Correct Answer - Option 4 : Statement (I) is false but Statement (II) is true
The air vessel in a reciprocating pump is a closed container in which the lower half is filled with fluid being pumped & the upper half filled with compressed air. Both the halves are separated by a flexible diaphragm which moves based on the pressure difference between the two fluids.
The air vessels are usually fitted near to the pump, in the discharge pipe to dampen out the pressure variations during discharge. They cannot increase the discharge in delivery pipe.
During operation, as the discharge pressure rises the air is compressed in the vessel & as the pressure falls, the air expands. The pressure energy is thus stored in the air & returned to the system when the pressure falls.
Apart from above, by using air vessels the acceleration & frictional heads are reduced. Thus the work in overcoming these losses is also reduced. This reduces the power required to drive the pump.