Repair of mechanical seals for pumps There are a wide variety of mechanical seals for pumps. The types of leaks are as follows:
(l) Sealing between shaft sleeves and shafts; (2) Sealing between moving rings and sleeves; (3) Sealing between moving and static rings; (4) Sealing between stationary ring and stationary ring seat; (5) Sealing between sealed end cap and pump body. In general, the leakage between shafts, seals, and housings of the shaft sleeves is relatively easy to detect and resolve, but careful observation is required, especially when the working medium is liquefied gas or high-pressure, toxic and hazardous gases. some. The remaining leaks are intuitively difficult to distinguish and judge, and on the basis of long-term management and maintenance practices, leakage symptoms must be observed, analyzed, and judged in order to draw correct conclusions.
First, the leakage analysis and judgment
1. Leak during installation test. After installation and debugging of mechanical seals, 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 the static ring seal ring; when the leakage amount is large, it indicates that there is a problem between the friction pair of the dynamic and static rings. After initially observing the leakage and judging the location of the leakage, observe the manual operation. If there is no significant change in the leakage, there will be problems with the static and dynamic ring seals; if there is a significant change in the amount of leakage, it can be concluded that it is moving, There is a problem with the static ring friction pair; if the leakage medium is ejected in the axial direction, there are many problems with the dynamic ring seal ring. If the leaking medium is injected around or leaks from the water cooling hole, the static seal ring is mostly ineffective. In addition, leakage channels can also exist at the same time, but generally there are differences between the primary and secondary, as long as the observation of detailed, familiar with the structure, we can certainly determine correctly. 2. Leakage during commissioning. After the mechanical seal of the pump is subjected to a static test, 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 trial operation after eliminating the shaft and the seal failure of the end cap is basically due to the damage of the friction and static pair of the dynamic and static rings. The main factors causing the failure of the friction pair seal are:
(1) During operation, due to evacuation, cavitation, squeezing, and other abnormal phenomena, a large axial force is caused and the moving and stationary ring contact surfaces are separated; (2) When the mechanical seal is installed, the amount of compression is too large, resulting in friction. The secondary end face is seriously worn and scratched; (3) The dynamic ring seal is too tight and the spring cannot adjust the axial float of the ring; (4) The static ring seal is too loose. When the ring moves axially, the stationary ring disengages. Stationary ring seats; (5) There are granular materials in the working medium, enter into the friction pair during operation, detect moving and static ring sealing the end face; (6) The design selection is wrong, the sealing end surface is lower than the pressure or the sealing material is cold shrinkage. Larger sex and so on. The above phenomenon often occurs during the test operation, and sometimes it can be eliminated by appropriately adjusting the stationary ring seat, but most of them need to be disassembled and replaced. 3. Sudden leakage during normal operation. A sudden loss of a few centrifugal pumps during operation is due to normal wear or has reached the service life, and most of them are caused by large changes in operating conditions or improper operation and maintenance. (1) The evacuation, cavitation, or long-time rolling pressure leads to the destruction of the seal; (2) The actual output of the pump is too small, a large number of medium pumps circulate, and heat accumulates, causing gasification of the medium, resulting in seal failure; (3) Return flow is too large, resulting in the bottom of the suction tube side of the container (tower, kettle, tank, tank) sediment raised, damage the seal; (4) for a longer time outage, restart without manual disk, friction due to adhesion and Cracked sealing surface; (5) Increased corrosiveness, polymerization, and gelling properties in the medium; (6) Rapid changes in ambient temperature; (7) Frequent changes or adjustments in operating conditions;
8) Abrupt power failure or downtime. Centrifugal pump suddenly leaks during normal operation. If it cannot be discovered in time, it often leads to a larger accident or loss. Attention must be given and effective measures must be taken.
Second, several errors in the maintenance of pump mechanical seals
1. The greater the amount of spring compression, the better the sealing effect. Actually, excessive spring compression can lead to rapid wear of the friction pair and instant burnout. Excessive compression causes the spring to lose its ability to adjust the end face of the moving ring, resulting in failure of the seal. 2. The tighter the dynamic ring seal, the better. In fact, the dynamic ring seal is too harmful and unhelpful. One is to intensify the wear between the seal ring and the sleeve and cause premature leakage; the other is to increase the axial adjustment of the moving ring and the resistance of the movement. When the operating conditions change frequently, it cannot be timely adjusted; third, the spring is fatigued and easily damaged; The deformation of the dynamic ring seal affects the sealing effect. 3. The tighter the ring seal, the better. The static ring seal is basically in a stationary state, and the relatively tight seal effect is better, but too tight is also harmful. The first is caused by excessive deformation of the static ring seal, affecting the sealing effect; Second, the static ring material is mostly graphite, generally more brittle, excessive stress can easily cause fragmentation; Third, installation, removal difficult, very easy to damage the static ring. 4. The tighter the impeller locks, the better. Leakage between shaft sleeves and shaft (leakage between shafts) is common in mechanical seal leakage. It is generally believed that the leakage between the shafts means that the impeller is not locked. In fact, there are many factors that lead to leakage between shafts. For example, the inter-shaft mat fails, there is misalignment between the shafts, and there are impurities in the shafts. The shaft and shaft sleeves have large shapes. Bit error, failure of the contact surface, gaps between the parts on the shaft, and excessive shaft thread length can cause shaft leakage. Locking the female lock excessively will only lead to premature failure of the inter-shaft pad. On the contrary, it will properly lock the lock mother so that the inter-axial pad always maintains certain compression and elasticity. In operation, the mother locks the locking automatically and timely so that the shaft is always in good condition. The 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 the old, but the quality or material selection of the new mechanical seal is not appropriate, and the larger dimensional error will affect the sealing effect; in polymeric and permeable media, the static ring is not excessive. Worn, or not replaced as well. Because the stationary ring is in a stationary state for a long time in the stationary ring seat, the polymer and the impurities are deposited as a whole, and a good sealing effect is achieved. 6. Renovation is better than not dismantling. Once the mechanical seal leaks, it will be eager to repair, in fact, sometimes the seal is not damaged, just adjust the working conditions or adjust the seal properly to eliminate the leak. This will not only avoid waste but also verify your own fault diagnosis capabilities, accumulate maintenance experience and improve the quality of maintenance.
Failure Analysis and Failure Analysis of Mechanical Seals
1. Corrosion failure Mechanical seals are subject to numerous failures due to corrosion. Common types of corrosion include the following: (1) Surface corrosion Corrosion of corrosive media can cause surface corrosion of mechanical seals. Corrosion perforation can also occur in severe cases. The parts are more pronounced, using stainless steel to reduce surface corrosion. (2) Point Corrosion Spring sleeves often have large area pitting or regional pitting, and some lead to perforation. Such localized corrosion will not cause serious consequences for sealing, but should also be replaced when overhauled. (3) Intergranular corrosion Tungsten carbide ring stainless steel ring seat is brazed. In use, the stainless steel seat is prone to intergranular corrosion. In order to overcome the effect of sensitization, the stainless steel should be solution treated. (4) Stress corrosion cracking Metal welding bellows, springs, etc., under the joint action of stress and medium corrosion, often occur fracture, due to the sudden rupture of the spring and the seal failure, generally using a larger spring wire to solve.
5) Crevice corrosion Between the inner hole of the moving ring and the bushing surface, between the screw and the screw hole, between the O-ring and the bushing, due to the difference of the concentration of the medium inside and outside the gap, the crevice corrosion is caused, besides the ceramic ring and the metal Crevice corrosion can also occur between the ring seats. Generally, ceramics are sprayed on the surface of the sleeve, and the surface of the ring is coated with a binder to reduce crevice corrosion. (6) Electrochemical corrosion Dissimilar metals often cause electrochemical corrosion in the medium. It loosens the insert ring and affects the seal. Generally, it is also adopted to overcome the problem by coating the joint with a binder. 2. Heat loss effect (1) Hot cracking If the sealing surface is exposed to dry friction, sudden cooling interruption, impurities entering the sealing surface, or evacuation, radial cracks may occur on the ring surface, causing the dual rings to wear abruptly and the leakage of the sealing surface rapidly increasing. Tungsten carbide ring hot cracking phenomenon is more common. (2) Foaming, carbonization If the graphite ring exceeds the allowable temperature during use, the resin will precipitate on the surface, and the resin near the friction surface will be charred. When there is a binder, it will be foamed and softened, resulting in increased leakage of the sealing surface. , seal failure. (3) Aging, cracking, and swelling When the rubber continues to be used in excess of the allowable temperature, it will rapidly age, crack, and harden. Swelling insults in organic media, all of which lead to seal failure. The key to seal failure due to thermal damage is to minimize frictional heat and improve heat dissipation so that no temperature change occurs at the sealing surface. 3. Wear failure If the friction pair has poor wear resistance, high friction factor, excessive end-face pressure (including spring specific pressure), and the seal surface entering solid particles, the seal surface wears too quickly and the seal fails. The use of balanced mechanical seals to reduce end-face specific pressure and proper reduction of spring pressure during installation can help to overcome the failure caused by wear. In addition, the use of good friction pair materials can reduce wear. In order of wear resistance, the materials are tungsten carbide, carbon graphite, hard alloy, carbon graphite, ceramics (alumina), carbon graphite, spray ceramics, carbon graphite, silicon oxide ceramics, carbon graphite, high-speed steel, carbon graphite, and surfacing. Carbide – carbon graphite. 4. Failure analysis due to installation, operation, etc. (1) Leakage during hydrostatic or hydrostatic test. Due to poor installation, leakage occurs when the mechanical seal is subjected to a water or static pressure test. Poor installation has the following aspects. a. The contact surfaces of the moving and stationary rings are uneven and there are bumps and damages during installation. b. The dimensions of the dynamic and static ring seals are incorrect, damaged or not compacted. c. Foreign objects on the surfaces of moving and stationary rings are caught. d. The direction of the V and V ring seals are reversed, or reversed when installed. e. The tightening screw is not tightened and the spring seat is retracted. f. Leakage at the sleeve, the seal is not installed or pressed tightly. g. If the axis leaks by hand, there are the following reasons: the spring force is not uniform, the single spring is not vertical, the lengths of multiple springs are different, or the number is small; the end face of the sealing chamber is not perpendicular to the axis. h. Static ring compression is not uniform. (2) Periodic leakage caused by installation, operation, etc.
In operation, if the pump impeller axial turbulence exceeds the standard, the shaft is periodically vibrated and the process operation is unstable, frequent changes in the pressure in the seal chamber can cause periodic seal leakage. (3) Frequent leakage a. Deformation of the moving ring and static ring contact end face will cause frequent leakage. If the end face is too large than the pressure, the frictional heat causes thermal deformation of the dynamic and static rings; the structure of the sealing parts is irrational, the strength is not enough to cause deformation; the residual deformation due to the processing of the materials; the uneven force of the parts during the installation, etc. are all sealed end faces. The main cause of deformation. b. Leakage at the joints of moving and stationary ring inserts or adhesives causes frequent leakage of the pump. Residual deformation due to irrational installation process, improper materials, unsatisfactory interference, and deterioration of the adhesive can all cause joints. leakage. c. The friction pair is damaged or deformed and cannot run and cause leakage. d. The friction pair traps particles. e. The spring is too small compared to the pressure. f. Incorrect seal material selection and failure of swelling. g. V-shaped seal installed anti. h. Dynamic and static ring seals face an axis with a non-perpendicular error. i. After the seal is pressed tightly, the drive pin and anti-rotation pin hold the part. j. The big spring is not rotating. k. Shaft vibration. l. Scale formed between the moving and stationary rings and the shaft sleeve cannot compensate for wear displacement. m. There is a groove or pit erosion in the bushing area where the seal is installed. n. The face pressure is too large and the surface of the ring is cracked. o. Poor ring floatability. p. Problems with the auxiliary device. 4. Abrupt Leaks A sudden leak in the pump seal occurs for the following reasons. (1) The pump vibrates violently and the evacuation destroys the friction pair. (2) The spring is broken. (3) The anti-rotation pin is lost or the drive pin breaks and loses its function. (4) The failure of the auxiliary device causes the sudden change of the hot and cold rings of the moving and stationary rings to cause the deformation or crack of the sealing surface. (5) As the temperature changes, the surrounding medium of the friction pair condenses and the crystallization affects the sealing. 5. When the pump is stopped for a period of time and then expenditure occurs, the solidification and crystallization of the medium near the leakage friction pair occur, and the friction pair has scale; the spring is rusted and clogged and loses elasticity, which can cause leakage when the pump is restarted.
What are the failures of mechanical seals? How to deal with?
Mechanical seal failure and handling methods are as follows: First, the mechanical seal failure performance on the part: 1, the sealing end of the failure: wear, hot cracking, deformation, damage (especially the non-metallic seal face). 2, spring failure: relaxation, fracture and corrosion. 3. Failure of auxiliary seals: Assembled failures include blocks, cracks, bumps, curls, and distortions; unassembled failures are deformed, hardened, broken, and deteriorated. Mechanical seal faults appear to vibrate, heat, wear, and eventually appear as media leaks. Second, mechanical seal vibration, heat analysis and treatment of the reasons 1, the static and dynamic ring end rough. 2. The gap between the static and dynamic rings and the seal chamber is too small, due to the impact caused by the oscillation. Treatment method: increase the inner diameter of the seal cavity or reduce the outer diameter of the rotating part, at least to ensure the gap of 0.75mm. 3. The corrosion resistance and temperature resistance of the sealed end face are poor, and the pairing of the friction pair is improper. Treatment method: Change the dynamic and static ring material to make it temperature resistant and corrosion resistant. 4. Insufficient cooling or end surfaces contain particulate impurities during installation. Treatment method: Increase the diameter of the coolant pipe or increase the hydraulic pressure. Third, the mechanical seal leakage analysis and treatment 1, leakage during static pressure test (1) seal face installation was bumped, deformed, damaged. (2) When the sealing face is installed, the cleaning is not clean and there are particulate impurities. (3) Sealed end face Because the set screw is loose or not tightened, the gland (static stationary ring assembly is the pressure plate) is not compressed. (4) The precision of the machine and equipment is not enough, so that the sealing surface is not completely fitted. (5) The static and dynamic ring seals are not pressed or compressed or damaged. (6) The direction of the V-ring seal is reversed. (7) If it is a bushing leak, the bushing seal assembly is not pressed or compressed or damaged when it is assembled. Treatment method: The inspection and cleaning during assembly should be strengthened and the assembly must be strictly in accordance with the technical requirements. 2. Periodic or paroxysmal leakage (1) The rotor assembly has too much axial turbulence. Treatment method: Adjust the thrust bearing so that the shaft’s turbulence is not more than 0.25mm. (2) Periodic vibration of the rotor assembly. Treatment method: Find out the reason and eliminate it. (3) The pressure in the sealed chamber often changes greatly. Treatment method: Stable process conditions. 3. Frequent leakage (1) Frequent leakage due to seal face defects. a. Spring compression (mechanical seal compression) is too small. b. The spring compression is too large and the graphite ring cracks. c. The width of the sealing face is too small and the sealing effect is poor. Treatment method: increase the width of the sealing face, and increase the spring force accordingly. d, compensating the floating performance of the sealing ring is too bad (the sealing ring is too hard or too long to use the hardening or compression is too small to compensate for the sealing ring gap is too small). Treatment method: To compensate the seal ring gap is too small, increase the gap of the compensation seal ring. e. Adhesion of the joints of inserting or bonding moving and stationary rings (poor setting process, residual deformation, uneven material, uneven adhesive, and deformation). f, moving, static ring damage or cracks. g. The seal face is seriously worn and the compensation ability disappears.
