204 Mechanical seal How can the manufacturer tell you to play a sealing role?

1. Push type mechanical seal and non push type mechanical seal

Push type mechanical seal refers to the mechanical seal whose auxiliary seal is pushed mechanically along the shaft or shaft sleeve to compensate for the wear of the sealing surface, which usually refers to the spring compression type mechanical seal.  

Non push type mechanical seal is used for mechanical seal with auxiliary seal fixed on shaft, which is usually bellows mechanical seal.

2. Balanced mechanical seal and unbalanced mechanical seal

The pressure in the seal chamber of the mechanical seal acts on the mechanical seal of the slurry pump on the moving ring to form a closing force, and the liquid film between the end faces forms an opening force. When the load factor K>1, the seal is a non-equilibrium mechanical seal. General unbalanced mechanical seal can only be used for low pressure. When the pressure is greater than a certain limit, the liquid film between the sealing surfaces will be squeezed out. Under the action of loss of liquid film lubrication and high load, the sealing face of mechanical seal will be damaged quickly. The non-equilibrium mechanical seal cannot balance the effect of liquid on the end face, and the end face specific pressure increases with the increase of fluid pressure.  

The load factor K<1, and the seal is a balanced mechanical seal. The steps on the built-in seal shaft of the balanced mechanical seal make the seal face move inward along the radial direction without reducing Mechanical seal for kettle Width of sealing surface. The opening force of the seal remains unchanged, but because the moving ring has a large area exposed to the liquid, the closing force is balanced to a considerable extent. The balancing method of external mechanical seal is the same as that of internal mechanical seal except that the force direction is exactly opposite. In this case, increase the hydraulic pressure share in the closing force to offset the opening force of the liquid film between non-standard mechanical seal faces at the mechanical seal end. Balanced mechanical seal can partially balance the effect of liquid on the end face, and the end face specific pressure rises slowly with the increase of fluid pressure. General non-equilibrium mechanical seal can only be used for low pressure, but for poor lubrication performance, low boiling point, easily vaporized medium and high-speed working conditions, even under low pressure, balanced mechanical seal should also be used. Because for non-equilibrium mechanical seals, when the pressure of the mechanical seal chamber rises, the liquid film between the seal faces will be squeezed out, which will quickly damage the sealing face of the mechanical seal. Balanced mechanical seal can be used in various pressure occasions.

3. Non pressure dual mechanical seal and pressure dual mechanical seal

The single end mechanical seal has only one pair of friction pairs, which is simple in structure and easy to manufacture and disassemble. Generally, only a flushing system is required, and no external sealing fluid system is required.  

Pressurized double mechanical seal (formerly called double end mechanical seal) refers to two pairs of friction pairs with complex structure, requiring external sealing fluid supply system. The sealing cavity of pressurized double mechanical seal is filled with isolation fluid 0.5~1.5bar higher than the medium pressure, which plays the role of sealing and lubrication, and the isolation fluid plays the role of lubrication for the inner seal. Non pressure double seal (formerly called series seal) refers to two pairs of friction pairs, which are complex in structure and require an external sealing fluid system. The buffer fluid in the non pressure double seal cavity is not pressurized, and the process medium plays a lubricating role in the inner seal.

Generally, single end mechanical seal shall be preferred because of its simple structure, convenient use and low price. However, in the following occasions, dual mechanical seal is preferred.

(1) Toxic and dangerous media.

(2) High concentration of H2S.

(3) Volatile low temperature medium (such as liquefied petroleum gas).  

With the society's increasing attention to health, safety and environmental protection, the use of pressureless dual mechanical seal is increasing year by year. The pressureless dual mechanical seal can be widely used in toxic, volatile and dangerous media such as vinyl chloride, carbon monoxide, light hydrocarbon, etc. The inner seal (the first seal) of the pressureless dual mechanical seal is the main seal, which is equivalent to a single end face built-in mechanical seal. The lubrication of the single end face mechanical seal is performed by the sealed medium. The sealing chamber is filled with liquid from the sealing liquid tank without pressurization. Once the inner seal of the pressureless dual mechanical seal fails and the pressure in the seal chamber increases, it can be displayed, recorded or alarmed by the pressure gauge of the seal liquid tank. At the same time, the outer seal of the pressureless dual mechanical seal can seal and contain the leaking liquid before maintenance.

For some toxic and granular media (or highly corrosive media), the following methods can generally be considered:

(1) Appropriate environmental control measures shall be taken, such as external flushing+pipeline flushing system with cyclone separator.  

(2) Pressure dual mechanical seal is adopted.

The pressure of the isolation liquid of the pressurized dual mechanical seal is higher than the medium pressure, so the pumped medium will not enter the seal chamber. The inner seal of the pressurized dual mechanical seal plays the role of preventing the spacer from entering the pump chamber. Therefore, when conveying such viscous, abrasive and high-temperature media, the inner seal of the pressurized dual mechanical seal can be made without expensive alloy because it is not exposed to the media. The outer seal of the pressurized dual mechanical seal only plays the role of preventing the isolation liquid from leaking into the atmosphere.

4. Internal mechanical seal and external mechanical seal

Built in mechanical seal: refers to the mechanical seal installed in the sealing chamber.

External mechanical seal: refers to the mechanical seal installed outside the seal chamber.

Since the internal mechanical seal is well stressed, and the specific pressure increases with the increase of medium pressure, the leakage direction of the internal mechanical seal is opposite to the direction of centrifugal force, so the internal mechanical seal is generally selected. External mechanical seal shall be considered only when the medium is highly corrosive and the pressure dual mechanical seal is not considered.

5. Rotating mechanical seal and static mechanical seal

Rotating mechanical seal refers to the mechanical seal whose compensation parts rotate with the shaft.

Static mechanical seal refers to the mechanical seal whose compensation parts do not rotate with the shaft.

Generally, rotating mechanical seals are used. However, when the shaft diameter is large, the rotating speed is high, and the seal circumferential speed reaches 25m/s, static mechanical seals should be used because of the large centrifugal force generated by springs and other rotating elements, high dynamic balance requirements, and large mixing power consumption. In addition, if the medium is easily crystallized due to strong agitation, static mechanical seal is also recommended.  

6. Single spring mechanical seal and multi spring mechanical seal

Single spring mechanical seal has a simple structure. The spring can also play a driving role, but the end face pressure is uneven, so it is not suitable for high-speed operation.  

Multi spring mechanical seal has a complex structure, and the spring can not also play a driving role, but the end face pressure is uniform, which is suitable for high-speed operation. In general, multi spring mechanical seal is recommended.