1. 4.6 Cavitation
Since NPSHav. is the absolute pressure available less the vapor pressure of the liquid, the NPSHav. should always be greater than the NPSHreq. If this were not the case, the pressure at some point in the pump suction area will be less than the vapor pressure of the liquid, and cavitation will occur. Cavitation is the formation of pockets of vapor, or bubbles, at a point inside the pump where the liquid pressure drops below its vapor pressure. These vapor bubbles are carried along to the higher pressure area of the pump, where they collapse. It is the violent collapse of the bubbles that cause the damaging effects of cavitation; noise, erosion, and short service life. Cavitation also reduces capacity and efficiency, as well as causes pulsations in the discharge pressure. If the NPSHav drops below that of required by the pumps design (i.e., NPSHav.˂ NPSHreq), the pressure within the impeller may be reduced to the vapor pressure of the fluid, Fig (4.9). If this occurs, the water will vaporize and a mixture of vapor and water will enter the pump.

2. Cavitation can be corrected by:
1) Increase the diameter of the pump suction pipe.
2) Decrease the pump speed.
3) Increase the static head on suction side.
4) Decrease of flow rate.

3. The barometric pressure (hb) is a function to the altitude of the pump
The barometric pressure (hb) is a function to the altitude of the pump. It can be calculated from the following table:
Table (4.2) Standard barometric pressure
And the following table used to calculate the vapor pressure.
Table (4.3) Standard vapor pressure of water

4. 4.5 Effects of speed and diameter of impeller on centrifugal pump
The rotational speed of an Impeller affects the operating characteristics of the pump. Equations give the relationship of pump discharge, head and power output with rotational speed:

6. Where
N1, N2: rotational speed of the pump, rpm
Q1, Q2: discharge corresponding N1, N2
TDH1, TDH2: total dynamic head corresponding N1, N2
Equations 4.11 – 4.13 give the relationship of pump discharge, head and power output with impeller diameter:

7. 4.6 Pumping stations
In general, pumping station can be classified as wet-pit or dry-pit. These classifications are based on the location of the pumps relative to the wet well or dry pit. In wet-pit pumping stations, the pumps are located in the wet well as illustrated in fig (4.6 a). In dry-pit pumping stations, pumps are located in a dry enclosure separated from the wet well, as shown in Fig (4.6 b).

8. 4.8 Pumping stations
In general, pumping station can be classified as wet-pit or dry-pit. These
classifications are based on the location of the pumps relative to the wet well or dry pit:
Wet-Pit Stations - In the wet-pit station, the pumps are submerged in a wet well involving the use of submersible pumps. The submersible pumps handle storm water very well and they allow for convenient maintenance in wet-pit stations because of easy pump removal. Submersible pumps are available in large sizes and should be considered for use in all station designs. Fig. (4.10 a)
ry-Pit Stations - Dry pit stations consist of two separate elements: the storage box or wet well and the dry well. Storm water is stored in the wet well, which is connected to the dry well by horizontal suction piping. Dry-pit stations are more expensive than the wet-pit stations. At dry-pit stations, centrifugal pumps are usually used. The main advantage of the dry-pit station is the availability of a dry area for personnel to perform routine and emergency pump and pipe maintenance. Fig. (4.10 b).