Difference between hard seal and soft seal

Personally, I think a hard seal should work like direct metal-to-metal contact for sealing, and is mainly used in high-temperature and high-pressure conditions； A soft seal is probably something like a rubber seal.

Hard seal: A seal where both sides of the sealing pair are made of metallic materials or other relatively hard materials is referred to as a “hard seal”. This type of seal has poor sealing performance, but it is resistant to high temperatures, wear, and has good mechanical properties. Examples include steel+steel; steel+copper; steel+graphite; steel+alloy steel (where the steel can also be cast iron or cast steel, and the alloys can be those applied through cladding or spraying). Soft seal: A seal where one side of the sealing pair is a metallic material while the other side is an elastic non-metallic material is called a “soft seal”. This type of seal offers better sealing performance, but it is not resistant to high temperatures, is prone to wear, and has poor mechanical properties. Examples include steel+rubber; steel+tetrafluoroethylene, etc. In a soft seal, one side of the sealing pair is made of a material with lower hardness. Generally, the soft seal seat is made of non-metallic materials that possess certain strength, hardness, and temperature resistance; it provides good sealing performance and can achieve zero leakage, but its lifespan and tolerance to temperature variations are relatively poor. Hard seals are made of metals, and their sealing performance is relatively poor, although manufacturers claim that they can achieve zero leakage. Soft seals cannot meet the process requirements for certain corrosive materials, but hard seals can solve this issue! These two types of seals can complement each other; in terms of sealing performance, soft seals are relatively better, but nowadays hard seals also meet the required standards! The advantage of soft seals is their good sealing performance, while the disadvantages are that they tend to age and wear out easily, resulting in a short service life. Rigid seals have a long service life, but their sealing performance is relatively inferior to that of soft seals.

The difference between hard sealing and soft sealing lies in the sealing material used for the valve seat. In general terms, hard sealing involves metal-to-metal contact, while soft sealing uses rubber materials such as PTFE for the sealing purpose. Hard sealing is used in high-temperature environments or when the process medium contains impurities; its leakage rate is typically at level 4-5. Soft sealing is used when strict leakage controls are required, with a leakage rate that can reach level 6

Rigid seals can handle more types of operating conditions, but their sealing performance is not as good as that of soft seals; the appropriate applications for soft seals made from different materials need to be determined on a case-by-case basis

Their sealing materials are different. Soft seals are generally made of materials such as rubber or polytetrafluoroethylene. A hard seal uses metals such as stainless steel

Soft and hard seals refer to the material used for the valve seat. A hard seal is what is commonly known as a metal-sealed valve seat, and it is typically used in environments with high temperatures and media that contain particles. A soft seal involves a valve seat made of non-metallic materials such as PTFE, and it is used in situations where the temperature is below 200 degrees, the pressure is low, and the media does not contain particles. This is just a general overview; the specific application depends on the actual conditions.

Current manufacturing techniques enable metal hard seals to achieve ANSI Class VI sealing performance, making them suitable for high-temperature and high-pressure conditions as well as those with particulate matter in the fluid; however, their cost is significantly higher.