Initally for OHSR’s verticles CAST IRON pipes were used.You can observe all OHSR’s before Mission Bhagiratha you find CI Verticles.

Why CI Verticles are used because they are corrosion resistance.

GI Was not used because it gets corroded very easily.

DI was not used because it is of high cost.

In Mission Bhagiratha,10&20 KL OHSR’s are more in number

And above this capacity are very less in number as a result by considering economy for less number of OHSR’s>20 KL.Best verticles that is DI are used ,they are corrosive resistance

Cast Iron also corrode with time but di not

So for ci regular maintanence is needed

And for 10&20 KL it was decided to used CI Verticles but as suppliers are not there for CI later it is decied to use GI Verticles