Accurate selection and application of mechanical seal of solid-liquid two-phase flow pump

Accurate selection and application of mechanical seal of solid-liquid two-phase flow pump [Student question] Accurate selection and application of mechanical seal of solid-liquid two-phase flow pump? [Answer] Because of the presence of abrasive particulate media in the solid-liquid two-phase flow, the selection of mechanical seals for autoclaves usually causes the following five types of damage: ①Aggravated wear on the seal end surface. Due to particles leaking between the sealing surfaces into the end surface, it plays an abrasive effect and accelerates the wear of the sealing surface. ②The particles on the media side are blocked. Because of the accumulation of particles, the bridge prevents the movement of the tension spring, pin and auxiliary sealing ring, and then causes the followability and buoyancy of the compensation ring to decrease. ③Clogging of particles on the atmospheric side. Because of the conventionally planned mechanical seal, the gap between the inner diameter of the sealing surface and the shaft (or sleeve) is small, and the leaked solid particles cannot be discharged in time, and are easy to accumulate and block, which prevents the movement of the auxiliary seal ring and causes the seal to fail. ④Abrasion. Refers to the partial erosion and tearing of the mechanical seal of the kettle due to the effect of abrasive particles on the surface of the sealing element. It usually occurs when softer steel or graphite materials are used, due to the impact of washing water or sealing liquid. In the presence of granular media, it will be more severe; ⑤ Wear of transmission components. Because these components of the drive pin are in a granular medium, the abrasive effect of the particles aggravates the wear of the components themselves during movement. When selecting the type, the mechanical seal product should be made to prevent the effect of particles as much as possible, so that these five types of failures will not occur. There are two major paths for the effect of mechanical seals in handling particulate media: one is to set up some additional internal structures for the mechanical seal or adopt auxiliary Methods (such as screw seal, lip seal, liquid sealing, flushing water, water tank or fuel tank to build liquid barrier to prevent particle accumulation, etc.), or external equipment (such as cyclone solid-liquid separator, magnetic filter, etc.), try to avoid the above 5 failures Presents and maintains outstanding working conditions of mechanical seals. This kind of path can be used in situations where the comparison is important and important equipment. However, regarding space constraints, perhaps because of the high cost of auxiliary equipment, and because in some occasions, materials do not allow sealing liquid or flushing water to enter the product, a new type of mechanical seal structure should be selected and planned. It is directly used in granular media to meet the sealing requirements of the production process. In order to make the mechanical seal achieve the intent of reliable sealing, long life, simple structure, convenient assembly and disassembly, easy adjustment, and low cost, the specific method is as follows: use a combination of spring and auxiliary seal ring. The main advantage is: high elasticity, and the tension spring does not touch the medium, which prevents the doubt that it is easy to be blocked by particles. In order to ensure that the conflict pair has the intent of wear resistance and abrasion resistance in the granular medium, the hardness of the conflict pair material must be higher than the hardness of the abrasive particles. Usually a hard-to-hard pair can be selected, and the raw material can be tungsten carbide or silicon carbide. Compared with tungsten carbide, silicon carbide has higher hardness, better thermal conductivity, better chemical stability, and self-smoothing properties, but the cost is higher. According to the conclusion drawn by A.I. GoLubiev (former Soviet Union) and others on the wear mechanism of high-hardness conflicts in abrasive granular media, the width of the conflicts should be wider than the usual mechanical seals in order to achieve a longer service life. The widths of the moving and static rings are equal, which is beneficial to prevent the particles from abrading the end face of the seal. Together, there is enough area to prevent large mismatch. Therefore, you can get used to much larger radial and axial runout than the end face of the usual mechanical seal. The mechanical seal for a mixed flow pump should be planned as an internal flow type. The particle medium is outside the seal ring. The effect of centrifugal force and inertial force causes particles and impurities to move outward and throw away from the sealing surface. Unlike normal mechanical seals, the gap between the shaft sleeve and the seal ring should be large, and when there is material leakage, it can be discharged in time to prevent the accumulation and blockage of particles. It is necessary for the planning of the sealed cavity to have appropriate space so that the materials in the sealed cavity can move without accumulation and precipitation, and are easy to cool and smoothly seal. In order to reduce the influence of the medium pressure in the pump on the specific pressure of the seal end face, a balanced mechanical seal structure is selected. The specific pressure of the end face is one of the most important factors affecting the sealing function and service life. In order to prevent particulate media from entering the end face of the seal, the amount of leakage will increase, and the end face wear will aggravate, causing the seal to fail, and the end face specific pressure should be larger than usual. However, if the end-face specific pressure is too large, the conflicting face will generate heat and wear aggravated, and the power consumption will increase. When planning, the specific pressure of the end face is about 0.3MPa. When selecting a single-end mechanical seal, in order to prevent damage from particulate impurities, it is necessary to adopt measures such as washing, filtering, separating, blocking, heat preservation and heating according to different conditions. The reliability and quality of the flushing fluid is the key to the success or failure of the seal. If the cleaning and flushing fluid is written from the outside, the sealed working environment can be improved, but it is necessary to consume the flushing fluid, and the prerequisite is that the transported fluid should be slightly diluted by the flushing fluid. When the double end face mechanical seal is selected, the sealing liquid is required to establish pressure in the sealed cavity, and perform sealing, smoothing and cooling cycles. The double end face seal has high reliability, but the forming cost and installation cost are higher. The selection of materials for most double end face seal groups has been standardized. In actual application, the conflict pair located at the medium end uses cemented carbide or silicon carbide pair; the pair located at the atmosphere end uses carbon graphite and nickel-chromium steel pair.