Reasons for the failure of the pump’s static sealing device

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In the sealing devices of pumps, static seals typically include types such as gasket sealing, O-ring sealing, and threaded sealing. (Shared by Hydraulic Engineer Li) The main reasons for leakage are twofold: one is the presence of gaps on the sealing surface. Secondly, there is a pressure difference on both sides of the sealing area; eliminating or reducing either of these factors can prevent or minimize leakage, thereby achieving a sealed condition. The design pressure and operating pressure of a pump are fixed values that cannot be reduced; therefore, the task regarding the pump’s sealing is to eliminate or reduce the gap between the sealing surfaces. Such gaps include the gaps between the sealing surfaces and the gaps within the seal device itself. O-ring sealing Rubber O-rings are commonly used in pumps. Due to its very simple shape, it is easy to manufacture and costs little. Regardless of the overall size of the O-ring, its cross-sectional dimension is small (only a few millimeters), resulting in low weight and reduced material consumption. It is simple to use, and easy to install and remove. An even more notable advantage is that the O-ring possesses excellent sealing capabilities, allowing it to be used in a wide range of applications. The operating pressure of static seals can reach over 100 MPa, while that of dynamic seals can reach 30 MPa. The applicable temperature range is -60 to 200°C, allowing it to meet the requirements of various media. Therefore, it is being used more and more widely in pump design. 　　The O-ring is installed between the groove and the surface to be sealed, with a certain degree of compression; the resulting rebound force applies an initial compressive stress to the smooth surface being sealed and to the bottom of the groove. Thus, it serves a sealing function. As the pressure of the sealed liquid increases, the deformation of the O-ring also increases, which in turn leads to an increase in the pressure applied to the sealing surfaces, thereby enhancing the sealing effect. This is why O-rings have good sealing capabilities. 　　Although O-rings provide reliable sealing, leaks can still occur if the conditions of use are not taken into account. The following situations are common: 1. The dimensions of the groove in which the O-ring is installed are out of spec; especially when the depth is too large, this results in insufficient compression and deformation of the O-ring after installation, thereby affecting its sealing ability. Generally, the compression deformation amount of an O-ring after installation should be between 18% and 22%. When the cross-sectional size is large, the relative compression deformation is smaller, whereas it is greater when the cross-sectional size is small. 　　2. The nominal size of the O-ring differs too much from its actual installation size, resulting in the O-ring operating with a reduced cross-sectional dimension after being stretched; this leads to insufficient compression deformation and thus leakage. 　　3. During installation, the O-ring may be scratched due to the lack of smooth chamfers or rounds on the sealing surface, which leads to leakage. 　　4. The material of the O-ring is not suitable for the sealing medium, and it will fail as a result of erosion. 5. O-ring seals deteriorate over time due to aging, and their elasticity decreases, causing them to fail; therefore, the O-rings are generally replaced during major equipment repairs. 　　In addition, the hardness of the O-ring, as well as the roughness of the grooves and sealing surfaces, also affect the sealing performance of the O-ring. 　　Gasket sealing Gaskets are the basic components for static sealing in centrifugal pumps, and they are used on a very wide range of applications. The selection of gaskets is primarily determined by factors such as the medium transported by the pump, temperature, pressure, and corrosivity. When the temperature and pressure are not high, non-metallic gaskets are generally used; at medium pressure and high temperatures, combined gaskets of non-metallic and metallic materials are employed. Non-metallic gaskets are the most commonly used in pumps, with materials typically including paper, rubber, and polytetrafluoroethylene. When the temperature does not exceed 120°C and the pressure is below 1.0 Mpa, green-shell paper or molded paper gaskets are generally used. If the conveying medium is oil and the temperature ranges from -30 to 110°C, nitrile rubber with good aging resistance is generally chosen. When the conveying medium is at -50 to 200°C, fluororubber is a more suitable choice. Because in addition to its oil and heat resistance, high mechanical strength is also one of its main characteristics. 　　In chemical pumps, polytetrafluoroethylene is generally used as the gasket material due to the corrosive nature of the media being transported. As pumps are used in an increasingly wide range of applications and more diverse types of media are transported, it is necessary to consult relevant information or conduct experiments before making the right choice regarding the material for gaskets. 　　The main reasons for gasket failure are as follows: 1. Uneven internal structure or thickness of the gasket material, as well as the use of cardboard with cracks or folds, can create gaps within the gasket itself. When the force applied to the gasket is not sufficient to cause an elastic deformation that fills these gaps completely, leakage becomes inevitable. 　　2. The material of the gasket is not suitable for the medium being transported. Due to the diverse chemical properties of the chemical products pumped by pumps, and the changes in certain properties of fuel resulting from the addition of small amounts of additives to improve its combustion value or alter its combustion products, it is not easy to select a gasket material that is suitable for the medium in question. As a result, leaks often occur due to erosion of the gaskets caused by incompatibility. 　　3. The pressure acting on the sealing gasket is insufficient. Since there are always microscopic irregularities on the sealing surface, and sometimes several annular grooves are also created on it, to ensure a good seal, a sufficient pressure must be applied to the gasket so that it undergoes elastic or plastic deformation in order to fill these gaps. The compressive force for various gasket materials is usually specified in the samples or product manuals provided by the gasket manufacturers, or it can be determined through experiments. The gasket can fail and cause leakage due to the failure to achieve the required compressing force on the gasket during assembly, a decrease in this compressing force as a result of vibration during normal operation that loosens the compressing bolts, or the loss of its original elasticity caused by aging and deformation of the gasket material. 　　Thread sealing 　 There are generally two forms of thread sealing used on pumps: one is sealing with thread connection gaskets, and the other is sealing using threads along with a filling material. Both methods are employed for sealing small-diameter threaded connections. In a threaded connection, the sealing element is the gasket; threads serve only to apply compressive force. The sealing effect depends not only on the performance of the gasket itself. The roughness of the sealing surface, as well as the precision of its relative geometric position with respect to the threaded hole, also have a significant impact on the sealing performance. When tightening the threads, the gasket is subjected not only to compressive forces but also to torque, which can cause the gasket to deform or even get damaged. Therefore, when the gasket is non-metallic, it is generally suitable only for applications with low pressures; whereas if the gasket is metallic, it can handle pressures of up to 30 Mpa or more. 　　The thread plugs commonly used on pumps represent another form of threaded sealing. Considering the economic aspects of plug manufacturing, relying solely on the thread fit generally is not sufficient for sealing; instead, fillers such as raw tape or sealant are often used to fill the sealing gaps in the threads. Its load-bearing capacity depends on the manufacturing precision and material of the thread, and has nothing to do with the fit between the plug and the threaded hole. Whether using a \"cone-to-cone\" or \"cylinder-to-cone\" design for threaded holes and plug valves, the sealing effect is the same; the difference lies only in the regions where they are used. Both forms are currently used equally in our country. A common cause affecting the sealing performance is an oversized diameter when machining the thread bottom hole, which shortens the thread profile and widens the thread crest, resulting in a reduced sealing surface. When the sealing pressure increases, leakage occurs, and in some cases the plug may even be pushed out. Leakages caused by the corrosion of the packing by the sealing medium also occur from time to time in pumps used in the chemical industry; therefore, the choice of packing is also one of the factors to consider for sealing.