The Dynamic Sealing of the Valve

The sealing of the valve cannot be ignored, because most of the valve's leakages occur due to sealing. The related information of dynamic sealing of valves is discussed in this article.

1. The dynamic sealing of the valve
The dynamic sealing of the valve mainly refers to the sealing of the valve stem. Preventing the medium in the valve from leaking due to the movement of the valve stem is the key to solving the problem of the valve's dynamic sealing.

(1) The form of the stuffing box

At present, the dynamic sealing of valves mainly refers to the stuffing box. The basic forms of the stuffing box are as follows:

The gland type: This is the most used form. The same form can be distinguished in many details. For example, from the perspective of compression bolts, they can be divided into T-shaped bolts used for low-pressure valves with pressure less than and equal to 16kg/cm²) and studs eyelet bolts. From the perspective of the gland, they can be divided into the integral type and combined type.
The compression nut type: Compression nuts have small dimensions, but their compression force is limited. They can only be used for small valves.

(2) Packing
In the stuffing box, the packing is in direct contact with the valve stem and fills the stuffing box to prevent leakages of media. The requirements for the packing are as the following:

Good sealing performance
Corrosion resistance
Small friction coefficient
Being adapt to the temperature and pressure of the medium

Commonly used fillers are as follows:

Flexible graphite: It is a new type of sealing material.
Asbestos packing: It has good temperature resistance and corrosion resistance, but when it is used alone, its sealing effect is not good. Therefore, it is always impregnated or other materials are added.
Oil impregnated asbestos packing: There are two basic structural forms; one is the twisted packing, and the other is the braided packing. It can be divided into round and square packing.
PTFE braided packing: The PTFE tape is woven into packing, which has excellent corrosion resistance and can be used in cryogenic media.
Rubber O rings: In a low-pressure state, they have a good sealing effect. The use temperature is restricted. For example, natural rubber can only be used at a temperature of 60°C.
Packing made by plastic molding: Generally, it is made into three pieces, but it can also be made into other shapes. Most of the plastics used are polytetrafluoroethylene, and nylon 66 and nylon 1010 are also used.
Other packing: In the steam valve at a temperature of 250°C, asbestos packing and lead rings are alternately used to reduce leakages of steam. For some valves, the medium is frequently changed. If asbestos packing and PTFE tapes are used together, the sealing effect will be better. In order to reduce the friction on the valve stem, molybdenum disulfide (MOS2) or other lubricants can be added in some cases.

(3) Bellows sealing
With the rapid development of the chemical and nuclear energy industry, the amount of flammable, explosive, highly toxic and radioactive substances has increased, and stricter requirements have been put forward on the sealing of the valve. In some cases, it is no longer possible to use packing to seal. Therefore, a new form of sealing, that is bellows sealing is produced. This kind of sealing does not require packing, so it is also called sealing without packing. The two ends of the bellows are welded to other parts. When the valve stem is raised and lowered, the bellows expands and contracts. As long as the bellows does not leak, the medium won't leak. A double seal of bellows and packing is often used just in case.

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Teresa

Teresa is a skilled author specializing in industrial technical articles with over eight years of experience. She has a deep understanding of manufacturing processes, material science, and technological advancements. Her work includes detailed analyses, process optimization techniques, and quality control methods that aim to enhance production efficiency and product quality across various industries. Teresa's articles are well-researched, clear, and informative, making complex industrial concepts accessible to professionals and stakeholders.