OpenHPL is an open-source hydropower library consisting of models for hydropower components that are developed based on mass and 1D momentum balance. It consists of mechanistic models for the flow of water in filled pipes (inelastic and elastic walls, incompressible and compressible water), a mechanistic model of a Francis turbine (including design of turbine parameters), friction models, etc. This paper includes an extension of OpenHPL with mechanistic models of different types of surge tanks (sharp orifice type, throttle valve surge tank, air-cushion surge tank) and draft tubes (conical diffusers and Moody spreading pipes). The simulated response of the models is presented using a case study of real hydro power plants.