In the industrial sector, the selection of valve gaskets is crucial for ensuring the safe operation of equipment and achieving effective sealing. A suitable gasket can effectively prevent leaks and ensure the smooth progress of production processes. So, how can we correctly choose the appropriate valve gasket based on the working conditions? This article will provide a detailed introduction to the key factors and steps in gasket selection, helping you make the right choice in practical work.
Selecting the right gasket is essential for achieving effective sealing. During the selection process, multiple factors need to be considered comprehensively, including temperature, medium, pressure, flanges, and bolts. These factors interact with each other and collectively determine the performance and applicability of the gasket.
Temperature is the first factor to consider when selecting a gasket. Different gasket materials have different performance characteristics within different temperature ranges. For example, when the system operating temperature approaches the maximum continuous operating temperature of a gasket material, a higher-grade material should be chosen. Generally, when the temperature is between 200°F (95°C) and 1000°F (540°C), the range of suitable gasket materials will quickly narrow. If the system temperature exceeds the tolerance range of the gasket material, the gasket may lose its sealing performance. Therefore, it is essential to select the appropriate gasket material based on the actual working temperature of the system.
The medium refers to the fluid flowing through the pipeline, which comes in a wide variety of types with different chemical properties. The corrosive, oxidative, and permeation characteristics of the medium have a significant impact on the selection of gasket materials. For example, ethylene propylene rubber (EPDM) has good corrosion resistance to strong acids, strong bases, and salt solutions, but it is not suitable for oils, solvents, aromatic hydrocarbons, and hydrocarbons. On the other hand, polytetrafluoroethylene (PTFE) has excellent corrosion resistance to most chemicals, but it performs poorly with free fluorine and alkali metals. Therefore, it is crucial to select a gasket material that is compatible with the medium's chemical properties to prevent corrosion and permeation of the gasket by the medium.
The pressure within the system is another important factor affecting gasket selection. Each gasket has a maximum pressure limit it can withstand. When the pressure exceeds this limit, the gasket may be extruded, deformed, or even ruptured. Additionally, the relationship between pressure and temperature should not be overlooked. Generally, an increase in temperature will reduce the material's strength, thereby decreasing its pressure-bearing capacity. Therefore, it is essential to select the appropriate gasket material based on the actual working pressure and temperature conditions of the system.
The type of flange and the bolts used are also significant factors influencing gasket selection. The size, number, and grade of the bolts determine the effective load, while the effective clamping area is calculated based on the gasket contact dimensions. The effective gasket sealing pressure can be derived from the bolt load and gasket contact surface. This parameter is crucial for selecting the appropriate gasket material.
In addition to the main factors mentioned above, the machining quality of the flange, the engagement of the bolt threads, the rotation of the flange, the tightening of the bolts, the additives in the medium, and the surface treatment of the flange can also affect the sealing performance of the gasket. Therefore, it is essential to consider these factors comprehensively during the selection process to ensure that the gasket performs optimally under actual working conditions.
After understanding the key factors that influence gasket selection, the next step is to determine the appropriate gasket by following a set of specific steps. These steps include determining the standard, nominal diameter and pressure, gasket category, form, material, and model. Each step is crucial to ensure that the selected gasket meets the requirements of the actual working conditions.
Generally, the standard used for the gasket is the same as that for the flange. Common standards include the Chinese National Standard (GB), the Mechanical Industry Standard (JB/T), the Chemical Industry Standard (HG), and the International Standard (ISO). These standards provide a unified specification for the design, manufacturing, and selection of gaskets.
The nominal diameter and pressure of the gasket should match those of the flange. For example, if the flange has a nominal diameter of DN50 and a nominal pressure of PN16, then the gasket should also have a nominal diameter and pressure of DN50-PN16.
Based on the nominal pressure and the maximum temperature of the medium, the category of the gasket to be used can be determined. Common gasket categories include flexible graphite, PTFE, rubber, non-asbestos fiber gaskets, and wound gaskets, as well as corrugated live-loaded gaskets, oval gaskets, and octagonal gaskets. Asbestos-based gaskets are gradually being phased out due to environmental and health concerns, and therefore, their use is not recommended.
The form of the gasket is mainly divided into four major categories: non-metallic soft gaskets, wound gaskets, metal composite gaskets, and solid metal gaskets. The specific form of the gasket should be selected based on the flange form and the technical parameters provided by the manufacturers.
The material of the gasket should be determined based on the temperature, pressure, and corrosive properties of the medium, and then the model of the gasket should be selected accordingly. The model of the gasket generally includes six parts: gasket form, material, nominal diameter, nominal pressure, standard number, and the manufacturer or brand.
After understanding the key factors and specific steps in gasket selection, it is necessary to become familiar with the performance characteristics of different gasket materials. These materials include metallic and non-metallic gaskets, each with its unique properties and application scope. The selection should be matched according to the specific working conditions.
Composition: 16-18% molybdenum, 13-17.5% chromium, 3.7-5.3% tungsten, 4.5-7% iron, with the remainder being nickel.
Recommended Maximum Working Temperature: Up to 1093°C.
Properties: It has excellent corrosion resistance, performing exceptionally well against various concentrations of cold nitric acid or up to 70% boiling nitric acid. It also shows good corrosion resistance to hydrochloric acid and sulfuric acid and has excellent stress corrosion resistance.
Brinell Hardness: Approximately 210.
Composition: 77% nickel, 15% chromium, and 7% iron.
Recommended Maximum Working Temperature: Up to 1093°C.
Properties: It has high strength at high temperatures and is commonly used in equipment that needs to address stress corrosion issues. It also has excellent machinability at low temperatures.
Brinell Hardness: Approximately 150.
Composition: 30% copper, with the remainder being nickel.
Recommended Maximum Continuous Working Temperature: Up to 815°C.
Properties: It has excellent corrosion resistance to most acids and alkalis, except for strong oxidizing acids. It is prone to stress corrosion cracking in hydrofluoric acid, mercuric chloride, and mercury media, so it is not suitable for these media. It is widely used in equipment for manufacturing hydrofluoric acid.
Brinell Hardness: Approximately 120.
Properties: It has good corrosion resistance to strong acids, strong bases, salts, and chloride solutions, but it is not suitable for oils, solvents, aromatic hydrocarbons, and hydrocarbons.
Recommended Working Temperature: -57°C to 176°C.
Properties: Pure graphite material without resins or inorganic substances can be divided into graphite materials with or without metal elements. It has excellent corrosion resistance to many acids, alkalis, salts, organic compounds, and high-temperature solutions. It sublimates at temperatures above 3316°C. Caution should be exercised when using it in strongly oxidizing media.
Applications: Besides being used as gaskets, it can also be made into packing and non-metallic winding tapes for wound gaskets.
Properties: It is an excellent gasket material for high-temperature and low-pressure conditions, as well as for lightweight flanges, with a recommended working temperature of 1093°C.
Applications: It can be used to make non-metallic winding tapes for wound gaskets.
Properties: It combines the advantages of most plastic gasket materials, with a temperature range of -95°C to 232°C. It has excellent corrosion resistance to chemicals, solvents, caustics, and acids, except for free fluorine and alkali metals. PTFE material can be filled with glass to reduce its cold flow and creep properties.
Selecting the appropriate valve gasket requires a comprehensive consideration of multiple factors, including temperature, medium, pressure, flanges, and bolts. By determining the standard, nominal diameter and pressure of the gasket, and choosing the right gasket category, form, material, and model based on specific working conditions, the gasket can perform optimally in practical applications, effectively prevent leaks, and ensure the safe operation of equipment. It is hoped that the introduction in this article will help you make the right choice in practical work.
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