When water is lifted from one level to another the height difference is called the static lifting height. The lift height can either equal the pressure head (in the case of a submerged pump) or it can be a combination of vacuum and pressure head depending on where the pump is placed. In addition to this, the pump has to overcome the head loss caused by the friction in the pipe on both sides of the pump. If a manometer that measures the pressure is con-nected to the pump outlet the measured pressure is the sum of the pressure head (static head) and head loss. When the water passes through the pump it needs to have certain velocity in order to flow; this is called the velocity head. The total pressure head is obtained by summing the manometric height and velocity head. The actual pressure head at the end of the pipe, in addition to the difference in level must also be considered, for example when the pump is required to deliver water to a pressurized tank.
To collect water from a lower level, a vacuum head must be overcome by removing air from the inlet pipe to create a vacuum inside it. The pressure of the atmosphere will force the surrounding water up the inlet pipe. For this reason the vacuum head must not exceed atmospheric pressure which is nor-mally 1013 mbar (10.3 mH2O), although it depends to some extent on the weather (low pressure, high pressure) and the altitude. When a pipe is com-pletely emptied of air, the water will therefore be forced up the pipe to a height of 10.3 m. The actual suction head of a pump is, however, lower than 10.3 mH2O, because of losses such as from the velocity head in the inlet pipe. To make a centrifugal pump self-suctioning it must include a mechanism, for instance a specially designed impeller, to remove the air from the inlet pipe.
If the vacuum into the pump is too high, the water may boil and vaporize. That the temperature of vaporization pressure dependent can easily be illus-trated with a pressure boiler, where the boiling temperature of the water is increased; similarly, below normal atmospheric pressure, the boiling temperature will decrease (see below). When a mixture of liquid and gas goes through a pump the boiling point will increase because the pressure around the water molecules increases from vacuum upwards. In changing from gas to liquid the bubbles undergo violent compression (implosion) and collapse creating very high local shock, i.e. a sharp rise and fall in the local pressure; the phenomenon is called cav-itation. If this happens in connections to a pump or in the impeller, small metal parts can be dislodged. Multiple indentations or dimples in the material can result. The same may occur on boat propellers where worm like holes may be observed in the material of the propeller.
Cavitation reduces the effectiveness of pumps and will also shorten pump life. A characteristic ‘hammer’ noise is produced inside the pump when it cavitates. Cavitation may also occur if there are leakages in the pipe or pipe connection on the suction side of the pump. If air leaks in here (known as ‘false air’), it will create air bubbles that enter the pump chamber with the water where they implode.
Cavitation can happen if the suction head is too high. When the pressure around the water molecules drops, the water will boil at a lower temperature (i.e. the boiling point of water is reduced). For example, if the pressure drops from atmospheric (10.3 mH2O) to 1 mH2O, water will boil at 46°C. This phenomenon can be observed when boiling water at high altitude, for instance in the Himalayas. Here the water will boil below 100°C because the atmospheric pressure is less than 10.3 mH2O. Atmos-pheric pressure also depends on the weather. The safe static suction head will also decrease with surface water temperature from 10.4 mH2O at 10°C to 7.1 mH2O at 21.1°C.12
If the pump is not self suctioning, the water level must be higher than the level of the pump. This means that the impeller needs a certain pressure to function optimally. The net positive suction head (NPSH) gives the absolute lowest pressure the water must have when flowing into the pump chamber, or (more easily) the actual height of water over the impeller. If the water pressure is lower than the NPSH the pump will cavitate. NPSH
depends on the water flow and increases with increasing flow; it can be described as follows:
hv=vapour pressure of the liquid at the operatingtemperature
hf=frictional losses due to fluid moving throughthe inlet pipe including bends
hh=pressure head on pumping inlet (negative if itis a static lift on the suction side of the pump).
A pump is to be chosen for a land-based fish farm. With an actual discharge (Q) and head (H), the nec-essary NPSH can be read from the pump perfor-mance curves to be 4 mH2O. The fish farm is situated close to the sea and the barometric pressure (hb) is measured to be 10.3 mH2O. The maximum tempera-ture during summer time is 30°C which corresponds to a vapour pressure ( hv) of 4.25 N/m2 equal to 0.44 mH2O. The friction loss in the inlet pipe includ-ing loss in fittings (hf) with the actual water velocity is 1.5 mH2O. The static suction lift (hh) is 2 m.
The NPSH can then be calculated as follows:
=10.3 − 0.44 − 1.5 + (−2)
This is higher than the NPSH value of 4 mH2O that the pump requires, which means that there will not be any problems regarding NPSH when using the pump.
The NPSH requirements of a specific pump are given in the pump diagram (see section 2.4.5). This value must be higher than the value calculated from the above equation. Remember that NPSH is given in units of pressure (mH2O, bar or pascal).