Valve Dynamic Seal  and Static Seal

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Valve sealing cannot be ignored and regarded as a important subject in valve applications , this is because most reasons of valve leakage phenomenon are related with it.Following is going to introduce them:

1. Dynamic Seal

Valve dynamic seal mainly means the stem seal,the key point to solve valve dynamic seal issues is making sure no medium leakage happens when valve stem do movements.

1.1 Packing Box Style

So far,the valve dynamic seal mainly basing on packing box, basic styles of packing box are:

1.1.1 Gland-stlye

This is the most used type and can be classified many styles, from compression bolt, there are T-shape bolt,double-end bolt and eyebolt; from gland, there are integral type and combined type.

1.1.2 Pressing nut style

Though the overall size f pressing nut is small, compressing force limited and subject to used be in small size vale.

1.2 Packing Features

The packing touch stem directly and fully filled in packing box, to prevent medium leaking.Packing need to have following characteristics:

1.2.1 Good sealing performance

1.2.2 Anti-corrosion

1.2.3 Small friction

1.2.4 applicable to medium pressure and temperature

1.3 Common Packing Materials

1.3.1 Flexible graphite

A new type of sealing material

1.3.2 Asbestos packing:

Good high-temperature resistance and anti-corrosion, but sealing effect is poor when used lonely, so always impregnated or attached to other materials.

1.3.3 Oil-soaked asbestos packing

Its basic structure in two forms, one is twisting style, the other is braided. And  also classified to round shape and square shape.

1.3.4 PTFE braided packing

This kind of packing has excellent corrosion resistance and can be used for cryogenic media.

13.5 Rubber O-ring

Its sealing effect is good when applied in low pressure situation. But working temperature is limited, such as natural rubber can only be used for 60 ℃.

1.3.6 Plastic molding packing

Generally made of three-piece, can also be made into other shapes. The plastic mostly adopt PTFE, but also use nylon 66 and nylon 1010.

1.3.7 Other packing form

Among 250 ℃ steam valves, if using asbestos packing and lead ring alternately as the packing, the leakage of steam will be reduced; some valves, the media often change, adopting asbestos packing and PTFE together as the apcking, sealing effect will be better. To reduce the friction on the stem, in ome occasions, you can add molybdenum disulfide (M0S2) or other lubricants.

1.3.8 Bellows seal

With the rapid development of the chemical industry and atomic energy industry, flammable, explosive, toxic and radioactive substances increased, more stringent requirements are exposed to the valve seal, and some occasions have been unable to use packing seal, thus resulting in a new seal type - bellows seal. This seal without packing, it is also called no-packing seal. Both ends of the bellows are welded to the other parts. When the stem lifts, the bellows telescopic, as long as the bellows itself does not leak, the media will not be able to leak. For insurance purposes, often using bellows and packing double seal.

2.Static Seal

A static seal usually refers to a seal between two stationary surfaces. The main method of static sealing is to use washer and gaskets.

2.1 Washer & Gasket Materials

2.1.1 Non-metallic gasket materials

Such as paper, hemp, leather, asbestos products, plastics, rubber and so on. However, material like paper, hemp, leather and the like, there are pores, easy to penetrate, when used to be impregnated with oil, wax or other impervious material. Rarely used in valves

2.1.1.1 Asbestos products and asbestos belt, rope, plate and asbestos rubber sheet. Asbestos rubber plate which has compact structure,good pressure temperature resistance performance, widely used in connection of valve-to valve and valve-to pipe.     

2.1.1.2 Plastic gaskets , have a good corrosion resistance and most widely used. Variety types like of polyethylene, polypropylene, soft PVC, Teflon, nylon 66, nylon 1010, and so on.

2.1.1.3 Rubber products have soft texture and a certain resistance ability of acid, alkali, oil, sea water. Types like natural rubber, styrene butadiene rubber, nitrile rubber, neoprene, isobutyl rubber, polyurethane rubber, fluorine rubber.

2.1.2 Metallic gasket materials

Generally speaking, metal materials, have high strength, high temperature resistance. However, lead is not the case, only to take its characteristics of resistant dilute sulfuric acid. Materials like brass, copper, aluminum, low carbon steel, stainless steel, monel alloy, silver, nickel and so on.

2.1.3 Composite gasket materials

Such as metal foreskin (internal asbestos) gaskets, combined wave washer, wound spiral gaskets and so on.

2.2 Performance of Gasket & Washer

As the case may be, the valve gasket frequently changes. Common gaskets are: rubber flat washers, rubber O-ring, plastic flat washers, PTFE gaskets, asbestos rubber gaskets, metal washers, metal shaped washers, metal foreskin washers, wave washers, wound spiral gaskets.

2.2.1 Rubber flat gasket & washer

Easy deformation and compression needs little force, but the pressure & temperature resistance ability are poor, only for low pressure & temperature application. Natural rubber has a certain acid and alkali resistance, working temperature should not exceed 60 ℃; neoprene is also resistant to certain acid and alkali, working temperature 80 ℃; nitrile rubber oil proof, working temperature 80 ℃; fluorine rubber has good corrosion resistance, temperature resistance performance better than the general rubber, can be used in the 150 ℃ medium.

 2.2.2 Rubber O-shaped gasket & washer

Its cross-section shape is round and has certain self-tightening effect, its sealing effect is better than the flat washer, with smaller compression force.

2.2.3 Plastic flat gasket & washer

The most prominent characteristic of plastic is good corrosion resistance, most of them with poor temperature resistance performance. PTFE ranking leading role among plastic ,not only excellent corrosion resistance, but also a relatively wide temperature range, range from -180 ℃ to +200 ℃.

2.2.4 PTFE wrapped gasket & washer

In order to make full use of  PTFE advantages while making up its shortcomings of poor flexibility,generally produce PTFE wrapped rubber or asbestos rubber gasket. In this way, it has same corrosion resistance as PTFE flat washer, but also good flexibility, enhance the sealing effect, reducing the compression force.

2.2.5 Asbestos rubber gasket & washer

It is made by cutting asbestos rubber sheet cut, having a good heat resistance, cold resistance, chemical stability, abundant supply and cheap. Its composition includes 60~80 % asbestos,10 ~ 20 % rubber, fillers, curing agents and so on. Asbestos rubber gaskets used, the clamping force should be too big. Because it can adhere to metal, so the better way is coating a layer of graphite powder on the surface, so as not to tear down strenuously.

Asbestos rubber sheet has four colors: gray, for low pressure (grade XB-200, pressure ≤ 16 kg / square centimeter, temperature 200 ℃); red for medium pressure (grade XB-350, 40Kg / cm2, temperature 350 ℃); purple, for high pressure (grade XB-450, pressure 100 kg / cm2 temperature 450 ℃); green for oil, pressure performance is also very good .

2.2.6 Metal flat gasket & washer

 Lead material, working temperature 100 ℃; aluminum 430 ℃; copper 315 ℃; low carbon steel 550 ℃; silver 650 ℃; nickel 810 ℃; monel nickel-copper 810 ℃, stainless steel 870 ℃.Among them, the lead with poor pressure resistance, , aluminum can be resistant to 64 kg / square centimeter, other materials can withstand high pressure.

2.2.7  Metal heteromorphosis gasket & washer

Lens washer: self-tightening effect, used in high-pressure valves.

Oval washers: belong to high-pressure self-tightening washers.

Tapered double gasket: for high pressure self-tighting seal.

In addition, there are other shapes, like square, diamond, triangle, tooth, dovetail, B-shaped, C-shaped, generally only used in high pressure valves.

2.2.8 Metal wrapped gasket & washer

Wrapping material are aluminum, copper, low carbon steel, stainless steel, Monel alloy. Inner filling materials are asbestos, PTFE, glass fiber. Metal has good temperature and pressure resistance properties, but also good flexibility.

2.2.9 Wave washer

Small compression force, good sealing effect. Often in the form of metal to non-metallic combination.

2.2.10  Spiral wound gaskets

It’s made of thin metal and non-metallic belt and tighten closly together, winding into a multi-layer circular, wave-shape cross-section, has good flexibility and sealing. Metal belt can be 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy production. Non-metallic materials are asbestos, PTFE and so on.

3.New Sealing Materials and Sealing Technology

3.1 Liquid seal

With the rapid development of polymer organic synthesis industry, in recent years there has been liquid sealant used in static sealing; this new technology, often called liquid seal. Liquid seal principle, is the use of liquid sealant adhesion, mobility and the effect of monomolecular film (the thinner membrane natural recovery tend to be greater), at the appropriate pressure, make it like a gasket to play the role. So the use of the sealant, also known as the liquid washer.

3.2 PTFE raw material seal

PTFE is a polymer organic compounds, before sintered into sealing products, called raw material. It’s soft and has monomolecular effect. The belt made of raw material called raw material belt, can be rolled and stored for long-term preservation. When come into use, forming freely, , as long as there is pressure, forming e evenly ring-shape film that has sealing function. As the gaskets of valve body and cover, no need to remove the disc or wedge, pry a gap, stuff the RM belt into the gap. Small compression force, not stick hand, nor stick flange surface, the replacement is very convenient, especially suitable for the tongue and groove flange.

3.3 Metal hollow O-ring

Good elasticity, small compression force,having self-tightening effect, a variety of metal materials available, which can be applicable from low temperature, high temperature and strong corrosive media can adapt

3.4 Graphite plate seal ring

In people's minds, graphite is brittle material, lack of flexibility and toughness, but after special treatment, it can be soft, have good elasticity. In this way, the heat resistance and chemical stability of graphite can be displayed in the gasket material. Moreover ,such gasket has samll compression force, the sealing effect is superior. Also can also made into belt combined with the metal belt, composed of excellent performance wound gasket.