Clapet valves are check valves which, through the action of a mechanical organ, block the reflux of the fluids conducted inside the system.
These valves, usually made of austenitic stainless steel of the AISI 316L series, are widely used when an almost total passage of the fluid is required.
As mentioned, the mechanical organ that blocks the back-flow assumes particular importance but also the use of plastic gaskets is not secondary neither for these particular clapet valves nor in all applications where a certain seal is required.
Gaskets: valves but not only!
"A gasket is an element that is interposed between two surfaces to prevent the passage of a fluid (liquid or gas) between them"
When it comes to gaskets, you come into contact with a varied and ever-expanding world. In everyday life it is easy to find oneself interacting with these components: it is not usual to replace clapet valves but, it can be, having to replace these elements to make household appliances work properly, for example!
Generally, the gaskets are divided between "static" and "dynamic" based on the fact that the surfaces with which they come into contact are stationary or find themselves in relative motion with each other. Correctly choosing the material with which to make the gaskets necessary for a component or system is essential to avoid any sealing problem and ensure a high level of safety. In fact, the case of the Challenger shuttle is famous: the wrong choice of take-off conditions led to the breakage of an O-Ring gasket and to the explosion of the spacecraft, causing the death of seven people.
The choice of the material for the gaskets
As can be seen from the example cited above, the choice of materials with which to make the gaskets can also be complex. The indications to follow are mainly the following:
- Having a certain degree of softness to adapt to surfaces;
- It must chemically resist the conveyed fluid and not attack the surfaces in contact;
- Having a certain resistance to temperatures (high or low they may be) or the pressures involved;
- In dynamic applications must withstand friction and wear.
Among the most used materials there are certainly:
- PTFE: polytetrafluoroethylene, more commonly known as Teflon. It is not the only usable polymeric material but it is the most common and can be used alone or with special additives that improve its properties;
- Elastomeric materials: for example compounds such as NBR widely used for O-rings;
- Metallic materials: stainless steel, aluminum, copper and others;
- Natural products: such as, for example, paper and hemp.
For the clapet valves, the most suitable and used material is certainly PTFE: this polymeric material guarantees a good compromise between mechanical resistance properties in temperature (up to about 260 ° C) and chemical properties, proving to be suitable for such applications. In addition, it is also an insulating material.
Swing check valves: the importance of seals
Operation of swing check valves
The swing check valves are devices that allow the passage of the flow from upstream to downstream but which block the reflux of fluids through a mechanical action. The engine underlying the operation of these valves is the speed of the fluid that regulates the opening of the "buffer" device, favoring the passage or blocking the flow. There are three main cases:
- Unequal level between the inlet and outlet channels in favor of the first: there is flow and the valve is open ensuring a complete passage;
- Level parity between the two channels: the valve closes as there is no flow;
- Unequal level between inlet and outlet channel in favor of the second: check valve closed in an optimal way and does not allow the passage of flow.
To ensure the correct functioning of the valve, in addition to a certified and high quality production process, it is required to prepare the connection walls of the channels which must be finished in such a way as to create a perfect and leak-free hydraulic gasket. For these reasons, in addition to legislative and safety reasons, the maintenance of pipes and valves is of fundamental importance.
PTFE: a polymer of extraordinary importance
As already described previously when talking about the various materials with which gaskets can be made, PTFE represents the best alternative for its characteristics and for the applications of interest.
What are the characteristics that make it suitable for check valve gaskets?
- Chemical inertia: this property has a double function. First of all, the gasket is not altered by the fluids with which it comes into contact but maintains its functional characteristics over time. Furthermore, it does not alter in any way the fluids with which it comes into contact, avoiding malfunctions to the system and guaranteeing durability over time.
- Insolubility: another fundamental characteristic for water systems or in the presence of organic substances.
- Very low friction coefficient: this allows for better smoothness and, therefore, greater efficiency of the system, reducing load losses.
- Invariability of characteristics in temperature: in fact, PTFE gaskets can work at very low temperatures (-80°C), but also at high temperatures (up to 250°C), guaranteeing a notable range of applicability that broadens the field of application of the gaskets and, therefore, of the valves themselves.
The greatest risks of this material concern the decompositions that it presents at even higher temperatures: in fact, around 500°C it does not melt but decomposes generating toxic gases. This is one of the greatest limitations of this material, even if there are various copolymers or compositions that reduce the effects while maintaining, however, excellent application characteristics.
The gaskets: an essential component
Speaking of hydraulic seal, one cannot fail to refer to the gaskets. They play a central role in many components with which we come into contact in everyday life and, in particular, in the clapet valves of our interest. Some characteristics are required for this:
1. Adaptability to surfaces: they must be able to conform properly with the surfaces in contact in order to avoid voids that can cause leaks;
2. Chemical resistance: coming into contact with fluids with different chemical characteristics, this property plays a central role;
3. Abrasion resistance: movements and flow can, over time, lead to damage that compromises correct functionality.
The choice of material allows you to "play" on these characteristics, preferring one or the other depending on the applications. In general, the most used material is PTFE (ie Teflon) which combines resistance, chemical, temperature resistance and mechanical resistance characteristics, favoring the functioning of the component.
A very important standard regarding the sealing of the gaskets for swing check valves is the DIN 19569-4 standard (and subsequent ones) which regulates the operation of wastewater systems.
As is usually the case for this type of component, the choice and correct installation are fundamental phases. Even a single mistake can compromise all the excellent characteristics provided by the other aspects. Thanks to its competence and experience, Intertubi represents a point of reference in the sector, providing the best solutions for different application areas.
Replacement of seals in swing check valves
Maintenance is a central phase in the life of any component, be it a valve or another mechanical, hydraulic, electronic component and so on. Very often, however, the concept of maintenance remains only theoretical, especially when it comes to predictive maintenance, and intervention is used exclusively when breakages or malfunctions occur with visible and easily verifiable effects.
Many times the seals can undergo breakages or incorrect functioning due to uses and conditions which change their efficiency and behavioral stability over time. In fact, the rubber can deteriorate more or less quickly, leading to a progressive loss of seal and allowing the passage of liquids which in turn can cause wear and corrosion phenomena which can partially or completely compromise the valve and, in the worst cases, the system on which it is installed.
To proceed with the replacement in a preventive manner, it is necessary to know the correct methods recommended by the manufacturer for replacing the seal, thus avoiding disassembly and assembly that is not consistent with the construction, further invalidating the component. Paying attention to all the steps indicated is the only way to restore the seal and allow the swing check valve to continue its action, thus extending its useful life.
