Soft sealing seat +PTFE

I. Core function of metal valve seat: structural support and foundation seal.

1. Provide high-strength structural support

• Material characteristics: metal valve seats (such as stainless steel, cobalt-based alloy, tungsten carbide, etc.) have the characteristics of high strength, high pressure resistance and impact resistance, and can withstand the mechanical load and medium pressure when the valve is opened and closed (such as the pressure above 10MPa in high-pressure pipeline).
• Typical scenario: In large-diameter valves (such as gate valves above DN500) or high-pressure working conditions, the metal valve seat is used as the main structural member to prevent the valve seat from failing as a whole due to pressure deformation.

2. Foundation seal and high temperature/wear resistance

• Hard sealing principle: the metal valve seat forms a smooth sealing surface through precision machining (such as grinding and surfacing hardened layer), and the initial sealing is realized by the plastic deformation of the metal itself (such as slight indentation of the stainless steel valve seat), which is suitable for harsh environments such as high temperature (such as steam above 300℃) and medium containing particles (such as pulp).

• Limitations:

  • The pure metal sealing surface is prone to “micro-leakage” under low or micro-pressure conditions (such as < ＜0.1MPa) due to insufficient pre-tightening;

  • Micro-defects on the metal surface (such as machining scratches and oxide layers) may lead to the sealing surface being not tightly attached.

II. the core function of PTFE sealing ring: strengthening sealing and making up metal defects.

1. Elastic compensation to achieve zero leakage sealing.

• Advantages of soft sealing: PTFE has low hardness (Shore hardness 50-60A) and high plasticity, which can fill micro-cracks (such as micron-scale scratches) on the surface of metal valve seat under the action of pre-tightening, and can form a tight fit even under low pressure, realizing “bubble-level seal” (leakage ≤ 10 PAM/s).

• Application scenario:

  • In natural gas pipeline valves, PTFE sealing ring can prevent methane gas from leaking slightly when it is transported at low pressure;

  • Clean valves in pharmaceutical industry need to meet the environmental protection requirements of “zero leakage” through PTFE sealing rings.

2. Corrosion resistance and low friction characteristics

• Corrosion resistance: PTFE is resistant to almost all chemical media (such as strong acid, strong alkali and organic solvent). In the scene where the metal valve seat may be corroded (such as hydrochloric acid media in chemical pipelines), PTFE sealing ring can independently undertake the sealing task to avoid direct contact between media and metal.
• Reduce the opening and closing torque: PTFE has a very low friction coefficient (0.05-0.1), which can reduce the friction resistance between the valve core and the valve seat, especially in rotary valves such as ball valves and butterfly valves, which can reduce the operating torque of the actuator and prolong the service life of the valve.

3. Sealing stability under temperature and pressure fluctuation

• Temperature adaptation range: PTFE can keep stable performance in the range of-200℃~260℃. When the metal valve seat expands due to temperature change, the elastic deformation of PTFE can compensate the gap change and prevent the sealing failure (for example, in a low-temperature liquefied natural gas valve, the PTFE sealing ring can resist the low-temperature contraction of-162℃).
• Pressure fluctuation compensation: under the condition of frequent pressure fluctuation (such as the outlet valve of reciprocating pump), the elasticity of PTFE can absorb pressure impact and avoid cracks on the metal sealing surface due to fatigue.

III. Synergistic logic of metal valve seat+PTFE sealing ring

IV. Structural Design and Installation Form

1.Common combination structure

• Inlaid: PTFE sealing ring is embedded in the annular groove of metal valve seat (such as the groove of ball valve seat) and fixed by metal pressure plate, and the sealing ring is pushed close to the valve core by medium pressure to form a “self-sealing” effect.
• Overlapping type: PTFE gasket (such as valve seat sealing surface of gate valve) is installed above the sealing surface of metal valve seat. Metal provides support and PTFE is responsible for sealing, which is suitable for occasions requiring double sealing.

2. Design key points

• PTFE size compensation: Considering the cold flow characteristics of PTFE (it will slowly deform under long-term pressure), the thickness of sealing ring should be reserved with 10%-15% compression to avoid sealing failure due to plastic deformation.
• Hardening treatment of metal valve seat: surfacing hard alloy (such as Stellit) or quenching to prevent long-term friction of PTFE sealing ring from causing metal surface wear.

V. Application Limitations and Alternatives

1. Limitations of PTFE

• Upper limit of high temperature: PTFE will gradually soften when it exceeds 260℃, so it is necessary to use metal winding pad or flexible graphite sealing ring (such as high temperature steam valve).
• Risk of granular media: the media containing hard particles (such as silt) may scratch the PTFE surface, so it is necessary to match the wear-resistant layer of metal valve seat (such as tungsten carbide) or use all-metal hard seal instead.

2. Substitution combination

• Metal valve seat+flexible graphite sealing ring: suitable for high temperature (≤600℃) and high pressure conditions (such as power station valves). The high temperature resistance of graphite is combined with the strength of metal.
• Metal valve seat+metal sealing ring: fully hard sealing structure, used for ultra-high pressure (> ＞30MPa) or scenes with strong scouring particles (such as oil cracking unit), but the sealing accuracy is lower than that with PTFE.