The pressure-time method is a discharge measurement technique widely used in hydropower plants to estimate the flow rate and indicate the turbine hydraulic efficiency. The flow in the pressure-time method undergoes the transient deceleration before the valve closure and water hammer after the complete valve closure. This paper investigates the physics behind the transient flow starting firstly from the water hammer phenomenon and then the flow in the pressure-time method considering a pipe with constant cross section area. The numerical analysis was based on 3D simulations considering k-ω SST turbulence model. The dynamic mesh technique was used for the valve closure modelling. The numerical results are compared with the experimental data. Moreover, the water hammer results showed that the flow could be considered axi-symmetric close to the valve. However, in the pressure-time method the asymmetry present in longer distance away from the valve. The behaviour of the velocity profiles of both water hammer and pressure –time method is also discussed and compared with each other.