Hydroelectric power is an important source of energy. This is particularly true for Quebec and some other provinces in Canada. In the event of a combination of power trip and wicket gate blockage, as an emergency response, it is necessary to close the intake gates in order to stop water flow through the penstock to the unit. Such emergency closure can cause air pressure inside the penstock chamber to drop so significantly that the safety risks to the power station structures and facilities become unacceptable. The purpose of this study is to develop analysis methods for the assessment of air pressure drop in emergency closure. The scope of this research work covers the determination of the following time-dependent quantities: water discharge beneath a sluice gate, dry air flow through air vents leading to the penstock chamber, amount of air entrained by turbulent water motions through the penstock, and the resultant changes of air pressure in the penstock chamber. The analyses are based on the energy principle and take into account a large number of variables including the upstream and downstream water levels, the geometry of the hydraulic passage, the time rate of gate closing, and features of downstream control structures. The analysis methods are applied to two cases of emergency closure of power generating stations in Quebec. The results of calculated air demand and pressure drop are in good comparison with field measurements. Emergency closure is shown to produce two significant impacts on penstocks and air vents: 1) intensified water jet in the first half of the time period it takes to close the gate; and 2) pressure drop in the last one third of the time period. Air entrainment by high-velocity flowing water is an important cause of pressure drop in emergency closure, and can be modeled using hydraulic jump entrainment equations. The values of air pressure drop calculated for the Isle-Maligne and La Tuque stations are below one third of the standard atmospheric pressure. However, there are significant air pressure fluctuations. This study has contributed to the development of quantitative framework and calculation procedures that can easily be extended for applications to other sites. The development is of engineering relevance to upgrade of existing air vents and the design of new air vents and to safe operations of emergency closure
