Metal gasket
The metal gasket for forced seal includes flat metal gasket, corrugated gasket, serrated gasket and ring gasket (including octagonal gasket and oval gasket). These gaskets are difficult to be manufactured. In particular for the gaskets with large diameter (such as ring gasket), multiple machining are needed such as the processes of winding,rapid prototyping, welding, flaw detecting and turning, etc. Besides, the requirements for accuracy and surface roughness are relatively higher, so the cost also is high. Metal gasket generally is used for high temperature and pressure and the services where the non-metal gasket and metal jacketed gasket are not qualified.
Based on the temperature and physical properties of medium, the materials of metal gaskets usually adopt aluminum, copper, soft steel, pure iron, stainless steel, titanium and Monel metal, etc. For example, for synthetic ammonia plant, aluminum and soft steel are usually adopted; for urea plant, 1Cr18Ni9Ti and 0Cr18Ni12Mo2Ti are generally adopted; under high temperature, 1Cr18Ni9T and Monel metal are adopted; under low temperature, 0Cr18Ni9 is adopted.
Metal flat gasket
The metal flat gasket generally used for the male flange, female and male face flange, tongued and grooved face flange.
Flat metal gasket and metal corrugated gasket are generally used for medium and high pressure valve, pipeline and flange of equipment with smaller diameter. The service pressure for the former is 1.568 31.36 Mpa and for the latter is 1.568~3.92MPa. The gasket materials should be chosen based on the medium and temperature.
Corrugated metal gasket
Corrugated metal gasket is the concentric waveform ring processed from a flat metal ring through molding, rolling or turning, with the nonmetals like the expanded graphite or PTFE pasted on the surface. Compared to the serrated gasket, its inside linings are of a variety of thickness, such as 3mm, 2mm, 1mm, 0.5mm, etc., and the pitch of waves differs in accordance with the width of metal sealing surface, generally being 3mm, 4mm, and 6mm.
Metal serrated gasket
Metal serrated gasket is processed into concentric grooves on both sides of stainless steel flat gasket by use of precision lathes. Flexible graphite, PTFE, non-asbestos sheets or other soft metals can be pasted on its two sides according to different mediums. Metal serrated gasket can be also directly used without sealing layer, and achieves a good sealing effect. But it will easily damage the surface of the flange under a high pressure.
After using the metal serrated gasket, the indentation is easy to be left on the flange sealing surface, so it is generally used for the position with scarce removal. The service pressure is 1.96~19.6 MPa. For the manhole of large-scale high pressure vessel which needs to be dismantled frequently, a layer of soft gasket needs to be attached to the both sides of metal serrated gasket, such as high pressure asbestos rubber sheet, flexible graphite sheet, aluminum sheet and copper sheet, etc. so as to not only protect the flange surface, but also drastically improve the sealing performance.
Metal ring gasket
Metal ring gasket is the sealing gasket of various forms processed from the solid metals of various materials through mechanical cutting by high precision CNC lathes. Featuring small installation area and high pretension pressure, metal ring gasket are applied in sealing parts of pressure vessels, valves and cylinders in the environments of high temperature, high pressure and high corrosiveness.
Metal octagonal gasket and metal oval gasket
The octagonal gasket (cross section of seal ring is octagonal) and oval gasket (cross section of seal ring is oval, also known as indigenous steel ring in oil refining industry) used for trapezoid flange with grooved surface have good sealing performance. On the conical surface of groove, the contact of octagonal gasket with the surface is surface contact, while the contact of oval gasket is line contact. Therefore, the oval gasket offers close fit with the surface under small fastening force, but needs second fastening; while for the octagonal gasket, the leakage is not easy to occur after first fastening. Their common weakness is that the relatively larger tightening force of bolt is required.
The octagonal metal gasket and oval metal gasket are suitable for circular-groove-face flange. Compared with oval metal gasket, the octagonal metal gasket is easy to be processed and widely used.
The octagonal metal gasket and oval metal gasket are suitable for circular-groove-face flange. Compared with oval metal gasket, the octagonal metal gasket is easy to be processed and widely used.
Metal jacketed gasket
The gasket adopts non-metal materials as core materials which are jacketed by thin metal sheet with the thickness of 0.25~0.5mm. Based on jacketed states, the gasket can be classified into four types: fully jacketed type, French style jacketed type, corrugated jacketed type and double jacketed type, etc.
The selection of thin metal sheet should be based on the elastoplasticity, heat resistance and corrosion resistance. The main materials include copper, galvanized iron, stainless steel, Monel metal etc. In general, copper, galvanized iron and stainless steel are widely used.
Asbestos sheet and asbestos rubber sheet are jacketed by thin metal plate. While being used, the asbestos sheet and asbestos rubber sheet do not directly contact medium, which prevents the strength of asbestos fiber from decreasing and avoids the leakage of seal. So the application range of asbestos rubber sheet can be enlarged. For asbestos gasket jacketed by metal, the service temperature generally is 450℃ (some can be up to 600~700℃, for example, when used in flue gas under atmospheric pressure to 0.16MPa), the service pressure is 4MPa, the maximum service pressure can be up to 6MPa. If the pressure further increases, the gasket is easy to flow laterally and the core material is squeezed out from overlaps.
If the temperature is greater than 450℃, the core material can adopt ceramic fiber and carbon fiber. Flexible graphite is the best choice for core material. The using effect of flexible graphite gasket jacketed by metal is superior to that of asbestos gasket jacketed by metal.
Metal jacketed gaskets can be made into various shapes and are widely used for various heat exchangers, the big cover of reactor, hole and manhole flange, etc. The iron jacketed gasket with the diameter of 2m had been produced in China and its using effect is good.
Metal spiral wound gasket
Metal spiral wound gasket is formed by alternatively winding thin metal corrugated strip and non-metal strips such as asbestos, flexible graphite into spiral shape, and connecting the beginning and the end of metal strip by point welding. In other countries, metal spiral wound gasket also is known as spiral gasket.
Metal spiral wound gasket not only takes advantage of heat-resistance, resilience and strength of metal materials, but also takes advantage of flexibility of non-metal materials, so the sealing performance is better. Among all metal spiral wound gaskets, the sealing performance of flexible graphite gasket spiral wound by stainless steel strip is the best. For the flexible graphite spiral wound gasket, the pre-tightening specific pressure is lower than that of asbestos spiral wound gasket and the leakage from capillary pore of asbestos fiber is avoided.
In the oil medium, metal strip material generally adopts 0Cr13. In other media, metal strip material is suggested to adopt 1Cr18Ni9Ti.
In the gas medium, the service pressure of flexible graphite gasket spiral wound by stainless steel strip is 14.7 (in China, the pressure of some gaskets can be up to 19.6MPa); in the liquid medium, the service pressure can be up to 30MPa. The service temperature is -190~+600 (can be up to 1000℃ under low pressure and absence of oxygen).
PTFE has good low-temperature resistance. The yield strength of PTFE at low temperature is greater than that at atmospheric temperature. So PTFE spiral wound gasket can be used for low temperature medium, such as liquid hydrocarbon and so on. At the same time, the thermal conductivity increases due to adding metal strip. The service temperature of PTFE spiral wound gasket can be up to 250℃. In the acid medium, PTFE spiral wound gasket can be used under the pressure of 9MPa and temperature of 200℃.
Spiral wound gasket is used for heat exchanger, reactor, pipeline, valve, inlet and outlet pump flange with drastic pressure and temperature fluctuations. For the above average pressure and the temperature over 300℃, the spiral wound gasket with inner ring, outer ring or the both should be considered to adopt. For the male-female flange, the spiral wound gasket with inner ring should be adopted.
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