BASIC KNOWLEDGE OF PUMP SEALS (4)

Sealing is to prevent the leakage of gas, liquid or solid and prevent outside particles and fluids from entering into the closed space or pipeline system. Parts that can play a sealing role are called seals, there are gland packings and mechanical seals, such as spring seals, elastomer bellow seals, etc. More complicated sealed connections are called sealed structures or sealed devices. This article mainly introduces the labyrinth seal, spiral seal and dry gas seal.

1. Labyrinth Seals

When the design is reasonable, the processing is excellent, the assembly is intact, and the rotation speed is high, the labyrinth sealing effect is very good. However, in practical applications, the resulted leakage situation of chemical pump seals are many, so it is not commonly used in chemical pump sealing. The main reasons are:

1) The matching clearance of the sealing pair (such as shaft and bearing gland) is too large, and this clearance is inversely proportional to the sealing effect. And the obvious spiral turning marks also increase the leakage tendency of the chemical pump in some cases.

2) Too much oil is injected into the bearing chamber of chemical pumps, and the overflow pressure exceeds the sealing resistance.

3) The installation position of the oil window or oil level gauge is wrong, which misleads people to make a correct judgment of the amount of lubricating oil in the oil chamber.

4) The increase in oil temperature during operation reduces its viscosity and increases the possibility of chemical pump leakage.

5) The size of the oil return tank or oil return hole is too small, or it is blocked by other obstacles, so that the blocked liquid cannot return smoothly and cause leakage.

The medium pumped by chemical pumps has the dangers of corrosion, volatilization, explosion, etc. The sealing performance requirements of the pump body are much higher than other types of pumps. The use of labyrinth seals increases the risk of leakage of materials transported by chemical pumps. Therefore, chemical pumps generally do not use this sealing method.

2. Spiral Seals

The spiral seal is also a form of power seal. It is to process a spiral groove on the rotating shaft or on the containing sleeve of the shaft, and a sealing medium is filled between the shaft and the sleeve. The rotation of the shaft causes the spiral groove to produce a pump-like delivery effect, thereby preventing the leakage of the sealed liquid. The sealing ability is related to the helix angle, pitch, tooth width, tooth height, tooth action length, and the gap between the shaft and the sleeve.

Because there is no friction between the seals, it has a long life, but due to the limitation of structural space, its spiral length is generally short, so its sealing ability is also limited. When the pump is used at a reduced speed, its sealing effect will be greatly reduced.

3. Dry Gas Seals

The dry gas seal is a new type of shaft end seal that uses slotted sealing technology for gas seals and it belongs to the non-contact seal.

The working principle of dry gas seals: When the dynamic ring with a fluid dynamic pressure groove (2.5-10µm) opened outside the end face, the fluid dynamic pressure groove pumps the high-pressure isolation gas on the outer diameter side (called the upstream side) between the sealing end faces, the gas film pressure from the outer diameter to the groove diameter gradually increases, while the gas film pressure from the groove diameter to the inner diameter gradually decreases.

The opening force generated by the increasing film pressure is greater than the closing force acting on the seal ring, forming a thin layer of gas film (1 ~ 3µm) between the friction pairs to make the seal work in a non-contact state. The formed gas film completely blocks the relatively low-pressure sealing medium leakage channel, and realizes zero leakage of the sealing medium.