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Analysis of the classification of the pump seal and the cause of its seal failure (Hebei Innovation Sealing Material Co., Ltd.)
There are two main types of pump sealing devices: one is static sealing and the other is dynamic sealing. Static seals are usually available in gasket seals, O-ring seals, and thread seals. Dynamic seals mainly include soft packing seals, oil seal seals, labyrinth seals, spiral seals, dynamic seals and mechanical seals.
Gasket seal (metal gasket, non-metallic gasket, stainless steel gasket)
The gasket is the basic component of the centrifugal pump static seal and is used in a wide range of applications. The selection of the gasket is mainly determined by factors such as the medium, temperature, pressure and corrosivity of the pump. Non-metallic sealing gaskets are generally used when the temperature and pressure are not high; non-metallic and metal composite gaskets are used when the medium-pressure high temperature is used. Non-metallic gaskets are most commonly used on pumps, and are typically made of paper, rubber and Teflon. When the temperature does not exceed 120 ° C, the pressure is below 1.0 Mpa, generally choose green paper or molded paper gasket. If the conveying medium is oil, the temperature is between -30 and 110 ° C, and the nitrile rubber with better aging resistance is generally used. When the medium is conveyed at -50 to 200 ° C, fluororubber is more suitable. Because it is resistant to oil and heat, its mechanical strength is also its main feature. In the chemical pump, because the medium to be transported is corrosive, PTFE is generally used as the gasket material. As the field of pump use is more and more extensive, and the types of media to be transported are more and more, when selecting the gasket material, the relevant materials should be consulted or the correct selection should be made after the experiment. (Metal Wound Gasket, Basic Metal Wound Gasket, Metal Wound Gasket with Inner Ring, Metal Wound Gasket with Outer Ring)
There are several reasons for the failure of the gasket:
1. The pressure acting on the gasket is insufficient. Since there are always microscopic irregularities on the sealing surface, a number of annular grooves are sometimes processed on the sealing surface. If the sealing is ensured, it is necessary to apply sufficient pressure to the sealing gasket to cause elastic or plastic deformation. Fill these gaps. The pressing force of various gasket materials is usually given in the gasket manufacturer's sample or product manual, and can also be determined experimentally. Since the pressing force required for the gasket is not achieved during assembly or the compression bolt is loosened due to the vibration during the normal operation, the pressing force is lowered and the original elasticity is lost due to the aging deformation of the gasket material. The film fails and a leak occurs.
2, the internal material or thickness of the gasket material is uneven, and the use of cardboard with cracks or wrinkles, so that the gasket itself forms a gap, when the force acting on the gasket makes the elastic deformation of the gasket is not enough When filling these gaps, the leak is inevitable.
3. The material of the gasket is not compatible with the medium to be transported. Due to the variety of chemical properties of the chemical products transported by the pump, and the addition of some small amounts of additives to the fuel to increase the combustion value of the fuel or to change its combustion products, certain properties of the fuel are changed. It is not easy to select a gasket material suitable for the medium to be transported. Therefore, it is often the case that the gasket is corroded and leaks due to incompatibility.
O-ring seal (stainless steel metal wound gasket ladle)
A rubber O-ring is commonly used in pumps. Because of its simple shape, it is easy to manufacture and low in cost. No matter the overall size of the O-ring, its cross-sectional size is very small (only a few millimeters), so it is light in weight, consumes less material, is easy to 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 static sealing working pressure can reach more than 100MPa, and the dynamic sealing can reach 30Mpa. Applicable temperature is -60～200°C, 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 mounted between the groove and the surface to be sealed with a certain amount of compression, and the resulting repulsive force imparts initial compressive stress to the smooth surface of the seal and the bottom surface of the groove. Thereby functioning as a seal. As 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 effect of the seal also increases. This is why the O-ring seal has a good sealing ability.
O-ring seals are reliable, but leaks can occur if you do not pay attention to the conditions of use. There are usually the following situations:
1. The groove size of the O-ring is out of tolerance, especially when the depth dimension is too large, so that the amount of compression deformation after the O-ring is installed is insufficient to affect the sealing ability. Generally, the compression deformation amount of the O-ring after installation should be between 18% and 22%. When the cross-sectional size is large, the compression relative deformation is small, and when the cross-sectional size is small, the compression relative deformation is large.
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 seal is installed, the O-ring is scratched due to the smooth chamfering or rounding of the inlet of the sealing surface.
4. The material of the O-ring is not suitable for the sealing medium and is eroded and then fails. 5. O-ring seals will deteriorate after aging for a long time, and the elasticity will decrease and then fail. Therefore, the O-rings are 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.
Thread seals Thread seals are generally available in two types on the pump. One is a threaded joint gasket seal, and the other is a threaded and filled seal. Both are used for sealing small diameter threaded joints. The seal of the threaded joint gasket is a gasket, and the thread only serves to provide a pressing force. The effect of the seal is in addition to the performance of the gasket itself. The roughness of the sealing surface and the relative geometrical position accuracy with the threaded holes have a great influence on the sealing effect. When the sealing gasket is used to tighten the thread, it not only bears the pressing force, but also withstands the torque, which causes the gasket to be deformed or even damaged. Therefore, when the gasket is non-metallic, it is generally only suitable for occasions where the pressure is not high, if the gasket For metals, the pressure can be up to 30Mpa. (Fluorine PTFE gasket expanded PTFE gasket soft PTFE gasket modified PTFE gasket polyethylene PTFE gasket)
The wire plug that is often used on pumps is another form of thread seal. Due to the economical manufacture of the wire plug, the threading alone cannot function as a sealing, and the sealing gap of the thread is often filled with a filler such as a raw tape or a sealant. Its pressure bearing capacity depends on the manufacturing precision and material of the thread, and has nothing to do with the form of the plug and the threaded hole. Threaded holes and plugs have the same sealing effect regardless of whether they are "cone-to-cone" or "column-to-cone", but only in different regions. China currently uses the two forms equally. A common cause of the effect of the sealing effect is that the size of the threaded bottom hole is too large, the thread shape is shortened, the crown is widened, and the sealing surface is reduced, and when the sealing pressure is increased, leakage occurs, and even the wire plug is pressed out. The phenomenon of leakage due to corrosion of the filler by the sealing medium also occurs in pumps for the chemical industry, so the choice of filler is also one of the factors considered for sealing.
Packing seal
The compressive and resilient filler is placed in the stuffing box and converted into a radial sealing force by the axial pressing force of the gland to seal. This method of sealing is called a packing seal and this packing is called a packing. Because the packing sealing structure is simple in form, convenient to replace, low in price, suitable for rotation speed, pressure, and wide medium, it is widely used in pump design. When conveying a medium temperature medium, the packing seal is generally provided with a packing ring, which is connected to the high pressure chamber of the pump or externally connected with a liquid medium having a certain pressure, which can be cooled, lubricated, sealed or flushed. Since the packing seal is a contact seal, there is bound to be friction and wear problems. The amount of friction and wear is mainly determined by the pressing force of the packing gland. The high pressure can improve the sealing effect, but it will increase the power consumption and wear of the bushing, and vice versa. 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 replenished if necessary. A reasonable leak of the packing seal is typically 10-20 ml/min. When introducing liquid from the outside, it should be ensured that the liquid has good chemical stability, neither contaminating the medium transported by the pump, nor reacting with the medium to produce precipitates and solid particles, and also having good impregnation with the filler. And long-lasting retention, which provides a good and long-lasting seal.
Dynamic seal
When the auxiliary impeller and the parking seal pump are in operation, the indenter generated by the auxiliary impeller balances the high pressure liquid at the main impeller outlet to achieve sealing. When parking, the auxiliary impeller does not work, so it is necessary to have a parking seal at the same time to solve the leakage that may occur when parking. (Asbestos gasket high pressure asbestos gasket medium pressure asbestos gasket low pressure asbestos gasket)
The auxiliary impeller has a simple sealing structure, reliable sealing, long service life, and can realize dripping without leakage during operation. Therefore, it is often used in pumps for conveying impurity-containing media. The good sealing effect of the impeller is conditional, ie the working pressure must not exceed the permissible working pressure. Once exceeded, a serious leak will occur. The main reason for the change of working pressure is the pressure change of the suction port of the pump. Therefore, the pump sealed by the auxiliary impeller must make strict regulations on the inlet pressure of the pump, otherwise the seal will fail. Because of the many forms of parking seals, they are easy to find after failure, and will not be described in this article.
Spiral seal
A spiral seal is also a form of dynamic seal that produces a helical groove on a rotating shaft or on a containment sleeve of the shaft with a sealing medium between the shaft and the sleeve. The rotation of the shaft causes the spiral groove to produce a pump-like action that prevents leakage of the sealant. The sealing capacity is related to the helix angle, the pitch, the tooth width, the tooth height, the length of action of the teeth, and the size of the gap between the shaft and the sleeve. Since the friction does not occur between the seals, the life is long, but due to the limitation of the construction space, the spiral length is generally short, and thus the sealing ability is also limited. When the pump is used for speed reduction, the sealing effect will be greatly reduced.
Labyrinth seal
The maze sealing effect is very good when the design is reasonable, the processing is excellent, the assembly is good, and the rotation speed is high. However, in practical applications, there are many leaks, and the main reasons for the analysis are:
1. The sealing pair (such as the shaft and the bearing gland) has too much clearance, and this gap is inversely proportional to the sealing effect. The mating surface is rough, and the obvious spiral turning tool marks also increase the leakage trend in some cases. (graphite pad metal graphite winding pad graphite composite pad graphite reinforced gasket)
2. The amount of lubricant 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 incorrect, which misleads people to correctly judge the amount of lubricating oil in the oil chamber.
4. The increase in oil temperature during operation reduces the viscosity and increases the possibility of leakage.
5. The size of the oil return groove or oil return hole is too small, or other obstacles are caused, so that the blocked liquid cannot return smoothly and cause leakage.
Oil seal
The oil seal is a self-tightening lip seal. It has a simple structure, small size, low cost, convenient maintenance and low resistance torque. It can prevent medium leakage, prevent external dust and other harmful substances from invading, and wear. It has a certain compensation capacity, but it is not resistant to high pressure, so it is generally used in low pressure applications. The oil seal should be mounted on a shaft with a manufacturing accuracy of h8 to h9 and a surface roughness of 1.6 to 0.8 μm and surface hardened. The sealing medium should not contain solid particles and impurities, which would cause rapid wear of the oil seal and shaft and void the seal.
Machinery Seal
Mechanical seals are the primary method of modern pump shaft seals, although it is not easy to use them to achieve complete leakage, but it is entirely possible to achieve a small, fully acceptable leak. However, in the operation of the pump, there are often embarrassing situations, so what is the cause of the mechanical seal failure?
1. The material selection of the mechanical seal is not suitable. The material of the mechanical seal does not match the medium being conveyed. At work, the sealing element is quickly corroded, dissolved or worn, thus losing its sealing ability. Therefore, the choice of the material of the mechanical seal according to the nature of the transport medium is a prerequisite for ensuring its sealing function and normal life. (Purchant gasket annealing copper gasket quenching copper gasket)
2. The flushing condition of the mechanical seal does not meet the design requirements. When transporting a medium that is easy to crystallize or has fine particles, it must be flushed with a certain pressure and a certain flow of flushing liquid. Otherwise, the crystal or particles will accelerate the wear of the sealing pair and affect the automatic compensation of the sealing pair after wear and leakage. Therefore, according to the nature of the transport medium, it is necessary to configure not only the corresponding flushing pipeline, but also the instrument and device with monitoring and regulating functions to ensure that the pressure and flow of the flushing fluid meet the design requirements, in order to maintain the normal operation of the machine seal, which is often Neglected by the user.
3. The pressure that each mechanical seal can withstand is limited. Because the pressure in the sealed chamber is not accurate, the pressure in the sealed chamber exceeds the pressure that the mechanical seal can withstand and leaks. 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 of the cooling pipeline, the cooling effect is often reduced due to insufficient flow of the cooling medium; in the design without the cooling pipeline, the mechanical seal is often in a dry friction state due to the air trapped in the sealed chamber. In both cases, the mechanical seal's moving seal pair operates too hot and accelerates wear, resulting in seal failure. (Rubber pad non-asbestos rubber pad silicone rubber pad natural rubber pad nitrile rubber pad)
5. When using a single spring mechanical seal, the correct combination of the direction of rotation of the spring and the direction of rotation of the pump rotor also occurs. Or the design is not stated, or the negligence during assembly, the spring force of the mechanical seal is increased due to the rotation of the rotor, but the opposite is caused, and as a result, the pressure of the moving ring and the static ring friction pair is insufficient to form a leak.
6. The serious wear or damage of the bearing causes the pump shaft to produce severe axial turbulence, which is also one of the causes of mechanical seal leakage. With the development of scientific technology, new sealing forms and sealing materials have emerged, which will inevitably have a direct impact on the pump sealing technology. A long-life, zero-leakage device or measure that prevents the working medium from leaking out of the pump or preventing external impurities or air from entering the pump is called a seal. The sealed medium is generally liquid, gas or dust.
The means or means for preventing the leakage of the working medium from the pump or preventing the intrusion of foreign matter or air into the interior of the pump is called a seal, and the medium to be sealed is generally liquid, gas or dust.
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 the use pressure of the pump are objectively and cannot be reduced, so the sealing of the pump solves the problem of eliminating or reducing the gap between the sealing faces. Such gaps include the gap between the sealing faces and the gaps within the body of the seal. (octagonal pad carbon steel octagonal pad stainless steel octagonal pad)
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