period as below;
|S.No.||Items||Design Period in Years|
|1||Storage by Dams||50|
|3 i||Pumping – Pump House Civil Works||30|
|3 ii||Electric Motors and pumps||15|
|4||Water Treatment Units||15|
|5||Pipe Connections to several treatment units and other small appurtenances||30|
|6||Raw Water and Clear Water Conveying Mains||30|
|7||Clear Water Reservoir at the head works & balancing tank and service reservoirs (overhead or ground level)||15|