Introduction of Shaft Seal

What Is A Shaft Seal

A shaft seal is a kind of friction seal or stuffing box, which is used to prevent liquid leakage between the shaft and bearing of the compressor or other fluid-conveying equipment, and it is also a sealing device to prevent leakage between the pump shaft and casing. Commonly used shaft seal types include packing seal, mechanical seal, and dynamic seal. The shaft seal of a reciprocating pump is usually a packing seal. When transporting a medium that does not allow leakage, a diaphragm reciprocating pump can be used. The shaft seals of rotary pumps including vane pumps and rotor pumps mainly include packing seals, mechanical seals, and dynamic seals. 

There are many brands of shaft seals on the market, such as Kobelco shaft seals, Hitachi shaft seals, Cummins shaft seals, and so on. 

Classification of Shaft Seals

Packing seal: 

The packing seal has a simple structure, a cheap price, and convenient maintenance. However, the leakage is large and the power loss is large. Therefore, the packing seal is used to convey general media, such as water, etc.; it is generally not suitable for petroleum and chemical media, especially in expensive, explosive, and toxic media.

Machinery Seal: 

The sealing effect of the mechanical seal is good, the leakage is small, and the service life is long, but the price is expensive, and the requirements for processing, installation, and maintenance are higher than those of ordinary seals.

Mechanical seals are suitable for conveying petroleum and chemical media and can be used for various viscosities, strong corrosion, and particle-containing media. The American Petroleum Institute standard stipulates that unless the user has specified, it should be equipped with a cartridge mechanical seal.

Dynamic seal:

Dynamic seals can be divided into back-leaf seals and vice-impeller seals. When the pump is working, the centrifugal force of the back blade makes the medium pressure at the shaft seal drop to normal pressure or negative pressure, so that the pump does not leak during use. Centrifugal force disappears when parking and the sealing function of the back blade become invalid. At this time, the sealing device plays a sealing role by parking. Packing seals are mostly used in the parking sealing device matched with the back leaf type. There are two kinds of stuffing seals: common type and mechanical elastic type. The ordinary packing seal is similar to the general packing seal pump, requiring a slight positive pressure at the shaft seal to avoid dry friction of the packing. The mechanical elastic packing seal uses a counterweight to loosen the packing when the pump is running and compress the packing when the pump stops.

The Function of Shaft Seal

The high-pressure liquid should be prevented from escaping from the pump and air should be prevented from entering the pump. Although the position of the shaft seal in the pump is not large, the normal operation of the pump is closely related to the shaft seal. If the shaft seal is not selected properly, it will not only require frequent maintenance during operation, but also leak a lot of transported liquid, and may cause fire, explosion, and poisoning accidents due to the leaked flammable, explosive, and toxic liquid, with disastrous consequences. Therefore, the shaft seal structure must be selected reasonably to ensure the safe operation of the pump.

Due to the unequal atmospheric pressure between the cylinder and the outside, the steam in the cylinder or the air outside the cylinder will inevitably leak or leak in along the radial gap between the main shaft and the cylinder, resulting in loss of working fluid, deterioration of the operating environment, and heating of the journal or use. Steam enters the bearing chamber, causing deterioration of the oil quality, leaking air, and destroying the vacuum, thereby increasing the suction load, which will reduce the efficiency of the unit, so there are steam seals at both ends of the rotor passing through the cylinder. The shaft end steam seal is called the shaft seal for short. The high-pressure shaft seal is used to prevent steam from leaking out of the cylinder, and the low-pressure shaft seal is used to prevent air from leaking into the cylinder.