Net positive suction head (NPSH) is the pressure at pumping temperature (expressed in feet of liquid) available at the pump suction over and above the vapor pressure of the liquid being pumped. In most cases, NPSH is a combination of two parameters: fluid head and pressure existing in the suction line. The pressure in the suction line, if the pump is above the liquid level in the suction tank, may be less than atmospheric. Large friction losses in the suction lines to a centrifugal pump may also decrease the pressure at the pump suction to a value less than atmospheric. Therefore, the net positive suction head may be viewed as the absolute pressure in the suction line at the pump.
NPSH conditions are checked for each pumping application to determine whether or not the fluid will vaporize inside the pump. Vaporization inside a pump is referred to as "cavitation." Cavitation is usually evidenced by excessive noise and vibrations and will sound like gravel being pumped through it.
Pressure on a fluid is reduced as it moves from the pump suction flange to the point where energy is imparted by the impeller. The pressure reduction must be compared to the liquid's vapor pressure, and to the absolute pressure on the fluid as it enters the pump, to determine if the fluid will vaporize.
The pressure reduction inside the pump is referred to as the "required NPSH." The difference between the absolute pressure acting on the fluid and its vapor pressure is called the "available NPSH." When the available NPSH is greater than the required NPSH the pump will not cavitate.
If the available NPSH is not greater than the required NPSH, cavitation, as well as many other serious problems can occur, such as a marked reduction in capacity, or even complete operational failure. Additionally, excessive vibration can occur when some sections of the impeller handle liquid while other sections handle vapor. This will cause pitting and erosion, which drastically reduces pump life. Pitting and erosion result from the collapse of vapor bubbles as they enter regions of higher pressure. As the vapor bubbles collapse, the adjacent walls receive a tremendous shock from the rush of liquid into the cavity left by the collapsed bubble, where small bits of metal actually flake off. Note that the erosion does not occur at the point of lowest pressure, but further upstream where higher pressures cause bubble collapse. Pitting or erosion often occurs on the impeller tips or volute, but are resultant from cavitation caused by insufficient NPSH at, or closer to, the pump suction.
Energy expended to accelerate liquid into voids left by collapsed bubbles causes a drop in the developed head. Consider that there is a volume increase of 50,000 times magnitude when water vaporizes at standard temperature. Even a slight amount of cavitation, therefore, will significantly reduce capacity.
Pumps operating with insufficient NPSH will often pump slugs or spurts of liquid. As the pump is started, liquid accelerates in the suction flange and impeller inlet "eye" until the pump reaches operating capacity. With acceleration of the liquid, friction losses increase and reduce the absolute pressure until the liquid flashes into vapor. When this occurs, pump capacities are reduced and flow decreases, or ceases altogether. In this case, losses are lower with the decreased flow, absolute pressure is higher, liquid ceases to vaporize, and the pump begins to transport fluid again.
The total suction head of the pump can be measured, and vapor pressures for common liquids and brines read from charts. The difference is the available NPSH.
The pump manufacturer determines the required NPSH for each pump by defined testing procedures. NPSH requirements are published on the standard composite performance curves for each pump.
NPSH requirements shown on performance curves are based on boiling water. As the water temperature decreases below the boiling point, less and less head is required to produce the same NPSH due to the difference in vapor pressure. With sufficient reduction in required NPSH, it is then possible to pull a suction lift. This is expressed as:
= Atmospheric pressure in feet of liquid, absolute = Suction head, feet of liquid = Liquid's vapor pressure, feet of liquid, absolute = Static suction head, feet of liquid = Friction head at suction, feet of liquid
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