Butterfly valve selection: How to choose between soft seal and hard seal?

Butterfly valve selection: How to choose between soft seal and hard seal?

In the selection of butterfly valves, the choice between soft sealed butterfly valves and hard sealed butterfly valves directly affects the system's sealing performance, service life, and cost. The core difference between the two lies in the sealing material and applicable scenarios, which need to be comprehensively judged from three aspects: medium characteristics, temperature and pressure, and opening and closing frequency.

The sealing pair of soft sealed butterfly valves often uses rubber (such as nitrile rubber, EPDM rubber) or fluoroplastic (such as PTFE), which has the advantages of zero leakage sealing and low opening and closing torque. When the medium is room temperature clean water, gas or weakly corrosive liquid (such as sewage, air conditioning circulating water), and the pressure is ≤ 1.6MPa, the soft sealed butterfly valve can achieve bi-directional bubble level sealing with its elastic sealing surface, especially suitable for occasions with strict leakage rate requirements (such as water supply pipelines, food processing). However, it should be noted that the upper limit of temperature resistance for rubber is usually 120 ℃, and for fluoroplastics it is 180 ℃. Exceeding this range can cause the sealing surface to harden or deform, leading to leakage.

Hard sealed butterfly valves achieve high temperature and high pressure resistance through metal (such as stainless steel, hard alloy) to metal or metal to ceramic sealing pairs. In the working conditions of steam, thermal oil, high-temperature gas (such as above 300 ℃) or particulate media (such as slurry, fly ash), the rigid sealing surface of the hard sealed butterfly valve can resist wear and erosion, and the service life is 3-5 times that of the soft seal. For example, double eccentric hard sealed butterfly valves are commonly used in boiler feedwater pipelines in the power industry. Their eccentric structure can reduce friction on the sealing surface, while balancing temperature resistance and opening and closing flexibility.

There are two major misconceptions to be aware of when selecting a product: firstly, one should not blindly pursue hard seals. If there are no high-temperature particles in the medium, the high cost and large opening and closing force of hard sealed butterfly valves become disadvantages; Secondly, soft sealed butterfly valves ≠ low pressure rating. By thickening the valve body and strengthening the sealing design, some soft sealed butterfly valves can withstand a pressure of 2.5MPa, but material certification (such as WRAS, CE) needs to be confirmed with the manufacturer.

Conclusion: Soft sealed butterfly valves are preferred for room temperature cleaning media, while hard sealed butterfly valves are preferred for high-temperature particulate media; If the operating conditions are between the two (such as hot water at 150 ℃), a composite butterfly valve with a metal sealing surface sprayed with ceramic or welded with hard alloy can be considered to balance performance and cost.