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How to choose a gasket?

DataTime:[2/14/2019]
There are many types of gaskets, and gaskets of different materials and shapes have different functions. If you choose the wrong gasket, not only can it not be sealed well, it may also become a safety hazard, and there is a possibility of danger at any time. Do you know what are the gaskets and what are their characteristics?
Industrial rubber sheet
Natural rubber is suitable for mediums such as water, sea water, air, inert gases, alkalis, salt aqueous solutions, but not resistant to mineral oils and non-polar solvents. The long-term use temperature does not exceed 90 ° C, and the low temperature performance is excellent, and can be used at -60 ° C. Used above.
Nitrile rubber is suitable for petroleum products such as petroleum, lubricating oil, fuel oil, etc. The long-term use temperature is 120 ° C, such as 150 ° C in hot oil and -10 ~ -20 ° C in low temperature.
Neoprene is suitable for seawater, weak acid, weak alkali, salt solution, excellent in oxygen and ozone aging resistance, oil resistance is inferior to nitrile rubber and superior to other general rubber. Long-term use temperature is lower than 90 °C, and the maximum use temperature is not more than 130 ° C, low temperature -30 ~ -50 ° C.
There are many varieties of fluororubbers, which have good acid and oxidation resistance, oil and solvent resistance. It can be used in almost all acid media as well as some oils and solvents. The long-term use temperature is below 200 °C.
As a flange gasket, the rubber sheet is mostly used for pipes or hand holes that are frequently disassembled, and the pressure does not exceed 1.568 MPa. Because in all kinds of gaskets, the rubber gasket is the softest, the fitting performance is good, and the sealing effect can be exerted under a small pre-tightening force. For this reason, when the internal pressure is applied, it is easily extruded because the gasket is thick or the hardness is low.
The rubber sheet is used in organic solvents such as benzene, ketone and ether, and is prone to swelling, weight gain, softening and stickiness, resulting in seal failure. Generally, the degree of swelling exceeds 30% and cannot be used.
In the case of low pressure (especially 0.6 MPa or less) and vacuum, it is suitable to use a rubber mat. The rubber material has good compactness and low air permeability. For example, fluororubber is the most suitable gasket for vacuum containers, and the vacuum can be up to 1.3×10-7Pa. When the rubber mat is used in the vacuum range of 10-1~10-7Pa, it needs to be baked and pumped.
Asbestos Rubber Sheet
The price is lower than other gaskets and it is easy to use. The biggest problem is that although the gasket material is added with rubber and some fillers, it is still impossible to completely fill the micropores that are colluded, and there is a slight infiltration. Therefore, in a highly polluting medium, even if the pressure and temperature are not high, it cannot be used.
When used in some high-temperature oil-based media, usually in the later stage of use, due to the carbonization of rubber and filler, the strength is reduced, the material becomes loose, and penetration occurs inside the interface and the gasket, and coking and smoking occur. In addition, the asbestos rubber sheet is easily bonded to the flange sealing surface at a high temperature, which causes a lot of trouble for replacing the gasket.
The pressure at which the gasket is used in various media in a heated state depends on the strength retention of the gasket material. Crystallized water and adsorbed water are present in the asbestos fiber material.
At 110 ° C, the adsorption water between the fibers has been precipitated by 2/3, and the tensile strength of the fiber is reduced by about 10%;
At 368 ° C, the adsorbed water is completely precipitated, and the tensile strength of the fiber is reduced by about 20%;
When the temperature exceeds 500 ° C, the crystal water starts to precipitate and the strength is lower. The influence of the medium on the strength of the asbestos rubber sheet is also great.
For example, in aviation lubricants and aviation fuels, the transverse tensile strength of No. 400 oil-resistant asbestos rubber sheets differs by 80%, which is due to the fact that aviation fuels swell the rubber in the sheet more than aviation lubricants. Taking into account the above factors, the recommended safe use range of domestic asbestos rubber sheet XB450: temperature 250 ° C ~ 300 ° C, pressure 3 ~ 3.5MPa; 400 oil-resistant asbestos rubber sheet use temperature should not exceed 350 ° C.
Asbestos rubber sheet contains chloride ions and sulfides. It is easy to form a corrosion original battery with water metal after water absorption. Especially the oil-resistant asbestos rubber sheet has several times higher sulfur content than ordinary asbestos rubber sheet, so it should not be used in non-oily medium. The gasket will swell in oil and solvent-based media, but within a certain range, it has little effect on the sealing performance.
For example, the No. 400 oil-resistant asbestos rubber sheet is subjected to a 24-hour immersion test in a normal-temperature aviation fuel, and the required increase in suction weight must not exceed 15%.
There is still a certain gap between domestic asbestos rubber sheets and foreign famous brand products. Statistics from a certain unit indicate that in oil media exceeding 350 °C, most of the asbestos rubber sheets are not leaking. Japanese Petroleum Industry Standard JPI-7S-71 "Asbestos Rubber Sheet Standards for Petroleum Industry", the use period is about one year.
PTFE
PTFE is easy to cold flow and creep under pressure and high temperature, so it is generally used for low pressure, medium temperature, strong corrosion and media that do not allow pollution, such as strong acid, strong alkali, halogen, medicine and so on. The safe use temperature is 150 ° C and the pressure is 1 MPa or less. Although the strength of the filled polytetrafluoroethylene is higher, the use temperature does not exceed 200 ° C, and the corrosion resistance is lowered. The maximum use pressure of the Teflon pad is generally not more than 2 MPa.
As the temperature rises, the material creeps, causing the seal pressure to drop significantly. Even if it does not heat up, the compressive stress of the sealing surface will decrease with the extension of time, resulting in “stress relaxation phenomenon. This phenomenon will occur in various gaskets, but the stress relaxation phenomenon of the Teflon pad is more serious. It should be noted.
The friction coefficient of PTFE is small (the compaction stress is more than 4 MPa, the friction coefficient is 0.035~0.04), and the gasket is easy to slide outward when pre-tightening, so it is better to use the concave-convex flange surface. When using a flat flange, the outer diameter of the gasket can be contacted with the bolt, and the bolt can be used to prevent the gasket from slipping out.
Since the glass-lined equipment is sintered after spraying a layer of enamel on the metal surface, the glaze layer is very brittle, coupled with uneven spraying and glaze flow, the flatness of the flange surface is poor. The use of metal composite gaskets is easy to damage the glaze layer, so it is recommended to use a PTFE sheet with a core material and a rubber wool board. The pad is easy to adhere to the flange surface and corrosion resistant, and the effect is good.
There are many factories in the strong corrosive medium with low temperature and pressure. The asbestos rubber sheet is wrapped with polytetrafluoroethylene raw material tape for manholes and pipes that are frequently disassembled. Because it is easy to make and use, it is very popular.
Asbestos resin sheet and impregnated asbestos sheet gasket
Pipes, pumps, valves, inlet and outlet flanges, which are mostly used in various acidic media, have a temperature of 80 ° C and a pressure of 0.6 MPa or less.
Asbestos-made gaskets are suitable for low-pressure and high-temperature conditions with a pressure below 0.1 MPa and a temperature not exceeding 800 °C. According to the specific requirements of the equipment, we can weave the gasket with the width, thickness and diameter.
Or cut the asbestos tape directly onto the flange surface. It is used in large sulfuric acid, nitric acid oxidation furnaces and some unprocessed equipment interfaces, far more effective than the original asbestos rope.
Metal asbestos pad
The asbestos sheet or the asbestos rubber sheet is covered by the metal sheet so as not to be in direct contact with the medium, the strength of the asbestos fiber is prevented from being lowered, and the leakage phenomenon is overcome, thereby expanding the use range of the asbestos rubber sheet.
Generally, the metal asbestos pad is used at a temperature of 450 ° C (individually, it can reach 600 to 700 ° C, such as in a flue gas of atmospheric pressure ~ 0.16 MPa), and the use pressure is 4 MPa, and the maximum is 6 MPa. If the pressure is increased again, the gasket is prone to cross flow and the core material is extruded from the lap joint.
Since the metal asbestos pad requires a large bolt tightening force, even when the operating pressure is lower than 2.45 MPa, a flange of pg 25 kg or less cannot be used. Otherwise, the rigidity of the flange and the bolt is insufficient, resulting in deformation and failure of the seal. Some people think that if the core material is changed to a synthetic rubber with better elasticity, the fastening force will decrease.
In fact, because the core material is softened, the fastening force is absorbed by the core material, and the fastening force required for the metal plate to be attached to the flange surface cannot be provided, and the pad is easily damaged. In addition, in the medium containing more chlorine ions and the acidic medium, the lap joint of the stainless steel pad and the iron pad is prone to crevice corrosion.
The temperature is higher than 450 ° C, and the core material can use ceramic fiber or carbon fiber. A steel plant uses metal-clad ceramic fiber gaskets for high temperature of 1100 ° C, and has not been damaged for two years. Flexible graphite is the most suitable core material. At present, metal-coated flexible graphite mats have been mass-produced in China. Its use is better than metal asbestos pads.
The metal pad can be made into various shapes, and is widely used for various heat exchangers, large covers of the reactor, loading and unloading holes, manhole flanges, and the like. The iron pad with a diameter of 2m has been produced in China and is in good condition.
A layer of flexible graphite sheet is applied on the surface of the metal pad. Compared with the uncoated metal pad on the surface, the pre-tightening pressure is smaller and the sealing performance is better.
At present, there is no such product in China, so some units have attached the existing flexible graphite wrinkle tape to the metal pad and metal flat pad, toothed pad and even asbestos rubber pad surface, solving many leakage problems. For example, a heat exchanger of a plant has a pressure of 5.88 MPa and a temperature of 450 ° C, and the medium is hydrogen/oil and gas.
I have used metal flat pads and toothed pads, all of which have leaks. After the flexible graphite wrinkle tape was attached to the flat pad, the problem was solved. It should be pointed out that this gasket form is a simple measure to solve the leakage of the flange gasket. The work quality of the flexible graphite tape directly affects the normal operation of the equipment. If a layer of glue is applied to the back of the strip, the quality of the overlay can be improved.
Metal winding mat
The metal wound mat skillfully utilizes the heat resistance, resilience and strength of the metal and the softness of the non-metallic material, so that the sealing performance is good, and the performance of winding the flexible graphite mat with the stainless steel strip is optimal. The pre-tightening pressure is smaller than that of the asbestos winding mat, and there is no disadvantage of leakage of asbestos fiber capillary pores.
In oil media, 0Cr13 is used for metal strips, while 1Cr18Ni9Ti is recommended for other media.
The stainless steel belt with flexible graphite winding mat is used in a gas medium with a pressure of 14.7 MPa (up to 19.6 MPa in China) and 30 MPa in liquid. Temperature -190~+600 °C (available to 1000 °C in the absence of oxygen and low pressure).
Teflon has good low-temperature resistance, and its yield strength at low temperatures is much higher than that at normal temperature. Therefore, the Teflon winding mat can be used for low temperature media such as liquid hydrocarbons. At the same time, the thermal conductivity is improved by the addition of the metal strip, and the temperature of the Teflon winding mat can reach 250 ° C, and can be used up to 9 MPa and 200 ° C in an acidic medium.
The winding pad is suitable for heat exchangers, reactors, pipes, valves, and pump inlet and outlet flanges with large pressure and temperature fluctuations. For medium to medium pressures and temperatures above 300 °C, the inner ring, outer ring or inner and outer rings should be considered. If a concave-convex flange is used, the winding mat with the inner ring is better.
A good sealing effect can also be obtained by attaching a flexible graphite sheet to both sides of the flexible graphite winding mat. A large-scale chemical fertilizer plant waste heat boiler is a key equipment for high temperature and high pressure. It uses a flexible graphite winding mat with an outer ring, which does not leak at full load and leaks when the load is reduced. A 0.5 mm thick flexible graphite plate was added to both sides of the gasket, which was cut into an arc shape, and the joint portion was overlapped by a slanting opening, and the use condition was good.
Metal flat pad, wave pad
And toothed pad metal flat pad, metal wave pad
Generally used for medium and high pressure valves, pipes and equipment flanges with small diameters. The use pressure is based on the temperature, the former is 1.568~31.36MPa, and the latter is 1.568~3.92MPa. The gasket material is selected based on the media and temperature.
Octagonal pad and oval pad
The octagonal pad and elliptical pad for the trapezoidal groove face flange (commonly known as "soil ring" in the refining industry) have good sealing performance. On the tapered surface of the groove, the octagonal pad is in face contact and the elliptical pad is in line contact.
Therefore, the elliptical pad has a good fit under a low fastening force, but needs to be tightened twice; and the octagonal pad is generally less likely to leak after being fastened once. Their shortcomings are that they require a large bolt tightening force. When used for low pressure and high temperature conditions, the flange grade must be above pg25kg.
Source: China Gaskets Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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