A closed tank of a fire engine is partly filled with water, the air space above being under pressure. A 5 cm hose connected to the tank discharges on the roof of building 2 m above the level of water in tank, the friction losses are 50 cm of water. What air pressure must be maintained in the tank to deliver 15 lit/sec on a roof.

 A closed tank of a fire engine is partly filled with water, the air space above being under pressure. A 5 cm hose connected to the tank discharges on the roof of building 2 m above the level of water in tank, the friction losses are 50 cm of water. What air pressure must be maintained in the tank to deliver 15 lit/sec on a roof. 

 Solution: 

 Discharge, Q= 15 lit/sec= 0.015 m 3 /sec 

Velocity in 5cm hose pipe= 0.015/[(π/4)*(0.05)2 

                                          = 7.64 m/sec 

Applying Bernoulli’s theorem to section 1 and 2, taking water surface level in the tank as datum

           (v1 2 /2g)+(p1/w)+y1 = (v2 2 /2g)+(p2/w)+y2+hf 

 The velocity v1 at the surface is zero. 

             0+(p1/w)+0 = [(7.64)2 /(2*9.81)]+0+2.05 

                    (p1/w)= 5.48 m of water

                  p1 = 0.548 kg f/cm2 (air pressure in tank)