Choosing Between Soft and Hard Sealing Butterfly Valves

What is a soft seal butterfly valve?

1.Soft  sealing  Butterfly Valves

A soft sealing butterfly valve is a valve where one side of the sealing pair is made of metal material and the other side is made of elastic non-metallic material. This type of sealing provides good sealing performance but is not resistant to high temperatures, wears easily, and has poor mechanical properties. Examples include steel + rubber and steel + PTFE.

The soft sealing seat is constructed from non-metallic materials with specific strength, hardness, and temperature resistance properties. It offers effective sealing performance and the potential for zero leakage, although its lifespan and temperature adaptability are relatively limited.

The most commonly used materials for the sealing surfaces of soft sealing butterfly valves include EPDM (ethylene propylene diene monomer rubber), NBR (nitrile butadiene rubber), VITON (fluoroelastomer), and PTFE (polytetrafluoroethylene). These four different materials are suitable for use with different media in pipelines.

EPDM Seat

EPDM is suitable for temperatures ranging from -40°C to 120°C. Its advantages include ozone resistance, heat resistance, and aging resistance, making it the most commonly used seat material for soft sealing butterfly valves. In normal temperature and pressure conditions, it provides excellent sealing performance. Soft sealing butterfly valves with EPDM seats are suitable for media such as freshwater, wastewater, seawater, and slurry containing impurities. They are employed in a multitude of applications within the construction, power generation, and water conservancy sectors.

NBR Seat

The NBR compound is suitable for use in temperatures ranging from -30°C to 90°C. The material exhibits notable advantages in terms of oil resistance, abrasion resistance, heat resistance, and strong adhesion. Nevertheless, it is not resistant to low temperatures, exhibits poor ozone resistance, displays low insulation performance, and displays slightly lower elasticity. Soft sealing butterfly valves with NBR sealing surfaces are employed in the lubrication of lubricating oil and engine oil. The most notable feature of this material is its excellent oil resistance, along with some abrasion and heat resistance. Soft sealing butterfly valves utilising NBR as the sealing surface material are suitable for use in oil media within petroleum pipelines. Nevertheless, NBR is unsuitable for freshwater and similar media due to its toxicity, which can have adverse effects on human health.

VITON Seat

VITON has a temperature range of -20°C to 250°C. It exhibits superior high-temperature resistance in comparison to silicone rubber, while also demonstrating excellent weather, ozone, and chemical resistance. However, it is less resilient to cold temperatures. It is resistant to the majority of oils and solvents, particularly acids, aliphatic hydrocarbons, aromatic hydrocarbons, and animal and vegetable oils. It is suitable for sealing requirements in diesel engines, fuel systems, and chemical plants.

PTFE Seat

PTFE is a fluoroplastic material that is suitable for temperatures ranging from -196°C to 200°C. The material's advantages include excellent heat and cold resistance, as well as resistance to general chemical solvents and almost all liquids. Soft sealing butterfly valves with PTFE seats are employed in a variety of industrial settings, including the chemical, refining, chemical fibres, phosphate fertilisers, non-ferrous metallurgy, acid production, and dyeing industries. Such valves are suitable for use in highly corrosive media, such as strong acids and bases.

Soft sealing butterfly valves are typically characterised by a small size, light weight, and excellent sealing performance, with the ability to achieve zero leakage. The soft sealing butterfly valve can be employed in a variety of settings. The selection of sealing material for this valve should be based on the specific operational conditions.

What is a hard seal butterfly valve?

2.Hard Sealing Butterfly Valve

A hard sealing butterfly valve is defined as a valve where both sides of the sealing pair are constructed from metal or other relatively hard materials.

Hard sealing butterfly valves are characterised by an eccentric metal sealing structure. The composition of the sealing surface materials typically comprises steel and steel, steel and copper, steel and graphite, or steel and an alloy steel (where the steel may be cast iron, cast steel, or alloy steel, and may involve overlay welding or sprayed alloy).

This type of sealing has relatively poor sealing performance, but it is resistant to high temperatures, wear, and has good mechanical properties.

The structural features of hard sealing butterfly valves are as follows:

The triple eccentric design eliminates friction between the sealing surfaces, thereby extending the valve's service life.

The sealing is achieved through the application of torque.

The innovative wedge design of the hard sealing butterfly valve enables an automatic sealing function that tightens as it closes, thereby providing compensatory sealing and zero leakage.

The material of the components can be altered to enable hard sealing butterfly valves to be adapted to a variety of media and to be lined for corrosion resistance.

What is the difference between soft seal and hard seal of butterfly valve?

The hard sealing material, which is typically metal, exhibits relatively poor sealing performance. The inability of soft sealing to meet the process requirements for some corrosive materials is a limitation that hard sealing can address. The two aforementioned sealing types can be employed in conjunction with one another. In terms of sealing performance, soft sealing is relatively superior, but hard sealing has also demonstrated an improvement in sealing performance to meet corresponding requirements.

Soft sealing is advantageous in terms of sealing performance, but it is susceptible to deterioration due to ageing, wear, and a shorter service life. Hard sealing has a longer service life than soft sealing, but its sealing performance is relatively poorer.

The principal structural distinctions between the two types of sealing can be summarized as follows:

Structural Differences:Soft sealing butterfly valves are predominantly of the centreline type, with large diameters employing double eccentric designs. Hard sealing butterfly valves are typically single eccentric, double eccentric, or triple eccentric.

Temperature Resistance:oft sealing is employed in normal temperature environments, whereas hard sealing can be utilised in low, normal, and high temperature environments.

Pressure:Soft sealing is typically employed in low to normal pressure conditions, while hard sealing can also be utilised in medium to high pressure conditions.

Sealing Performance: Both soft-sealing and triple-eccentric hard-sealing butterfly valves demonstrate satisfactory sealing performance. It can be demonstrated that triple eccentric butterfly valves are capable of maintaining effective sealing in high-pressure and high-temperature environments.

Concluding Remarks

In light of the aforementioned characteristics, soft sealing butterfly valves are well-suited for bidirectional opening, closing, and regulation in ventilation and dust removal pipelines, as well as in industries such as water treatment, light industry, petroleum, and chemicals. Hard sealing butterfly valves are more commonly employed in applications involving heating, gas supply, and environments with a high concentration of gases, oils, and acidic or alkaline substances. United Valves is a supplier of butterfly valves that offers a range of types and specifications. Should you require further information, we request that you contact us without delay.