Cause analysis of seal failure of the hottest pump

Cause analysis of pump seal failure

the device or measure to prevent the working medium from leaking out of the pump or prevent external impurities or air from invading into the pump is called sealing, and the sealed medium is generally liquid, gas or dust

there are two main reasons for 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 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 sealing of the pump is to eliminate or reduce the gap between the sealing surfaces. This gap includes the gap between the sealing surfaces and the gap inside the sealing device

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

gasket seal

gasket is the basic element of centrifugal pump static seal, which is widely used. The type selection of gaskets is mainly determined by factors such as pump delivery medium, temperature, pressure and corrosivity. When the temperature and pressure are not high, non-metallic sealing gaskets are generally used; At medium pressure and high temperature, non-metallic and metal composite gaskets shall be selected. Non metallic gaskets are most commonly used in pumps, and their materials are generally paper, rubber and polytetrafluoroethylene. When the temperature does not exceed 120 ℃ and the pressure is below 1.0MPa, green paper or molded paper gaskets are generally selected. If the transmission medium is oil and the temperature is -30 ~ 110 ℃, NBR with good aging resistance is generally selected. When the transmission medium is -50 ~ 200 ℃, fluororubber is more suitable. Because in addition to oil resistance and heat resistance, it is also characterized by high mechanical strength. In chemical pumps, due to the corrosive medium conveyed, polytetrafluoroethylene is generally used as gasket material. As the pump is used in more and more fields, there are more and more types of media conveyed. Therefore, when selecting gasket materials, you should consult relevant data or make correct choices after passing experiments

the main reasons for gasket failure are the following:

1. Insufficient pressure acting on the sealing gasket. Since there are always micro irregularities on the sealing surface, sometimes several annular grooves are machined on the sealing surface. If sealing is guaranteed, enough pressure must be applied to the sealing gasket to make it elastic or plastic deformation to fill these gaps. The compression force of various gasket materials is usually given in the samples or product instructions of the seal gasket manufacturer since the 1990s, and can also be determined through experiments. If the pressing force required by the gasket cannot be reached during assembly, or the pressing force is reduced due to the loosening of the pressing bolt due to the vibration during normal operation, and the original elasticity is lost due to the aging and deformation of the gasket material, the gasket will fail and leak

2. The uneven internal structure or thickness of the gasket material and the use of cardboard with cracks or wrinkles make the gasket itself form a gap. When the force acting on the gasket makes the elastic deformation of the gasket insufficient to completely fill these gaps, leakage is inevitable

3. The material of the gasket is not suitable for the transmitted medium. Due to the diversity of chemical properties of the chemical products delivered by the pump, and the change of some properties of the fuel oil after adding a small amount of additives to the fuel oil in order to improve the combustion value of the fuel oil or change its products after combustion, it is not easy to select the gasket material suitable for the transmission medium, so the gasket erosion and leakage phenomenon often occur due to incompatibility

o-ring seal

rubber O-ring is commonly used in pumps. Because of its simple shape, it is easy to manufacture and low cost. No matter how large the overall size of the O-ring is, its cross-section size is very small (only a few millimeters), so it is light in weight, consumes less materials, has simple use methods, and is convenient for installation and disassembly. The more prominent advantage is that the O-ring has good sealing ability and a wide range of applications. The working pressure of static seal can reach more than 100MPa, and that of 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

o-ring seals are reliable, but if you don't pay attention to the service conditions, leakage will also occur. There are usually the following situations:

1. The size of the groove where the O-ring is installed is out of tolerance, especially when the depth size is too large, so that the compression deformation of the O-ring after installation is insufficient, which affects the sealing ability. Generally, the compression deformation of O-ring after installation should be between 18% and 22%. When the section size is large, the compression relative deformation is small, while when the section size is small, the compression relative deformation is large

2. There is too much difference between the nominal size of the O-ring and the actual installation size, resulting in the O-ring working under the condition of reduced section size after stretching, resulting in insufficient compression deformation and leakage

3. During the installation of the O-ring, the O-ring was scratched due to no smooth chamfer or rounding at the inlet of the sealing surface, resulting in leakage

4. The material of the O-ring is not suitable for the sealing medium and becomes invalid after being eroded. 5. The O-ring is aged and deteriorated after being used for too long, and becomes invalid after its elasticity decreases. Therefore, the O-ring is generally replaced during equipment overhaul

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

thread seal

generally, there are two forms of thread seal on the pump, one is the seal of threaded connection gasket, and the other is the seal of thread plus filling. Both of them are used in the sealing occasions of small-diameter thread connection. The sealing element sealed by the threaded connection gasket is the gasket, and the thread only plays the role of providing compression force. The sealing effect is in addition to the performance of the gasket itself. The roughness of the sealing surface and the relative geometric position accuracy with the threaded hole also have a great impact on the sealing effect. When tightening the thread, the sealing gasket not only bears the compression force, but also bears the torque, causing the gasket to deform or even damage. Therefore, when the gasket is non-metallic, it is generally only suitable for occasions with low pressure. If the gasket is metal, the applicable pressure can reach more than 30MPa

the screw plug often used on the pump is another form of thread sealing. Considering the economy of screw plug manufacturing, generally, the cooperation of threads alone cannot play a sealing role. Raw tape, sealant and other fillers are often used to fill the sealing gap of threads. Its pressure bearing capacity depends on the manufacturing accuracy and material of threads, and has nothing to do with the matching form of plugs and threaded holes. The sealing effect of threaded holes and plugs is the same whether they are "cone to cone" or "column to cone", but they are used in different regions. At present, the two forms are adopted equally in China. The common reason that affects the sealing effect is that the size is too large when processing the thread bottom hole, which makes the thread tooth deformation short and the tooth top wider, resulting in the reduction of the sealing surface. When the sealing pressure increases, leakage occurs, and even the screw plug is pressed out. The leakage phenomenon caused by the corrosion of the filler by the sealing medium also occurs from time to time in the pumps in the chemical industry, so the selection of the filler is also one of the factors considered for sealing

packing seal

put the compressible and resilient packing into the stuffing box, and convert the axial compression force of the gland into radial sealing force, so as to play a sealing role. This sealing method is called packing sealing, and this packing is called sealing packing. Packing seal is widely used in pump design because of its simple structure, convenient replacement, low price, wide adaptability to speed, pressure and medium. When transporting normal temperature media, the packing seal is generally equipped with a packing ring, which is either connected with the high-pressure chamber of the pump or externally connected with a certain pressure of liquid media, which can play the role of cooling, lubrication, sealing or flushing. As the packing seal is a contact seal, there must be friction and wear problems. The size of friction and wear mainly depends on the compression force of the packing gland. High pressure can improve the sealing effect, but it will increase the power consumption and wear of the shaft sleeve, otherwise it will produce large leakage. Therefore, the pressing force of the gland should be adjusted according to the leakage volume 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 ml/min. When introducing liquid from the outside, it should be ensured that this liquid has good chemical stability, which will not pollute the medium delivered by the pump, nor react with the medium to produce sediment and solid particles. It should also have good impregnation and lasting retention with the filler, so as to have a good and lasting sealing effect

dynamic seal

when the auxiliary impeller plus parking seal pump is running, the pressure head generated by the auxiliary impeller balances the high-pressure liquid at the outlet of the main impeller, so as to realize sealing. During shutdown, the auxiliary impeller does not work, so the shutdown sealing device must be equipped at the same time to solve the possible leakage during shutdown

the auxiliary impeller seal has the advantages of simple structure, reliable seal, long service life, and no leakage during operation. Therefore, it is often used in pumps conveying media containing impurities. 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 with auxiliary impeller seal must be attached: the upper clamp 40X40X10mm is used for stretching, and the lifting belt is connected to m8x Φ The inlet pressure of the pump must be strictly regulated for 5 pieces with length ≥ 50mm, otherwise the seal will fail. Due to many forms of parking seals, it is easy to find them after failure, so this article will not repeat it

spiral seal

spiral seal is also a form of power seal. It processes spiral grooves on the rotating shaft or on the bearing sleeve of the shaft, and the sealing medium is filled between the shaft and the sleeve. The rotation of the shaft makes the spiral groove produce a conveying effect similar to that of the pump, so as to prevent the leakage of sealing fluid. Its sealing capacity is related to the screw angle, pitch, tooth width, tooth height, tooth action length and the gap between shaft and sleeve. Because there is no friction between seals, it has a long service life, but due to the limitation of structural space, its spiral length is generally short, so its sealing ability is also limited. When the pump is used at reduced speed, its sealing effect will be greatly reduced

labyrinth seal

when the design is reasonable, the processing is excellent, the assembly is intact, and the speed is high, the labyrinth seal effect is very good. However, in practical application, there are many leaks caused by this. The main reasons are analyzed as follows:

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

2. Bearing chamber