Basically, the following valves come under this category:

• Check valves,

• One-way valves,

• Quick exhaust valves,

• Shuttle valves (OR valves) and

• Dual pressure valves (AND valves).

Check valves are used very frequently in pneumatics. Applications range from the simple shutting off of flow directions and speed control through to logic operations. They are also used in cases where individual components are to be isolated in a circuit in order to eliminate mutual pressure induced effects or to prevent the exhausting of units or entire system sections in the event of pressure failure. Check valves can prevent water hammer. Lift check valve, swing check valves, ball check valve, diaphragm check valve or wafer check valves go under this category. See below the check valve types:

• Pressure controlled check valves without spring assistance (load-free non-return valves).

With this design, the seal element usually is an elastomer diaphragm. Here the pressure acting in the high resistance direction is used in order to close the seal seat. In the opposite direction, the air flow deforms the diaphragm and thereby opens the valve in the direction of flow.

Since in this case the working pressure is used to switch the valve, the valves are also referred to as pressure-controlled valves, although usually simply as check valves. The sealing element is a ductile diaphragm of simple design, which is insensitive to dirt particles, etc.

• Pressure controlled check valves with spring assistance.

Whereas in the case of check valves without spring assistance, the pressure in the direction needs to be only slightly higher than in the high-resistance direction, so as to overcome the spring pressure.

However, this additional force is extremely slight and is therefore not separately indicated by manufacturers. The valve opens in the direction of movement as soon as the pressure generates a force on the sealing element in this direction, which is greater than the spring force.

These valves are used primarily for reasons of safety. The valve must be connected directly at the air connection of the cylinder in order to prevent other risks.

Piloted non-return valves are not permissible for use as safety elements on presses. There are two reasons for this:

First the switching time from the pressure dropping until the actual closing of the valve is 10 to 20 ms, depending on size. During this time, the mass can therefore still drop and traverse a distance which leads to corresponding damage.

Secondly, these valves are not 100% leak-proof. This means that the downward movement may still continue after the valve has closed. These are not negligible amounts by any means. Therefore, a cylinder with a diameter of 63 mm, provided with a 3/8" non-return valve should be considered as safe for a maximum of 30 minutes. A time period which is, for instance, too short to rescue trapped personnel.

In such a case, a fast-switching 2/2-way valve (stop valve) would be a better solution. However, apart from the pneumatic aspects, there are also a number of electrical factors to be considered. Special rules therefore apply for press controls.

A swing check valve is a type of check valve that is designed to allow fluid to flow through it in only one direction. It consists of a movable disc or clapper, which is attached to a hinge, and a seat around the perimeter of the valve.

With many applications, it is important to obtain the highest possible speeds in order to assume the start position for a working cycle.

In such cases here it is often a disadvantage that the air escaping from the cylinder has to travel the "long and narrow" way through the exhaust duct, a one-way flow control valve, the directional control valve and a silencer. This costs time and as such speed, since a cylinder can only be filled with air on the one piston side according to the corresponding air volume being evacuated on the other piston side.

In such cases a type of "short-cut" is used to vent the cylinder, i.e. the quick exhaust valve. Basically, this functions like a non-return valve where the air escapes laterally to atmosphere in the closed position.

A double check valve, also known as a double check valve assembly (DCVA) or dual check valve, is a type of check valve that consists of two independently operating check valves connected in series. It is designed to provide an extra level of protection against backflow in piping systems, pumps, and other mechanical equipment. This valve is suitable for prevention of back pressure and back siphonage.

Double check valves are commonly used in drinking water applications to prevent the backflow of contaminated water into the drinking water systems. They are also used in other types of pump systems to prevent reverse flow and protect against damage or malfunction and prevent contamination.

Double check valves work by allowing fluid or gas to move in only one direction. When the flow is in the proper direction, the first check valve opens, and the second check valve remains closed. When the flow reverses direction, both check valves close, sealing off the flow direction.