Due to the presence of abrasive particulate media in solid-liquid two-phase flow, the use of a common mechanical seal often constitutes the following five hazards:
â‘ Sealing face of the increased wear and tear. Between the sealing surface due to particle leakage into the end, plays the role of abrasive, accelerated sealing surface wear.
â‘¡ media-side particle blockage. Due to the accumulation of particles, bridging hinders the movement of the spring, the pin and the auxiliary sealing ring, resulting in a reduction of followability and floating of the compensating ring.
â‘¢ air-side particle blockage. Due to the conventional design of the mechanical seal, the gap between the inner diameter of the sealing surface and the shaft (or sleeve) is small, and the leaking solid particles can not be discharged in a timely manner, which can easily accumulate and block the movement of the auxiliary sealing ring, thereby causing the sealing failure.
â‘£ abrasion. Refers to the sealing element surface due to the role of abrasive particles generated by local bite, tear. It usually occurs when using softer steel or graphite materials due to the impact of flushing water or sealant. In the case of granular media, the more serious;
⑤ transmission wear components. Since these components of the drive pin are in a granular medium, the abrasion of the component itself is exacerbated by the grinding action of the particles during the movement.
In the selection, should make the mechanical seal products try to avoid the role of particles, will not produce these five kinds of failure, the mechanical seal to solve the problem of the role of particulate media has two ways: First, the mechanical seal set some additional internal structure or take the auxiliary Measures such as spiral seals, lip seals, sealing fluids, flushing water, tank or tank build up liquid slugs to prevent particle build up, etc., or external devices such as cyclones, magnetic filters, etc., Appear, maintain a good working condition mechanical seal. This approach for more important occasions, important equipment can be used. However, due to space constraints or due to excessive consumption of auxiliary equipment, and because of some occasions, the material is not allowed to seal fluid or flush water into the product, you should choose to design a new type of mechanical seal structure, can be directly Used in granular media to meet the sealing requirements of the production process.
In order to seal the mechanical seal can *, long life, simple structure, easy assembly and disassembly, easy adjustment, low cost, the specific methods are as follows:
Combination of spring and auxiliary seal. The main advantages are: high elasticity, and the spring is not in contact with the medium, to avoid the problem of particle blockage.
In order to ensure that the friction pair has the purpose of abrasion resistance and abrasion resistance in the granular medium, the hardness of the friction material must be higher than the hardness of the abrasive particles. Usually hard and hard pairs can be used, the material can be tungsten carbide or silicon carbide. Compared with tungsten carbide, silicon carbide has higher hardness, better thermal conductivity, good chemical stability, as well as self-lubricating, but the cost is higher.
According to the conclusion of AIGoLubiev (former Soviet Union) et al. On the wear mechanism of high hardness friction pair in abrasive granular media, the width of the friction pair should be wider than the average mechanical seal to obtain a higher service life. Dynamic and static ring width equal to help prevent the particles on the sealing end wear and tear, at the same time, there is sufficient area to avoid large mismatch. Therefore, much larger radial and axial runout than conventional mechanical seal faces can be accommodated.
Francis pump mechanical seal should be designed for inflow, particulate media outside the seal ring, the centrifugal force and inertia force of the particles, impurities outward movement, throw away from the sealing surface.
Different from the general mechanical seal, the gap between the sleeve and the seal ring should be large. When there is material leakage, it can be discharged in time to avoid the accumulation and obstruction of particles. The design of the seal chamber must have adequate space to allow the material in the seal chamber to flow without settling and cooling and lubricating the seal. In order to reduce the media pressure on the seal end face pressure than the impact of the use of balanced mechanical seal structure.
End pressure is one of the most important factors affecting the sealing performance and service life. In order to prevent the particle medium from entering the sealing end surface, the amount of leakage increases and the wear of the end surface aggravates, resulting in the failure of the sealing. The pressure on the end surface should be made larger than that of the normal one. But the end pressure is too large, will result in friction surface heating and wear increased, power consumption increases. Design, the end pressure is about 0.3MPa.
When using single-end mechanical seal (Figure 1), in order to avoid the hazards of particle impurities, according to different situations need to take flushing, filtration, separation, isolation, insulation and heating and other measures. The reliability and quality of the rinse solution is the key to the success or failure of the seal. If the cleaning solution is injected externally, the working environment of the seal can be improved, but the cleaning solution must be wasted, and the precondition is that the fluid to be transported is allowed to dilute slightly with the processing solution.
When using a double-ended mechanical seal (Figure 2), the seal fluid is required to establish pressure in the seal chamber for plugging, lubrication, and cooling cycles. Double seal can be * high, but the manufacturing cost and installation costs are higher. The choice of materials for most double-ended seal sets has been standardized. In actual use, the friction pair on the media end uses carbide pairs or silicon carbide pairs; the carbon graphite and nickel-chromium steel pairs on the atmosphere end are used.
Shanxi Aluminum Plant currently used B173-125 type (Figure 3) the selection of mechanical seals to consider the above aspects. Its working parameters are as follows.
Medium: a small amount of aluminum hydroxide solid particles
Pressure: 0.5MPa
Speed: 480r / min
Temperature: 120 ~ 150 ℃
B173-125 type mechanical seal is a single end, inflow machine seal. Use a rinse solution (Figure 3).
Friction deputy material selection of tungsten carbide on tungsten carbide. The performance parameters are as follows.
Grade: YG6
Linear expansion coefficient: 4.5 ~ 5.0 × 10-6 / ℃
Temperature range: -10 ~ 450 ℃
Hardness: 89.5HRA
Weight: 14.6 ~ 15g / cm3
Flexural strength: 1421MPa
Tungsten carbide is characterized by high hardness and strength, good wear resistance, high temperature resistance.
The mechanical seal is a cartridge seal, complete assembly, into the sealed chamber, installation and no special requirements. Operation, first flush valve open, then open the medium valve, and then start running. As the mechanical seal in the selection, considering the above conditions, therefore, the use of the process is very simple, just to ensure the continuity of external washing cooling, and the use of good results.
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