Simplification of water supply networks is an indispensible design step to make the original network easier to be analysed. The impact of networks’ simplification on water hammer phenomenon is investigated. This study uses two loops network with different diameters, thicknesses, and roughness coefficients. The network is fed from a boundary head reservoir and loaded by either distributed or concentrated boundary water demands. According to both hydraulic and hydraulic plus water quality equivalence, three simplification levels are performed. The effect of demands’ concentration on the transient flow is checked. The transient flow is initialized by either concentrated or distributed boundary demands which are suddenly shut-off or released. WHAMO software is used for simulation. All scenarios showed that both hydraulic equivalence and demands’ concentration simplifications increase the transient pressure and flow rate. However, hydraulic plus water quality equivalence simplification produces an adverse effect. Therefore, simplifications of the networks should be done carefully. Also, it was found that pump shut-off gives the same trend of valve shut-off or release.