Mechanical seal auxiliary seal material and selection

Auxiliary seals include dynamic and static ring seals, which are mostly made as O-rings, V-rings, rectangular rings, wedge rings or trapezoidal rings.

Their main functions are: to prevent leakage between the moving ring and the rotating shaft, and the static ring and the gland; to compensate for the deflection and vibration of the sealing surface, so as to ensure a good fit of the dynamic and static ring end faces.

The materials of the auxiliary sealing ring are mainly elastomers (such as rubber), plastics (such as tetrafluoroethylene), fibers (such as asbestos, carbon fiber), inorganic materials (such as expanded graphite), and metals (such as copper, aluminum, stainless steel, etc.).

The requirements for the physical and mechanical properties of the auxiliary sealing ring material are related to the sealing surface material.

1.Synthetic rubber

Rubber auxiliary seal is the most widely used auxiliary seal. Common rubber seals include nitrile rubber, fluorine rubber, silicone rubber, and neoprene.

(1) NBR Nitrile is the most commonly used rubber. It is a copolymer of butadiene and acrylonitrile. It is divided into several types according to the content of acrylonitrile: low acrylonitrile (butyronitrile-18), medium acrylonitrile (butyronitrile-26) and high acrylonitrile (butyronitrile-40). The higher the acrylonitrile content, the better its oil resistance, tensile strength, hardness and abrasion resistance, increased water resistance, and reduced air permeability. As a result, its solubility in polar solvents increases, and its corrosion resistance is affected. Elasticity and cold resistance also deteriorate. Nitrile rubber is not resistant to flexion and tear resistance. Generally, the acrylonitrile content in the nitrile rubber seal ring is 26% to 50%.

Nitrile rubber has excellent corrosion resistance to mineral oil, animal and vegetable fats and oils, and is widely used in equipment that contacts gasoline and other oils. It is resistant to alkali and non-oxidative dilute acid corrosion, non-oxidative acid (such as nitric acid, chromic acid, etc.), aromatic hydrocarbons, lipids, ketones, ethers, halogenated hydrocarbons and other corrosion.

Recently, hydrogenated nitrile rubber (HNBR) has been developed, which has better performance than nitrile rubber. The operating temperature range is -40 ~ 150 ℃, the oil resistance is better than nitrile rubber, and the hydrogen sulfide resistance is better than fluorine rubber. It is second only to ethylene-propylene rubber in 200 ℃ steam.

(2) FPM Fluorine rubber has the advantages of high temperature resistance, oil resistance and chemical resistance. Can be used in concentrated sulfuric acid, nitric acid, phosphoric acid, caustic soda and other media. However, as the temperature increases, its corrosion resistance decreases, and the maximum use temperature is 200 ° C.

The most widely used fluorine rubbers in China are fluorine-containing olefin copolymers, which are mainly fluorine-23 type and fluorine-26 type.

Type 23 fluororubber is an amorphous rubber-like copolymer made by vinylidene fluoride and trifluorochloroethylene at room temperature and a pressure of about 3.3 MPa by suspension polymerization. Type 23 fluorine rubber is equivalent to foreign Kel-F fluorine rubber and can be used for strong acid.

There are two types of type 26 fluorine rubber: fluorine rubber-26, which is an emulsion copolymer of vinylidene fluoride and hexafluoropropylene, which is equivalent to foreign Viton fluorine rubber; fluorine rubber-246 series of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene. Terpolymer.

In recent years, a perfluororubber (FFKM perfluoropolymer elastomer) has been developed, which is equivalent to foreign Kalrez perfluororubbers. Its anti-aging performance is excellent. There is no obvious aging phenomenon after 112 days of use at 260 ° C. The tensile strength is still about 90%, and it can be used continuously at 288 ° C and short-term at 310 ° C. Its oil resistance, abrasion resistance and corrosion resistance of mixed organic matter are good, but its expansion coefficient is nearly twice that of nitrile rubber (320 * 10-6 / K), and it is not suitable for use below zero.

(3) Silicone rubber MVQ Silicone rubber has good high temperature resistance and low temperature resistance. The safe operating temperature range is -70 ~ 200 ℃. There is no obvious corrosion in dilute sulfuric acid, hydrochloric acid, acetic acid, caustic soda, ethanol, mineral oil and other media. Silicone rubber is a polymer of polar polymers formed by polymerizing dimethylsiloxane and other silicone monomers in the presence of an acidic or basic catalyst. Silicone rubber has high thermal stability, but it is polar, and it is prone to ionic cracking under the action of acid and alkali, so it has poor corrosion resistance. Not suitable for organic solvents such as petroleum solvents (such as benzene, toluene, etc.), acetone, ketones, ethers. Silicone rubber has a low breaking strength and breaking elongation (only 1/3 of nitrile rubber).

Recently, fluorosilicone rubber (MFQ) has been widely used in gasoline, petroleum oils and solvents.

(4) EPM EPM EPM is a polymer of ethylene and propylene, which is divided into binary and terpolymers. It is particularly resistant to phosphate ester hydraulic oils, ketones, alcohol solutions and acids and bases. It is also resistant to high pressure steam, weather resistance and ozone resistance. However, it swells in mineral oil and diester-based lubricants, so it cannot be used in these media.

2.Polytetrafluoroethylene PTFE

Teflon has better heat resistance, oil resistance and corrosion resistance than ordinary rubber. It is commonly used in mechanical seals to make V-shaped rings and wedge-shaped rings. Compared with rubber, polytetrafluoroethylene has greater rigidity, lower elasticity and cold flow. Polytetrafluoroethylene has a high expansion coefficient and a large change with temperature, especially with a peak at normal temperature, which hinders its application in mechanical seals. However, polytetrafluoroethylene has a large range of use (-150 ~ 250 ℃), extremely low friction coefficient (f = 0.05 ~ 0.1 at low speed) and self-lubricity, the surface is not adhered, and the chemical stability is good. Resistant to chloride, boron trifluoride, high boiling point solvents, ketones, esters, ethers, boiling nitric acid, aqua regia and sodium hydroxide, hydrofluoric acid, etc. The only thing that will attack PTFE is molten metal and fluorine under high pressure. Under the load, creep (ie cold flow) occurs at any temperature. When it exceeds 8) 83, it will sublimate to produce toxic fumes.

3.Other materials

Other materials used as auxiliary seals are metal, filled polytetrafluoroethylene, expanded graphite (flexible graphite), asbestos, and rubber-plastic composite materials. These materials are mainly used in high temperature applications.