Ever heard of a one way check valve? While it sounds like the scale adder, it is actually a very simple and widely-used device in many systems. The one way check valve is a very specific type of the valve, it lets things such as water and gas flow through but in only an mechanical operation going forward A control uses differential pressure to allow fluid supply upstream (or not) downstream. It is like a door that opens if you pull, but try to push it and it stays closed.
A cheap and readily available type of one way check valves are used all over the place for many different purposes. These might be in water pipes that bring things to your home or gas for the stove. They are necessary because they prevent something called backflow. Especially useful with this protector from water flow for phase backflow of the liquid. That can be a major irritant, as those types of things flowing the wrong way have been known to cause pipes and machinery damage or even create hazardous situations.
Backflow can occur in a ton of different situations and it brings with many problems. Let us take an example, consider a bathtub with the drain plugged due to some hair and soap. When this occurs, the water will no more flow down but instead start to run back up. This can quickly become a disaster and possibly flood your bathroom... that is NOT pretty!
Check Valves: Check valves are a one way valve, only allowing something to flow in that direction. So, whenever water tries to reverse flow back up the drain pipe, it must pass through a one way check valve which works upon the prinicipal of oppurtunity knocks only once and this will prevent any further advancements. That is a very useful thing as it ensures that all the water from your bathtub gets drained and keeps your bathtub perfectly dry so no damage will appear at least in this part of the house.
Ball Check Valves : It contains a small ball which stops the flow of fluid when it moves in wrong direction. Ball check valves are most commonly used in pipes that transport gas or fluids, which aid in ensuring that everything keeps flowing and so no harmful chemicals get into your home.
Swing Check Valves: This type of valves is a little bit different. Their notion is that of a swinging door, which... flaps unceasingly open and shut upon itself (26:355) -- opening only in one direction when an element wants to pass through it, then closing before anything behind tries to follow. Swing Check Valves are common and versatile ubiquitous product used mainly in waterway application to ensure proper flow of waste water etc.
Diaphragm Check Valves-- These valves are similar to ball check valves, but instead of a sphere, they have a flexible rubber or silicone piece called a diaphragm that opens when something flows through in the correct direction. But it shuts down after something liquid attempts to backtrack. They are used in medical equipment that needs to control the flow of air, such as blood pressure cuffs.