A penstock is a channel used to feed or carry away water. The flow of the water through the penstock can be controlled with a sluice or gate that is raised and lowered. This term, and the technology, originate in water mills that used the flow of water to rotate a wheel that in turn generated energy for milling grains. Similar technology can be seen in use in hydroelectric dams, as well as flood control systems that are designed to trap and regulate the flow of water to prevent sudden changes in water level.
Penstocks can take the form of pipes or long channels, depending on the facility. Historically, channels were most commonly used and they were dug to connect with an existing waterway. When the sluice is fully open, water flows freely through the penstock. When it is closed, the water is limited and less water enters. A grate or filter may be used to trap large debris like branches and other floating materials so that the debris does not enter the channel and clog it.
Controlling the amount of water in the penstock allows people to regulate how much water is released at the other end. For things like flood control, penstocks can be wide open in the dry season to allow water through, and they can be partially closed during wet times of the year to hold water back. A containment tank or pond may be located behind the penstock to hold water that would otherwise build up and put pressure on the sluice.
Being able to completely close the gate or sluice allows people to drain penstocks so they can be inspected, serviced, and repaired. These channels usually need regular cleaning to remove debris that accumulates over time and to scrub down the sides to clear away algae and bacterial mats. Holes, cracks, and other problems may develop and require periodic repair. Routine maintenance is used to prevent catastrophic failures that might cause flooding and other problems.
In addition to being used to direct the flow of water into or through something, a penstock can also be used to control outflows. A common example can be seen at landfills and sewage treatment plants. Fluids from both facilities need to be released but the rate of release must be controlled for safety. In the event of contamination, the ability to close a gate and completely cut off the release of water is also important for protecting the surrounding environment.