What Is Motive Seal of Valves?
Motive seal of valves mainly means stem seal. Media inside valves cannot leak out with stem moving. It is also the center of motive seal of valves. Currently, motive seal of valves is mainly with stuffing box. Basic form of stuffing box is classified into gland and gland nut.
1. Gland
Gland is in common use.
The same type also has differences on various details. For example, compression bolts are classified into T-type bolts (used for low pressure valves whose pressure bellows 16 kg per square centimeter), studs, eyelet bolts, etc. Gland is classified into integral type and combined type.
2. Gland nut
Gland nut has small boundary dimension. However, for pressing force being limited, gland nut is only used for small valves.
Packing:
Inside stuffing box, packing contacts directly with stem and fulfils the whole stuffing box, in case of media leakage. Packing should have following properties:
(1) Good sealing;
(2) Corrosion resistance;
(3) Low coefficient of friction;
(4) Adapt to temperature and pressure of media.
Packing in common use:
(1) Asbestos packing: Asbestos packing has good temperature resistance and corrosion resistance. However, it has poor sealing when being used alone. Nevertheless, asbestos packing is also infused or added with other materials. Oil immersed asbestos packing: It has two types of basic structure: torsional and knitted. And it can also be classified into round and square.
(2) Polytetrafluoroethylene wire rod: Polytetrafluoroethylene bandeaux are knitted into packing. The packing has excellent corrosion resistance, and it can be also used for cryogenic media.
(3) Rubber O ring: Under low pressure, rubber O ring has good sealing. However, rubber is limited by temperature. For example, natural rubber is only used under 60℃.
(4) Plastic molding packing: Generally, plastic molding packing is made into three pieces, and also can be made into other shapes. Polytetrafluoroethylene is mainly used. Nylon 66 and nylon 1010 are also adopted.
Bellows sealing
For chemical industry and nuclear industry highly developing, materials having flammability, explosion, toxicity and radiation are increasing, having higher requirements for valves seal. Packing seal cannot be used in some occasions. So, new type of sealing came out – bellows seal. Bellows seal does not need packing, which is also called as packless seal.
Two ends of bellows are welded with other parts. When stem rises and falls, bellows stretch out or draw back. Bellows have no leakage, so media cannot leak out. For insurance, double seal consisting of bellows and packing are adopted.
