Water hammer in water-supply networks and irrigation networks may give rise to high pressures, due to the superposition of reflected pressure waves. The problem is relevant nowadays for the common use of automatic valves in networks regulation. When large variations in user demand happen often, as in distribution networks, water hammer occurs daily. In irrigation networks, its effect was found to concur to high rate of pipe failures, which these networks usually suffer. This paper investigates the effect of sudden variation of water demands in pipes network on transient pressures variations through the network. WHAMO software was used in the analysis which uses the implicit finite difference scheme for solving the momentum and continuity equations at unsteady state case. The study was applied on a two loops pipe network composed of eight pipes of different diameters and elevations. The flow is fed in the network by elevated storage tank. The results showed that rapidly changing the demand could significantly increase or decrease the pressure head in distribution system and cause flow reversal in a network system. In this simulation, shorter increasing/decreasing time of demand change causes the greater magnitude of transient fluctuation. Therefore, it is very obvious that the operations of distribution system should be done with very careful caution. The slower opening rates significantly reduce the impact of hydraulic transients. When the distribution system is designed, many different cases of simulation with different scenarios should be performed for the economical and safe design.