Classification of Pump Seals and Analysis of the Cause of Sealing Failure

Devices or measures that prevent the working medium from leaking out of the pump or prevent foreign matter or air from entering the interior of the pump are called seals and the medium to be sealed is usually liquid, gas or dust.

Leakage caused by the main There are two aspects: First, there is a gap on the sealing surface. Second, the pressure difference exists on both sides of the seal site, eliminate or reduce any one factor can prevent or reduce leakage, to seal the purpose. Pump design pressure and the use of pressure is an objective existence can not be reduced, so the pump seal solution to eliminate or reduce the gap between the sealing surface. This gap includes the clearance between the sealing surfaces and the gap inside the sealing device.

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

Gasket seal

Gaskets are the basic components of centrifugal seals, the use of a very wide range. Gasket selection mainly based on pump delivery medium, temperature, pressure and corrosion and other factors. When the temperature and pressure is not high, generally use non-metallic gaskets; medium pressure and high temperature, the choice of non-metallic and metal combination gasket. Non-metallic gaskets are most commonly used in pumps, and are typically paper, rubber, and polytetrafluoroethylene. When the temperature does not exceed 120 ℃, the pressure below 1.0Mpa, the general choice of green shell paper or molded paper gasket. If the transmission medium is oil, the temperature is -30 ~ 110 ℃, the general selection of good aging resistance of nitrile rubber. When the transmission medium at -50 ~ 200 ℃, the choice of fluorine rubber is more appropriate. Because it is in addition to oil heat, the mechanical strength is also its main feature. In the chemical pump, due to the corrosive media, so the general choice of PTFE as a gasket material. As the pump is used more and more widely in the field, the types of media transported more and more, so the choice of gasket material should consult the relevant information or through the experiment before making the right choice.

Gasket failure mainly due to the following situations:

1, the role of pressure on the gasket is not enough. Due to the microscopic unevenness always present on the sealing surface, sometimes a number of annular grooves are machined in the sealing surface. If the sealing is to be ensured, sufficient pressure must be applied to the gasket so that it is elastically or plastically deformed Fill these gaps. Compression force of various gasket material is usually the size of the gasket manufacturer samples or product specifications are given, but also by experiment. Due to the assembly of the gasket can not reach the required pressure or due to the vibration during the operation of the compression bolts to loosen the compression force is reduced as well as the gasket material aging deformation loss of the original elasticity will make pad Leakage failure.

2, gasket internal structure or thickness of non-uniform, and the use of a crack or wrinkle of the cardboard, the gasket itself formed a gap, when the gasket on the force of the spacer to produce the elastic deformation is not sufficient Leakage is inevitable when filling these gaps.

3, the material of the gasket does not match with the transported medium. Due to the chemical nature of the chemical products transported by the pump and the change in certain properties of the fuel resulting from the addition of a small amount of additive to the fuel to increase the value of the fuel or to alter its combustion products, It is not easy to select the material of the gasket that is suitable for the transportation of the medium. Therefore, the gasket is often eroded due to the incompatibility and the leakage occurs.

O-ring seal

Pump is commonly used in rubber O-ring. Because of its simple shape, it is easy to manufacture and has low cost. It has the advantages of light weight, less consumables, easy to use, easy installation and disassembly, no matter how big the O-ring is. , The more prominent advantage is that O-ring has a good sealing ability, the use of a wide range. Static sealing working pressure up to 100MPa above, dynamic sealing up to 30Mpa. Applicable temperature is -60 ~ 200 ℃, can meet the requirements of a variety of media. Therefore, the design of the pump is more and more widely used.

The O-ring is installed between the groove and the surface to be sealed with a certain amount of compression. The resulting rebound force gives the sealed smooth surface and the groove bottom an initial compressive stress. Thus play a sealing role. As the pressure of the liquid being sealed 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 a good seal.

O-ring seal although reliable, but if you do not pay attention to the conditions of use, there will be leakage, usually the following situations:

1, installed O-ring groove size tolerance, especially when the depth is too large, so that the O-ring after the installation of compression deformation is not enough to affect the sealing capacity. Generally O-ring compression deformation after installation should be between 18% ~ 22%, when the cross-sectional size of the compression relative deformation is small, while the smaller cross-sectional compression relative deformation is larger.

2, O-ring nominal size and the actual installation of the size difference is too much, the formation of O-ring in the drawing after the reduction of cross-sectional size under the conditions of work, resulting in insufficient amount of compression deformation and leakage.

3, O-ring in the installation, due to the import of the sealing surface is not smooth chamfering or rounding and O-ring scratch and produce leakage.

4, O-ring material is not suitable for sealing medium and erosion after failure. 5, O-ring used for too long after the aging deterioration, the flexibility to reduce after the failure, so the general overhaul of the equipment O-ring replacement.

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

Thread seal

Threaded seals on the pump generally have two forms, one is a threaded connection gasket seal, one is filled with thread filling economy seal, both for small diameter threaded connections sealing occasions. Threaded Gaskets Sealed seals are shims, and threads only act to provide compression. Sealed effect in addition to the gasket's own performance. The roughness of the sealing surface and the relative geometrical position accuracy of the threaded hole have a great influence on the sealing effect. In the tightening of the gasket by tightening the screw not only under pressure, but also to bear the torque, the gasket deformation or even damage, so when the gasket is non-metallic, generally only suitable for low pressure occasions, if the gasket For metal pressure is up to 30Mpa or more.

Pump often used on the plug, it is another form of thread seal. Due to the economy of thread plugging, thread sealing alone can not be used as a sealing function, and a packing of thread tape, sealant or the like is often used to fill the thread sealing gap. Its bearing capacity depends on the thread manufacturing accuracy and material, and thread plug and threaded hole with the form does not matter. Threaded holes and plugging Whether the use of "cone on the cone" or "column on the cone," the same sealing effect, but the use of different geographical only. At present, our country adopts the same two forms. The common reason that affects the sealing effect is that when the thread bottom hole is processed, the size is too large, the thread profile is shortened, the crest of the thread is widened, the sealing surface is reduced, leakage occurs when the sealing pressure is increased, and even the thread plug is pushed out. As the filler is corroded by the sealing medium and the phenomenon of leakage occurs in the pump in the chemical industry also occur from time to time, so the choice of filler is also one of the sealing considerations.

Packing seal

The compressive and resilient packing into the stuffing box, rely on gland axial compression into radial sealing force, which play a sealing effect. This method of sealing is called packing sealing, which is called packing. As the packing seal structure is simple, easy to replace, low cost, to adapt to speed, pressure, medium wide and in the pump design is widely used. When transporting normal temperature media, the packing seal is generally provided with a packing ring, which is communicated with the high-pressure chamber of the pump or an externally connected liquid medium with a certain pressure to perform functions of cooling, lubricating, sealing or flushing. As the packing seal is a contact seal, there must be friction and wear problems. The size of the friction and wear, mainly depends on the packing gland packing pressure. Large pressure can improve the sealing effect, but it will increase the power consumption and sleeve wear, otherwise it will have a larger leak. Therefore, according to the size of the leakage and the temperature of the leakage medium to adjust the pressure on the gland, if necessary, the filler should be replaced or added. A reasonable leak of packing seal is generally 10-20ml / min. When introducing liquid from the outside world, should ensure that the liquid has good chemical stability, neither to pollute the pump transport medium, and does not react with the media to produce sediment and solid particles, but also with the filler has a good impregnation And lasting retentivity, so that you can play a good and lasting seal effect.

Power seal

Vice impeller plus parking seal pump In operation, the pressure generated by the deputy impeller balance the main impeller outlet high pressure liquid, in order to achieve the seal. When parking, the auxiliary impeller does not work, so it must be equipped with a parking seal at the same time to solve the possible leakage during parking.

Vice impeller seal structure is simple, reliable sealing, long service life, operation can be drip leaks, so the pump containing impurities in the medium often used. A good impeller sealing effect is conditional, that is, the working pressure must not exceed the allowable working pressure. Once exceeded, there will be a serious leak. The main reason for the change of working pressure is the pressure change of the suction inlet of the pump. Therefore, the pump adopting the aero-impeller seal must strictly regulate the pressure of the pump inlet or the seal will fail. Due to the many forms of parking seals, failure is also easy to find, this article will not repeat them.

Spiral seal

Spiral seal is also a form of power seal, it is in the rotation of the shaft or in the sleeve on the sleeve made of spiral groove, filled with a sealing medium between the shaft and the sleeve. Rotation of the shaft causes the spiral groove to function like a pump, preventing leakage of the sealing liquid. The sealing capacity of the size and helix angle, pitch, tooth width, tooth height, the role of tooth length and the gap between the shaft and the sleeve size. As the seal does not occur between the friction, and thus long life, but due to structural constraints, the spiral length is generally shorter, so its sealing ability is also limited. The pump slowdown in use, the sealing effect will be greatly reduced.

Labyrinth seal

In a well-designed, well-processed, well-assembled, high speed, the labyrinth seal works well. However, in actual application, there are many leaks, so the main reasons for the analysis are as follows:

1, seal pair (such as shaft and bearing gland) with the gap is too large, and this gap is inversely proportional with the sealing effect. Rough with the surface, the obvious helical tool marks in some cases also increased the leakage trend.

2, bearing indoor oil injection too much, the overflow pressure exceeds the sealing resistance.

3, oil windows or oil level meter installed in the wrong location, misleading people on the oil indoor lubricant the correct judgment.

4, the operation of the oil temperature increases its viscosity decreases, increasing the possibility of leakage.

5, back to the tank or back to the oil hole size is too small, or by other obstacles, so that the liquid can not be blocked back to return and cause leakage.

Oil seal

The oil seal is a self-tightening lip seal which has the advantages of simple structure, small size, low cost, convenient maintenance and small resistance torque, which can prevent media leakage and prevent external dust and other harmful substances from entering, A certain degree of compensation, but impatience high pressure, it is generally used in low pressure occasions. Oil seal should be installed in the manufacture of precision h8 ~ h9, the surface roughness of 1.6 ~ 0.8μm and surface hardened shaft. Sealing media should not contain solid particles and impurities, otherwise it will cause rapid wear of oil seals and shaft seal failure.

Machinery Seal

Mechanical seals are the primary method of modern pump shaft seals. Although it is not easy to achieve full leakage without using them, it is entirely possible to achieve a small, perfectly acceptable amount of leakage. But in the pump operation often appear embarrassing situation, then the mechanical seal failure is what is it?

1, mechanical seal material selection is not appropriate. The mechanical seal does not match the material being delivered. At work, the sealing element quickly corrodes, dissolves or abrades, losing its sealing ability. Therefore, the choice of 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 status of the mechanical seal flushing does not meet the design requirements. In the transport of easily crystallized or small particles of media, there must be a certain pressure and a certain flow of flushing fluid, or crystals or particles will speed up the seal pair wear and tear, and affect the sealing pair wear automatically compensate for leakage. Therefore, according to the nature of the transport medium not only to configure the corresponding flushing pipeline, but also to install the monitoring and regulation functions of the instruments and devices to ensure that the pressure and flow of flushing fluid to meet the design requirements in order to maintain the normal operation of the seal, which is often Neglected by users.

3, each mechanical seal can withstand the pressure is limited, due to the pressure inside the sealed chamber is not accurate, resulting in sealed chamber pressure exceeds the mechanical seal can withstand the pressure generated leaks, but also a common seal One of the reasons for failure.

4, the working temperature of the mechanical seal can not exceed its specified value. In the design of the cooling pipe, the cooling effect is usually reduced due to insufficient flow of the cooling medium. In the design without the cooling pipe, the mechanical seal is often in a dry friction state in the sealed cavity due to the stored air. In either case, the sealing of the seal will fail due to the operating temperature of the sealing seal of the mechanical seal being too high to accelerate wear.

5, in the use of a single spring mechanical seal, ignoring the rotation of the spring and pump rotor rotation of the right combination also occurred. Or design did not specify, or assembly of negligence, not because of the rotation of the rotor and the mechanical seal to increase the spring force but the opposite, resulting in moving ring and static ring friction pressure is insufficient to form a leak.

6, the bearing serious wear or damage, so that the shaft has a serious axial movement, but also one of the causes of mechanical seal leakage. With the development of science and technology, new sealing forms and sealing materials appear one after another, which inevitably has a direct impact on the pump sealing technology and promotion. Long-life, zero-leakage pump products will be more and more occasions to be promoted and applied.