The expression "NPSH" represents the energy in absolute height of the liquid in the suction flange of the pump above the vapor pressure of this liquid at the pumping temperature, with reference to the centerline of the pump. Therefore, the practical purpose of NPSH is the imposition of limitations to the conditions of suction, in order to maintain the pressure at the intake of the pump's rotor above the vapor pressure of the liquid pumped.
AVAILABLE NPSH (NPSHavail)
Is a characteristic of the facility in which the pump operates, and of the available pressure of the liquid at the suction side.
NPSHavail can be calculated by the following equation:
Where:
Ps [kgf/cm²] |
Pressure at suction flange |
Patm [kgf/cm²] |
Atmospheric pressure |
Pv [kgf/cm²] |
Vapor pressure of the liquid at the pumping temperature |
g [kgf/dm³] |
Specific weight of the fluid |
Vs [m/s] |
Flow speed at the suction flange |
Zs [m] |
Distance between pump and pressure gauge centerlines |
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NPSH REQUIRED (NPSHreq)
Usually the characteristic curves of a pump include the NPSHreq curve as a function of the flow. This curve is a characteristic of the pump itself and can be obtained experimentally at the manufacturer's testing benches.
In the test to obtain the NPSHreq a 3% drop in the manometric height for a certain flow is used as a criterion. Both the HI (Hydraulic Institute) and API 610 (American Petroleum Institute Standard 610) adopt this criterion.
CAVITATION
Cavitation is a term used to describe the phenomenon that occurs in a pump when there is insufficient NPSH available in relation to the NPSH required.
Small bubbles or vapor pockets are formed when the pressure of the liquid is reduced to a value equal or lower than its vapor pressure. As these bubbles move ahead of the rotor blades to a higher pressure zone, they burst rapidly.
The burst is so abrupt that it generates a violent noise, as if the pump were pumping gravel. Therefore the easiest way to know if the pump is cavitating is by paying attention to the noise of the pump.
Another consequence of the collapse of the bubbles is the removal of surface material (pitting) from where the implosions occur, causing principally, depending on the intensity and duration, erosion of the rotor.
In addition to damage to the rotor, cavitation usually results in a reduction of the pump's capacity due to the presence of vapor, reduction and instability of the manometric height, vibration and mechanical defects.
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