In this paper, through the introduction of the principle of mechanical seal structure and the analysis of the reasons for the failure of mechanical seal leakage, summed up the correct installation and use of mechanical seals to extend its service life experience and methods.
[preface]
Mechanical seal (mechanical seal) is composed of at least a pair of end faces perpendicular to the axis of rotation of the fluid pressure and the elasticity of compensation organizations (or force) the role of the device and auxiliary seal with maintaining relative sliding postmerger and constitutes to prevent fluid leakage.
Mechanical seal structure (typical structure principle is shown in Figure 1)
1, rotating ring (ring)
2, elastic components
3, spring seat
4, set screws
5, rotating ring auxiliary seal ring
6, static ring auxiliary seal ring
The anti rotation pin 7 is fixed on the cover plate 9 to prevent the static ring from rotating.

Fig. 1 Schematic diagram of mechanical seal
Mechanical seal with reliable sealing performance, small leakage and long service life, low power loss and wide application range, has been widely used in various fields of technology, especially suitable for high speed, high pressure process medium of bad working conditions and expensive or toxic and corrosive. At the same time, mechanical seal is one of the weakest parts of the equipment.
In order to extend its service life, the correct installation and maintenance can also play an important role in the choice of the appropriate friction material and the appropriate end face pressure.
In this paper, the main causes of the failure of mechanical seal are analyzed through the practice of daily maintenance.

Cause analysis of mechanical seal failure leakage
Chemical equipment used in a variety of mechanical seals, different models, but there are five main leak:
Seal between shaft sleeve and shaft;
Ring and shaft seal;
Dynamic and static ring seal;
The seal between the static ring and the static ring seat;
Sealing between sealing end cover and pump body.
Mechanical seal fluid leakage may be the way in Figure 1 in the A, B, C, D, the four channel.
1 dynamic and static ring wear caused by mechanical seal leakage
No matter what type of mechanical seal, the most important feature is that the sealing surface is perpendicular to the axis of rotation of the end surface, which is easy to leak the axial seal is not easy to leak the end face seal. Therefore, the mechanical seal failure is the main form of static and dynamic wear failure between the rings. The dynamic and static ring end friction pairs are pressed by the spring thrust to prevent leakage. Dynamic and static ring pressure the more tight the more difficult to leak, but the friction between them will also increase, the dynamic and static ring contact end of the mirror in the role of large friction will soon wear, the final failure of leakage.
2 process instability and poor mechanical seal leakage caused by poor installation
The process conditions are not stable and the installation of equipment, vibration caused by poor vacuum vaporization moment will lead to disconnection between mechanical seal ring on the membrane damage, so that the mechanical seal &ldquo under unlubricated condition; dry ” sealing ring operation, the temperature rose rapidly, some directly burned, or when the pump return to normal working condition is rapid cooling, the formation of thermal shock and fragmentation. Flushing fluid and poor flushing conditions will also form a thermal shock, resulting in radial cracks in the sealing ring, increasing the wear and tear of dynamic and static ring. At the same time, when the graphite ring over the use of temperature, the surface of the crystal will be precipitated, in the vicinity of the higher temperature of the friction occurred in the vicinity of the particles, the particles into the friction pairs of dynamic and static ring wear and tear.
3 mechanical seal seal failure is the main cause of seal leakage
Sealing ring assembly skew matched with a sealing ring; the shaft or sleeve surface finish is not enough, or with small size; the sealing ring and the sealing medium physical or chemical reaction, corrosion, deformation and aging, which can lead to leakage.
4 improper assembly leads to mechanical seal leakage
In front of the assembly mechanical seal, the sealing component is not clean, the components are damaged or scratched; the assembly is not in place; the spring is installed, the fastening screw is not fastened, and the damage of the disassembly is the cause of the failure of the mechanical seal.
5 mechanical seal design selection caused by improper sealing leakage
Due to diversity of process medium characteristics in chemical equipment design, mechanical seal to seal the improper selection, too small, too large pressure or sealing material such as shrinkage of the larger, easy to make mechanical seal failure caused the leakage.

[solution]
According to the above analysis, the following measures should be taken to prolong the service life of the mechanical seal and the maintenance method.
1 spring compression adjustment
Adjust the compression of the spring is to adjust the mechanical seal of the end face than the pressure, related to the sealing performance and service life of the important parameters, and the sealing structure of the type, the size of the spring and the medium pressure. End face than the pressure will damage the friction pair; than the pressure is too small is easy to leak, often given by the manufacturer for a suitable range, generally by 3 ~ 6kg/cm2 value. The free length of the spring A, the spring stiffness of the K (unit load generated when the amount of compression load), the requirements of the specific pressure P, these are the parameters given by the manufacturer. Compressed size is B, then P/ (A-B) =k, B=A-P/k, which is the size of the spring after the installation of compressed. If the size of the spring is too large, the thickness of the adjusting pad can be increased between the spring seat and the spring, and the thickness of the adjustment can be reduced when the size is too small.
2 ring seal
Dynamic tight ring seals harmful, is a severe wear ring and sleeve, premature leakage; two is to increase the dynamic axial adjustment of the resistance movement, frequent changes in working conditions can not adjust; three spring fatigue is easy to damage; the four is to make dynamic ring seal deformation, influence sealing effect. In order to ensure the floating ring, the inner diameter of large shaft diameter of 0.5 ~ 1mm, with compensation axis of vibration and deflection, but the gap is not too large, otherwise it will make the dynamic ring seal ring is clipped into the damaged mechanical seal mechanical properties. The tightness of the sealing ring can be pressed with one hand when the lubricant is coated.
3 static ring seal tightness
Ring sealThe circle is basically in a static state, the relatively tight sealing effect will be better, but too tight will cause: one is caused by static seal due to excessive deformation, affecting sealing effect; two static ring materials are mostly of graphite, are generally more brittle, vulnerable to excessive stress caused by fragmentation; three is to install, difficult to remove, easy to damage static ring. Therefore, the inner diameter of the ring is generally larger than the shaft diameter of 1 ~ 2mm, the tightness of the sealing ring to be coated with lubricant can be pressed into the hands of the quasi. Hand can lighter pressure into the loose, hands pressed into it too tight.
4 seal new and old
Relatively speaking, the use of new mechanical seal effect is better than the old, but the new mechanical seal of the quality or material selection is not at that time, with the size of the error than the General Assembly affect the sealing effect. In the polymerization of deep and medium, such as static ring without excessive wear (when the cracks, sealing off angle, scratches, pits, burr and abrasion, scratches, pits throughout the seals called excessive wear), or not replaced at all. Because the static ring in the static ring seat for a long time in a static state, so that polymer and impurities deposited as a whole, played a better sealing effect.
5 demolition
Mechanical seal leakage occurs, the first rush to repair, and sometimes the seal may not be damaged, simply adjust the working conditions or the appropriate adjustment of the seal can eliminate leakage. Such as adjusting the compression of the spring, eliminating the vibration of the running equipment, removing the scale of the spring and the scale of the friction ring of the dynamic and static rings. In this way, it can not only avoid waste but also solve practical problems, at the same time, it can also verify the fault judgment ability, accumulate experience and improve the quality of maintenance.
6 points of attention
Mechanical seal is a precision component, manufacturing and installation precision are very strict, if improper assembly will affect the sealing performance, shorten the service life, so we must pay attention to the following requirements:
6.1 install mechanical seal parts of the shaft (or sleeve) radial runout tolerance according to table 1. The surface roughness Ra value should not be greater than 3.2um, outside diameter tolerance h6. When the pump is installed with a mechanical seal or other similar rotating machinery, the axial displacement of the rotor is not more than 0.3mm. The tolerance of the end face of the seal cavity and the sealing end cover to the surface of the shaft (or sleeve) is shown in table 1.

6.2 installation of rotary ring auxiliary seal shaft (or sleeve) end should be in accordance with figure 2 chamfer to smooth transition.

Fig. 2 chamfer angle of the shaft of the sealing ring
6.3 the end of the hole of the sealing end cover (or shell) of the mechanical seal ring is shown in Figure 3 and table 2. The surface roughness of the contact surface of the sealing end cover (or shell) and the auxiliary seal ring is shown in figure 3.

Figure 3 sealing end cover (or shell) installation requirements

6.4 sealing parts, shaft surface, sealed cavity must be cleaned, and to ensure that the sealing end of the flushing fluid pipeline.
6.5 before installation, it is necessary to confirm the product model and specifications and equipment requirements. When the medium temperature is too high, too low, or the impurity particles, flammable, explosive, toxic, must take the appropriate sealing, washing, cooling, filtering and other measures.
6.6 in the installation of the sealing shaft, the cavity and the cover and auxiliary sealing ring at the same place of the oil evenly (Note: ethylene propylene rubber, or the media is not allowed to inject lubricating oil, can be applied to vegetable oil or soap water).
6.7 in the installation of the sealing ring, do not allow the use of grease seal to prohibit the use of paint, otherwise it will make the seal swelling larger, so that the mechanical seal leakage.
6.8 installation should check the sealing installation size, generally single spring seal axial installation size maximum allowable difference is 1.0mm, more than ± spring; 0.5mm.
6.9 mechanical seal on the shaft, according to the design of the installation of the work height.

6.10 through the way of diagonal cross tightening, the whole seal and seal cavity (pump cover) connection bolts tightened.
6.11 seal with auxiliary system, according to the correct connection.
6.12 after the installation of manually rotating ring, ensure flexible rotation, and some floating. Then the static pressure test and dynamic pressure test can be put into use.
7 installation of mechanical seal
The assembled mechanical seal, mechanical seal has been factory compression adjustment after the mechanical seal, and mechanical operation tests were carried out, so we are not allowed in the site installation or re adjustment of disintegration machine sealing machine sealing compression.
8 mechanical seal installation of corrugated pipe
For the corrugated pipe mechanical seal in the installation of kerosene before the use of corrugated pipe leak detection, to ensure that the bellows without leakage, can be installed. At the same time due to bellows mechanical seal bellows easily damaged, in the installation process must be careful.
9 strict process control and process control
When the flow is adjusted and the pump is stopped, the phenomenon of surge, pump evacuation and flushing fluid flow are avoided. Sealed filter impurities in the medium, reasonable using self balanced washing or external forced circulation washing mode, improve washing cooling conditions, and increase the flushing system restriction orifice aperture, increase the amount of flushing liquor, washing fluid and ensure steady supply of adequate, so that the mechanical seal lubrication and cooling conditions have been improved, to prevent mechanical the seal due to overheating phenomenon of burning, rupture, crack, crystallization, prolong the service life of mechanical seal.
10 mechanical seal selection
The mechanical seal in the selection, we must provide detailed technical parameters of equipment operation and technical requirements for mechanical seal manufacturers, to avoid the selection of mechanical seal is not suitable for the site conditions and reduce its service life.

[Conclusion]
To sum up, as long as in strict accordance with the technical requirements of mechanical seal installation, maintenance and use of not only can achieve good sealing effect, but also greatly extend the service life of mechanical seals.
This article source: chemical 707;
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