How to calculate pressure at certain position of water pipe

Since the chiller unit is on the top roof, and the AHU/FCU are on the lower floors, it implies that there is an elevation between there units. The chilled water is distributed to other units by gravity. Therefore, the pressure inside the pipe is the hydraulic pressure by water due to elevation. By definition, every 10m of elevation, water exerts 1 bar at the bottom of the 10 m elevation. (P = rho x g x h; 20m = 2 bar, 30m =3 bar ....) For exact pressure at certain point along the pipe line, the pressure due to the piping, fittings, turns and elbows, reducers has to be estimated. There is a lot of engineering software / table availables giving you the pipe loss. By deducting the pipe loss from the hydraulic pressure, the pressure at certain point along the pipe line is thereby given.

Chilled water system is a closed loop circulation system with a expansion tank connected. Pressure at any point can be calculated by:

Elevation of expansion tank + pressure head of chilled water pump - pressure loss of equipment (e.g chiller), valves and pipeline & fittings + elevation between calculated point and chiller plant.

By the way, it's not necessary to add pressure reducing valve along chilled water circuit. Pressure reducing valve use mainly in open loop circuit, e.g. fresh water system to cater for excess pressure head at lower floor water point.

The pressure at the supply side of a close loop chilled water system have to cater also for the return path pressure loss, therefore adding a pressure reducing valve is useless but increase the overall pressure loss of the critical path and thus increase the pumping head and pump power of the chilled water pump. This waste energy.