In hydro terminology, a penstock is an enclosed pipe, shaft or tunnel used to carry water to the powerhouse. In rare cases, the penstock might be a low-head, low-velocity conveyance (sometimes made from wood staves). Normally, however, the penstock carries high pressure water at fairly high velocity (typically 5 to 7 m/s).
For any particular flow discharge, the smaller the penstock, the higher the flow velocity. Since head losses increase with the square of the velocity, an undersized penstock can cause a large reduction in the water power delivered to the turbine. On the other hand, the larger the penstock, the more it costs to build. The size of the penstock is optimized by striking a balance between the loss of power generation revenue over the life of the project (expressed as a Net Present Value) and the initial capital cost of building the penstock.
Penstocks carry high pressures and, unlike pipelines, are made of special steel, welded in accordance with the Pressure Vessel Code, which requires certified pressure-vessel welders, special welding techniques and a high standard of quality assurance.
While an unlined tunnel or shaft may be used as a penstock in some locations and in good rock, a steel liner is always used near the powerhouse to reduce the hydraulic gradient (and hence the seepage of water into the powerhouse) and to prevent hydro-jacking. Steel liners use ductile steel that allows them to expand and share the internal pressure load with the surrounding rock. They are designed with stiffener rings or sufficient thickness so as not to buckle under the external groundwater pressure when the penstock is dewatered.