Selection of flanges and gaskets

Common gaskets are divided into three main categories: 1. Non-metallic gaskets: these are mostly flat gaskets, suitable for pressures not exceeding 2.5 MPa; asbestos-rubber sheet gaskets can be used in temperatures ranging from -50 to 400 degrees； The temperature for PTFE gaskets should not exceed 150 degrees. 2. Semi-metallic gaskets: wound gaskets and metal-clad gaskets; they can withstand a maximum pressure of 25 MPa and temperatures up to 600 degrees. 3. Metal gaskets: generally used in high-pressure applications, with their applicable temperature range varying depending on the material used.

Reply 1# fengboy: Bolts, flanges, and gaskets together form a sealing structure. In this structure, the gaskets are compressed against the flanges by the pre-tightening force of the bolts, causing the gaskets to undergo elastoplastic deformation. This deformation fills the microscopic geometric gaps between the flanges and the gaskets, increasing the flow resistance of the medium and thus preventing or reducing medium leakage. a) Non-metallic gaskets: These are PTFE-coated gaskets, suitable for use in pipelines where corrosion resistance, anti-adhesion properties, and high cleanliness are required. Their maximum nominal pressure is 5.0, and their maximum operating temperature is 150°C. The use of asbestos or rubber-asbestos gaskets is prohibited. b) The semi-metal gasket, specifically the wound-type semi-metal gasket, is the most ideal and widely used type of gasket among semi-metals. It features good compressive elasticity and high strength, enabling it to absorb changes in pressure and temperature as well as mechanical vibrations. It maintains excellent sealing performance under high and low temperatures, impact vibrations, and alternating loads. It is generally used in pipelines containing highly toxic or flammable media, or those operating at high temperatures, with large temperature differences, and subject to mechanical vibration or pressure fluctuations. The selection of wound gaskets shall comply with the following requirements: 1) Flanges with raised surfaces shall use wound gaskets with an outer ring ； 2) Concave-convex face flanges shall use wound gaskets with an inner ring ； 3) Grooved face flanges shall use basic type wound gaskets ； 4) Flanges with a raised face and a nominal pressure of 15.0 MPa or higher shall use wound gaskets with inner and outer rings. c) Metal gaskets are used in high-temperature, high-pressure pipelines. It is often used in combination with boss surfaces, recessed surfaces, and tenon-mortise surfaces. Octagonal metal gaskets and elliptical metal gaskets are often used in conjunction with ring-grooved face flanges. Under normal circumstances, the material of the metal gasket should be selected in conjunction with the flange material, and it is required that the hardness of the gasket be lower than that of the flange sealing surface (not less than HB30).

Reply to 3# APPLEPINE: Thank you. I’ve learned it; in such cases, non-metallic gaskets are sufficient

Look for the flange standards and read them several times; there are indicated the methods for selecting the appropriate gaskets as well as the standard numbers corresponding to them. Only by practicing can one remember well!

When selecting gaskets, the following factors should be fully considered: (1) They must have good elasticity and resilience, enabling them to adapt to changes in pressure and temperature fluctuations； (2) It has appropriate flexibility, allowing it to fit well against the surface in contact ； (3) Does not contaminate the process medium ； (4) Have sufficient toughness to avoid failure under pressure and tightening forces ； (5) Does not harden at low temperatures, with low shrinkage ； (6) Good workability, easy to install and compress ； (7) Non-stick sealing surface, easy to disassemble ； (8) Low price and long service life. In the use of gaskets, pressure and temperature influence each other; as the temperature rises, after the equipment has been in operation for some time, the gasket material softens, undergoes creep, and experiences stress relaxation, which leads to a decrease in its mechanical strength and a reduction in the sealing pressure. The same is true in reverse. For example, the manual specifies that the high-pressure asbestos rubber sheet XB450 can be used in water and steam environments at a temperature of 450°C and a pressure of <6 MPa (when testing its sealing properties, this material is exposed to steam at 440°C–450°C and 12 MPa for 30 minutes). However, in long-term practical use, when the temperature reaches 450°C, the pressure that can be sealed is only 0.3~0.4 MPa. For gas media with high permeability, it is only 0.1~0.2 MPa. The above circumstances should be fully considered when making a selection. Industrial rubber sheets made of natural rubber are suitable for use in media such as water, seawater, air, inert gases, alkaline solutions, and saltwater solutions. However, they are not resistant to mineral oils and non-polar solvents. The maximum operating temperature is 90°C; they exhibit excellent performance at low temperatures and can be used at temperatures above -60°C. Nitrile rubber is suitable for petroleum-based products such as oil, lubricants, and fuel oils. Its maximum operating temperature is 120°C; it can withstand temperatures of up to 150°C in hot oil, while its minimum operating temperature ranges from -10 to -20°C. Neoprene is suitable for seawater, weak acids, weak bases, and salt solutions. It exhibits excellent resistance to oxygen and ozone aging, its oil resistance is inferior to that of nitrile rubber but better than that of other general-purpose rubbers. The recommended operating temperature is below 90°C, with a maximum operating temperature of 130°C; it can function at low temperatures ranging from -30 to -50°C. There are several types of fluororubber, each possessing good acid and oxidation resistance as well as oil and solvent resistance. It can be used in almost all acidic media as well as in certain oils and solvents, with a maximum operating temperature of below 200°C over the long term. Rubber sheets, used as flange gaskets, are commonly applied to pipes or manholes and handholes that are frequently disassembled, in applications where the pressure does not exceed 1.568 MPa. Among various types of gaskets, rubber gaskets are the softest and have excellent fitting properties, enabling them to provide a sealing effect even with a low pre-tightening force. For this reason, when subjected to internal pressure, it is prone to being extruded due to the thick gasket or its low hardness. Rubber sheets used in organic solvents such as benzene, toluene, and ethers are prone to swelling, weight increase, softening, and stickiness, which leads to seal failure. Generally, it cannot be used if the swelling degree exceeds 30%. In low-pressure conditions (especially below 0.6 MPa) and in a vacuum environment, rubber gaskets are more suitable. Rubber materials have good density and low air permeability. For example, fluororubber is the most suitable material for gaskets in vacuum vessels, with a vacuum level that can reach up to 1.3×10-7 Pa. When rubber gaskets are used in a vacuum range of 10-1 to 10-7 Pa, they need to be baked and evacuated. Asbestos rubber sheets are cheaper than other gaskets and are easy to use ； The biggest problem is that, despite the addition of rubber and some fillers to the gasket material, it is still not possible to completely fill those tiny interconnected pores, resulting in slight leakage. Therefore, in highly polluted media, it cannot be used even if the pressure and temperature are not high. When used in high-temperature oil media, over time, due to the carbonization of the rubber and fillers, the strength decreases and the material becomes looser, which leads to penetration at the interfaces and within the gaskets, resulting in coking and smoking. Furthermore, asbestos rubber sheets tend to stick to the flange sealing surface at high temperatures, causing many difficulties in replacing the gaskets.