BASIC KNOWLEDGE OF PUMP SEALS (6)

When the chemical pump is in operation, the pressure generated by the auxiliary impeller balances the high-pressure liquid at the outlet of the main impeller, thereby achieving sealing. During stopping, the auxiliary impeller does not work, so it must be equipped with a stopping sealing device to solve the chemical pump leakage that may occur during stopping.

The seals of auxiliary impellers have simple structure, a reliable sealing performance, and a long service life. It can achieve no dripping during the operation of the chemical pump, so it is often used on chemical pumps that transport impurities.

Power seals include centrifugal seals, spiral seals, and magnetic fluid seals, among which spiral seals are very promising. Fully enclosed seals include diaphragm type and shield type.

1. Centrifugal Power Seals

The centrifugal power seal is to use the centrifugal force to throw the liquid medium out in the radial direction to prevent the liquid from entering the leakage gap, thereby achieving the purpose of sealing. Centrifugal seals are only suitable for liquid, but not for gaseous. Therefore, if airtightness is also required where centrifugal seals are used, a combination seal type that combines centrifugal seals with other types of seals must be accepted.

The most frequently used centrifugal seal is the oil swing pan, which is widely used in various transmission devices to seal lubricating oil or other liquids. The seal of the oil-swing pan is not limited by speed. As long as the oil-swing pan has sufficient strength, the higher the rotation speed, the better the oil-swing seal effect. On the contrary, if the rotation speed is too low or the oil-swing pan does not rotate, the seal is invalid. In addition, the seal of the oil-swing pan is not limited by high temperature.

Therefore, the oil-swing pan seal is very suitable for high temperature and high speed pump seals, such as thermal oil pumps. However, this type of seal cannot be used for high pressure, and is generally used in applications where the pressure difference is zero or closer to zero.

The centrifugal oil swing pan has a simple structure, low cost, no friction power consumption, no wear and it does not need to maintain, so it is widely used.

The characteristic of the centrifugal power seal is that it has no friction pair that directly contacts, and can accept a larger sealing gap, so it can seal the medium containing solid impurities, with reasonable design, it can achieve zero leakage. But the ability to overcome the pressure difference is low, the power consumption is large, and even up to 1/3 of the pump's useful power. In addition, because it is a power seal, the sealing ability is lost immediately upon stopping, for which a stopping seal must be supplemented.

2. Spiral Power Seals

The working principle of the spiral power seal is equivalent to a screw displacement pump. The right spiral is cut out on the shaft (or the spiral groove is engraved on both of the casings). The rotation direction of the shaft is clockwise when viewed from right to left, the friction between the liquid medium and the housing is counterclockwise.

The medium pushes the liquid to the right as if the nut loosen along the screw. As the capacity continues to shrink, the pressure gradually increases, so that the established sealing pressure is balanced with the pressure of the fluid being sealed, thereby preventing leakage.

When designing the spiral sealing device, pay special attention to the direction of the oil guiding. If the direction is wrong, it will not only fail to seal, but on the contrary, it will guide the liquid to the direction of leakage, which greatly increases the amount of leakage.

Obviously, the spiral seal is also a kind of power seals, which will lose the sealing function during stopping or at a low-speed state, so it is often necessary to assist with the stopping seal, which complicates the device and requires sufficient axial dimensions.

Certainly, in addition to power seals, there are gland pakings and mechanical seals, common mechanical seals include spring seals, elastomer bellow seals, etc.