The using scheme of mechanical seal of fluorine lined centrifugal pump in different environments

Fluorine lined centrifugal pump is one of the most widely used pumps in the market today. Fluorine lined centrifugal pump mainly uses the form of mechanical seal. According to the different transport medium, the use of seal is different. What is the use plan for the mechanical seal under the conditions of high temperature, low temperature, particles, corrosive liquid and so on? In the pump unit, the mechanical seal of the motor equipment accounts for about 80%-85% of the total number of dynamic seals. In the machine pump failure, the failure of mechanical seal accounts for about half of the entire maintenance workload. The mechanical seal can be seen in the position of the machine pump maintenance. Because the medium transported by the pump varies greatly, the process conditions are different, so it is very important to understand these conditions for the selection and maintenance of the mechanical seal. Combined with years of experience in mechanical seal selection and maintenance, the working characteristics and matters needing attention of mechanical seal under several special working conditions are introduced.

First. Mechanical seal in corrosive environment

Some mechanical seals in a corrosive environment, we should not only pay attention to the corrosion resistance of the various parts of the mechanical seal itself, but also prevent the combination of them. For mechanical seal in corrosion resistant environment, corrosion-resistant materials should be chose.In addition, the external mechanical seal can be selected.If the corrosion is extremely strong, easy to cause safety accidents, it can be protected by double-ended seal.

Second. Mechanical seals in granular media

Some mechanical seals may work in the medium containing solid particles. When solid particles enter the friction end, it is easy to produce sharpening, resulting in rapid failure of the sealing end face.In addition, solid particles can make the ring seal can not move along the axial direction, so that the ring can not float, loss compensation, resulting in mechanical seal failure. A more effective way to solve this is to add an external flushing device. Because the washing pressure of the sealing chamber is higher than the medium pressure of the sealing chamber, the particle medium can be effectively prevented from entering the sealing chamber. In addition, double end mechanical seal can be selected to add sealing fluid in the middle of the sealing cavity.External mechanical seal can also be selected.

Third. Mechanical seal in low temperature

In refineries and chemical plants, low temperature transmission medium usually has the characteristics of small gap, poor lubricity, easy gasification, gasification process medium absorption of heat, resulting in a sharp drop in temperature, resulting in the condensation of water and gas in the air, resulting in the end face and its surrounding freezing, resulting in end face paste leakage.At the same time, due to poor lubrication, the heat generated by the end friction is more, if not transferred in time, it will aggravate the gasification of the end face, resulting in a vicious cycle, and accelerate the early failure of the sealing surface. In addition, low temperature seal auxiliary seal rings are prone to leakage when low temperature hard loss of ammunition. For low temperature sealing, in addition to choosing the appropriate auxiliary sealing ring material, but also pay attention to the following points.

1. The corresponding measures should be taken in the process.Such as reducing the water content of the medium to a greater extent;Keep the pressure of the sealed chamber relatively stable;If possible, a washing and cooling device is provided.Before starting the equipment, properly preheat the sealing components.

2. Improve the sealing structure and choose the correct material of friction pair. The practice shows that the open circular groove on the static ring surface can form hydrodynamic seal, increase the bearing capacity of the fluid film, improve the lubrication condition of the end face, and effectively prolong the service life of the mechanical seal. In the aspect of end matching, tungsten carbide -M106K graphite base is superior to other materials because of its good lubricity, low friction coefficient and good thermal conductivity.

3. Select the right end specific pressure. Due to poor end lubrication, the low temperature meahanical seal is easy to gasification, icing, so under the premise of ensuring reliable sealing, the specific pressure of the end face can be appropriately reduced. The specific pressure value of the general mechanical seal is recommended to be 0.29-0.59MPa, and the lower limit of the low temperature seal is recommended. For the metal bellows mechanical seal, because the moving ring has no axial friction, the specific pressure of the end face can even be lower than the lower limit.

Fourth. Mechanical seal in high temperature

In general, medium temperature over 120℃ is considered to be high temperature seal. At this time, the main problems of mechanical seal are:

(1) With the increase of the temperature of the end face of the friction pair, the gasification of the liquid film between the sealing end faces increases, the friction coefficient increases, the wear increases, the temperature increases, the sealing ring becomes hot deformation or hot crack, and lead to failure.

(2) The auxiliary sealing ring material in the mechanical seal is generally rubber or PTFE, which is easy to age and decompose for a long time in the high temperature environment.

(3)The elastic parts of mechanical seal are prone to fatigue and creep at high temperature, resulting in seal failure.                                                    (4) High temperature will aggravate the corrosion of metal materials and shorten the life of secret seals..

Take the following measures to ensure the normal operation of the mechanical seal at high temperature.

1. Add cooling and washing units to the mechanical seal.

2. Choose metal bellows for mechanical seals.

In recent years, metal bellows seal has been widely used in high temperature seal, and has achieved good results. In this kind of mechanical seal, the metal bellows replace the spring in the ordinary mechanical seal, remove the auxiliary sealing ring, do not need to overcome the friction and wear of the rotating shaft in the dynamic ring compensation, so it can generally achieve good results at high temperature.

Choose high temperature resistant materials. Choose different materials according to different operating temperatures.

The maximum temperature of NBR is 80℃, silicone rubber and fluorine rubber is 200℃, PTFE is 250℃. In addition, the non-metallic friction pair in the mechanical seal is made of graphite impregnated material, so different types of impregnated graphite should be selected according to different operating temperatures. Suitable temperature ranges are less than 150 ° C and 170-200 ° C. Graphite impregnated with copper, aluminum and lead can be used under working conditions less than 400 ° C. Moving ring components should choose materials with similar expansion coefficient as far as possible to prevent the connection between moving ring and moving ring seat from loosening under high temperature.

Fifth. Mechanical seal at high speed

When the speed exceeds 3000r/min or the linear speed of the friction pair exceeds 25m/s, it is considered as high speed seal.Under the condition of high speed, because of the high line speed of the friction end face, the heat generated by friction is more, and the liquid film on the end face is not easy to form, and the wear is aggravated. High speed is also easy to cause the vibration of the rotating ring and the centrifugal force, which is not conducive to the formation and maintenance of the liquid film between the end faces.Therefore, in order to reduce the influence of high-speed centrifugal force and vibration on the rotating parts, the rotating parts should be reduced as far as possible and the spring static structure should be selected.It can also enhance the cooling and lubrication of the end face of the friction pair.Alternatively, a high value [PV] friction pair material can be selected to reduce the contact width of the end face or to control the mechanical seal of the membrane.In recent years, a type of mechanical seal called differential ring has also been used in high-speed equipment.The linear velocity of the friction pair can be reduced by half by adding a differential ring in the middle of the original static and static ring, and a good effect is obtained.

Sixth. Mechanical seal under high pressure

The working pressure of the mechanical seal exceeds 4-5MPa, which belongs to the range of high pressure seal.Under the condition of high pressure, because the specific pressure value of the sealing end face is too large, the end face is heated, thus destroying the formation of the liquid film and causing abnormal wear. High pressure can also lead to deformation and fracture of friction pairs. High pressure seals are generally selected balance type structure, in order to make the specific pressure of the end face in a suitable range. However, considering the reliability of the seal, the balance coefficient cannot be increased without limits, so in some cases, although the balance structure is chosen, but the specific pressure on the end face may be larger. It is difficult to meet the requirements of the use of balanced structure alone. In this case, multi-end machine seal or controlled membrane mechanical seal can be selected.

At the present stage, mechanical seal has become a common form of seal in the use of centrifugal pumps in oil refining and chemical equipment. At the same time, with the continuous improvement of the overall level of design and manufacturing and the continuous development of new materials, its application scope will be further expanded.

For more technicial questions please feel free to contact Kelly~

Kelly - International Trade Pumps Engineer

Anhui Tenglong Pump And Valve Manufacturing Co., Ltd.(TEFLOW)

No. 30 fortune east road, Jingkai district Jing County, Xuancheng City, Anhui Province, China

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