A complete solution to the failure causes of various sealing methods if pump happen leakage

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

 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. 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 conveying 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 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.

Oil Seal

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 seal

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 through experiments.

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. Because 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 the loss of original elasticity due to the aging and deformation of the gasket material, the gasket will be damaged. Leakage due to chip failure.

O-ring Seals

Rubber O-rings are commonly used in pumps. Because of its very simple shape, it is easy to manufacture and low in cost. No matter how big the overall size of the O-ring is, its cross-sectional size is very small (only a few millimeters), so it is light in weight, consumes less materials, is simple in use, and is easy to install and disassemble. , The more prominent advantage is that the O-ring has a good sealing ability and a wide range of use. The working pressure of the static seal can reach more than 100MPa, and the dynamic seal can also reach 30Mpa. The applicable temperature is -60 ~ 200 ℃, which can meet the requirements of various media. Therefore, it is more and more widely used in the design of pumps.

The O-ring is installed between the groove and the sealed surface with a certain amount of compression, and the resulting rebound force gives the sealed smooth surface and the bottom surface of the groove an initial compressive stress. So as to play a sealing role. When the pressure of the sealed liquid increases, the deformation of the O-ring also increases, so that the pressure transmitted to the sealing surface also increases, and the sealing effect also increases. This is why O-rings have good sealing ability.

Although the O-ring seal is reliable, if you do not pay attention to the conditions of use, leakage will occur, usually in the following situations:

1.The size of the groove where the O-ring is installed is out of tolerance, especially when the depth is too large, the compression deformation of the O-ring after installation is insufficient and the sealing ability is affected. Generally, the compression deformation of the O-ring should be between 18% and 22% after installation.

2.The nominal size of the O-ring seal is too different from the actual installation size, and the O-ring is formed to work under the condition that the cross-sectional size is reduced after stretching, resulting in insufficient compression deformation and leakage.

3.When the O-ring is installed, the O-ring is scratched and leaks due to the lack of smooth chamfering or rounding at the inlet of the sealing surface.

4.The material of the O-ring seal is not suitable for the sealing medium and will fail after being eroded.

5.The O-ring will be aged and deteriorated after being used for too long, and the elasticity will be reduced and then it will fail. Therefore, the O-ring is generally replaced when the equipment is overhauled.

In addition, the hardness of the O-ring, the roughness of the groove and the sealing surface also affect the sealing effect of the O-ring.

Gland Packing Seal

The compressible and resilient packing is put into the stuffing box, and the axial compression force of the gland is converted into a radial sealing force, thereby playing a sealing role. This sealing method is called packing sealing, and this packing is called sealing packing. The packing seal is widely used in pump design due to its simple structure, easy replacement, low price, and wide adaptability to rotational speed, pressure and medium. When conveying medium at room temperature, the packing seal is generally equipped with a packing ring, which is either communicated with the high-pressure chamber of the pump, or externally connected to a liquid medium with a certain pressure, which can play the role of cooling, lubricating, sealing or flushing.

Since the packing seal is a contact seal, there are bound to be friction and wear issues. The size of friction and wear mainly depends on the pressing force of the packing gland. High pressure can improve the sealing effect, but it will increase the power consumption and the wear of the shaft sleeve, otherwise it will cause a large leakage. Therefore, the pressing force of the gland should be adjusted according to the amount of leakage and the temperature of the leaking medium, and the packing should be replaced or supplemented if necessary. The reasonable leakage of packing seal is generally 10-20ml/min. When the liquid is introduced from the outside, it should be ensured that the liquid has good chemical stability, neither pollutes the medium conveyed by the pump, nor reacts with the medium to produce precipitates and solid particles, and should also have good impregnation with the filler. And long-lasting retention, so it can have a good and long-lasting sealing effect.

Spiral seal

Spiral seal is also a form of dynamic seal, which is to machine a spiral groove on the rotating shaft or on the containing sleeve of the shaft, and the sealing medium is filled between the shaft and the sleeve. The rotation of the shaft causes the helical grooves to act like a pump to prevent leakage of the sealing fluid.

The size of its sealing ability is related to the helix angle, pitch, tooth width, tooth height, action length of the tooth and the size of the gap between the shaft and the sleeve. Since there is no friction between the seals, the life is long, but due to the limitation of the structure space, the length of the helix is generally short, so the sealing ability is also limited. When the pump is used at a reduced speed, its sealing effect will be greatly reduced.

Labyrinth seal

 When the design is reasonable, the processing is excellent, the assembly is complete, and the rotation speed is high, the labyrinth sealing effect is very good. However, in practical applications, there are many leaks caused by this. The main reasons for analysis are as follows:

1.The matching clearance of the sealing pair (such as the shaft and the bearing gland) is too large, and this clearance is inversely proportional to the sealing effect. The mating surface is rough, and the obvious helical turning tool marks in some cases also increase the leakage tendency.

2.The amount of lubricating oil injected into the bearing chamber is too much, and the overflow pressure exceeds the sealing resistance.

3.The installation position of the oil window or oil level gauge is wrong, which misleads people to correctly judge the amount of lubricating oil in the oil chamber.

4.The increase of oil temperature during operation reduces its viscosity and increases the possibility of leakage.

5.The size of the oil return tank or oil return hole is too small, or other obstacles prevent the blocked liquid from returning smoothly and cause leakage.

Dynamic Seal

When the auxiliary impeller plus the stop seal pump is running, the pressure head generated by the auxiliary impeller balances the high-pressure liquid at the outlet of the main impeller, thereby achieving sealing. When parking, the auxiliary impeller does not work, so it must be equipped with a parking seal at the same time to solve the leakage that may occur during parking.

The auxiliary impeller has a simple sealing structure, reliable sealing, long service life, and can achieve no leakage during operation, so it is often used in pumps that transport impurity-containing media. The good sealing effect of the auxiliary impeller is conditional, that is, the working pressure is not allowed to exceed the allowable working pressure. Once exceeded, serious leakage will occur. The main reason for the change of working pressure is the pressure change at the suction port of the pump. Therefore, the pump using the auxiliary impeller seal must make strict regulations on the inlet pressure of the pump, otherwise the seal will fail.

Thread Seals

There are generally two types of thread seals on the pump, one is the threaded connection gasket seal, the other is the thread filling seal, both of which are used for the sealing occasions of small diameter threaded connections. The seal of the threaded joint gasket seal is the gasket, and the thread only serves to provide the pressing force. The effect of sealing is in addition to the performance of the gasket itself. The roughness of the sealing surface and the relative geometrical position accuracy of the threaded hole also have a great influence on the sealing effect. Because the gasket not only bears the compression force but also bears the torque when tightening the thread, so that the gasket is deformed or even damaged. Therefore, when the gasket is non-metallic, it is generally only suitable for occasions where the pressure is not high. For metal, the applicable pressure can reach more than 30Mpa.

Plugs, often used on pumps, are another form of threaded seal. Due to the economy of the manufacture of screw plugs, generally the cooperation of the threads alone cannot play a sealing role, and the sealing gaps of the threads are often filled with fillers such as raw tape and sealant. Its pressure bearing capacity depends on the manufacturing accuracy and material of the thread, and has nothing to do with the matching form of the screw plug and the threaded hole. Whether the threaded hole and the plug are “cone-to-cone” or “column-to-cone”, the sealing effect is the same, but the use area is different. At present, the two forms are equally used in my country. The common reason that affects the sealing effect is that the size of the thread bottom hole is too large when processing the thread bottom hole, which shortens the thread shape and widens the tooth crest, which reduces the sealing surface. The phenomenon of leakage due to the corrosion of the packing by the sealing medium also occurs in pumps used in the chemical industry from time to time. Therefore, the choice of packing is also one of the factors to be considered in sealing.