NPSH is a dirty word? There is enough fear of it to suggest it is. But why? Because some people will not accept that pumps don't suck! If you accept that a pump creates a partial vacuum and atmospheric pressure forces water into the suction of the pump, then you will find NPSH a simple concept.
NPSH(a) is the Nett Positive Suction Head Available, which is calculated as follows: NPSH(a)= p + s - v - f NPSH(a) must exceed NPSH(r) to allow pump operation without cavitation. (It is advisable to allow approximately 1 metre difference for most installations) The other important fact to remember is that water will boil at much less than 100 deg C if the pressure acting on it is less than it's vapour pressure, ie water at 95 deg C is just hot water at sea level, but at 1500m above sea level it is boiling water and vapour. The vapour pressure of water at 95 deg C is 84.53 kPa, there was enough atmospheric pressure at sea level to contain the vapour, but once the atmospheric pressure dropped at the higher elevation, the vapour was able to escape. This is why vapour pressure is always considered in NPSH calculations when temperatures exceed 30 to 40 deg C. |
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