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There are various types of check valves' clacks since check valves can be divided into several categories.

The clacks of swing check valves
The clacks of swing check valves can be divided into the conventional type and the tilting type, and usually, the conventional type clacks are the default choices of many valve manufacturers.
 
For the tilting type clacks, due to the balanced action of the assistant valve plates, there will exist the time-lag action when the check valves are opened or closed, which will greatly reduce the impact force of the clacks on the valve seats. Thus, the tilting type clacks are recommended for the following working conditions except the conditions when the equipment needs to be closed because it is sensitive to the flow back of the media.

1. The pipelines transporting the liquid with DN400 and above DN400
2. The pipelines transporting the gas with Class300 and above Class300
3. The working conditions with frequently fluctuated pressure and valves with DN200 and above DN200

When the tilting type clacks are used, it is needed to indicate it in the valve data sheet.

The clacks of lift check valves
The clacks of lift check valves can be classified into three kinds, and they are the flat plate type, the plug type and the ball type. These three kinds can be further divided into two categories, the type with the restoring spring and the type without the restoring spring.

Since all check valves have the non-mandatory seal and form the seal pressure through the pressure of media, there are not obvious differences among the sealing performance of the flat plate type clacks, the plug type clacks and the ball type clacks, especially for the sealing performance of the flat plate type clacks and the plug type clacks. Thus the valve manufacturers usually determine the clack type to be used according to their customs. The ball type clacks have relatively better sealing performance when they are compared with the other two types, but they also have the relatively higher prices, which make them often used when there are strict requirements on the internal leakage of the check valves, for example, the working conditions containing the extremely toxic media.
 
Except the ball type clacks, the flat plate type clacks and the plug type clacks are the default choices of many valve manufacturers. If they are used, it is not needed to be indicate it in the valve data sheet.
 
Whether clacks are equipped with restoring springs or not is decided by the importance of the equipment to be protected and the operating pressure of the media. When the outlet check valves of the protected equipment need to be closed fast and timely, the restoring spring can be used to assist the closing of check valves. However, for the low pressure medium conditions, the restoring spring of the clacks will increase the resistance of the media and decrease the flow of the media, which may make the check valve unable to be opened. Therefore, the selection of the restoring spring must be paid attention to, and if necessary, the medium pressure and the specified opening pressure of the check valves should be indicated in the valve data sheet.
 
The clacks of the wafer check valves
The frequently used clack types of wafer check valves are the axial-flow type, the swing type and the dual plate type.

The axial-flow type clack needs the assistance of the restoring spring, and is often used for the check valve with DN40 and below DN40. It has the advantage of the simple structure, but it also has the disadvantage of taking up too much space of the runner. Thus, the axial-flow type clacks are not recommended for the medium conditions with large diameter pipes and high pressure such as with Class600 and above Class600.
 
The swing type clacks are also called the singe plate type clacks, which do not need the help with the restoring springs and are usually used for the check valves from DN25 to DN200. Compared with the dual plate type clacks, they have relatively simpler structures and leave the middle space of the runner, which make fluid have better flow performance. However, for the check valves with large diameters, the clacks will stretch into the flanges of pipes after they are opened, which requires the adaptability of the flange to the clacks to be considered. It is possible that the clacks can not be opened due to the restriction of the flanges.
 
The dual plate type clacks are generally used for the wafer check valves with DN50 and above DN50, and they are the prior and default choices of many valve manufacturers because they overcome the shortcoming of the swing type clacks, that is, the collision between the swing type clacks and the flanges. However, it is still supposed to indicate it in the valve data sheet when the dual plate type clacks are used.