Correct Selection and Use of Mechanical Seals for Solid-Liquid Two-Phase Flow Pumps

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Proper selection and use of mechanical seals for solid-liquid two-phase flow pumps: Due to the presence of abrasive particles in solid-liquid two-phase flows, the use of ordinary mechanical seals can lead to the following five types of damage: ① Increased wear on the sealing surfaces. Particles that leak into the end faces between the sealing surfaces act as abrasives, accelerating the wear of those sealing surfaces. ②Particle blockage on the medium side. Due to the accumulation of particles and bridging, the movement of the springs, pins, and auxiliary sealing rings is hindered, which results in a reduced followability and floatability of the compensation ring. ③Particle blockage on the atmospheric side. In mechanical seals with conventional design, the gap between the inner diameter of the sealing surface and the shaft (or sleeve) is small; as a result, solid particles that leak in cannot be removed in time, and they tend to accumulate and get blocked, hindering the movement of the auxiliary sealing ring and thus leading to seal failure. ④Abrasion. It refers to the localized erosion and tearing on the surface of the sealing element caused by abrasive particles. It usually occurs when softer steel materials or graphite materials are used, as a result of the impact from flushing water or sealing fluid. It becomes more severe in the presence of particulate media ； ⑤Wear of the transmission elements. Since components such as drive pins are located in a granular medium, the abrasive action of the particles during movement exacerbates the wear of these components themselves. When selecting a mechanical seal, efforts should be made to protect it from the effects of particles, thereby preventing these 5 types of failures. There are two main ways in which mechanical seals can deal with the impact of particulate media: one is to incorporate additional internal structures or adopt supplementary measures (such as spiral seals, lip seals, sealing fluids, flushing water, and liquid barriers provided by tanks or oil tanks to prevent particle accumulation), or to use external devices (such as cyclone solid-liquid separators and magnetic filters) in order to minimize the occurrence of the aforementioned 5 failures and maintain the seal’s optimal operating condition. This approach can be used for more important occasions with critical equipment. However, in cases where space is limited, or where the cost of auxiliary facilities is too high, or in situations where it is not allowed for sealing fluids or washing water to enter the product, it is necessary to design a new type of mechanical seal structure that can be used directly in granular media, thereby meeting the sealing requirements of the production process. To achieve reliable sealing, a long service life, a simple structure, easy installation and removal, ease of adjustment, and low costs for mechanical seals, the following approach is adopted: the use of springs in combination with auxiliary sealing rings. The main advantages are: it has high elasticity, and the spring does not come into contact with the medium, thus avoiding the problem of being clogged by particles. To ensure that the friction pair is resistant to wear and corrosion in a particulate medium, the hardness of the friction pair material must be higher than that of the abrasive particles. A hard-to-hard pairing is usually available, with materials that can be tungsten carbide or silicon carbide. Compared to tungsten carbide, silicon carbide has higher hardness, better thermal conductivity, better chemical stability, and self-lubricating properties, but it is more expensive. Based on the conclusions drawn by A. I. Golubiev (Former Soviet Union) and others from their studies on the wear mechanism of high-hardness friction pairs in abrasive particle media, the width of the friction pair should be greater than that of conventional mechanical seals in order to achieve a longer service life. The widths of the moving and stationary rings are equal, which helps to prevent particles from wearing out the sealing surfaces; at the same time, this provides sufficient area to avoid significant misalignment. Therefore, it can accommodate much larger radial and axial runout than that of conventional mechanical seal faces. The mechanical seal used for mixed-flow pumps should be designed as an internal-flow type, with the particulate medium located outside the sealing ring; centrifugal and inertial forces cause these particles and impurities to move outward, away from the sealing surface. Unlike conventional mechanical seals, the gap between the shaft sleeve and the sealing ring should be larger, so that any leakage of material can be discharged promptly, preventing the accumulation and blockage of particles. The design of the sealing chamber must provide sufficient space to allow the material within it to flow, preventing accumulation and sedimentation, as well as to facilitate cooling and lubrication of the seal. To reduce the influence of the pressure of the medium inside the pump on the specific pressure at the sealing surface, a balanced mechanical seal structure is employed. The end-face specific pressure is one of the most important factors affecting sealing performance and service life. To prevent particulate media from entering the sealing face, which would increase leakage and accelerate face wear leading to seal failure, the specific pressure on the face should be higher than usual. However, an excessive end-face specific pressure will cause the friction surface to heat up and wear more severely, leading to increased power consumption. During design, the end-face specific pressure is around 0.3 MPa. When a single-face mechanical seal is used (Figure 1), to avoid the damage caused by particulate contaminants, measures such as flushing, filtering, separation, isolation, insulation, and heating must be taken depending on the specific circumstances. The availability and quality of the flushing fluid are key to the success of sealing. If a cleaning flush fluid is introduced from the outside, the sealed working environment can be improved, but this requires a consumption of the flush fluid; moreover, a prerequisite is that the liquid to be delivered can be slightly diluted by the flush fluid. When a double-end face mechanical seal is used (Figure 2), it is required that the sealing fluid generate pressure within the seal chamber to carry out sealing, lubrication, and cooling cycles. Double-end face seals offer high reliability, but their manufacturing cost and installation expenses are high. The selection of materials for most double-end face seal pairs has been standardized. In practical use, the friction pair at the medium side is assembled using cemented carbide or silicon carbide ； The carbon graphite and nickel-chromium steel selected for the atmospheric side are paired together. The selection of the B173-125 type (Figure 3) mechanical seal currently used by Shanxi Aluminum Plant took these various factors into account. Its operating parameters are as follows. Medium: Contains a small amount of solid aluminum hydroxide particles. Pressure: 0.5 MPa. Rotational speed: 480 rpm. Temperature: 120–150°C. The B173-125 type mechanical seal is a single-face, internal-flow type mechanical seal. An external flushing fluid is used (Figure 3). Tungsten carbide is selected as the material for the friction pair. Its performance parameters are as follows. Grade: YG6. Coefficient of linear expansion: 4.5–5.0×10⁻⁶/°C. Temperature range: -10–450°C. Hardness: 89.5 HRA. Density: 14.6–15 g/cm³. Flexural strength: 1421 MPa. Tungsten carbide is characterized by high hardness and strength, as well as excellent wear resistance and heat resistance. This mechanical seal is a modular type; after being assembled as a complete set, it is placed in the sealing chamber, and no special requirements exist for its installation. During operation, first open the flushing fluid valve, then open the medium valve, and finally start the machine. Since the selection of this mechanical seal took all the aforementioned conditions into account, it is very simple to use; only continuity of external flushing cooling needs to be ensured, and it delivers excellent performance.