Analysis of leakage causes of mechanical seals for pumps

Pumps have a wide range of mechanical seals and different models, but there are five main leak points:
(l) a seal between the sleeve and the shaft;
(2) sealing between the moving ring and the sleeve;
(3) Seal between moving and static rings;
(4) sealing between the stationary ring and the stationary ring seat;
(5) Sealing between the sealed end cap and the pump body.
In general, the leakage between the shafts of the bushings, the seal end caps and the pump body is easier to find and solve, but it needs to be carefully observed, especially when the working medium is liquefied gas or high pressure, toxic and harmful gas, it is relatively difficult. some. The remaining leaks are difficult to distinguish and judge intuitively. It is necessary to observe, analyze, and judge the leak symptoms on the basis of long-term management and maintenance practices in order to draw correct conclusions.
I. Analysis and judgment of leakage causes
1. Leak when installing the static test. After the mechanical seal is installed and debugged, it is generally necessary to perform a static test to observe the amount of leakage. If the leakage is small, there are problems with the moving ring or static ring seal; when the leakage is large, it indicates that there is a problem between the friction pairs of the moving and stationary rings. On the basis of preliminary observation of the leakage amount and judging the leakage location, the vehicle can be manually observed. If there is no significant change in the leakage amount, the static and dynamic ring seals have problems; if there is a significant change in the leakage amount during the cranking, it can be determined that it is moving. There is a problem with the static ring friction pair; if the leakage medium is sprayed in the axial direction, there are many problems with the moving ring seal ring, and the leakage medium is sprayed to the periphery or leaks from the water cooling hole, and the static ring seal ring is mostly invalid. In addition, the leakage channel can also exist at the same time, but there are generally primary and secondary differences. As long as the observation is meticulous and familiar with the structure, it can be judged correctly.
2. Leakage that occurs during commissioning. After the static test of the mechanical seal of the pump, the centrifugal force generated by the high-speed rotation during operation will suppress the leakage of the medium. Therefore, the leakage of the mechanical seal during the test run is basically caused by the failure of the friction pairs of the moving and stationary rings after the failure of the seal between the shafts and the end cap seal. The main factors causing the failure of the friction pair seal are:
(l) During operation, due to abnormal phenomena such as evacuation, cavitation, and rolling, a large axial force is caused to separate the moving and static ring contact surfaces;
(2) The amount of compression is too large when installing the mechanical seal, resulting in severe wear and abrasion of the end face of the friction pair;
(3) The moving ring seal ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring;
(4) The static ring seal ring is too loose, when the moving ring is axially floating, the static ring is separated from the static ring seat;
(5) There are granular materials in the working medium, and the friction pair is entered during operation to detect the moving and static ring sealing end faces;
(6) The design selection is wrong, the sealing end face is lower than the pressure deviation or the sealing material is more cold-shrinkable. The above phenomenon often occurs during trial operation, and can sometimes be eliminated by appropriately adjusting the static ring seat, etc., but most of them need to be reassembled and replaced.
3. Sudden leakage during normal operation. A small amount of sudden leakage of the centrifugal pump during operation is due to normal wear or has reached the service life, and most of them are caused by large changes in working conditions or improper operation and maintenance.
(1) evacuation, cavitation or rolling for a long time, resulting in seal damage;
(2) The actual output of the pump is too small, a large number of medium pumps circulate, heat accumulates, causing gasification of the medium, resulting in seal failure;
(3) The return flow is too large, causing the sediment at the bottom of the suction side container (tower, kettle, tank, pool) to rise and damage the seal;
(4) For a longer period of time, there is no manual turning when restarting, and the friction pair tears the sealing surface due to adhesion;
(5) Increased corrosive, polymerizable, and gelatinous substances in the medium;
(6) The ambient temperature changes drastically;
(7) Frequent changes or adjustments in working conditions;
(8) Sudden power failure or downtime. The centrifugal pump suddenly leaks during normal operation. If it cannot be found in time, it will often cause a big accident or loss. It must be taken seriously and effective measures taken.
Second, several misunderstandings in the mechanical seal inspection of pumps
1. The greater the amount of spring compression, the better the sealing effect. In fact, if the spring compression amount is too large, the friction pair may be sharply worn and burned instantaneously; excessive compression causes the spring to lose the ability to adjust the end face of the moving ring, resulting in seal failure.
2. The tighter the seal diagram of the moving ring, the better. In fact, the dynamic ring seal is too tight and harmful. First, the wear between the seal ring and the bushing is intensified, and the leakage is premature; secondly, the resistance of the moving ring in the axial adjustment and movement is increased, and the adjustment cannot be made timely when the working conditions change frequently; the third is that the spring is excessively fatigued and easily damaged; It is to deform the moving ring seal and affect the sealing effect.
3. The tighter the seal ring of the static ring, the better. The static ring seal is basically at rest, and the relatively tight seal will be better, but too tight is also harmful. First, the static ring seal is caused by excessive deformation, which affects the sealing effect. Secondly, the static ring material is mostly graphite, which is generally brittle. Excessive force is easy to cause chipping. Third, it is difficult to install and disassemble, and it is easy to damage the static ring.
4. The tighter the impeller lock is, the better. In mechanical seal leaks, leakage between the bushing and the shaft (interaxial leakage) is more common. It is generally believed that the leakage between the shafts is that the impeller locks are not locked. In fact, there are many factors causing leakage between the shafts, such as the failure of the inter-axis pads, the offset, the impurities between the shafts, and the larger shape of the shaft and the sleeve. Position errors, contact surface damage, gaps between components on the shaft, and excessive length of the shaft threads can cause leakage between the shafts. Excessive locking of the lock nut will only lead to premature failure of the inter-axis pad. On the contrary, the lock nut is moderately locked, so that the inter-axis pad always maintains a certain compression elasticity. During operation, the lock nut will automatically lock in time, so that the shaft is always in good condition. Sealed state.
5. The new one is better than the old one. Relatively speaking, the effect of using a new mechanical seal is better than that of the old one, but the quality or material selection of the new mechanical seal is not appropriate. The large dimensional error will affect the sealing effect; in the polymerizable and permeable medium, the static ring is not excessive. Wear or not replace it as well. Because the static ring is in a static state for a long time in the static ring seat, the polymer and impurities are deposited as one body, which has a good sealing effect.
6. The repair is better than not. In the event of a mechanical seal leak, it is urgent to dismantle. In fact, sometimes the seal is not damaged. It is only necessary to adjust the working condition or properly adjust the seal to eliminate the leak. In this way, it avoids waste and can verify its own fault judgment ability, and accumulates maintenance experience to improve the quality of maintenance.