The sealing device of the centrifugal pump is mainly divided into two categories: one is static seal, the other is dynamic seal. Static seal usually has gasket seal, O-ring seal, thread seal and other types. Dynamic seal mainly includes soft packing seal, oil seal seal, labyrinth seal, screw seal, power seal and mechanical seal of centrifugal pump.
There are two main reasons for the leakage: first, there is a gap on the sealing surface. Second, there is pressure difference between both sides of the sealing part. Eliminating or reducing any factor can prevent or reduce the leakage and achieve the purpose of sealing. The design pressure and service pressure of the pump are objective and cannot be reduced, so the solution of pump sealing is to eliminate or reduce the gap between sealing surfaces. This kind of gap includes the gap between the sealing surfaces and the gap inside the sealing device.
Mechanical seal is the main method of modern pump shaft seal, although it is not easy to use it to achieve no leakage at all, but to achieve a small, completely acceptable leakage is completely possible. But in the operation of the centrifugal pump is often embarrassing situation, so what is the reason of mechanical seal failure?
1. The material selection of mechanical seal of centrifugal pump is not appropriate. The material of the mechanical seal does not match the medium delivered. During operation, the sealing element is corroded, dissolved or worn quickly, thus losing the sealing ability. Therefore, selecting the material of mechanical seal according to the nature of the conveying medium is the prerequisite to ensure its sealing function and normal life.
2. The flushing condition of mechanical seal of centrifugal pump does not meet the design requirements. When transporting the medium which is easy to crystallize or has small particles, it must have a certain pressure and a certain flow of flushing liquid to wash, otherwise the crystal or particles will accelerate the wear of the seal pair, and affect the automatic compensation of the wear of the seal pair, resulting in leakage. Therefore, according to the nature of the transmission medium, it is not only necessary to configure the corresponding flushing pipeline, but also to install instruments and devices with monitoring and regulating functions to ensure that the pressure and flow of the flushing fluid meet the design requirements, so as to maintain the normal operation of the mechanical seal, which is often ignored by users.
3. The pressure of centrifugal pump that each kind of mechanical seal can bear is limited. Due to the inaccurate calculation of the pressure in the seal cavity, the pressure in the seal cavity exceeds the pressure that the mechanical seal can bear, resulting in leakage, which is also one of the common reasons for seal failure.
4. The working temperature of mechanical seal of centrifugal pump shall not exceed its specified value. In the design of cooling pipeline, the cooling effect is often reduced due to the insufficient flow of cooling medium; in the design of no cooling pipeline, the mechanical seal of centrifugal pump is often in dry friction state due to the air in the seal cavity. In both cases, the working temperature of the moving seal pair of the mechanical seal will be too high and the wear will be accelerated, resulting in seal failure.
5. When using the mechanical seal of centrifugal pump with single spring, the correct combination of the rotation direction of the spring and the rotation direction of the pump rotor is often ignored. Or the design did not explain, or the assembly negligence, did not make the mechanical seal spring force increase because of the rotation of the rotor, but on the contrary, resulting in the dynamic ring and the static ring friction pair pressure is insufficient, resulting in leakage.
6. The serious wear or damage of the bearing causes serious axial movement of the centrifugal pump shaft, which is also one of the reasons for the leakage of the mechanical seal. With the development of science and technology, new sealing forms and sealing materials appear one after another, which will inevitably have a direct impact on and promote the sealing technology of centrifugal pumps. Pump products with long life and zero leakage will be popularized and applied in more and more occasions.