At a thermal power station where the boiler pressure is 80 bar and the required flow rate is 130 tonnes/ hour, two multistage centrifugal pumps are used in series to pump water from the condenser to the boiler. Each pump is required to produce a head of approximately the total head and run at 1450 r.p.m. If the impellers in stages are identical and the specific speed of each impeller is not less than 15, find : (i) The head developed per stage and required number of stages in each pump. (ii) The required impeller diameters, assuming the speed ratio based on the outer tip diameter to be 0.98 and the shaft power input, if the overall efficiency of each pump is 0.76. What would you expect if the discharge valve is closed and pump is switched on ? What safety device would you recommend ?