Does pressure remain same in closed water circuit of chilled water piping network?

No. the pressure near the pump will be higher than farthest point of critical path.

ya working pressure is same 8bar

but it happen on my project, there is a difference of 1~2 bar pressure on most highest point of circuit....

if it is not like this than why air vent always installed on the highest point???

yes it remains same in the close circuit as it does not have any atmospheric pressure variations