When choosing between a soft-sealed or hard-sealed butterfly valve, the key factor is the working conditions, rather than the price or brand. In simple terms, it is a balancing process of "requiring both tight sealing and long service life". 

Detailed selection logic (three-step method) 

1. Look at "temperature" and "pressure" (hard threshold) 

This is the physical limit and there is no room for negotiation. 

Normal temperature and low pressure (water, air): The preferred seal type is soft seal. 

If it is tap water, sewage, or HVAC (with temperatures usually around 90℃), soft seals (such as EPDM ethylene propylene diene monomer rubber) not only provide excellent sealing but are also inexpensive and easy to operate. 

High temperature and high pressure (steam, hot oil): Hard seals must be selected. 

If the temperature exceeds 120℃ or the pressure exceeds 1.6 MPa, the soft-sealed rubber will rapidly age, deform and lose its elasticity, resulting in leakage. At this point, a metal-to-metal hard-sealed butterfly valve (such as a three-sealed eccentric butterfly valve) must be used, which can withstand temperatures of up to 400℃ or even higher. 

2. Examine the "medium" characteristics (the key factor determining lifespan) 

Clean medium: Choose soft seals. 

If it is water, gas, cooking oil, etc., the elasticity of the soft seal can achieve a perfect "zero leakage". 

Harmful medium: Choose hard sealing. 

With particles/mud: such as coal powder, mineral slurry. The soft-sealed rubber can be scratched or worn down by the particles. Once the sealing surface is damaged, the valve is rendered unusable. The hard seal (such as tungsten carbide clad welding) is extremely wear-resistant and has a lifespan more than five times that of the soft seal. 

Strong corrosive/organic solvents: Some organic solvents can cause the rubber to swell and lose its effectiveness. In such cases, hard-sealed valves made of corrosion-resistant metals (such as stainless steel, Hastelloy) need to be used. 

High-temperature steam: This is the domain of hard seals. Soft seals will rapidly carbonize under high-temperature steam. 

3. Review "Sealing Requirements" and "Budget" 

Select soft seals. 

In some sensitive situations where leakage is a concern (such as in laboratories, food and pharmaceutical industries), the soft-sealing elastic seal has inherent advantages. 

Long-term maintenance-free / high reliability: Choose hard sealing. 

Although the initial cost of hard seals is high and the processing accuracy is demanding, in high-temperature and high-pressure environments, their total life cycle cost may be lower because there is no need to frequently replace the valve seats. 

Typical scenario example 

Scene A: Heating pipes in the residential area 

Operating condition: Hot water, temperature 90℃, pressure 0.8 MPa. 

Option: Soft-sealing butterfly valve (high cost-effectiveness, excellent sealing performance, sufficient for use). 

Scene B: Steam Pipeline of the Chemical Plant 

Operating condition: High-temperature steam, temperature 250℃, pressure 2.5 MPa. 

Option: Hard-sealed butterfly valve (it must be of the three-sealed eccentric structure; otherwise, the valve will get stuck or leak). 

Scene C: Sewage Treatment Plant 

Condition: Mud-containing wastewater, at normal temperature, under low pressure. 

Option: Usually, soft seals (such as EPDM rubber) are chosen. However, if the wastewater contains a large amount of sharp particles, it is recommended to upgrade to hard seals. 

Summary in one sentence: For normal temperature and normal pressure, choose soft seals (cheaper and more convenient); for high temperature and high pressure, or with particles, choose hard seals (more durable and sturdy). 

The differences between soft-sealed and hard-sealed valves and the selection guide.