Sealing form of forged valve and its sealing performance
Seals will also be a vital component in forged valves. The sealing performance of the forged valve means ability of the sealing parts of the cast valve to stop the leakage in the medium. It is an essential technical performance index in the forged valve.
You can find three sealing elements of the cast valve:
This is the contact between your sealing surfaces with the frequent lowering and raising member as well as the valve seat the joint with the packing together with the valve stem along with the stuffing box the text between your forged valve body and the forged valve cover. The leakage inside the former place is called inner leakage, that is known as having less tightness, and can affect the ability of the solid valve to slice from the medium. For forged shut off valves, internal leakage is not allowed. The latter two leakages these are known as outside leakages, that’s, the medium leakage internally in the forged valve facing outward of the forged valve.
External leakage could cause material loss, pollute the environment, and cause serious accidents in severe cases. For flammable, explosive, toxic or radioactive media, external leakage just isn’t allowed, so the forged valve have to have a dependable sealing performance.
A forged valve dynamic seal mainly means valve stem seal. This doesn’t let the medium in the forged valve to leak together with the movement in the valve stem, the actual main subject of forged valve dynamic sealing.
Stuffing box form: forged valve dynamic seal, mainly stuffing box. The basic form of the stuffing box is:
1. Gland type: This is the most commonly used form.
The unified form can be distinguished in many details. For example, as far as the compression bolt is concerned, T-bolts (for low pressure forged valves with working pressure less than 16kg/cm2), double end bolts and swing bolts are available for use. In terms of the gland, it can be divided into integral type and combined type.
2. Compression nut type: This seal form is of small size and limited compression force, so only applicable to small forged valves.
Stuffing: Inside the stuffing box, the stuffing is directly in contact with the valve stem and the stuffing box is filled with the stuffing to prevent leakage of the medium. The following requirements are imposed on the stuffing:
- Good sealing;
- Corrosion resistant;
- Small friction coefficient;
- Comply with media temperature and pressure.
With the increase of flammable, explosive, highly toxic, and radioactive materials, the chemical industry and the atomic energy industry have grown rapidly. As a result, there are more stringent requirements for forged valve sealing. Some places have no way to use packing seals, and a new seal form has emerged. – Bellows seal. This type of seal does not require a packing, which is also called a packing-free seal.
The two ends of the bellows are welded to other parts. When the valve stem is lifted and lowered, the bellows expands and contracts. As long as the bellows themselves does not leak, the medium cannot be released outwards. For the sake of safety, a double seal of bellows and packing is often used.
What is a static seal? A static seal is usually a seal between two stationary faces. The sealing method is mainly to use a gasket.
Regular use of gasket performance:
When using a forged valve, the original gasket is often replaced as the case may be. Commonly available gaskets include: rubber flat gaskets, rubber O-rings, plastic flat gaskets, polytetrafluoroethylene packing gaskets, asbestos rubber gaskets, metal flat gaskets, metal special-shaped gasket, metal sheathed gaskets, wave gaskets, wraparound entangled gaskets and soon.
1. Rubber flat gasket: It is easy to deform, not tightened when pressed, and has poor pressure resistance and temperature resistance. It is only used for low pressure and low temperature. Natural rubber has certain acid and alkali resistance, the working temperature should not exceed 60 °C; neoprene can also resist certain acids and alkalis, with its working temperature of 80 ° C; butadiene-acrylonitrile rubber is oil resistant, and can be used at temperatures up to 80 ° C; fluororubber has excellent corrosion resistance, with good temperature resistance performance also better than common rubber and can be used in the medium of 150 °C.
2. Rubber O-shaped gasket: the cross-sectional shape is a perfect circle, there is a certain self-tightening effect, the sealing effect is better than that of the flat gasket, and the pressing force is smaller.
3. Plastic flat gaskets: The most obvious characteristic of plastics is corrosion resistance. The temperature resistance of the plastics is not good. To some extent, PTEF is the best among plastics, excellent in corrosion resistance, with wide temperature range, and can be used continuously within temperature of -180 ° C to + 200 ℃.
4. PTEF coated gasket: PTFE wrapped rubber or asbestos rubber gasket enriches and demonstrates the advantages of PTFE, at the same time making up for its poor flexibility. In this way, it is as resistant to erosion as PTFE flat gaskets, and has excellent elasticity, which enhances the sealing effect and reduces the compression force.
5. Asbestos rubber gasket: it is cut from asbestos rubber sheet. Its composition is 60-80% 25 asbestos and 10-20% rubber, and fillers, vulcanizing agents and the like. It has good heat resistance, cold resistance, chemical stability, abundant supply and low price. When used, the pressing force does not need to be very large. It can adhere to the metal, and it is best to be coated with a layer of graphite powder in order to avoid the trouble of disassembly.
6. Metal flat hot ring: lead has the capacity to withstand the temperature of 100 %uFFFD C aluminum 430 %uFFFD C copper 315 %uFFFD C low carbon steel 550 %uFFFD C silver 650 %uFFFD C nickel 810 %uFFFD C Monel (nickel copper) alloy 810 %uFFFD C, and stainless-steel 870 %uFFFD C. Included in this, pressure resistance of lead is poor, aluminum can withstand 64 kg per square centimeter, and other materials can withstand ruthless.
7. Metal special shaped gasket:
Lens gaskets: Self-tightening for top pressure valves.
Elliptical gaskets: also belongs to high pressure self-tightening gaskets.
Cone double gaskets: employed for ruthless inner self-care sealing.
Furthermore, you’ll find square, rhombus, triangle, tooth-shaped, dovetail-shaped, B-shaped, C-shaped, etc., generally only found in high and medium pressure valves.
8. Metal sheathed gasket: Metal has excellent temperature and pressure resistance and excellent elasticity. The sheath material is aluminum, copper, low carbon steel, stainless steel, Monel and so on. The filling materials are asbestos, PTFE, fiberglass and so on.
9. Wave gasket: The nation’s characteristics of small pressing force and good sealing effect. A combination of metal and non-metal combinations will often be used.
10. Wraparound gasket:
The very thin metal strip and also the non-metallic strip are closely attached together, and therefore are in the middle of entangled right into a multi-layered circular shape using a wave-shaped cross section, which includes very excellent elastic and sealing performance. The metal strip might be manufactured from 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, monel or perhaps the like. The non-metallic strip materials can be asbestos, polytetrafluoroethylene, and stuff like that.
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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