Problems with contact mechanical seals-

Problems with contact mechanical seals

　2017-08-10　　

Problems with contact mechanical seals:

With the development of modern industrial production, the working environment of mechanical seals is becoming more and more severe, and the requirements for sealing performance such as sealing, safety, and reliability are getting higher and higher. At present, the mechanical seals used for rotary pumps (mainly centrifugal pumps) at home and abroad are basically ordinary contact mechanical seals. Due to the direct solid friction and wear between the end faces of the seal, the temperature of the seal is too high and excessive due to end face friction. Early failure due to wear and tear. Under the harsh conditions of the sealing environment, such as the sealing is easy to vaporize, high risk, high corrosiveness, high solid particle medium, high pv value work, etc., there are a large number of liquid phase leakage and gas phase escape to pollute the environment. , The reliability and stability of the seal are greatly reduced, and the probability of failure is significantly increased. It is difficult to meet the objective requirements of modern industrial production such as long-term operation, pollution-free, and low energy consumption.

The defects of contact mechanical seals are mainly reflected in the following aspects:

(1) The sealed medium is easy to leak, causing material loss and environmental pollution.

Sealing principle: Under the premise that there is a pressure difference between the sealed fluid and the outside (usually the pressure is higher than the outside pressure), by maintaining a certain end-face contact pressure, the axial direction between the end faces of the sealing friction pair is reduced as much as possible Clearance to reduce the leakage of the sealed fluid. It can be seen that it is difficult for the contact mechanical seal to achieve zero leakage of the sealed fluid, and it is even more impossible to achieve zero leakage. With the continuous improvement of modern environmental protection requirements, the application of ordinary contact mechanical seals will be more and more restricted.

(2) There is friction and wear on the sealing end face, and the service life is short.

There is direct solid friction and wear between the seal friction pairs, and the result of accumulation must be the limited service life of the seal.

Generally, the service life of ordinary contact mechanical seals does not exceed one year, and for some sealing fluids with poor lubricity or sealing fluids containing a certain amount of solid particles, the service life of the seal is only a few months or even a few days.

(3) The working state of the seal is unstable, and the probability of failure is greatly increased.

The friction temperature rise between the seal end faces has a great influence on the working state of the seal friction pair, and it may cause the end face liquid film to vaporize (that is, phase change, especially for easily vaporized media such as high temperature oil pumps, hot water pumps, etc.) Light hydrocarbon pumps, etc.), change from single-phase lubrication to vapor-liquid mixed-phase lubrication, which makes the sealing friction pair in an unstable working state, causing 'screaming' and 'steam injection' phenomena, and the probability of failure is greatly increased.

(4) The seal fails due to thermal deformation, thermal cracking, and thermal expansion.

The rapid temperature rise between the sealing end faces may also cause large thermal deformation of the seal ring due to excessive temperature rise, serious friction and wear, and reduce the mechanical performance of the sealing material, which is easy to cause the seal ring (mainly refers to hard Ring) thermal cracking, thermal expansion and aging of the auxiliary seal ring, etc., make the seal early failure.

A considerable proportion of the failure records of mechanical seals are found to be caused by the excessive temperature rise of the seal end faces.

(5) The seal has poor ability to adapt to variable conditions.

When the fluid machinery is under variable working conditions (variable pressure, variable speed, frequent start and stop, etc.), the seal is always in an unstable operating state-running-in state, and it is prone to early failure.

(6) Change the properties of the media and affect the quality of the product.

The direct solid friction and wear between the end faces of the sealing friction pair will inevitably produce a certain amount of solid abrasive particles, which will cause different degrees of pollution to the process products, and directly affect the product quality and market competitiveness. Therefore, as the quality requirements for products, especially deep-processed foods, biomedical products, and fine chemical products, become increasingly stringent, the scope of use of ordinary contact mechanical seals will be affected to varying degrees.

(7) High operating costs and low economic benefits.

In order to improve the working environment, measures such as cooling and flushing are often used for the seal, which consumes a large amount of cooling oil and reduces the volumetric efficiency of the pump and the production efficiency of the process equipment. The cooling method is often to directly cool the seal with water and discharge it naturally, which wastes a lot of water resources and increases the burden of sewage treatment.

For example, high-temperature wax oil pumps and residual oil pumps in refineries generally adopt measures such as cooling self-washing or low-temperature wax oil external washing to improve the sealed working environment, resulting in huge energy waste.

(8) The auxiliary system is complex and poor in reliability.

For the double-end mechanical seal, it is necessary to block the fluid circulation and control system, which greatly increases the one-time investment cost of the sealing device. For the centrifugal compressor, the cost of the double-end contact mechanical seal, the sealing oil circulation, and the control system is about It accounts for 20%-40% of the investment of the entire unit. In addition, the oil-sealing cycle and control system double the operating cost of the seal, and the reliability of its own operation directly affects the reliability of the seal.

(9) Narrow range of use.

Contact mechanical seals are not suitable for rotating machinery shaft seals under high pressure, high speed, and high pv conditions. For sealing process fluids with high risk and high solid particles, the reliability and stability of the seal Not satisfactory.

M3200 sand mill mechanical seal 　　549.html,

The control of mechanical seal deformation 　　 the basic requirements for mechanical seal design