Common gasket performance - and new materials and technologies

Common gasket performance - and new materials and technologies

1） Common gasket performance

When using valves, the original gasket is often replaced according to the specific situation. Common gaskets include: rubber flat gasket, rubber O-ring, plastic flat gasket, PTFE wrapped gasket, asbestos rubber gasket, metal flat gasket, metal irregular gasket, metal foreskin gasket, waveform gasket, winding gasket, etc.

(1) Rubber flat washer: easy to deform, easy to press, but has poor pressure and temperature resistance, only used in places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, and its usage temperature should not exceed 60 ℃; Chloroprene rubber can also withstand certain acids and alkalis, with a usage temperature of 80 ℃; Nitrile rubber is oil resistant and can be used up to 80 ℃; Fluororubber has excellent corrosion resistance and stronger temperature resistance than ordinary rubber, and can be used in media at 150 ℃.

(2) Rubber O-ring gasket: The cross-sectional shape is perfectly circular, with a certain degree of self tightening effect. The sealing effect is better than that of a flat gasket, and the clamping force is smaller.

(3) Plastic flat washer: The biggest characteristic of plastic is its good corrosion resistance, while most plastics have poor temperature resistance. Polytetrafluoroethylene is the crown of plastics, which not only has excellent corrosion resistance, but also has a wide temperature range and can be used for a long time within -180 ℃ to+200 ℃.

(4) PTFE wrapped gasket: In order to fully utilize the advantages of PTFE and compensate for its poor elasticity, a gasket made of PTFE wrapped rubber or asbestos rubber is used. In this way, it is corrosion-resistant like polytetrafluoroethylene flat washers and has good elasticity, enhancing the sealing effect and reducing the clamping force. The cross-sectional shape is shown in Figure 4-20.

(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60-80% asbestos and 10-20% rubber, as well as fillers, vulcanizing agents, etc. It has good heat resistance, cold resistance, chemical stability, and abundant supply at a cheap price. When in use, the clamping force does not need to be very strong. Due to its ability to adhere to metals, it is best to apply a layer of graphite powder on the surface to avoid difficulties during disassembly.

There are four colors of asbestos rubber sheet: gray, used for low voltage (grade XB-200, pressure resistance ≤ 16 kg/cm2, temperature resistance 200 ℃); Red, used for medium voltage (brand XB-350, with a pressure resistance of up to 40 kg/cm2 and a temperature resistance of 350 ℃); Purple red, used for high voltage (brand XB-450, pressure resistance of 100 kg/cm2, temperature resistance of 450 ℃); Green, used for oils, also has good pressure resistance.

(6) Metal flat heating ring: lead, temperature resistance of 100 ℃; Aluminum 430 ℃; Copper 315 ℃; Low carbon steel at 550 ℃; Silver 650 ℃; Nickel at 810 ℃; Monel (nickel copper) alloy at 810 ℃, stainless steel at 870 ℃. Lead has poor pressure resistance, aluminum can withstand 64 kg/cm2, and other materials can withstand high pressure.

(7) Metal anisotropic gasket: Lens gasket: has self tightening effect and is used in high-pressure valves.

Elliptical gasket: also belongs to high-pressure self tightening gasket.

Cone double washer: used for high-pressure internal self sealing.

In addition, there are also square, diamond, triangle, tooth, dovetail, B-shape, C-shape, etc., which are generally only used in high and medium pressure valves.

(8) Metal foreskin gasket: Metal has both good temperature and pressure resistance, as well as good elasticity. The foreskin materials include aluminum, copper, low-carbon steel, stainless steel, Monel alloy, etc. The filling materials inside include asbestos, polytetrafluoroethylene, fiberglass, etc.

(9) Wave washer: It has the characteristics of low compression force and good sealing effect. Often in the form of a combination of metal and non-metal.

(10) Wrapped gasket: It is a method of tightly attaching thin metal and non-metal strips together, winding them into multiple layers of circular shapes with a wavy cross-section, providing excellent elasticity and sealing properties. Metal strips can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non metallic strip materials include asbestos, polytetrafluoroethylene, etc.

Above, when discussing the performance of sealing gaskets, some numbers were listed. It must be noted that these numbers are closely related to the flange form, medium conditions, installation and repair techniques, etc. Sometimes they can be exceeded, sometimes they cannot be achieved, and the pressure resistance and temperature resistance performance are also mutually transformed. For example, as the temperature increases, the pressure resistance often decreases. These subtle issues can only be experienced in practice.

2） New materials and new technologies

The sealing gasket introduced above is still not comprehensive, and the sealing technology is developing rapidly. Here are some examples to introduce new materials and technologies.

(1) Liquid sealing: With the rapid development of the polymer organic synthesis industry, liquid sealants have emerged for static sealing; This new technology is commonly referred to as liquid sealing. The principle of liquid sealing is to utilize the adhesion, fluidity, and monolayer effect of liquid sealant (thinner film naturally tends to recover), and under appropriate pressure, make it function like a gasket. So for the sealant being used, it is also called a liquid gasket.

(2) PTFE raw material sealing: PTFE is also a high molecular weight organic compound, which is called raw material before being sintered into products. It has a soft texture and also has a monolayer effect. The tape made from raw materials is called raw material tape, which can be rolled into a disc for long-term storage. When in use, it can be freely formed with any joint, and as soon as there is pressure, it forms a uniform annular film that plays a sealing role. As a gasket between the valve body and the valve cover in the valve, it can be pried open a gap without removing the valve disc or gate, and then stuffed into the raw material belt. The clamping force is small, it does not stick to hands or flange surfaces, making replacement very convenient. For mortise and tenon flanges, it is most suitable. Polytetrafluoroethylene raw material can also be made into tube and rod shapes for sealing purposes.

(3) Metal hollow O-ring: good elasticity, low clamping force, self tightening effect, can be made of various metal materials, and can adapt to low temperature, high temperature, and strong corrosive media.

(4) Graphite plate sealing ring: In people's impression, graphite is a brittle substance that lacks elasticity and toughness, but specially treated graphite has a soft texture and good elasticity. In this way, the heat resistance and chemical stability of graphite can be demonstrated in the gasket material; Moreover, this type of gasket has low compression force and exceptionally superior sealing effect. This type of graphite can also be made into strips, which can be combined with metal strips to form high-performance wound washers. The emergence of graphite plate sealing rings and graphite metal wound gaskets is a major breakthrough in high-temperature corrosion-resistant sealing. This type of gasket has been widely produced and used abroad.