Question

A pump is put into service at the coast where the barometric pressure is 760 mm Hg. The conditions of service are : Flow rate 0,08 m3/s, suction lift 3,5 metres, suction pipe friction loss 0,9 metres, water temperature 65°C, water velocity 4 m/s. Under these conditions of service, the pump requires an NPSH of 2,1 metres. Assuming the density of water as 980,6 kg/m3, establish whether it will operate satisfactorily.

Answer 1

The pump operates satisfactorily.

Step-by-step explanation:

According to the NPSH available definition:

`NPSHa =  (P_(a) )/(density*g) + (V^(2) )/(2g) - (P_(v))/(density*g)`

Where:

`P_(a) absolute pressure at the inlet of the pump`

`V velocity at the inlet of te pump = 4m/s`

`g gravity acceleration = 9,8m/s^(2)`

`P_(v) vapor pressure of the liquid, for water at 65°C = 25042 Pa`

The absolute pressure is the barometric pressure Pb minus the losses: Suction Lift PLift and pipe friction loss Ploss:

To convert the losses in head to pressure:

`P = density*g*H`

So:

`P_(b)  = 760 mmHg = 101325 Pa`

`P_(lift)  = 33634,58 Pa`

`P_(loss)  = 8648,89 Pa`

The absolute pressure:

`P_(a) = P_(b) - P_(lift) - P_(loss) = 59044,53 Pa`

replacing on the NPSH available equiation:

`NPSHa =  6,14 m + 0,816 m - 2,6 m = 4,356 m`

As the NPSH availiable is higher than de required the pump should operate satisfactorily.