Net Positive Suction Head
The professional engineer must be able to properly determine net positive suction head in order to avoid cavitation. Cavitation occurs when the suction pressure (head) is less than the vapor pressure of the water. If the suction pressure is lower than the vapor pressure, then small vapor bubbles form and when these bubbles reach the pump where the pressure is increases, the bubbles implode causing damage to the impellers and other parts of the pump. This is what is known as cavitation.
Suction head is defined as the pressure at the inlet of the pump and net positive suction head is the difference between the suction head at the inlet and the vapor pressure of the water at the inlet of the pump.
Net Positive Suction Head Available=Suction Head_(inlet of pump)-Vapor Pressure_water
Net positive suction head is the total amount of head or pressure at the inlet of the pump. This value is found by determining all the pressures acting upon the fluid whether positive or negative. The following figure best describes all the pressures that can be acting upon a pump.
(1) P_abs: This pressure refers to the absolute pressure acting on the fluid. If the tank is pressurized, then the value is pre-determined. If the tank is open to the atmosphere, then the pressure is equal to 1 atmosphere [atm] or 14.7 psia or 33.9 ft of water.
(2) P_elev: This pressure identifies the elevation difference between the top surface of the liquid and the pump centerline. This value can be positive or negative and is measured in ft of head.
(3) P_fric: The friction pressure or head is the amount of pressure lost due to friction in the piping, fittings, equipment, valves, etc. leading from the fluid source to the pump.
(4) P_vel: The velocity head pressure is the pressure due to the flowing liquid.
(5) P_suction: Finally, all of the pressures leading to the pump are summed and the resulting value is the suction pressure at the pump.
The vapor pressure of the water is found by simply looking up water (or pumping fluid) tables and finding the vapor pressure at the operating temperature. In the HVAC and Refrigeration field, water is the most common fluid used in pumping systems and a table of corresponding vapor pressure and temperatures are shown below.
From the table above, it can be seen that as the temperature of the water increases, the pressure at which vaporization will occur also increases. Thus cavitation becomes even more critical at higher temperatures.