4. (a) (5 points) What is the physical meaning of the Net Positive Suction Head (NPSH)? (b) (5 points) Explain the sharp change of capacity for low values of NPSH (Fig. 3). (c) (5 points) Why low values of NPSH are undesirable for the pump operation? Capacity NPSH Figure 3: General shape of capacity vs. NPSH curve. NPSH NPSH Can be defined as 2 parts NPSH Available The absolute pressure at the Suction post the pump. NPSH - Required - The minimum pressure required at the Suction port of the pump to keep the pump from Cavitating (a) NPSH (net positive suction head) is the least suction head (pressure drop that is created in the pump to lift the water from a certain depth) that should be created in a centrifugal pump to avoid cabitation. If the value of suction falls below the value of NPSH, cavitation will occur which whill cause damage to the impeller. The required NPSH should be calcualted before designing and installing the pump and it should be ensured that the available head does not fall below the calculated value. (b) For low values of NPSH, the capacity will drop. Because, the volumetric flow rate of fluid will reduce as the vapour bubbles start forming at low values of NPSH and they occupy certain volume in the flowing fluid instead of the fluid itself and the capacity will drop. (c) As explained in (a), if the value of suction head drops below the NPSH value, the liquid will start boiling and vapour bubbles start forming in the fluid and these vapour bubbles will collapse on the impeller and casing creating sudden low pressure at various points and hence causing damage to the pump (this is called pitting) and also vibration.