mechanical seal
1 Introduction
A mechanical seal is a device for preventing fluid leakage that is composed of at least a pair of end faces perpendicular to the axis of rotation under the action of fluid pressure and elasticity (or magnetic force) of the compensation mechanism and the cooperation of the auxiliary seal to maintain adhesion and relative sliding
The mechanical seal structure (see Figure 1 for typical structural principles) is mainly composed of a stationary ring (static ring)
1. Rotating ring (moving ring)
2. Elastic element
3. Spring seat
4. Set screw
5. Auxiliary sealing ring of rotating ring
6. Auxiliary seal ring of stationary ring
The mechanical seal has the advantages of reliable sealing performance, small leakage, long service life, low power loss and wide application range. It is widely used in various technical fields, especially suitable for working conditions with high speed and high pressure difference and expensive or toxic and strong Corrosive process media. At the same time, the mechanical seal is one of the weakest links of the equipment.
In order to prolong its service life, in addition to selecting the appropriate friction pair material and the appropriate end surface specific pressure, correct installation and maintenance can also play an important role.
2 Reason analysis of mechanical seal failure leakage
There are many types and types of mechanical seals used in chemical equipment, but there are five main leak points:
① Seal between shaft sleeve and shaft;
②The seal between the moving ring and the shaft sleeve;
③ Sealing between dynamic and static rings;
④ Sealing between static ring and static ring seat;
⑤Seal between the end cap and the pump body.
The possible leakage paths of the fluid in the mechanical seal are shown in the four channels of A, B, C and D in Figure 1.
2.1 The wear of the end face of the dynamic and static ring causes the mechanical seal to leak
Regardless of the type of mechanical seal, the most important feature is that the sealing surface is the end surface perpendicular to the axis of rotation, that is, the axial seal that is easy to leak is changed to the end seal that is not easy to leak. Therefore, the main form of mechanical seal failure is the wear failure between the static and dynamic rings. The friction pairs of the end faces of the moving and static rings are mainly pressed by the spring thrust to prevent leakage. The tighter the dynamic and static rings are, the less likely it is to leak, but the friction between them will also increase. The mirror surface of the contact ends of the dynamic and static rings will wear out quickly under the action of large friction and eventually leak.
2.2 Unstable process conditions and poor installation lead to mechanical seal leakage
Vibration caused by unstable process conditions and poor installation, and instantaneous interruption of equipment evacuation and vaporization will lead to the destruction of the liquid film between the dynamic and static rings of the mechanical seal, so that the mechanical seal operates in a "dry state" without lubrication, and the temperature of the seal ring rises rapidly Some are directly burned, and some are cooled sharply when the pump returns to normal working state, forming a thermal shock and cracking. Poor flushing fluid and poor flushing conditions can also cause thermal shocks, which can lead to radial cracks in the seal ring and exacerbate wear and tear on the dynamic and static rings. At the same time, when the graphite ring exceeds the use temperature, crystals will precipitate on the surface, carbonization occurs near the friction pair with a higher temperature, and its particles enter the friction pair to make the dynamic and static ring wear and tear.
2.3 The failure of the seal ring of the mechanical seal is also the main reason for the seal leakage
The assembly of the dynamic and static ring seals is skewed; the surface finish of the shaft or sleeve matched with the seal ring is not enough, or the size of the fit is too small; physical or chemical reactions between the seal ring and the sealing medium, corrosion, deformation, aging, etc. can cause leakage.
2.4 Improper assembly leads to mechanical seal leakage
Before the mechanical seal is assembled, the mechanical seal components are not cleaned, the components are scratched or scratched; the assembly is not in place; the spring is biased, the fastening screws are not tightened; and the damage during disassembly is the reason for the mechanical seal's early failure.
2.5 Mechanical seal leakage caused by improper mechanical seal selection design
Due to the diversity of process media characteristics in the chemical plant, the mechanical seal design is not properly selected, so that the seal end face is too small, too large, or the seal material is cold shrinkable, etc., which is very likely to cause the mechanical seal to fail and cause its leakage.
3 Solutions
According to the above analysis, the following measures and maintenance methods should be taken to extend the service life of the mechanical seal
3.1 Spring compression adjustment
Adjusting the compression of the spring is to adjust the specific pressure of the end face of the mechanical seal, which is an important parameter related to the sealing performance and service life, and is related to the structural type of the seal, the size of the spring, and the medium pressure. If the specific pressure is too large, the friction pair will be damaged; if the specific pressure is too small, it will be easy to leak. A suitable range is often given by the manufacturer, generally according to 3 ~ 6kg / cm2. The free length A of the spring, the spring stiffness K (the load to bear when a unit of compression is generated), and the required specific pressure P are specified by the manufacturer. The size after compression is B, then P / (A-B) = k, and B = A-P / k is obtained, which is the size of the spring after compression. If the size of the spring after installation is too large, the thickness of the adjustment pad can be increased between the spring seat and the spring. If the size is too small, the thickness of the adjustment pad is reduced. The thickness of the adjustment pad is measured with a micrometer.
3.2 Moving ring seal ring is tight
Over-tightening of the moving ring sealing ring is harmful and unhelpful. First, it increases the wear between the sealing ring and the bushing and leaks prematurely. Second, it increases the resistance to the axial adjustment and movement of the moving ring. It cannot be adjusted in time when the working conditions change frequently; The third is that the spring is easily fatigued due to excessive fatigue; the fourth is to deform the moving ring sealing ring and affect the sealing effect. In order to ensure the floatability of the moving ring, its inner diameter is larger than the shaft diameter by 0.5 to 1 mm to compensate for the vibration and deflection of the shaft, but the gap should not be too large, otherwise the moving ring seal ring will get stuck and cause mechanical performance of the mechanical seal damage. The tightness of the sealing ring is based on the fact that it can be pressed firmly with one hand after the lubricant is applied.
3.3 The static ring seal ring is tight
The static ring sealing ring is basically in a static state, and the relatively tight sealing effect will be better, but excessive tightening will cause: First, the static ring sealing will be affected due to excessive deformation, which will affect the sealing effect; Excessive force is very easy to cause cracking; third, it is difficult to install and disassemble, and it is easy to damage the static ring. Therefore, the inner diameter of the static ring is generally 1 to 2 mm larger than the shaft diameter. The tightness of the seal ring is based on the fact that it can be pressed in with both hands after applying lubricant. The hand can be pressed lightly, it is too loose, and the hands cannot be pressed too tightly.
Relatively speaking, the effect of using the new mechanical seal is better than the old one, but the quality or material of the new mechanical seal is not selected properly, and the larger size error will affect the sealing effect. In the polymerizable and deep-permeable medium, the static ring is not excessively worn (when cracks, corners, scratches, pits, burrs, burrs and partial wear occur on the sealing surface, the scratches and pits are called excessive Wear), or not replaced as well. Because the static ring is in a static state for a long time in the static ring seat, the polymer and impurities are deposited as a whole, which plays a good sealing role.
3.5 Dismantling
Once the mechanical seal leaks, don't rush to repair it. Sometimes the seal may not be damaged. Just adjust the working conditions or adjust the seal to eliminate the leak. Such as adjusting the compression of the spring, eliminating the vibration of the running equipment, removing the scale that makes the spring bounce, and the scale on the dynamic and static ring friction pair. This not only avoids waste but also solves actual problems, and at the same time verifies your own fault judgment ability, accumulates maintenance experience and improves the quality of maintenance.
