speed of water=2.52km/hr=2520m/hr

length = 2520m/hr

let the radius of pipe = r

and radius of tank =40 cm=4/10m

length of tank in half hour = 3.15m

length of tank in an hour = 2*3.15m =6.3m

now,

volume of pipe = volume of tank

pie *r ^2 * 2520= Pie *4/10*4/10*6.3

r^2 =1/2500 m

r= 1/50 m

diameter = 2/50 m = 200/50 cm = 4 cm