Mechanical seal is widely used in rotating equipment. The sealing effect of mechanical seal will directly affect the operation of the whole machine, and serious accidents will occur. From the perspective of internal and external conditions of mechanical seal, several factors influencing the sealing effect and reasonable measures should be taken:

I. principle of mechanical seal

Mechanical seal, also known as end face seal, it is a kind of rotating machinery shaft sealing device, its main purpose is to easy to leak the axial seal change is difficult to let the cat out of the end face seal, it is formed by at least a pair of end face perpendicular to the rotation axis in the liquid pressure and elastic (or magnetic force) the role of compensation institutions and with the cooperation of the auxiliary seal keep post merger relative sliding and constitutes a device to prevent fluid leaks. At present, it is widely used to seal rotary shaft of pump, kettle, compressor and other similar equipment.

2. Function of mechanical seal

Mechanical seal usually consists of moving ring, static ring, compression element and sealing element. The moving ring rotates with the pump shaft, and the moving ring and the static ring are closely attached to form the sealing surface to prevent the media from leaking. The moving ring can achieve the purpose of sealing by pressing the end face of the liquid in the sealing chamber tightly on the end face of the static ring and producing an appropriate specific pressure on the end face of the two rings and maintaining a very thin liquid film. The pressure produced by the compression element can make the pump in the non-running state, and also keep the end face sticking, so as to ensure that the sealing medium does not leak out, and prevent impurities from entering the sealing end face. The sealing element ACTS as the sealing clearance between the moving ring and shaft, the static ring and the gland, and the elastic element ACTS as a buffer against the vibration and impact of the pump. In actual operation, mechanical seal is combined with other parts of pump to operate. The normal operation of mechanical seal has a lot to do with its own performance and external conditions. However, first of all, we guarantee the performance of our own parts, auxiliary sealing devices and installation technical requirements, so that the mechanical seal plays its due role.

What are the manifestations of mechanical seal failure

1. Failure of mechanical sealing parts; During the operation of rotating equipment, the sealing end face often suffers from abrasion, thermal cracking, deformation, damage, etc., and the spring will also relax, break and corrode after long use. The auxiliary sealing ring will also appear cracks, distortion and deformation, rupture and other situations.

2. Mechanical seal vibration and heating failure; During the rotation of the equipment, the end surface of the static and dynamic ring will be rough, and the clearance between the dynamic and static ring and the sealing cavity will be too large, which will cause vibration due to the collision caused by the vibration pendulum. The vibration and heating of the mechanical seal may also occur due to poor corrosion resistance and temperature resistance of the sealing end face, insufficient cooling or particle impurities in the end face during installation.

4. Failure causes of leakage of mechanical sealing media

1. Large mechanical seal vibration; Mechanical seal vibration is too large, resulting in loss of sealing effect. But the cause of mechanical seal big vibration often is not only the reason of mechanical seal itself, other parts of the pump is also the source of vibration, such as the unreasonable design of pump shaft, the cause of the processing, poor coupling balance, big radial force, etc.;

2. Regular leakage of mechanical seal; There are many reasons for frequent leakage of mechanical seals. A. Regular leakage caused by defects in the sealing end face; B. Regular leakage caused by auxiliary sealing ring; C. leakage caused by spring defect. Among other things, also including the rotor vibration caused by leakage, drive, and thrust parts quality is bad cause leaks, mechanical seal auxiliary mechanism caused by leakage, due to the problems caused by the frequent leakage of medium etc;

3. Periodic or sporadic leakage; Mechanical seal rotor component periodic vibration, axial flow momentum too large, will cause leakage. The sealing surface of mechanical seal must have certain specific pressure, so as to play a sealing role, this requires mechanical seal springs to have a certain amount of compression. Give the sealing end face a push, rotate up to make the sealing surface produce the required specific pressure. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft should not have too large amount of channeling, generally within 0.25mm. But in the actual design, because the design is not reasonable, often pump shaft produces a large amount of channeling, the use of mechanical seal is very adverse.

4. Leakage during static pressure test; Mechanical seal not careful when installation, tend to make the seal face is bruised, deformation, damage, cleaning not net, with granular impurities, or due to the positioning screw loosening, gland without pressure, precision machinery, equipment is not enough, make the sealing surface not fully fit, can cause the medium leakage. If the sleeve is leaking, the sleeve seal ring is not compressed or compressed enough or damaged during assembly.

5. measures taken to deal with failures

If the mechanical seal parts failure, you need replacement parts or improve the machining precision of parts, improve the machining precision of the mechanical seal itself and other pump body parts machining accuracy for mechanical seal effect is very good. In order to improve the sealing effect, the requirements for the smoothness and roughness of the friction surface of the static and dynamic ring are high. The width of the friction surface of the rotating and rotating ring is not large, generally between 2- 7 mm.

There are generally four improvements:

1. Handling of mechanical seal vibration and heating; If the clearance between the static and static ring and the sealing cavity is too small, the inner diameter of the sealing chamber shall be increased or the outer diameter of the rotating chamber shall be reduced, and the clearance shall be at least 0.75mm. If the friction pair is not matched properly, the material of dynamic and static ring should be changed to make it heat and corrosion resistant. This reduces the vibration and heat of the mechanical seal.

2. Handling of mechanical seal leakage; Mechanical seal leakage is caused by a variety of reasons, we need to deal with specific problems. In order to minimize leakage, the mechanical seal shall be installed in strict accordance with the technical requirements, and the following matters shall be noted:

A. The assembly should be clean and smooth. Mechanical seal parts, tools, lubricants, cleaning materials to be very clean. The sealing end face of the static and static ring should be wiped with soft gauze.

B. finish chamfering and rounding. The chamfer Angle of shaft and seal end cover should be dressed smooth, and the relevant rounded Angle of shaft and end cover should be sanded and polished.

C. When assembling the auxiliary sealing ring, the rubber auxiliary sealing ring shall not be bathed in gasoline and kerosene for washing, so as to avoid swelling and deformation and premature aging. After the assembly of the static and static rings, press the compensation rings by hand to check whether they are in place and flexible. Is the elastic opening ring located reliably? After installing the moving ring, it must be ensured that it moves flexibly on the axis.

3. Handling of pump shaft channeling amount; The balancing device of axial force is designed reasonably to eliminate axial flow. In order to meet this requirement, for multistage centrifugal pump, the design scheme is: balance disc + axial thrust bearing, the balance disc axial force balance, the pump shaft by the axial thrust bearing axial spacing.

4. Add auxiliary washing system; Sealing cavity sealing medium particles, impurities, must to wash, otherwise it will because of crystallization precipitation particles, impurities deposition, make the mechanical seal failure of the spring, if the particles into the friction pair, can lead to rapid destruction of the mechanical seal. Therefore, the auxiliary washing system of mechanical seal is very important. It can effectively protect the sealing surface and play the role of cooling, lubricating and washing away debris.

More than a simple mechanical seal is studied in the application of rotating equipment and failure, failure of mechanical seal problems later, first of all to consider the influence factors of the mechanical seal itself, then consider some of mechanical seal external influencing factors. For example, when analyzing the causes of mechanical seal quality accidents, we should consider the impact of other parts of the pump on the operation of mechanical seal, and take measures to improve the effect of mechanical seal continuously.