Correct selection and use of mechanical seals for solid-liquid two-phase flow pumps

Due to the presence of abrasive particulate media in solid-liquid two-phase flow, the use of general mechanical seals often constitutes the following five hazards:

1 The sealing end face is intensified. The sealing surface enters the end surface due to particle leakage, which acts as an abrasive and accelerates the wear of the sealing surface.

2 The media side particles are clogged. Due to the accumulation of particles, bridging hinders the movement of the spring, the pin and the auxiliary sealing ring, resulting in a decrease in the followability and floatability of the compensating ring.

3 Atmospheric side particles are clogged. Due to the mechanical seal of the conventional design, the gap between the inner diameter of the sealing surface and the shaft (or the bushing) is small, and the leaked solid particles cannot be discharged in time, which is easy to accumulate and block, hindering the movement of the auxiliary sealing ring, thereby causing the sealing failure.

4 abrasion. Refers to the localized bite and tear of the surface of the sealing element due to the action of abrasive particles. 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, it occurs more seriously;

5 wear of the transmission components. Since the components of the drive pin are in the particulate medium, the wear of the components is exacerbated by the grinding action of the particles during the movement.

In the selection, the mechanical seal products should be avoided as much as possible to avoid the action of particles. The five kinds of failures should not occur. The mechanical seal solves the problem of the function of the granular medium. There are two major ways: First, install some additional internal structures or assist the mechanical seal. Measures (such as spiral seals, lip seals, seals, flushing water, tanks or tanks to establish liquid barriers to prevent particle build-up, etc.), or external devices (such as cyclone-solid-liquid separators, magnetic filters, etc.), to avoid the above five failures Appears to maintain a good working condition of the mechanical seal. This approach can be used for important equipment for more important occasions. However, due to space constraints, or because the auxiliary facilities are too expensive, and because some occasions, materials are not allowed to have liquid sealing or flushing water into the product, the choice should be to design a new type of mechanical sealing structure, which can directly Used in granular media to meet the sealing requirements of the production process.

In order to achieve mechanical seals with reliable sealing, long life, simple structure, convenient assembly and disassembly, easy adjustment and low cost, the specific measures are as follows:

A combination of a spring and an auxiliary seal is used. The main advantage is that it has high elasticity and the spring is not in contact with the medium, avoiding the problem of being susceptible to particle clogging.

In order to ensure that the friction pair has wear resistance and abrasion resistance in the granular medium, the hardness of the friction pair material must be higher than the hardness of the abrasive grain. Hard and hard pairs are usually available, and the 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-lubricating, but higher cost.

According to the study by AI GoLubiev (former Soviet Union) et al. on the mechanism of high hardness friction pair wear in abrasive granular media, the width of the friction pair should be wider than that of a general mechanical seal to obtain a higher service life. The widths of the moving and stationary rings are equal, which helps to prevent the particles from wearing on the sealing end face, and at the same time, has sufficient area to avoid large mismatch. Therefore, it is possible to accommodate much larger radial and axial runouts than the general mechanical seal end faces.

The mechanical seal of the mixed flow pump should be designed as an inflow type, and the granular medium is on the outside of the seal ring. The action of centrifugal force and inertial force causes the particles and impurities to move outward and throw away from the sealing surface.

Unlike general mechanical seals, the gap between the bushing and the seal ring should be large. When there is material leakage, it can be discharged in time to avoid particle accumulation and blockage. The design of the sealed chamber must have a suitable space to allow the material of the sealed chamber to flow without depositing deposits and to easily cool and lubricate the seal. In order to reduce the influence of the medium pressure in the pump on the specific pressure of the sealing end face, a balanced mechanical seal structure is adopted.

End face specific pressure is one of the most important factors affecting sealing performance and service life. In order to prevent the particulate medium from entering the sealing end face, the leakage amount is increased, the end face wear is intensified, and the sealing failure is caused, so that the end face specific pressure is larger than the general one. However, if the end face is too large, the friction surface will be heated and the wear will be aggravated, and the power consumption will increase. When designing, the specific pressure of the end face is about 0.3 MPa.

When using a single-end mechanical seal (Fig. 1), in order to avoid the harm of particulate impurities, measures such as washing, filtering, separating, isolating, heat preservation and heating should be taken according to different conditions. The reliability and quality of the rinse is the key to the success of the seal. If the cleaning rinse is injected from the outside, the working environment of the seal can be improved, but the rinse must be lost, and the prerequisite is that the delivered liquid is allowed to be diluted by the rinse.

When using a double mechanical seal (Fig. 2), the sealant is required to establish pressure in the sealed chamber for sealing, lubrication and cooling cycles. Double-end seals are highly reliable, but have high manufacturing and installation costs. The choice of materials for most double seal groups has been standardized. In actual use, the friction pair at the end of the medium uses a pair of cemented carbide pairs or a group of silicon carbide; at the atmospheric end, a pair of carbon graphite and nickel-chromium steel are used.

The selection of the B173-125 type (Fig. 3) mechanical seal currently used by Shanxi Aluminum Plant takes into account 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°C

The B173-125 mechanical seal is a single-end, in-flow seal. Use an external rinse solution (Figure 3).

The friction pair material is selected from tungsten carbide to tungsten carbide. Its performance parameters are as follows.

Grade: YG6

Linear expansion coefficient: 4.5～5.0×10-6/°C

Temperature range: -10 to 450 °C

Hardness: 89.5HRA

Specific gravity: 14.6 ~ 15g / cm3

Bending strength: 1421MPa

Tungsten carbide is characterized by high hardness and strength, good wear resistance and high temperature resistance.

The mechanical seal is a cartridge type machine seal. After assembly, it is placed in a sealed chamber. There is no special requirement for installation. When running, first open the flushing fluid valve, then open the media valve, and then start the machine. Since the mechanical seal is selected in consideration of the above conditions, the use process is very simple, and only the continuity of the external flushing cooling is required, and the use effect is good.