Leakage point analysis and maintenance scheme of m

2022-07-30
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Leakage point analysis and maintenance scheme of mechanical seals for pumps

mechanical seals for pumps have a wide variety of types and models, but there are mainly five leakage points:

(L) seal between shaft sleeve and shaft

(2) sealing between moving ring and shaft sleeve

(3) sealing between dynamic and static rings

(4) sealing between the stationary ring and the stationary ring seat

(5) seal between the end cover and the pump body

generally speaking, the leakage between the extended shafts of the shaft sleeve and between the seal end cover and the pump body is easy to find and solve, but it needs careful observation, especially when the working medium is liquefied gas or high-pressure, toxic and harmful gas, it is relatively difficult. Other leaks are difficult to distinguish and judge intuitively. Correct conclusions can only be drawn by observing, analyzing and judging the leakage symptoms on the basis of long-term management and maintenance practice

I. cause analysis and judgment of leakage

1. Leakage during installation static test. After the mechanical seal is installed and debugged, it is generally necessary to conduct a static test to observe the leakage. If the leakage is small, there are problems with the moving ring or static ring sealing ring; When the leakage is large, it indicates that there is a problem between the dynamic and static ring friction pairs. On the basis of preliminary observation of the leakage amount and judgment of the leakage position, manually turn the gear for observation. If there is no obvious change in the leakage amount, there is a problem with the static and dynamic ring seals; If there is obvious change in the leakage during turning, it can be concluded that there is a problem in the friction pairs of the moving and static rings; If the leakage medium is sprayed along the axial direction, most of the problems exist in the moving ring sealing ring. If the leakage medium is sprayed around or leaked from the water cooling hole, most of the problems are due to the failure of the stationary ring sealing ring. In addition, leakage channels can also exist at the same time, but there are generally primary and secondary differences. As long as you observe carefully and are familiar with the structure, you can make a correct judgment

2. Leakage during test run. After the static test of the mechanical seal for pump, the centrifugal force generated by high-speed rotation during operation will restrain the leakage of the medium. Therefore, the mechanical seal leakage during the test run is basically caused by the damage of the dynamic and static ring friction pairs after the failure of the seal between the shafts and the end cover is eliminated. The main factors causing the seal failure of the friction pair are:

(L) during operation, due to abnormal phenomena such as evacuation, cavitation and pressure holding, large axial force is caused, which separates the contact surfaces of the dynamic and static rings

(2) when installing the mechanical seal, the compression is too large, resulting in serious wear and abrasion of the end face of the friction pair

(3) the sealing ring of the movable ring is too tight, and the spring cannot adjust the axial floating amount of the movable ring

(4) the sealing ring of the stationary ring is too loose. When the movable ring floats axially, the stationary ring will be separated from the stationary ring seat

(5) there are granular substances in the working medium. Friction pairs are introduced during operation, and the sealing faces of the moving and stationary rings are inspected

(6) wrong design and model selection, low specific pressure of sealing face or large cold shrinkage of sealing material, etc. The above phenomena often occur during the test run, and sometimes can be eliminated by properly adjusting the static ring seat, but most of them need to be disassembled and replaced

3. Sudden leakage during normal operation. The sudden leakage of centrifugal pump during operation is caused by normal wear and tear or service life, while most of it is caused by large changes in working conditions or improper operation and maintenance

(1) evacuation, cavitation or pressure holding for a long time, resulting in seal damage; (2) The actual output of the pump is too small, a large number of media circulate in the pump, and heat accumulates, causing media gasification and sealing failure

(3) the return flow is too large, causing the bottom sediment of the vessel (tower, kettle, tank, pool) at the suction pipe side to float up and damage the seal

(4) for long-term shutdown, there is no manual turning when restarting, and the friction pair is torn off due to adhesion

(5) more corrosive, polymerizable and gelling substances in the medium

(6) sharp change of ambient temperature

(7) frequent change or adjustment of working conditions

(8) sudden power failure or fault shutdown, etc. Sudden leakage of centrifugal pump during normal operation, if not found in time, will often lead to major accidents or losses, which should be paid attention to and effective measures should be taken

II. Several misunderstandings in the maintenance of mechanical seals for pumps

1. The greater the spring compression, the better the sealing effect. In fact, if the spring compression is too large, the friction pair will be rapidly worn and burned instantly; Excessive compression makes the spring lose the ability to adjust the end face of the moving ring, resulting in seal failure

2. The tighter the sealing diagram of the moving ring, the better. In fact, too tight sealing ring of moving ring is harmful. One is to aggravate the wear between the seal ring and the shaft sleeve, resulting in premature leakage; Second, the resistance to axial adjustment and movement of the moving ring is increased, which can not be adjusted in time when the working conditions change frequently; Third, the spring is easily damaged due to excessive fatigue; The fourth is to deform the moving ring sealing ring and affect the sealing effect

3. The tighter the stationary ring seal, the better. The static ring sealing ring is basically in a static state, and the sealing effect will be better if it is relatively tight, but too tight is also harmful. First, the static ring seal is excessively deformed, which affects the sealing effect; Second, the static ring is mainly made of graphite. First, to sum up, the utilization scope and functional characteristics of the metal tensile testing machine are relatively brittle, and excessive stress is very easy to cause fragmentation; Third, it is difficult to install and disassemble, and it is very easy to damage the static ring

4. The tighter the impeller nut, the better. In mechanical seal leakage, the leakage between shaft sleeve and shaft (inter shaft leakage) is common. It is generally believed that the leakage between shafts is caused by the failure to lock the impeller nut. In fact, there are many factors that cause the leakage between shafts, such as the failure and offset of the inter shaft gasket, shaft 5 If there are impurities in the shaft without abnormal reasons, there is a large form and position error at the fitting position between the shaft and the shaft sleeve, the contact surface is damaged, there is a gap between the components on the shaft, the shaft head thread is too long, etc., all these will lead to the leakage between the shafts. Excessive locking of the lock nut will only lead to premature failure of the inter axle pad. On the contrary, locking the lock nut appropriately will keep the inter axle pad in line with the current challenges faced by the personal hygiene products market and the high level of consumer demand. All partners in the value chain of hygiene products have accelerated the research and development process, launched more innovative product solutions and set the compression elasticity. During operation, the lock nut will automatically lock in time, Keep the shafts in good sealing condition

5. The new one is better than the old one. Relatively speaking, the effect of using the new mechanical seal is better than that of the old one, but when the quality or material of the new mechanical seal is not selected, the matching dimension error will affect the sealing effect; In polymerizable and permeable media, if the static ring is not excessively worn, it is better not to replace it. Because the stationary ring is in a stationary state in the stationary ring seat for a long time, the polymer and impurities are deposited as a whole, playing a better sealing role

6. It is better to dismantle and repair than not to dismantle. In case of mechanical seal leakage, it is urgent to disassemble and repair. In fact, sometimes the seal is not damaged, and the leakage can be eliminated by adjusting the working condition or properly adjusting the seal. In this way, we can not only avoid waste, but also verify our fault judgment ability, accumulate maintenance experience and improve maintenance quality

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