Air-pressure drop in a penstock in emergency closure of intake gates

Samuel Li, Michel-Olivier Huard, Yannick Bosse

Friday 3 july 2015

11:30 - 11:45h at Asia (level 0)

Themes: (T) Water engineering, (ST) Hydraulic machinery and industrial flows, (ST) River and coastal engineering

Parallel session: 15C. Engineering - Industrial

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. 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 are unacceptable. The purpose of this paper is to develop analysis methods for the assessment of pressure drop in emergency closure. The scope of the paper covers the determination of water discharge beneath a sluice gate, dry air flow through air vent leading to the penstock chamber, amount of air entrained by turbulent water flow through the penstock, and the resultant change of air pressure in the penstock chamber. The analysis methods are based on the energy principle. They are applied to a case of emergency closure of a power generating station in Quebec, Canada. 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: 1) an intensified water jet in the first half of the time period of gate closure; and 2) a significant 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 modelled using hydraulic jump entrainment equations. The values of air pressure drop calculated are below one third of the standard atmospheric pressure. However, there are significant air pressure fluctuations. This paper has contributed to the development of quantitative framework and calculation procedures that can easily be extended for application to other stations.