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A Brief Discussion on Leakage in Centrifugal Pump Seals and Its Prevention 1 Introduction Pumps are machines used to transport liquids or to increase their pressure. It transfers the mechanical energy of the prime mover or other external energy to the liquid, thereby increasing the energy of the liquid. Pumps are primarily used to transport water, oil, acids, alkalis, emulsions, suspensions, and liquid metals, as well as liquid-gas mixtures and liquids containing suspended solids. Therefore, it is widely used in various sectors such as petroleum, chemicals, power, metallurgy, mining, shipbuilding, agriculture, and defense, occupying an important position in the national economy. In the production of the chemical and petroleum industries, raw materials, semi-finished products, and finished products are mostly in liquid form. Converting raw materials into semi-finished products and finished products requires complex processing steps, and pumps play a role in transporting these liquids as well as providing the pressure flow necessary for chemical reactions. 2 Regarding centrifugal pumps, they have notable advantages such as a wide range of applicable performance parameters (including flow rate, head, and adaptability to the properties of the medium), small size, simple structure, easy operation, uniform flow rate, few failures, long service life, as well as low purchase and operating costs. For these reasons, they are widely used in chemical production. The basic components of a centrifugal pump are a high-speed rotating impeller and a fixed spiral-shaped pump casing. The impeller, which has several (usually 4 to 12) backward-curving blades, is fixed to the pump shaft and rotates at high speed along with the pump shaft driven by the motor. The impeller is the component that directly does work on the liquid inside the pump, and it serves as the energy-providing element of the centrifugal pump. The suction inlet at the center of the pump casing is connected to the suction pipe, the bottom of which is equipped with a non-return foot valve. The outlet located on the side of the pump casing is connected to the discharge pipeline equipped with a control valve. When the centrifugal pump is started, the pump shaft drives the impeller to rotate at high speed, forcing the liquid pre-filled between the blades to spin. Under the effect of inertial centrifugal force, the liquid moves radially from the center of the impeller toward the periphery. During its flow through the impeller, the liquid gains energy; its static pressure increases and its flow velocity rises. After the liquid leaves the impeller and enters the pump casing, it slows down due to the gradually widening flow channels within the casing; part of its kinetic energy is converted into static pressure energy. Finally, it flows tangentially into the discharge pipe. Therefore, the volute casing is not only a component that collects the liquid flowing out of the impeller but also an energy conversion device. As the liquid is flung from the center of the impeller toward the periphery, a low-pressure area forms at the center of the impeller. Due to the difference in total potential energy between the liquid surface in the tank and the center of the impeller, the liquid is drawn into the center of the impeller. Thanks to the continuous rotation of the impeller, the liquid is continuously drawn in and discharged. The mechanical energy obtained by the liquid in a centrifugal pump is ultimately manifested as an increase in static pressure energy. 3 Common problems during the operation of centrifugal pumps: Since the pump shaft rotates while the pump casing remains fixed, there is inevitably a certain gap at the point where the shaft meets the pump casing. Although pumps are equipped with shaft sealing devices to prevent high-pressure liquid inside the pump from leaking through gaps, or to stop outside air from entering the pump from the opposite direction, there is no such thing as absolute zero leakage; leaks inevitably occur. Sealing can be divided into two main categories: static sealing between relatively stationary mating surfaces, and dynamic sealing between relatively moving mating surfaces. Static seals mainly fall into three categories: point seals, adhesive seals, and contact seals. Based on operating pressure, static seals can be divided into medium and low-pressure static seals and high-pressure static seals. For medium and low-pressure static seals, softer materials and gaskets with a wider width are commonly used, while for high-pressure static seals, harder materials and metal gaskets with a very narrow contact width are employed. Dynamic seals can be divided into two basic types: rotary seals and reciprocating seals. Seals can be classified into contact seals and non-contact seals, depending on whether the seal comes into contact with the components that move relative to it. Generally speaking, contact seals offer good sealing performance, but are limited by frictional wear and are suitable for applications where the linear speed of the sealing surfaces is low. Contactless seals have poor sealing performance and are suitable for applications at higher speeds. The common location of leaks in centrifugal pumps is at the seal between the pump casing and the shaft; this is a type of contact rotary seal, and mechanical seals are generally used for this purpose. A mechanical seal is a shaft sealing device that prevents leakage by relying on a pair or several pairs of end surfaces that slide relative to each other perpendicular to the axis; these surfaces remain in contact under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, along with the assistance of additional sealing elements. In addition, some types of centrifugal pumps are also equipped with bearing housings, and a labyrinth seal is generally used between the bearing housing cover and the shaft. A labyrinth seal consists of several annular sealing teeth arranged in sequence around the rotating shaft; gaps and expansion chambers are formed between these teeth. As the fluid to be sealed passes through these intricate labyrinthine gaps, a throttling effect occurs, thereby preventing leakage. Leaks can also occur here easily. 4 Analysis of the causes of seal leakage: For the sealing of pump shafts and bearing housings, labyrinth seals are generally used. There are various reasons for leaks in labyrinth seals. It could be due to an excessive amount of lubricant and high oil pressure, or it might be caused by excessive sealing gaps; other factors such as the viscosity of the lubricant could also be at play. The sealing between the motor casing and the end cover is a type of static seal. Water ingress usually occurs due to failed gaskets or loose end cap bolts. In this way, water from the outside will seep into the motor through the casing along the sealing surface, causing the motor to fail. For the sealing of the pump shaft and pump casing, mechanical seals are generally used. Mechanical seals have many advantages, such as reliability, a long service life, and low wear. 5 Prevention of seal leaks (1) Regularly clean and sanitize the pump. The pump is installed outdoors, where it is prone to accumulating dust. Dust on the sealing surface, as the pump shaft rotates, accelerates the wear of that sealing surface and causes the sealing gap to widen. (2) For the lubricating oil in the bearing housing, it is necessary to choose an oil that is suitable for the environment in which the pump operates and for the application it is used in. It is also advisable to replace the oil regularly in order to maintain its purity, thereby preventing wear of the sealing surfaces due to impurities present in the oil. (3) During inspections, pay more attention to monitoring the temperature of the shaft and bearings while the pump is in operation. Do not overload the pump or allow its shaft temperature to become too high; switch the pumps on and off at appropriate intervals to prevent any single pump from operating for too long. If the temperature of the shaft rises very quickly, the pump must be stopped to check for issues such as damaged bearings or inadequate lubrication. (4) Carry out proper inspection and turning operations. Since the pumps shut down while operating at high speeds, their temperature is relatively high and the oil film becomes thin. After cooling down, if the pumps remain idle for an extended period, the sealing surfaces tend to adhere to each other; in severe cases, seizure may occur. Under such circumstances, starting the pump may not only cause an instantaneous excessive load on the motor, but also lead to dry friction between the sealing surfaces, thereby damaging the seals. Regular cranking ensures that there is a sufficient amount of lubricating oil on the bearings and seals of the pump prior to startup. This facilitates pump operation and helps protect the seals. (5) For seals such as gaskets, sealing rings, and sealing end caps, regular replacement, inspection, or tightening should be carried out. It’s best not to wait until a leak occurs before taking action. 6 Conclusion Centrifugal pumps play a very important role in chemical production; their proper operation is directly related to the continuity of production. Therefore, it is necessary to gain more knowledge about sealing techniques, accumulate experience, observe carefully during regular operations, and carry out proper maintenance.