How to deal with seal leakage in the ethylene pump in the tank area

I really don’t know how to handle this; it seems there’s no backup plan in the design either

I wonder if a combustible gas alarm is installed around the pump? On-site management is very important, and attention should be paid to strengthening inspections.

Reply to 3# WSL01218: Everyone knows that combustible gas detectors are concerned – a sudden release of ethylene can form a mist (vapor), which can lead to an explosion in a short time.

Hehe, it’s not that exaggerated. It doesn’t explode that easily. You just need to switch to another pump and then repair or replace it – that’s all

Reasons for failure and countermeasures of mechanical seals for pump shafts. A mechanical seal is a device that achieves sealing by having two sealing elements in contact with each other on their smooth and straight surfaces perpendicular to the axis, while rotating relative to one another. As shown in Figure 1. It usually consists of components such as a stationary ring, a rotating ring, a spring loading device (including a push ring, springs, spring seats, fixing screws, and drive pins), as well as auxiliary sealing rings (rotating ring sealing ring and stationary ring sealing ring). The anti-rotation pin is fixed to the gland to prevent the stationary ring from rotating. Due to its advantages such as reliable sealing performance, low leakage, long service life, low power consumption, and the need for minimal maintenance, as well as its ability to meet the sealing requirements in environments involving production automation, high/low temperatures, high pressures, high vacuums, high speeds, as well as various flammable, explosive, corrosive, and aggressive media, mechanical seals have seen faster development and adoption compared to other types of seals, increasingly replacing packing seals and other sealing methods. The following are an introduction to the early failure phenomena, causes, and remedial measures for common mechanical seals in our factory: 1. Causes of early wear of the new sealing surface of mechanical seals and solutions. 1.1 Whether the material selection for the rotating ring and the stationary ring of the mechanical seal is appropriate – The friction pair formed by the rotating ring and the stationary ring is the most important component of a mechanical seal. Both the lifespan of the seal and its performance quality (leakage index) are directly related to it. The materials for the friction pairs of mechanical seals must be selected based on factors such as the properties of the fluid handled by the pump, operating pressure, temperature, and rotational speed. This is especially true when converting packing seals into mechanical seals in existing products or upgrading existing mechanical seals; it is very important to choose the appropriate materials for mechanical seals in such cases. When a mechanical seal is in a corrosive medium, it is subjected to chemical corrosion and electrochemical corrosion; the corrosion rate is particularly high on the friction surfaces, as the corrosion products formed on those surfaces are destroyed by friction as soon as they form (which would otherwise reduce the corrosion rate). This periodic cycle of the formation and wearing away of a corrosion-resistant surface is commonly referred to as erosion. Its erosion rate is about 10 to 50 times that of a surface without friction. Therefore, the friction pair should be made of materials that offer both good corrosion resistance and wear resistance. Generally, graphite impregnated with phenolic resin is resistant to acids but not to bases; that impregnated with furan resin is resistant to both acids and bases; that impregnated with epoxy resin has good resistance to bases; while those impregnated with polytetrafluoroethylene resin or ceramics can withstand highly corrosive media. (1) When a mechanical seal is exposed to high-pressure media, if the surface pressure caused by these media exceeds its acceptable range, it can lead to the breakdown of the liquid film, resulting in heating and wear ； High pressure can cause the friction pair to deform and crack, thereby leading to the failure of the mechanical seal. Materials for friction pairs operating under high pressure must possess sufficient stiffness and strength; it is necessary to consider using materials such as cemented carbides, ceramics, and spray-coated ceramics, which are resistant to high pressures and have high strength and stiffness. (2) At high rotational speeds, due to the high linear velocity, the frictional heat and wear on the sealing surface increase; therefore, materials for the friction pairs should be ceramics impregnated with copper or graphite, which have a low coefficient of friction and good thermal conductivity. (3) When the mechanical seal is in a high-temperature environment, improper handling can lead to aging of rubber components and deformation of parts, resulting in leakage of the mechanical seal. Therefore, materials resistant to high temperatures should be used, such as sintered alumina ceramics (corundum ceramics) or cemented carbide. 1.2 The quality of the graphite static ring in machine 1.2 does not meet the design requirements; the design specifies that the graphite static ring should be impregnated with resin, and that there should be a certain degree of impregnation depth. Sometimes, with graphite static rings, either the impregnation depth is insufficient or very little of the impregnated layer remains after machining ； The design requires a certain level of surface finish on the end faces, but the surface finish of spare parts often fails to meet this requirement ； Design requirements specify that graphite should be of pure quality and possess a certain level of hardness, whereas spare parts often contain hard particles and lack sufficient hardness. As a result, the wear of graphite is quite severe. 1.3 Improper selection of the spring pressure ratio in the machine: The main sealing point of a mechanical seal – the dynamic sealing point – is formed by the mutual contact between the end face of the rotating dynamic ring and the end face of the stationary ring, which are perpendicular to the axis of rotation. The compressive force exerted by the spring on the sealing surface, namely the spring specific pressure, is a necessary condition to ensure that the sealing surface remains in close contact during low-pressure startup, operation, and shutdown, thereby maintaining a good sealing condition. When the sealing end face wears out, the spring preload overcomes the friction between the seal ring and the shaft, allowing the moving ring to move axially in order to compensate for the wear of the end face and maintain sealing. Since the value of the spring specific pressure has a direct impact on the reliability of the seal, if this value is too high, the sealing surfaces are pressed too tightly, leading to increased wear ； If the spring specific pressure is too low, the sealing surfaces cannot fit together properly; the spring force is not sufficient to counteract the pressure of the internal gas medium, nor can it compensate for wear at the end faces, resulting in leakage in the mechanical seal. Therefore, selecting an appropriate spring ratio value is key to ensuring reliable sealing. The spring compression is insufficient; the following two methods can be used to eliminate leakage: (1) Add a shimming behind the moving ring to compensate for the spring force. (2) Use a scraper and sandpaper to scrape and polish the areas where the sealing chamber meets the spring seat and the \"V\"-shaped ring, so that the spring force acts to ensure a tight fit between the friction pairs. 1.4 Wear caused by impurities entering the sealing surface: When the contact and friction at the sealing surface are normal, the solid impurities that can enter this surface are extremely fine and are easily washed away by the sealing fluid; therefore, they have no effect on seal leakage. The situation in which larger solid particles enter the sealed end face and cause increased wear occurs whenever the sealing function is not operating properly. It can generally be divided into the following situations: (1) At the beginning of commissioning after shutdown or maintenance, due to dirty pipelines or poor quality of the sealing fluid, solid impurities can easily enter the sealing surfaces. Therefore, the friction surfaces of the mechanical seal wear out quickly. (2) With a double-faced mechanical seal, when the pressure of the sealing water fluctuates or even drops below the pressure inside the pump or machine, the chemical medium containing impurities in the pump enters the sealing chamber through the inner sealing surface; some of these impurities remain on the sealing surface, leading to increased wear. With a single-faced mechanical seal, due to the fluctuations in medium pressure, the likelihood of impurities reaching the sealing surface is much lower, resulting in a longer service life. (3) Impurities enter the sealed end face for various reasons. The end faces of peroxide mechanical seals suffer severe wear at an early stage. From the perspective of wear mechanisms, particle wear is the main cause, while adhesive wear only occurs in individual cases; therefore, we will discuss only particle wear. The wear rate of peroxide particles is very high, **which reduces the service life of mechanical seals. Particle wear not only causes wear on the soft sealing ring (graphite ring) but also on the hard sealing ring (mainly the cemented carbide rotating ring); the surface affected by particle wear features obvious, roughly regular scratches and grooves. The essence of particle wear is the process in which hard particles, under the action of the vertical component of force, penetrate into the friction surface, while moving relative to it under the action of the tangential component of force; this leads to micro-cutting and ploying of the friction surface. Particle wear is related not only to the hardness of the material and the end-face specific pressure, but also to factors such as the material’s microstructure, the test temperature, the nature and magnitude of the load, the size and hardness of the abrasive particles, and the chemical properties of the friction surfaces. The difference in hardness between the mating materials of the friction pair has a significant impact on particle wear. When the hardness difference between the friction pairs is large, particle wear occurs much less frequently, and the rate of frictional wear is also lower ； However, the difference in hardness is small, resulting in more particle wear. The main reason for this is that when particles enter the friction pair, and the hardness of the moving ring is high, grooves are only formed on or embedded in the graphite ring; the hard ring remains unscathed and retains its original smoothness. Over time, the particles may also be washed away by the sealing fluid. When particles penetrate the friction surface and the hardness of the moving ring is low, the hard ring gets scratched; under the effects of vibration and eccentricity, it acts like a file against the graphite ring, resulting in severe wear. At the same time, the abrasions on the moving ring itself are also becoming increasingly severe. After running in, the grooves on the surface of the rotating ring gradually align concentrically with those on the surface of the graphite ring, at which point the leakage decreases. Furthermore, at the leakage entry points on the end faces of the rotating or stationary ring, if the chamfering is too large, larger particles can enter the friction surface along the inclined surface, leading to premature wear. For special cases, special measures should be adopted. At the beginning of operation or when the quality of the sealed water is poor, in order to prevent premature wear and failure of the mechanical seal, temporarily increasing the end-face pressure can yield significant results. An appropriate increase in the spring pressure does not significantly increase the wear of the friction pair; on the contrary, it prevents premature particle-induced wear and extends the service life. 2 Causes of wear failure at the new end faces and remedial measures: Sometimes, after maintenance of the mechanical seal, there is no leakage when a static pressure test is conducted, but leakage becomes significant once the seal starts to rotate. Once the running-in period has passed, the leakage does not decrease, forcing further maintenance to be carried out. During maintenance, it is often found that there is uneven wear on the sealing surface of the graphite static ring; that is, a part of the static ring’s surface does not make contact with the surface of the dynamic ring, and there are no signs of friction, which allows the sealing fluid to leak. Based on statistics from multiple cases of eccentric wear, such wear is primarily caused by the eccentricity of the moving ring or stationary ring (not being aligned with the axis centerline) and the skewness of the moving ring or stationary ring (not being perpendicular to the axis centerline). There are several situations of misalignment: the rotating ring is not aligned with the axis, and the stationary ring is not aligned with the axis. Due to the uneven pressure distribution on the friction surface of the stationary ring caused by eccentricity, the stress on the rubber seal or PTFE seal of the stationary ring becomes uneven as well, resulting in different degrees of compression. As the rotating ring moves, it causes the center line of the stationary ring to deviate from the axis, resulting in a deviation angle γ. At low speeds of the rotating ring (if it rotates a few to several dozen revolutions per minute), the spring force is sufficient to overcome this deflection angle ； At near 3000 r/min, the spring force can no longer overcome it, and the mechanical seal will leak. Eccentric wear is mainly caused by the following reasons: (1) The quality of manufacturing of sealing elements such as rubber or PTFE seals, moving rings, and stationary rings is inadequate; the allowable deviations in concentricity and perpendicularity are too large, or there are significant manufacturing errors. (2) The manufacturing or installation of the end cover of the sealed chamber is not centered or perpendicular to the axis. The end cover is the component used for installing and positioning the stationary ring. If the positioning surface of the end cover is not aligned with or perpendicular to the axis, it will inevitably cause the stationary ring to be misaligned or perpendicular to the axis, leading to uneven wear. Additionally, at the junction between the end cover and the flange of the sealing chamber, there is usually a non-metallic seal gasket; if the bolt tension on the flange is not even during installation by a fitter, this can also result in the end cover being misaligned or perpendicular to the axis, thereby causing uneven wear. It can be seen that, in addition to the high requirements for concentricity and perpendicularity when designing and manufacturing the end caps of the sealing chamber, ； When installing the end cover of the sealed chamber, the bolt tension must be even. At this flange, non-metallic gaskets should be chosen as thin as possible, provided that this ensures a leak-free seal. In short, the causes of uneven wear are mostly due to poor manufacturing quality of seals, end caps, etc., as well as incorrect maintenance and installation of mechanical seals. 3 Causes of cracked new sealing rings and preventive measures Cracked sealing rings are one of the common phenomena of early failure in mechanical seals. The graphite static ring is mainly fractured, while the ceramic and tungsten carbide dynamic rings are primarily affected by cracks and deformation in their sealing surfaces. The graphite ring is broken, resulting in severe leakage; the seal water pressure cannot be maintained at a level higher than the pressure of the medium. Although the rotating ring has cracks that increase leakage, it can still remain operational in most cases. There are various reasons for the fracture of graphite rings, mainly the following. (1) The rubber seal ring or PTFE V-ring is too tightly fitted with the graphite ring and the stationary ring seat, causing the graphite to be compressed and break under dynamic conditions. (2) Excessive force was applied during the installation of the sealing chamber and the pump body, or the pump body’s sealing chamber is severely tilted. (3) The shaft’s swing and backlash exceed the specified values. (4) The supply of sealing water is interrupted, causing dry friction between the friction pairs; this leads to excessive temperature rise, resulting in thermal cracking of the sealing ring. When sealing water is supplied suddenly again, the sealing ring contracts sharply and breaks apart. The main causes of cracks in the rotating ring are as follows: (1) The cemented carbide rotating ring is subject to high stress, and cracks occur during its operation due to the combined effect of external and internal forces. These cracks are dense, fine cracks that diverge radially. (2) Temperature effect: When non-metallic materials are used as sealing rings (such as ceramics, WC), poor heat conduction leads to a highly uneven temperature distribution within the ring. Even under normal conditions, the temperature difference can reach several hundred degrees. This temperature difference causes thermal deformation, which in turn generates thermal stress. When thermal stress reaches a certain level, it causes the stress to exceed the material’s strength limit. At this point, surface cracks often occur in the sealing rings of brittle materials, which in turn increases frictional wear on the sealing surfaces and leads to failure. Thermal deformation also causes distortion in the gap between the bonded sealing surfaces, thereby increasing leakage. When cemented carbide is used for the end face of the rotating ring, due to the low thermal expansion coefficients of these materials, there is a significant difference compared to the thermal expansion coefficient of the base material (stainless steel has a thermal expansion coefficient 3–4 times higher than that of cemented carbide). As a result, when the temperature rise during sealing exceeds a certain value, it can cause deformation or cracking in the cemented carbide seal ring. An effective measure to address dynamic and static damage, cracking, and deformation is, in addition to strictly controlling the quality of spare parts processing, performing careful installation and maintenance, and improving operational management, ensuring that the sealed end surfaces receive adequate lubrication and cooling. However, in practice, it is not always possible to ensure adequate lubrication and cooling; fluctuations in the pressure of the sealing water, interruptions in its supply, and workers’ mistakes can all result in the seal not receiving sufficient lubrication and cooling. 4 Leakage of new sealing rings and cause analysis: Sometimes, mechanical seal leakage is not caused by end-face leakage, but rather by leakage at the sealing rings of the rotating ring and stationary ring, or at the shaft sleeve sealing ring. The reasons are as follows: 4.1 Leakage at the rubber seals of the machine. The “O”-ring rubber seals used in the moving and stationary rings can also cause sealing leakage due to issues such as insufficient rubber smoothness, inappropriate hardness, poor elasticity, high friction coefficient, inadequate geometric dimensions, as well as poor heat resistance and corrosion resistance. 4.2 Leakage at the contact point between the shaft sleeve and the rotating ring seal: After a shaft sleeve has been used multiple times, rough abrasion marks and spots appear on its surface at the contact point with the rotating ring seal, which often leads to sealing leakage. The main reasons are: (1) Due to the slight relative movement between the rotating ring seal and the shaft sleeve, abrasive particles or other harder impurities get trapped within the softer seal. It generates a rubbing friction on the bushing, and if the surface of the bushing has not been hardened or the hardening process was of poor quality, wear occurs as a result. (2) As a result of rubbing and friction, the passivation film on the surface of the bushing is damaged; a potential difference exists across the metal surface of the bushing, leading to electrochemical corrosion. Furthermore, since both the contact area itself and the condition of micro-leakage provide conditions for crevice corrosion, the corrosion rate is even higher. Furthermore, on the side of the dynamic ring seal in contact with the atmosphere, the presence of oxygen creates more complex and severe corrosion conditions, resulting in more noticeable wear marks and spots. Polytetrafluoroethylene sealing ring sets are used on the moving ring; due to their lower friction coefficient compared to rubber, higher surface finish, and easier movement along with the moving ring, they are superior to rubber sealing rings. Rubber seals are used on the stationary ring; due to their good elasticity and low hardness, they offer better sealing reliability compared to PTFE seals. Therefore, it is also important to choose the material of the sealing ring appropriately. Based on factors such as corrosion, we have made the following adjustments to the mechanical seals: polytetrafluoroethylene rings or corrosion- and temperature-resistant rubber rings are used for the static ring on the medium side, while rubber rings are used for the static ring on the atmosphere side ； Dynamic rings generally use polytetrafluoroethylene seals, which yield fairly good results. To eliminate fretting wear of the bushing, in addition to using coating materials with high hardness, good corrosion resistance, strong adhesion to the base material, and high surface finish on the bushing surface, as well as employing better coating processes, it is also necessary to pay attention to the degree of wear of the bushing during installation. New bushings and new seals should be replaced at appropriate times to ensure sealing reliability from multiple aspects. 5 New Discussions 5.1 Comparison between mechanical seals and packing seals: Mechanical seals offer advantages such as reliable sealing, long service life, low power consumption, and a wide range of applications. Some mechanical seals experience premature failure and have a short service life; the main shortcomings lie in design and manufacturing issues, which are often caused by improper selection, substandard quality of spare parts, inappropriate choice of materials for the friction pairs, incorrect installation and maintenance, as well as poor quality of the water used for sealing. To extend the lifespan of mechanical seals, concerted efforts are required in all aspects such as design, manufacturing, maintenance, and operation. 5.2 The design and configuration of mechanical seals must take into account the actual production conditions; materials for the friction pairs and static seal rings that offer reliable sealing performance and a long service life should be preferred wherever possible. If the inner end of a double-end face mechanical seal is in contact with the medium, corrosion-resistant materials should be preferred ； The outer end does not come into contact with the medium, so materials with high sealing reliability are preferred, such as WC rubber rings. It is not necessary to use the same materials for both the inner and outer ends; this approach not only helps reduce costs but also enhances sealing reliability. 5.3 Strict requirements must be imposed on the machining quality of mechanical seal components. The instructions and maintenance procedures should specify the technical requirements for the installation and use of mechanical seals, so that workers have clear guidelines to follow. 5.4 During the operation of mechanical seals, it is not necessary to carry out maintenance just because the leakage amount is slightly high. Sometimes, after operating for a period of time, the dirt that had accumulated on the sealing surfaces is washed away, the quality of the sealing fluid improves, the friction pairs become well-conditioned, temperature-induced deformation is eliminated, and the leakage rate decreases significantly as well. For double-end face mechanical seals, as long as the pressure can be maintained, it is entirely possible to wait for a period of time before carrying out maintenance.