A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways.Valves are technically fittings, but are usually discussed as a separate category.
In an open valve, fluid flows in a direction from higher pressure to lower pressure.
Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems. Źródło: https://en.wikipedia.org/wiki/Valve
Water supply and sanitation has been a primary logistical challenge since the dawn of civilization.
Where water resources or infrastructure or sanitation systems are insufficient for the population, people fall prey to disease, dehydration, and in extreme cases, death. Major human settlements could initially develop only where fresh surface water was plentiful, such as near major rivers.Over the millennia, technology has dramatically increased the distances across which water can be relocated, but the availability of clean and fresh water remains a limiting factor on the size and density of population centers, and is expected to remain so into the foreseeable future.Źródło: https://en.wikipedia.org/wiki/History_of_water_supply_and_sanitation