A pressure vessel is installed to prevent transient vacuum and overpressure in centrifugal pump integrated system. In order to study the transient response of the pressure vessel with multichannels and improve design approach, an integrated system with two centrifugal pumps and a pressure vessel is presented. Based on the water hammer method of characteristics (MOC), the integrated numerical model and program are established by combining pumps, valves and pressure vessels in the integrated systems. Transient pressure process and gas volume variation are simulated for the pressure vessel. The Oscillation amplitude and frequency are obtained, and then the extreme hydraulic transient pressures are analyzed and compared. An optimal design method is provided to determine the safe and economic mass (SEM) of gas (nitrogen) and corresponding optimal safe and economic volume (SEV) of pressure vessel.