Page 166 - Solar Power in Building Design The Engineer's Complete Design Resource

136 SOLAR POWER GENERATION PROJECT IMPLEMENTATION

Then the tabulated pressure drop from the piping manual for a specific length of
pipe is compared to the length and pressure drop calculated by proportion:

Pressure drop = pressure drop for specific length of pipe
× equivalent pipe length/pipe
Typically, end suction pumps are selected and are of the close-coupled type (where
the pump impeller fastens directly to the shaft and the pump housing bolts directly to
the motor) for up to about 15 hp and the base-mounted type (where the separate pump
and motor fasten to a base and are connected by a coupling) is used for larger sizes.
The static lift is typically the distance between the operating level in the cold water
basin and the reservoir inlet near the top of the towers.
When selecting a pump, it is important to make sure the net available suction head
exceeds the required net suction head. This ensures the application will not cause
water to vaporize inside the pump causing a phenomenon called cavitation.
Vaporization inside the pump occurs when small water particles essentially “boil” on
the suction side of the pump. These “bubbles” collapse as they pass into the high-
pressure side producing the classic “marbles sound” in the pump. If operated under
this condition, pumps can be damaged.
Pumps are also required to operate under net positive suction head (NPSH) condi-
tions, which means that the pump lift must be able to cope with the local barometric
pressure and handle the friction losses in the suction line and vapor pressure of the
water being pumped. Figure 5.16 depicts a large-scale solar pumping system in
Imperial Valley California.

Figure 5.16 Large-scale solar pumping system in Imperial Valley
California. Photo courtesy of WorldWater & Power Corporation.