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Principle and form of dynamic seals

Update:2018-11-07

A seal between the relative motion seals in the machine (or equipment). Dynamic seals are divided into two types: reciprocating dynamic seals and rotary dynamic seals. Reciprocating dynamic seals have molded packing seals, stuffing box seals and expansion ring seals. 

The rotary dynamic seal has a contact type dynamic seal in which the motion seal is in direct contact with the stationary seal; a non-contact type dynamic seal which is not directly in contact with each other; and a semi-fluid dynamic seal which is sealed by the special property of adding a fluid in the seal gap. 

The semi-fluid seal has a magnetic fluid seal. Some machines and equipment must be sealed, and some allow a small amount of leakage. Therefore, the sealed structure should be designed according to the working conditions of the working medium, temperature, pressure and relative speed, and sealing performance requirements.

First, the stuffing box form

At present, the valve dynamic seal mainly based on stuffing box. The basic form of the stuffing box is:

(1) gland type

This is a much more used form.

The same form can be distinguished by many details. For example, from the compression bolts, T-bolts (for low pressure valves with a pressure of ≤ 16 kg / cm² ), studs and joint bolts can be divided. From the gland, it can be divided into integral and combined.

(2) compression nut type

This form has a small form factor but is limited in pressing force and is used only for small valves.

Second, the filler

In the stuffing box, the filler is in direct contact with the valve stem and filled with a stuffing box to prevent leakage of the medium. The following requirements apply to the packing:

1. Good sealing;

2. Corrosion resistance;

3. Small coefficient of friction

4. Adapt to medium temperature and pressure

Common fillers are:

Asbestos packing: Asbestos packing have good temperature and corrosion resistance, but when used alone, the sealing effect is not good, so always dipping or attaching other materials. Oil-impregnated asbestos packing: There are two basic structural forms, one is twisting and the other is knitting. Can be divided into round and square.

PTFE braided packing: The PTFE strip is woven into a packing, which has excellent corrosion resistance and can be used for deep cooling media.

Rubber O-ring: The sealing effect is good under low pressure. The use temperature is limited, such as rubber can only be used at 60 ° C

Plastic molding filler: generally made of three pieces, can also be made into other shapes. The plastic used is mostly PTFE, and nylon 66 and nylon 1010 are also used.

In addition, the use of units according to their own needs, often explore a variety of effective filler forms. For example, in a 250 ° C steam valve, asbestos packing and lead ring are alternately stacked, the air leakage will be reduced; some valves, the medium often changes, such as the use of asbestos packing and PTFE raw materials, the sealing effect will be Better. In order to reduce the friction on the valve stem, in some cases, molybdenum disulfide (MOS2) or other lubricants may be added.

At present, new fillers are being explored. For example, the fiber is impregnated with PTFE emulsion, and after pre-oxidation, it is sintered and pressed in a mold to obtain a molding filler having excellent sealing performance; and a stainless steel sheet and asbestos are used to form a wave filler, which can withstand high temperature, high pressure and corrosion.

Third, the bellows seal

With the rapid development of the chemical industry and the atomic energy industry, flammable, explosive, highly toxic and radioactive substances have increased, and there are stricter requirements for valve sealing. In some cases, packing seals have not been used, thus creating new ones. Sealed form - bellows seal.

This seal does not require a packing, so it is also called a packing-free seal.

Both ends of the bellows are welded to other parts. When the valve stem is lifted and lowered, the bellows expands and contracts, and as long as the bellows itself does not leak, the medium cannot be discharged. For the sake of safety, double seals of bellows and packing are often used.

Dynamic seals can be divided into two basic forms: rotary seal and reciprocating seal. According to the sealing conditions, the parts that are in relative motion with the sealing conditions can be divided into contact sealing and non-contact sealing. 

Generally speaking, the sealing performance of the contact sealing is good, but it is limited by friction damage, and is suitable for the sealing surface speed. Low occasions. Non-contact seals have poor sealing performance and are suitable for higher speeds! The essential difference between a dynamic seal and a static seal is whether there is relative motion.