A check valve is a type of valve that allows fluid to pass in only one direction.
To put this another way, it's a valve that prevents the backflow, reverse flow, or reflux of fluid into an outlet.
Check valves are used in many areas of industry and everyday life. They're often employed to keep water flowing in the proper direction through household pipes or to keep water forward flow in ponds and water gardens. Other common check valves are found in the gas tanks of vehicles, where they prevent dangerous gases from flowing backward into the fuel tank. Check valves can also be used in irrigation systems to keep water moving only in the desired direction.
Reverse flow is prevented due to an obstruction inside the check valve known as a 'seat.' When water (or another liquid) flows through, it enters an inlet and then passes into an outlet underneath. The cracking pressure is the pressure differential between the inlet and outlet. This upstream pressure pushes the valve body to open automatically. The inlet pressure creates suction that pulls the water down into the outlet pipe.However, any air bubbles present will rise to the top of the device but only after they pass through the seat. Because this seat is extremely narrow, air bubbles are stopped, which prevents the flow of water in the opposite direction. For liquids to flow back up into an outlet pipe, there would need to be a differential pressure between the top and bottom of the valve and such a scenario is highly unlikely.
In hydraulic systems, check valves work automatically. Check valves are often employed to maintain fluid flow in the intended direction. This can be done with pressurized or non-pressurized systems and through a variety of piping systems. Examples include check valves used in aquariums, pond filters, pumps, siphons, toilets and irrigation systems.
Check valves allow water (or other liquids) to pass in only one direction when the cracking pressure is attained. They are non return valves. They do this by the use of a seat that is so narrow that any air bubbles cannot pass through, but low pressure water can pass through easily. This allows for liquids to flow in one direction and stop the reverse flow (or backflow).Check valves are used for applications like emergency shut-off equipment, fountains, irrigation systems, pumps, and motors.
There are three main types of check valves: Swing Check Valves, Lift Check Valves and Diaphragm Check Valves.
The swing check valve is the most common type of check valve, and it's usually used in non-pressurised systems. Swing check valves consist of an enclosed casing that contains a ball with two openings opposite each other. This check valve allows the flow to go one way and blocks flow in the opposite direction (backflow) when installed in piping systems. When water begins flowing through a pipe system, pressure begins to build up behind the ball, which opens one hole while blocking the other opening. This allows water to pass through but not back into the piping system. As long as the water pressure remains powerful enough, this process will continue, and the flow of the water will be unimpeded. A variation of the swing check valve is the tilting disc check valve. It has the pivot fitted at the centre, allowing it to open at approximately 50 degrees, unlike the swing shaft where the disc opens at 90 degrees.
The reverse flow is impeded by a spherical ball placed in the seat housing in the ball check valves. The ball's diameter is a few dimensions larger than the through-hole. When the cracking pressure is attained, the ball moves above the inlet hole. When the pressure differential is higher on the outflow, the ball falls back and sits on the valve seat to prevent reverse flow. A ball check valve is recommended for horizontal and vertical lift check valve installations.
A lift check valve is very similar to a swing check valve, but it operates slightly differently. This type of check valve uses weights instead of fluid pressure to push open one opening while blocking the other. Lift check valves are also non-pressurised systems, but they're typically used for higher flow rates. So whereas a swing check valve can be used anywhere from 2 - 4 feet below the surface where water isn't pressurised by any means (like irrigation), lift check valves are usually installed about 16 inches below the surface because there's still some degree of water pressure at that depth even though it's very limited. In addition to the weights, lift check valves also have a 'float' that sets up an added pressure level inside the valve when water begins to flow. This float gets lifted by the water and pulls on a rod that opens one hole while blocking another opening. Once again, this allows for only a single direction of flow and stops backflow in its tracks.
Diaphragm check valves are typically used in pressurised systems (like irrigation), where fluid pressures reach as high as 100 psi or more. A diaphragm check valve consists of rubber diaphragms on either side of an enclosure that contains rollers attached to levers. When water (or other liquid) begins flowing through, the minimum upstream pressure on the inlet side of the diaphragm causes it to deflect and allow water inside. The rollers also lift levers that open one hole while blocking another -- allowing for only a single flow direction.
It is very important to check the direction of the flow of water before installing a valve. A check valve should be installed in a location where the flow would always be from the same side, whenever it is being used or not. It will ensure that there's no backflow of water into the system and that the valves are working properly at all times. In other words, if you have your main shut-off valves outside your house and they get turned off, you don't want any water backing up inside your house because it can cause problems with flooring or any appliances fitted inside the house. If there is a problem with any of these valves causing backflow, then some check valve needs to be fitted in-line before any other devices. In addition, there are some places where a check valve should not be installed. Check valves should not be used in a line that's subject to rapid or variable pressures. For example, this would include any standpipes, pressure tanks and other parts of the system that maintain pressure. In these cases, a swing check valve would be better because it doesn't have any diaphragms, which can become displaced if the water pressure changes drastically. When installing a new pump or well system into a home that doesn't already have one, then you'll need to install at least two check valves - one below the pump to ensure there's no backflow from the irrigation lines and one above the pump to ensure there's no back.
Hardware failure is another reason why check valve slam can occur, but it shouldn't be assumed that this is always the cause of check valve slam. Using small-diaphragm check valves below 1 inch isn't recommended because they are subject to greater forces on them. The noise you hear with these types of smaller valves may not be actually caused by 'check valve slam' - it could very well be coming from something else entirely.
For example, one type of common noise that's mistaken for 'check valve slam' is called 'water hammer'. Water hammer occurs when there's a big change in water pressure that creates an impact on a pipeline. When this happens, a loud noise is produced, and the water pressure inside the pipe bounces back immediately - much like a hammer hitting something hard.
There are three main ways to reduce or eliminate check valve slam: select the right check valve type, increase the size of the pipe, and use an isolation valve before your backflow preventer (i.e., below the pump). The key reasons for this situation are either because there isn't enough room between each sprinkler or if too much water pressure is being used on smaller irrigation lines. One other cause for this could be due to contaminants or debris getting lodged inside the valve and preventing it from fully closing. Or both of these things combined could cause this problem as well. For most diaphragm check valves, you are supposed to have 1/8-1/4 inch gap between each sprinkler within itself. If they are too close together, sometimes there isn't enough room for the water pressure coming out of your irrigation system to properly open up all of these valves correctly without one slamming shut on top of another.
If that is the case, then you can always try increasing this gap size by adding more piping behind each individual sprinkler head, but also keep in mind that if you do increase the gap size, then there will be an increased amount of rainwater flowing through at once which can sometimes cause the water pressure to increase as well. If you are using a good amount, or more than 2-3 GPM per sprinkler, then this could also be the reason why these valves are slamming shut on one another. If this is the case, then you can try increasing your pipe size for each individual line that runs out from your irrigation manifold before they split off into smaller lines and head towards each sprinkler.
When using diaphragm check valves, it's best to install them directly after the main flow control device, which will often be either a flow control manifold, an impact wrench with multiple nozzles, or even just one nozzle attached to your main irrigation line with four outputs coming out of it (one for each quarter.
The design of some check valves by itself causes the slam or water hammer. This is a common problem with the swing check valve because the disk swings on a shaft. The opening of the flap slams on the walls of the pipe. The closing action of the return flow slams the shaft on the seat. There is no shortage of check valves whose mechanism reduces or eliminates valve slam. With the guidance of your plumber, you can get a suitable valve from quality check valve manufacturers. The spring loaded check valves have an inline spring and movable disc fitted. When the specific cracking pressure exceeds the spring force, fluid flows into the outlet pipe. When the inlet pressure drops, the spring pushes the diaphragm back to the closed position to prevent backflow. The action of the spring is fast. It shuts the inlet pipe before fluid flow reversal, which prevents the water hammer effect. Another alternative is the silent check valves, also known as nozzle check valves. These ones have a stem disc and compression spring. They are fitted inline. The cracking pressure must be strong enough to open the compression spring. Another option is the foot valve that works in the same spring principle in addition to a strainer that serves to sift out the debris. Besides, you can also choose the piston check valve. The opening and closing of piston check valves are aided by the action of a piston and cylinder. The piston valve works best with the globe valve and the angle valve.
A vacuum check valve is a two-way valve with a spring in the middle. On one side of the vacuum check valve is water, and on the other side, air. When the pressure from the water pushes down on the diaphragm, it moves into the cavity. The spring then presses the diaphragm up, which makes it push down on the other side and close off that cavity. When the water pressure is reduced, the force of the spring opens that cavity back up and allows water to flow through again.