In industrial production, valves are indispensable and crucial equipment, and the sealing surface is the key part of the valve. The quality of the sealing surface directly determines the service life and sealing performance of the valve. Therefore, selecting the appropriate sealing surface material is of great significance. This article will provide a detailed introduction to the commonly used sealing surface materials and their performance characteristics, discuss how to choose the right materials according to different working conditions in practical applications, and analyze the causes of sealing surface damage and preventive measures.
The sealing surface material is the key to the sealing performance of the valve. To better understand how to select the appropriate material, we need to first understand the classification and performance characteristics of sealing surface materials. Sealing surface materials are mainly divided into two categories: soft sealing materials and hard sealing materials.
Nitrile Rubber (NBR): Nitrile rubber is renowned for its excellent oil resistance, with better heat resistance than natural rubber and styrene-butadiene rubber. It also has good air tightness and water resistance. Depending on the acrylonitrile content, it is divided into NBR-18, NBR-26, and NBR-40. The higher the acrylonitrile content, the better the oil and heat resistance, but the elasticity decreases. It is suitable for temperatures ranging from -60℃ to 120℃ and is commonly used for sealing in petroleum products, benzene compounds, water, and acidic and alkaline media.
Fluororubber (FKM): Fluororubber has characteristics of high-temperature resistance, acid and alkali resistance, oil resistance, and steam resistance. It has low permanent compression deformation and good air tightness, maintaining sealing performance in harsh chemical environments. It is applicable in the temperature range of -30℃ to 220℃ and is commonly used for sealing in petroleum products, water, acids, and alcohols, especially in the chemical industry for strong corrosive media.
Polytetrafluoroethylene (PTFE): Polytetrafluoroethylene is resistant to high temperatures and chemical corrosion, with a low friction coefficient, which can reduce wear and extend the service life of the valve. However, it has insufficient mechanical strength and elasticity and is prone to creep. It is suitable for strong corrosive media at temperatures up to 170℃ and is commonly used in sealing applications of chemical equipment.
Copper Alloys: Copper alloys are corrosion-resistant and wear-resistant, commonly used for water and steam media at pressures up to 1.6MPa and temperatures up to 200℃. Common grades include ZCuAl10Fe3 (aluminum bronze) and ZCuZn38Mn2Pb2 (cast brass), mainly used for medium and low-pressure water valves.
Chromium Stainless Steel: Chromium stainless steel has good corrosion resistance and is suitable for water, steam, and oil products at temperatures up to 450℃. Common grades are 2Cr13 and 1Cr13, widely used in valves under medium pressure and temperature conditions.
Stellite Hard Alloy: Stellite hard alloy combines corrosion resistance, erosion resistance, and anti-scuffing properties. It is applicable in the temperature range of -268℃ to 650℃ and is suitable for various media, especially in highly corrosive environments. Due to its high cost, it is often processed by cladding and is commonly used in high-pressure, highly corrosive working conditions for valves.
Nickel-based Alloys: Typical materials include Monel, Hastelloy B, and Hastelloy C. Monel is resistant to hydrofluoric acid corrosion and is suitable for alkalis, salts, food, and acidic solvents at temperatures ranging from -240℃ to 482℃. Hastelloy is resistant to strong corrosion and is suitable for mineral acids, sulfuric acid, wet HCl gas, and strongly oxidizing media at temperatures ranging from 371℃ to 538℃. They are widely used in the chemical industry for strong acid and alkali environments.
When selecting sealing surface materials, a comprehensive consideration of various factors is necessary to ensure the reliability and service life of the valve under actual working conditions.
Corrosion is the process by which the surface of the sealing surface is damaged by the medium. If the sealing surface material is not corrosion-resistant, the seal cannot be guaranteed. Therefore, the sealing surface material must have good corrosion resistance. The corrosion resistance of the material mainly depends on its composition and chemical stability. For example, when dealing with acidic media, materials resistant to acid corrosion, such as fluororubber or nickel-based alloys, should be selected; when dealing with alkaline media, materials resistant to alkali corrosion, such as nitrile rubber or chromium stainless steel, should be chosen.
During the relative motion of the sealing surface, the damage caused by friction is called scuffing. This damage can destroy the sealing surface, leading to seal failure. Therefore, the sealing surface material must have good anti-scuffing performance, especially for valves that require frequent opening and closing, such as gate valves. The anti-scuffing property of the material is often determined by its internal properties. For example, Stellite hard alloy has excellent anti-scuffing performance and is suitable for valves that require frequent opening and closing.
When the medium flows at high speed over the sealing surface, it can cause damage to the sealing surface, known as erosion. This phenomenon is more evident in throttling valves and safety valves used in high-temperature, high-pressure steam media, and it has a significant impact on the seal. Therefore, erosion resistance is also an important requirement for sealing surface materials. For example, nickel-based alloys and Stellite hard alloy have good erosion resistance and are suitable for valves in high-speed media.
The sealing surface material should have a certain hardness, and its hardness should not decrease significantly at the specified working temperature. If the hardness is insufficient, the sealing surface is prone to wear; if the hardness is too high, it may lead to poor fit between the sealing surface and the body. For example, chromium stainless steel has moderate hardness and is suitable for medium pressure and temperature conditions.
The coefficient of linear expansion of the sealing surface material and the body material should be similar, which is particularly important for structures with inlaid sealing rings. If the difference in the coefficient of linear expansion is too large, additional stress and loosening can occur at high temperatures. For example, the coefficient of linear expansion of copper alloys and aluminum bronze is similar to that of some common valve body materials, making them suitable for structures with inlaid sealing rings.
When used at high temperatures, the sealing surface material must have sufficient oxidation resistance, thermal fatigue resistance, and the ability to withstand thermal cycling. For example, nickel-based alloys and Stellite hard alloy have good high-temperature performance and are suitable for valves in high-temperature working conditions.
Although sealing surface materials have good properties, in actual use, the sealing surface may still be damaged due to various reasons. Understanding the causes of sealing surface damage and taking corresponding preventive measures can effectively improve the quality and service life of the valve sealing surface.
Welding and Heat Treatment Defects: Cracks, porosity, and inclusions on the sealing surface are usually caused by improper selection of welding and heat treatment specifications and poor operation. For example, during the welding process, if the welding parameters are not properly controlled, pores and inclusions may appear in the weld seam, affecting the quality of the sealing surface. Moreover, the hardness of the sealing surface being too high or too low may be due to improper material selection or heat treatment. For example, if the heat treatment temperature is too high or too low, the hardness of the sealing surface may not meet the requirements.
Inhomogeneous Material Composition: The uneven hardness and poor corrosion resistance of the sealing surface are mainly caused by the dilution of the sealing surface alloy composition due to the underlying metal being blown onto the surface during the welding process. For example, during the welding of nickel-based alloys, improper operation may cause the underlying metal to mix into the sealing surface, reducing its corrosion resistance.
Improper Selection: Not selecting the valve according to the working conditions, such as using a globe valve as a throttling valve, can lead to excessive closing pressure, too fast or incomplete closure, causing erosion and wear of the sealing surface. For example, in conditions requiring throttling, a throttling valve should be selected instead of a globe valve to avoid sealing surface damage.
Improper Installation and Maintenance: Incorrect installation and inadequate maintenance can lead to abnormal operation of the sealing surface, causing the valve to run with faults and prematurely damaging the sealing surface. For example, if the sealing surface is not properly aligned during valve installation, it may lead to uneven force on the sealing surface, accelerating its damage.
Chemical Corrosion: The medium around the sealing surface directly chemically reacts with the sealing surface without generating an electric current, corroding the sealing surface. For example, when dealing with acidic media, if the sealing surface material is not acid-resistant, the sealing surface may be corroded.
Electrochemical Corrosion: The contact between sealing surfaces, the contact between the sealing surface and the closure member and valve body, as well as differences in medium concentration and oxygen concentration, can create potential differences, leading to electrochemical corrosion and causing the anodic sealing surface to corrode. For example, when dealing with media containing chloride ions, electrochemical corrosion may occur, leading to sealing surface damage.
Medium Erosion: The result of medium flow causing wear, scouring, and cavitation on the sealing surface. For example, in high-speed media, suspended fine particles in the medium can impact the sealing surface, causing local damage; high-speed flowing media can directly scour the sealing surface, causing local damage; when the medium is mixed and locally vaporized, bubble burst impacts the sealing surface, causing local damage. The combined effect of medium erosion and chemical corrosion can strongly corrode the sealing surface.
Mechanical Damage: During the opening and closing process of the sealing surface, damages such as scuffing, bumping, and pinching can occur. For example, under high-temperature and high-pressure conditions, atomic interpenetration can occur between the two sealing surfaces, leading to adhesion. When the two sealing surfaces move relative to each other, the adhesion points are prone to tearing. Additionally, during the valve closing process, the valve disc may bump and pinch the sealing surface during the reseating process, causing local wear or indentation on the sealing surface.
In long-term use, the sealing surface may develop fatigue under the action of alternating loads, resulting in cracks and delamination. Rubber and plastics are prone to aging after long-term use, leading to deteriorated performance. For example, in valves that are frequently opened and closed, the sealing surface is subjected to alternating loads and is prone to fatigue damage.
To effectively enhance the quality and service life of the sealing surface, we need to take comprehensive measures from multiple aspects. These measures include the rational selection of materials, optimization of structural design, and standardized use and maintenance. The following will provide a detailed introduction to these specific measures.
Selecting the appropriate sealing surface material according to different working conditions is the key to improving the quality and service life of the sealing surface. For example, when dealing with highly corrosive media such as strong acids and alkalis, materials with good corrosion resistance, such as nickel-based alloys or fluororubber, should be selected; in high-speed media, materials with good erosion resistance, such as Stellite hard alloy, should be chosen; for valves that require frequent opening and closing, materials with good anti-scuffing performance, such as Stellite hard alloy, should be selected.
A rational sealing structure and processing method can effectively improve the quality of the sealing surface. For example, during the welding process, welding parameters should be strictly controlled to avoid defects such as cracks, porosity, and inclusions. During the heat treatment process, the appropriate heat treatment process should be selected according to the material's performance requirements to ensure that the hardness and performance of the sealing surface meet the requirements. Additionally, the surface roughness of the sealing surface also affects its performance and should be selected according to the actual working conditions.
Proper use and maintenance of valves can effectively extend the service life of the sealing surface. For example, during valve installation, the sealing surface should be properly aligned to avoid uneven force. During use, the appropriate valve type should be selected according to the working conditions to avoid improper selection leading to sealing surface damage. During maintenance, defects in the sealing surface should be promptly identified and addressed to prevent the valve from running with faults.
Taking effective measures to prevent corrosion and wear can effectively extend the service life of the sealing surface. For example, when dealing with corrosive media, protective measures such as coatings and linings can be adopted to reduce the corrosion of the medium on the sealing surface. In high-speed media, a streamlined sealing surface structure can be used to reduce the scouring of the medium on the sealing surface. Additionally, regularly checking the corrosion and wear conditions of the sealing surface and promptly repairing or replacing it can effectively extend its service life.
The sealing surface is a key part of the valve, and its quality directly determines the service life and sealing performance of the valve. Selecting the appropriate sealing surface material, rational sealing structure and processing methods, and proper use and maintenance of the valve are important measures to improve the quality and service life of the sealing surface. In practical applications, various factors should be comprehensively considered according to different working conditions to select the most suitable sealing surface material and processing methods, ensuring the safe and reliable operation of the valve.
