DIFFERENCE BETWEEN PUMP DOUBLE MECHANICAL SEALS AND PUMP DRY GAS SEALS

In coal-based petrochemical devices, there are many toxic, harmful, flammable and explosive media, such as methanol, ethylene, etc. When using pumps to transport such media, there is a high requirement for the seal of the pump. According to modern management requirements, the medium is not allowed to leak.

At present, shielding pumps, magnetic pumps, diaphragm pumps, etc. are often used to transport such media. If it is necessary to use centrifugal pump, usually two pump seals are used, namely double mechanical seals or dry gas seals. Non contacting mechanical seals, gland packings, spring seals, elastomer bellow seals are not suitable for this situation.

So which is better, double mechanical seal or dry gas seal? The following describes the selection and comparison of the two seals.

Double Mechanical Seal

Double mechanical seals are commonly used for better sealing when transporting toxic media. According to the pressure of sealing fluid system, it can be divided into pressurized double mechanical seal and pressure-free double mechanical seal.

Pressurized double mechanical seals generally have a pressure of 0.01-0.03MPa higher than that before the primary seal. When leakage occurs in the primary seal, the sealing liquid will enter the pump, ensuring that the medium in the pump will not leak to the outside. As the sealing liquid enters the pump, the pressure and liquid level of the sealing liquid system decrease, so that leakage monitoring and alarm of the seal can be realized.

The pressure of pressure-free double mechanical seals is usually atmospheric pressure, which is lower than that of sealing chamber before primary seal. When the leakage occurs in the primary seal, the medium in the pump will enter into the sealing fluid system, causing the pressure and level of the sealing fluid system to rise. Leak monitoring and alarm can be realized by pressure switch or level switch.

Dry Gas Seal

If the medium conveyed by centrifugal pump is liquid, double-end dry gas seal and series dry gas seal can be used according to different operating conditions.

The double-end dry gas seals can be used on the shaft seals of most centrifugal pumps. They use pressurized sealing gas instead of pressurized sealing liquid to ensure that the medium does not leak. The whole sealing system operates without contact and its power consumption is only 5% of that of traditional double-end seal, and its service life is more than 5 times longer than that of traditional seal. However, the pump double-end dry gas seal requires a certain pressure of air supply, and the pressure of air supply is at least 0.2MPa higher than the medium pressure.

The series dry gas seal has the following characteristics: dry gas seal is used in series with contact mechanical seal, mechanical seal is the primary seal and dry gas seal is the secondary seal. Nitrogen is introduced between the dry gas seal and the primary seal to ensure a certain back pressure of the primary seal, which greatly prolongs the service life of the primary seal. After the primary seal fails, the dry gas seal acts as the primary seal for a short time to prevent large leakage of medium to the atmosphere.