Internal and external leakage
There are three sealing parts of the valve: (1) The contact between the opening and closing parts and the two sealing surfaces of the valve seat; (2) The fit between the packing, the valve stem and the stuffing box; (3) The connection between the valve body and the valve cover. The first leakage is called internal leakage, which is commonly referred to as lax sealing. It will affect the valve's ability to cut off the medium. For cut-off valves, internal leakage is not allowed. The latter two leaks are called external leakage, that is, the medium leaks from inside the valve to outside the valve. Leakage will cause material loss, pollute the environment, and even cause accidents in severe cases. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.
- Dynamic seal
Valve dynamic seal mainly refers to valve stem seal. Preventing the medium in the valve from leaking with the movement of the valve stem is the central issue of valve dynamic sealing.
- Filling letter form
The dynamic seal of the valve is mainly stuffing box. The basic form of a stuffing letter is:
(1) Gland type
This is the most used form. There are many detailed distinctions in the unified form. For example, from the perspective of compression bolts, they can be divided into T-shaped bolts (for low-pressure valves with pressures ≤16 kg/cm²), stud bolts, and articulated bolts. From the perspective of gland, it can be divided into integral type and combined type.
(2) Compression nut type
This type of valve has small dimensions but limited pressing force, so it is only used for small valves.
- Filling
Inside the stuffing box, the stuffing box is in direct contact with the valve stem and filled with stuffing box to prevent the medium from leaking out. There are following requirements for fillers:
(1) Good sealing;
(2) Corrosion resistance;
(3) Small friction coefficient;
(4) Comply with medium temperature and pressure.
- Commonly used fillers include:
(1) Asbestos packing: Asbestos packing has good temperature and corrosion resistance, but when used alone, the sealing effect is not good, and it is always impregnated or attached with other materials. Oil-impregnated asbestos packing: It has two basic structural forms, one is twisted and the other is braided. It can be divided into round and square.
(2) PTFE braided packing: PTFE thin strips are woven into packing, which has excellent corrosion resistance and can be used in cryogenic media.
(3) Rubber O-ring: Excellent sealing effect under low pressure. The use temperature is limited, for example, natural rubber can only be used at 60°C.
(4) Plastic molding filler: Generally made into three pieces, it can also be made into other shapes. Most of the plastics used are polytetrafluoroethylene.
In addition, for example, in a 250°C steam valve, asbestos packing and lead rings are alternately superimposed, and the steam leakage will be reduced; with valves, the medium is often changed, such as using asbestos packing and polytetrafluoroethylene raw material tape together. , the sealing effect will be better. In order to reduce friction on the valve stem, molybdenum disulfide (MoS2) or other lubricants can be added in some places.
Some new fillers are also constantly being explored. For example, polyacrylonitrile fiber is impregnated with polytetrafluoroethylene emulsion and pre-oxidized before being sintered and pressed in a mold to obtain a molded filler with excellent sealing performance; another example is a corrugated filler made of stainless steel sheets and asbestos, which can withstand high temperatures and high pressures. and erosion.
- Bellows seal
The rapid development of the chemical industry and the atomic energy industry, and the increase in flammable, explosive, highly toxic and radioactive materials, have stricter requirements for valve seals. Packing seals can no longer be used in some places, and a new sealing form - bellows seal has emerged. This type of seal does not require fillers, also called filler-free seals.
The middle ends of the corrugated pipe are welded to other parts. When the valve stem rises and falls, the bellows expands and contracts. If the bellows itself does not leak, the medium cannot leak. For safety reasons, double sealing with bellows and packing is often used.