Performance Requirements of Control Valve Sealing Materials

In industrial production, the importance of control valve sealing elements is self-evident. They not only ensure the tightness of control valves, effectively preventing the leakage of liquids, gases, or dust, but also protect the integrity of the medium, avoiding environmental pollution and energy waste. So, how can the quality of a control valve sealing material be evaluated? This article will provide you with an in-depth and detailed explanation from multiple perspectives, including key performance indicators of sealing materials, factors affecting sealing performance, and comprehensive performance requirements, to help you find the most suitable sealing solution among numerous options.

Key Performance Indicators of Sealing Materials

To ensure that control valve sealing elements can maintain stable sealing performance under various working conditions, the properties of the sealing material itself are crucial. The following are the key performance indicators to measure the quality of sealing materials, which directly determine their performance in actual applications.

1. Tensile Properties

Tensile properties are the first consideration for sealing materials, including tensile strength, stress at definite elongation, elongation at break, and permanent deformation after break. Tensile strength refers to the maximum stress when the specimen is stretched to fracture, which is like the "strength limit" of the material; the higher the value, the greater the force the material can withstand under tension without breaking. Stress at definite elongation (modulus at elongation) refers to the stress reached at a specified elongation, reflecting the material's "tensile resistance" at a certain elongation. Elongation is the deformation caused by a specified tensile force, expressed as the ratio of the elongation increment to the original length, which reflects the material's "flexibility." Elongation at break is the elongation rate when the specimen is pulled apart, while permanent deformation after break refers to the residual deformation between marked lines after fracture; the smaller this is, the stronger the material's ability to recover after being stretched.

2. Hardness

Hardness indicates the ability of sealing materials to resist indentation by external forces and is one of the basic properties of sealing materials. Hardness is related to other properties to some extent. Generally speaking, the higher the hardness, the greater the strength, the smaller the elongation, and the better the wear resistance, but the poorer the low-temperature performance. This is like a balance: when materials perform well in certain aspects, they may have shortcomings in others.

3. Compression Properties

Rubber sealing elements are usually in a compressed state. Due to the viscoelasticity of rubber materials, when compressed, the pressure decreases over time, which is manifested as compression stress relaxation; after the pressure is removed, the inability to return to the original shape is manifested as permanent compression set. This phenomenon is more significant in high-temperature and oil media. This property directly relates to the durability of sealing ability. If the sealing element cannot recover well after compression, leakage and other issues are likely to occur.

4. Low-Temperature Properties

There are mainly two test methods to measure the low-temperature characteristics of rubber sealing elements. One is the low-temperature retraction test, where the sealing material is stretched to a certain length, fixed, rapidly cooled below freezing, released after equilibrium, and then heated at a certain speed while recording the temperatures at which the specimen retracts by 10%, 30%, 50%, and 70%, respectively denoted as TR10, TR30, TR50, and TR70. Standards usually use TR10 as the index, which is related to the brittleness temperature of rubber. The second is low-temperature flex test, where specimens are frozen at specified low temperatures for a given time, then repeatedly bent at a specified angle, to evaluate sealing performance under dynamic load at low temperature. In cold environments, if sealing materials cannot maintain good properties, they may crack and fail.

5. Medium Resistance

Besides petroleum-based, diester, and silicate oils, sealing materials in the chemical industry may also encounter corrosive media such as acids and alkalis. In such media, sealing materials may not only be corroded but also expand and lose strength at high temperatures, leading to a reduction in hardness. At the same time, plasticizers and soluble substances in sealing materials may leach out, causing weight loss, volume shrinkage, and leakage. Generally, after immersion in media for a certain time under specific temperature, changes in weight, volume, strength, elongation, and hardness are measured to evaluate oil resistance or medium resistance. This is like a "chemical endurance test" for sealing materials, and only materials that pass can maintain reliable performance in complex environments.

6. Aging Resistance

When exposed to oxygen, ozone, heat, light, moisture, and mechanical stress, sealing materials degrade, which is referred to as aging. Aging resistance (also known as weather resistance) can be evaluated by changes in strength, elongation, and hardness after aging. The smaller the rate of change, the better the resistance. Weather resistance refers to phenomena such as fading, discoloration, cracking, chalking, and strength reduction in plastic products due to sunlight, temperature changes, and weather. Among these, ultraviolet light is the key factor promoting aging. Just like people age over time, sealing materials are affected by external factors, but excellent materials can maintain their properties longer.

Factors Affecting Control Valve Sealing Performance

In this section, we will analyze the various factors affecting the sealing performance of control valves. These include not only the properties of the sealing material itself but also valve design, manufacturing process, and operating environment.

1. Sealing Surface Quality

The sealing surface of a control valve refers to the part where the valve seat and closure member contact to achieve shutoff. When the contact pressure on the sealing surface is below 40 MPa, the quality of the sealing surface plays a decisive role. This is because when contact pressure is low and surface roughness is poor, leakage increases rapidly. When contact pressure is high, surface roughness has less effect.

2. Sealing Surface Width

The width of the sealing surface determines the length of capillary pores. When width increases, fluid flow path lengthens, and resistance increases. Increasing width can reduce erosion wear in high-pressure valves. However, increased width lowers contact pressure under the same sealing force, increasing leakage probability. This is like a balancing act: you must find the optimal point, not endlessly increase width.

3. Sealing Surface Material and Treatment State

The material and treatment state of the sealing surface greatly affect leakage. Residual gap size depends on microscopic unevenness. If the same material sealing rings are used, greater contact pressure is needed compared to brass sealing rings. Surface treatment states such as wave crest deformation, dimension changes, and gap variations all occur on the surface layer, which differs from base material properties. Machining may affect a surface layer of 50 μm. At low pressure, this effect is more prominent. When pressure exceeds 20 MPa, roughness influence decreases while material influence increases. Thus, microscopic differences in surface treatment significantly impact sealing performance.

4. Medium Properties

The properties of the medium also significantly influence sealing performance. For liquids, leakage depends mainly on viscosity; higher viscosity leads to less leakage under the same conditions. The difference between gases and liquids is even greater (except saturated steam), as saturated steam easily ensures sealing. This is like facing different "opponents," requiring different strategies: sealing is easier with viscous liquids but requires refined design for gases or low-viscosity liquids.

5. Surface Hydrophilicity

Surface hydrophilicity affects leakage due to capillary action. A thin oil film on the sealing surface requires increased water pressure to pass through gaps. Since metal surfaces are hydrophilic, kerosene easily penetrates casting and sealing joints. Thus, kerosene is often used in hydraulic seal tests, equivalent to a water pressure test of about 0.3–0.4 MPa. This is like a "gatekeeper" at the micro-level, controlling fluid penetration and influencing sealing performance.

Performance Requirements for Control Valve Sealing Elements

After understanding key performance indicators and influencing factors, we must also clarify comprehensive performance requirements to ensure excellent sealing performance in practice.

1. Excellent Sealing Performance

The primary task of control valve sealing elements is to ensure tightness, effectively preventing leakage of liquids, gases, or dust. This not only protects the medium's integrity but also avoids pollution and energy waste. Material selection, structural design, and manufacturing precision play decisive roles. This is like a strong "guardian," protecting the medium and preventing leakage.

2. Outstanding Pressure Resistance

In high-pressure environments, sealing elements must withstand huge pressures. Excellent pressure resistance ensures safety. Superior materials maintain integrity and reliability under pressure, avoiding leakage or rupture. Structural design also enhances pressure resistance. This is like a reliable "protector," ensuring stability under pressure.

3. Excellent Temperature Resistance

Sealing elements must maintain stable and reliable performance in varying temperatures. Whether extreme heat or cold, good temperature resistance ensures elasticity and sealing effect. Selecting proper materials, heat treatments, and coatings enhances resistance, enabling adaptation to all temperature conditions. This is like a "stabilizer," ensuring consistent performance despite temperature changes.

4. Strong Corrosion Resistance

Some media are highly corrosive, such as acid-alkali solutions and oxidizers. Corrosion-resistant materials and protective measures must be chosen. Good corrosion resistance extends service life, reduces maintenance, and ensures safe system operation. This is like a strong "shield", protecting against chemical attacks.

5. Excellent Low-Friction Properties

During opening and closing, sealing elements need low friction coefficients for flexible movement and long-term reliability. Low friction reduces energy use, operating force, and wear. Proper lubrication, low-friction materials, and surface treatments improve performance. This is like adding "lubrication oil," ensuring smooth operation and stability.

Conclusion

The performance indicators of control valve sealing materials and the factors influencing sealing performance are diverse. Only by comprehensively considering these factors, selecting suitable materials, designing reasonable structures, and applying precise manufacturing processes can sealing elements perform excellently under various conditions, providing reliable assurance for the safe and efficient operation of industrial production.