Types of packings
Packings of valves can be divided into soft packings and hard packings.
Packings of valves can be divided into soft packings and hard packings.
Soft packings: they are cotton ropes wovening from mariajuana, flax, cotton and jute, or having metal wire inside with asbestos fibers and coating with graphite powder on the outside. There are also packings formed by pressing, as well as flexible graphite packings developed in recent years. Plant packings are often used for low-pressure valves below 100°C; mineral packings can be used for valves with temperatures of 450 to 500°C. In recent years, the temperature of the structural medium that uses rubber O-rings as packings is generally limited to below 60°C. The packing on the high-temperature and high-pressure valve is also made of pure asbestos and flake graphite powder.
Hard packings: it is the packing made of metal, metal mixed with asbestos and graphite, and formed by pressing and sintering of polytetrafluoroethylene. Metal packings are used less.
The selection of the packing
The selection of packing should be based on the medium, temperature and pressure. The commonly used materials are as follows:
The selection of packing should be based on the medium, temperature and pressure. The commonly used materials are as follows:
Oil-impregnated asbestos ropes
Rubber asbestos ropes
Graphite asbestos ropes: The asbestos rope is coated with graphite powder, and the usable temperature is above 450℃; the pressure can reach 16Mpa, which is generally suitable for high-pressure steam. Recently, the herringbone-shaped packing has been gradually adopted, which is placed in a single ring and has good sealing performance.
Polytetrafluoroethylene: This is a widely used packing. It is especially suitable for corrosive media, but the temperature should not exceed 200℃. It is generally made by pressing or bar turning, and the shape is shown in Figure 5-1.
Gaskets
The gasket is used to fill all the unevenness between the two joint surfaces such as the sealing surface between the valve body and the valve bonnet to prevent the medium from leaking from the joint surface.
Requirements for gaskets
The gasket has a certain elasticity, plasticity and sufficient strength at working temperatures to ensure sealing. At the same time, it must have good corrosion resistance.
Types and selection of gaskets
Gaskets can be divided into two types, that is, soft gaskets and hard gaskets. Soft materials are generally non-metallic materials, such as cardboard, rubber, asbestos rubber sheets, PTFE and so on. Hard gaskets are generally made of metal or asbestos wrapped with metal, entangled metal and asbestos. There are many shapes for gaskets, including flat, round, oval, toothed, lens-shaped and other special shapes. Figure 5-2 shows the asbestos coated with metal and metal wound asbestos wound gaskets. Figure 5-3 is a rubber O-ring. Figure 5-4 shows the trapezoidal grooved metal gasket. Figure 5-5 shows the concave-convex metal tooth-shaped gasket on the sealing surface. Figure 5-6 shows the lens metal gasket on the sealing surface.
The materials of metal gaskets are generally 08, 10, 20 high-quality carbon steel and 1Cr13, 1Cr18Ni9 stainless steel, which require high machining accuracy and good surface finish, and are suitable for high-temperature and high-pressure valves. Non-metallic gaskets generally have good plasticity, and can be sealed with a small pressure, which is suitable for low-temperature and low-pressure valves. Materials for gaskets can be selected according to Table 5-4.
Table 5-4
| Materials for gaskets | Media | Scope of application | |
| Pressure(Mpa) | Temperatures (℃) | ||
| Thick cardboard | Water and oil | Less than and equal to 10 | 40 |
| Oil-impregnated cardboards | Water and oil |
Less than and equal to 10 | 40 |
| Rubber boards | Water and air | Less than and equal to 6 | 50 |
| Asbestos boards | Steam and gas | Less than and equal to 6 | 450 |
| Teflon | Corrosive media | Less than and equal to 25 200 |
|
| Rubber asbestos board XB-450 | Water vapor, air and gas | Less than and equal to 60 | 450 |
| XB-350 | Water vapor, air and gas | less than and equal to 40 | 350 |
| XB-200 | Water vapor, air and gas | Less than and equal to 15 | 200 |
| Oil-resistant rubber asbestos boards | Oil | 160 | 30 |
| 08 steel with XB-450 filling | Steam | 100 | 450 |
| 08 steel and XB-350 filling | Steam | 40 | 350 |
| 1Cr13, 0Cr13 asbestos filling | Steam | 100 | 600 |
| 08 Steel and oil resistant rubber asbestos filling | Oil | 100 | 350 |
| Copper | Water vapor and air | 100 | 250 |
| Aluminum | Water vapor and air | 64 | 350 |
| 10 steel, 20 steel | Water vapor, oil | 200 | 450 |
| 1Cr13 | Steam | 200 | 550 |
Table 1-2 Common materials for internal parts of valves
| Materials for internal parts | Applicable Temperatures/℃ | Materials for internal parts | Applicable Temperatures/℃ |
| 304 | -268 to 316 | 440 stainless steel 60RC | -29 to 427 |
| 316 | -268 to 316 | 17-4PH | -40 to 427 |
| Bronze | -273 to 232 | No. 6 alloy steel (Co-Cr) | -273 to 816 |
| Inconel | -240 to 649 | Nickel Plating | -268 to 427 |
| K Monel | -240 to 482 | Chrome plating | -273 to 316 |
| Monel alloy | -240 to 482 | Nitrile rubber | -40 to 93 |
| Hastelloy B | -198 to 371 | Fluorine rubber | -23 to 204 |
| Hastelloy C | -198 to 538 | Teflon | 200 |
| Titanium alloy | -29 to 316 | Nylon | -73 to 93 |
| Nickel base alloy | -198 to 316 | Polyethylene | -73 to 93 |
| Alloy 20 | -46 to 316 | Neoprene | -40 to 82 |
| 416 type stainless steel 40RC | -29 to 427 |
Attached table 1-3 Common materials and applicable temperatures of valve sealing surfaces
| Materials for sealing surfaces | Applicable temperatures/℃ | Hardness | Applicable media |
| Bronze | -273 to 232 | Water, sea water, air, oxygen, saturated steam, etc. | |
| 316L | -268~316 | 14HRC | Steam, water, oil, gas, liquefied gas, and other slightly corrosive and non-erosive media |
| 17-4PH | -40 to 400 | 40 to 45HRC | Medium with slight corrosion but erosion |
| Cr13 | -101 to 400 | 37 to 42HRC | Medium with slight corrosion but erosion |
| Stellite alloys | -268 to 650 | 40 to 45HRC (room temperatures) 38HRC（650℃） |
Erosive and corrosive media |
| Monel alloy KS | -240 to 482 | 27 to 35HRC 30 to 38HRC |
Alkali, salt, food, acid solution without air, etc. |
| Hastelloy CB | 371 538 |
14HRC 23HRC |
Corrosive mineral acid, sulfuric acid, phosphoric acid, wet hydrochloric acid gas, chlorine-free acid solution and strong oxidizing medium |
| Alloy 20 | -45.6 to 316 | -253 to 427 | Oxidizing media and sulfuric acid of various concentrations |
Next: Stainless Steel with Low Carbon Content Used for Valves