A complete solution to the failure causes of various sealing methods

 The device or measure to prevent the leakage of the working medium from the pump or to prevent the intrusion of external impurities or air into the pump is called sealing, and the sealed medium is generally liquid, gas or dust.

The sealing device of the pump is mainly divided into two categories: one is static seal, and the other is dynamic seal. Static seals usually have gasket seals, O-ring seals, thread seals and other types. Dynamic seals mainly include soft packing seals, oil seal seals, labyrinth seals, screw seals, dynamic seals and mechanical seals.

There are two main reasons for the leakage: First, there is a gap on the sealing surface. Second, there is a pressure difference on both sides of the sealing part. Eliminating or reducing any one factor can prevent or reduce leakage and achieve the purpose of sealing. The design pressure and operating pressure of the pump are objective and cannot be reduced, so the solution of the pump seal is to eliminate or reduce the gap between the sealing surfaces. Such gaps include gaps between the sealing surfaces and gaps within the Bunsen of 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, it is completely possible to achieve a small and completely acceptable leakage amount. But there are often embarrassing situations in the operation of the pump, so what is the reason for the failure of the mechanical seal?

1. The material selection of the mechanical seal is inappropriate. The material of the mechanical seal does not match the medium being conveyed. During operation, the sealing elements are quickly corroded, dissolved or worn, thus losing their sealing ability. Therefore, selecting the material of the mechanical seal according to the nature of the conveying medium is a prerequisite for ensuring its sealing function and normal life.

2. The flushing condition of the mechanical seal does not meet the design requirements. When transporting media that is easy to crystallize or has fine particles, there must be a certain pressure and a certain flow of flushing liquid for flushing, otherwise the crystals or particles will accelerate the wear of the sealing pair, and affect the automatic compensation of the sealing pair after wear and leakage occurs. Therefore, according to the nature of the conveying medium, it is not only necessary to configure the corresponding flushing pipeline, but also to install instruments and devices with monitoring and adjustment functions to ensure that the pressure and flow of the flushing liquid meet the design requirements, so as to maintain the normal operation of the mechanical seal. This is often ignored by users.

3. The pressure that each mechanical seal can withstand is limited. Due to the inaccurate measurement of the pressure in the sealing chamber, the pressure in the sealing chamber exceeds the pressure that the mechanical seal can withstand, resulting in leakage. It is also a common seal. One of the reasons for failure.

4. The working temperature of the mechanical seal cannot exceed its specified value. In the design with cooling pipeline, the cooling effect is often reduced due to insufficient flow of cooling medium; in the design without cooling pipeline, the mechanical seal is often in a dry friction state due to trapped air in the sealing 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 a single-spring mechanical seal is used, the correct combination of the rotation direction of the spring and the rotation direction of the pump rotor is sometimes ignored. Or there is no description in the design, or the negligence in the assembly, the spring force of the mechanical seal is not increased due to the rotation of the rotor, but the opposite, resulting in insufficient pressure of the friction pair between the dynamic ring and the static ring, resulting in leakage.

6. The serious wear or damage of the bearing causes serious axial movement of the 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 and promotion on the sealing technology of the pump. Pump products with long life and zero leakage will be promoted and applied in more and more occasions. The oil seal is a self-tightening lip seal with simple structure, small size, low cost, convenient maintenance and small resistance torque. It has a certain compensation ability, but it is not resistant to high pressure, so it is generally used in low pressure occasions. The oil seal should be installed on the shaft with a manufacturing accuracy of h8 to h9, a surface roughness of 1.6 to 0.8 μm and a surface hardening treatment. The sealing medium should not contain solid particles and impurities, otherwise it will cause the rapid wear of the oil seal and the shaft and make the seal invalid.

Gasket is the basic element of static seal of centrifugal pump and is widely used. The selection of gaskets is mainly determined by factors such as pump conveying medium, temperature, pressure and corrosion. When the temperature and pressure are not high, non-metallic gaskets are generally used; when medium pressure and high temperature are used, non-metallic and metal combined gaskets are used. Non-metallic gaskets are most commonly used in pumps, and their materials are generally paper, rubber and PTFE.

When the temperature does not exceed 120°C and the pressure is below 1.0Mpa, green shell paper or molded paper gaskets are generally used. If the conveying medium is oil and the temperature is between -30 and 110 °C (public number: Pump Butler), NBR with better aging resistance is generally used. When the conveying medium is at -50～200℃, it is more suitable to use fluororubber. Because in addition to oil resistance and heat resistance, its mechanical strength is also its main feature.

In chemical pumps, because the transported medium is corrosive, polytetrafluoroethylene is generally used as the gasket material. As the pump is used in more and more fields, more and more types of media are transported. Therefore, when selecting the gasket material, you should consult the relevant information or make the correct choice after passing the experiment.

The main reasons for gasket failure are as follows:

1. The internal organization or thickness of the gasket material is uneven, and the cardboard with cracks or wrinkles is used, so that the gasket itself forms a gap. When the force acting on the gasket makes the elastic deformation of the gasket insufficient to completely When filling these gaps, leaks are inevitable.

2. The material of the gasket is not suitable for the medium being conveyed. Due to the diversity of chemical properties of the chemical products delivered by the pump, and the addition of a small amount of additives to the fuel in order to improve the combustion value of the fuel or change its combustion products, some properties of the fuel have changed, so It is not easy to select the gasket material suitable for the conveying medium, so it often occurs that the gasket is eroded and leaked due to incompatibility.

3. The pressure acting on the gasket is insufficient. Since there are always microscopic unevenness on the sealing surface, sometimes a number of annular grooves are machined on the sealing surface. Fill these gaps. The compression force of various gasket materials is usually given in the gasket manufacturer's sample or product manual, and can also be determined through experiments. Due to the failure to achieve the required pressing force of the gasket during assembly or the reduction of the pressing force due to the loosening of the pressing bolt due to vibration during normal operation, and losing original elasticity due to the aging and deformation of the gasket material, the gasket will be damaged. Leakage due to chip failure.

This article is transferred from ShipParts.com