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Structural features of a mechanical seal and a mechanical seal <br> <br> principle widely used is a rotary shaft seal assembly movable member, also known as face seals. It consists of at least one pair of end faces perpendicular to the axis of rotation, which acts on the end face by the fluid pressure and the compensating mechanism elastic force (or magnetic force), and the cooperation of the auxiliary seals causes the joint surface to be attached and kept relatively sliding, thereby preventing fluid leakage. the goal of. The mechanical seal works by forming a very thin liquid film between the rotating ring and the shaft, preventing the medium from leaking and allowing the end face to be lubricated, thereby achieving the sealing effect. Mechanical seals are commonly used in rotary machines such as pumps, compressors, and reaction stirred tanks.
There are two common types of pump sealing: mechanical seals and soft packing seals. They each have different characteristics and are also suitable for different occasions. Proper selection is beneficial to reduce costs and improve work efficiency. Mechanical seals have the following advantages over soft-fill seals:
(1) The seal is reliable. In the long-term operation, the sealing state is stable, the leakage is small, and the leakage is about 1% of the soft packing seal. (2) Long service life. It can be used in oil and water media for more than 1~2a, and can be used stably for 6 months in chemical media.
(3) Friction power consumption is small. The friction power is only 10% to 50% of the soft packing seal, and the wear on the shaft or sleeve is basically negligible.
(4) The maintenance period is long. After the end face is worn, it can be repaired within a certain range and can still meet the requirements for use.
(5) Good earthquake resistance. The vibration, yaw of the rotating shaft and the deflection of the shaft to the sealing chamber are not sensitive.
Of course, compared with the soft packing seal, the structure of the mechanical seal is complicated, the processing requirements of the parts are high, and the cost is high; the requirement for the personnel is that the operator needs to have a certain technical level during installation and replacement, master the basic knowledge of the fitter, and understand the machine. The structural characteristics of the seal, especially the handling of sudden accidents in the operation of the pump, require the maintenance personnel to have certain business knowledge and experience accumulation.
2 factors affecting the use of mechanical seals for pumps
The main factors affecting the use of mechanical seals for pumps are selection, installation, and daily maintenance.
2.1 Reasonable selection of the type is the premise of safe and normal operation of the pump. Mechanical seal selection is based on parameters such as working pressure, temperature, shaft diameter, media characteristics, installation environment, and allowable leakage. In the process of using the factory, various standard mechanical seals for pumps can be selected according to JB1472-75.
2.2 Installation After the mechanical seal is selected reasonably and correctly, the installation is an important part to ensure the mechanical seal works. The first thing to emphasize during the installation process is:
(1) The installation process should be kept clean at all times. In particular, the surface of the rotating ring and the stationary ring and the surface of the auxiliary sealing ring should be free from impurities and dust. Never use an unclean cloth to wipe the sealing surface. A small amount of mechanical oil can be applied to the friction surface of the static and dynamic sealing ring to prevent damage to the sealing surface when the water test pump is empty.
(2) It is not allowed to strike the seal with iron tools. The rotating parts should be carefully placed on the shaft, and the fixing bolts should be tightened evenly several times. After the Zui, press the rotating ring backwards to see if the rotating ring is Can float axially. It is particularly important to note that the positive pressure between the moving ring and the stationary ring is adjusted to just seal the working medium, preventing the mechanical seal from being damaged in a short period of time.
(3) The sealing surface of the mechanical seal during installation shall have a certain specific pressure (0.4~0.6MPa), so as to achieve the purpose of sealing, the tightness of the two end faces can be adjusted by the spring. In order to ensure a certain specific pressure, the axial movement of the pump shaft is required to be ≤ 0.5 mm.
(4) The winding of the pump shaft should not be too large. The force between the two sealing end faces of the mechanical seal caused by the assembly is not uniform, the liquid film cannot be formed, and the sealing effect is lost.
(5) Since the vibration will damage the mechanical seal of the pump, appropriate damping measures should be taken for the pump seat.
2.3 Daily maintenance (1) The mechanical seal should be thoroughly inspected before starting, and the attachments and pipelines should be inspected and installed in place as required.
(2) The static pressure test is to be carried out before starting, and the general static pressure is 0.2 to 0.3 MPa. The static pressure test is qualified, the sealed chamber is filled with liquid, and the vehicle is manually driven.
(3) For the mechanical seal using the pump external sealing oil system, the oil sealing system should be started first, and the oil sealing system should be stopped after the parking. If there is a slight leak after the pump is started, it should be observed for a while. If the leakage does not decrease after 4 hours of continuous operation, the pump should be stopped.
(4) Do a good job of daily inspection and recording. Special attention should be paid to the fact that the operating pressure of the pump should be stable and the pressure fluctuation is recommended to be 0.1Mpa.
(5) Inspection of sealing condition, when the leakage exceeds the standard, heavy oil ≯ 5 drops / min, light oil ≯ 10 drops / min, if there is no signs of improvement after 2 days of observation, the pump should be stopped to check the sealing device.
(6) Pay attention to the measures such as lubrication, flushing and cooling during operation.
3 pump mechanical seal common faults and maintenance methods
3.1 Leakage that often occurs during operation (1) Leakage caused by deformation of the contact surface of the static and dynamic rings. When the end face is over-pressured, the friction heat is likely to cause thermal deformation of the static and dynamic ring; of course, if there is uneven force in the process of installing the parts, the end face will be deformed. Disposal method: Disassemble the mechanical seal and check whether the end face of the static and dynamic ring is overheated. If it is, it needs to be replaced. If it is assembled, it can be reassembled according to the specifications.
(2) Impurities such as crystal particles are trapped in the friction pair. Treatment method: After disassembling, clean it, check the end face wear, and repair according to the specific conditions.
(3) The auxiliary seal is not swelled by the working medium and loses its elasticity due to improper material selection. Solution: Replace the new seal and select the matching mechanical seal.
(4) The static and dynamic ring seals face the axis and the verticality error is too large. Solution: Check the winding of the pump shaft and repair or replace it if it exceeds the standard.
(5) The formation of scale by the static and dynamic rings and the round kit cannot compensate for the wear displacement in time. Treatment: clean the scale.
(6) Long-term operation causes severe wear on the end face of the static and dynamic ring. Solution: Replace the mechanical seal.
In short, the leakage caused by the above reasons should be carefully checked and analyzed for damage during the inspection of the equipment, and the symptoms should be dealt with.
3.2 Mechanical seals produce sudden leakage phenomenon The sudden failure of mechanical seals is generally caused by the following reasons: such as the pump's strong vibration, vacuuming and other reasons destroy the working state of the friction pair, the compensation spring breaks, the anti-rotation pin falls off or If the drive pin is broken and the auxiliary device is faulty, the thermal and thermal ring of the static and dynamic ring will suddenly deform or cause cracks. In the event of the above failure, the mechanical seal must be updated before use, and the problems in the operation process can be found out to facilitate the prevention work.
3.3 Leakage occurred when the pump is stopped for a period of time. When any medium near the friction pair is solidified, crystallized, fouled, and spring rust is broken, any such situation will cause leakage when the pump starts running. To completely prevent such phenomena from happening, it is necessary to do all the checks and preparations before the pump starts.
3.4 Leakage occurred during water or static pressure test If the mechanical seal is not installed according to the specifications, leakage will occur when adding water or static pressure test. The installation is not standardized and has the following performances:
(1) The contact surface of the static and dynamic ring is not flat, and there is a bump or damage during installation, resulting in failure of the seal;
(2) The size of the static and dynamic ring seal is incorrect, damaged or not pressed;
(3) There are foreign objects such as rust or sand dust on the surface of the static and dynamic ring;
(4) The seal ring on the static and dynamic ring is leaked or forced into the installation to cause damage;
(5) The moving ring tightening member is not installed in place, so that the spring seat is retracted;
(6) When the pump shaft is moved by hand, if the leakage has obvious directionality, it may be due to uneven spring force or excessive verticality of the sealed cavity and the shaft.
Leakage caused by the above reasons should be handled according to the cause.
3.5 Periodic leakage due to installation, operation, etc. If the pump impeller axial turbulence exceeds the standard or the shaft periodically vibrates and the process parameters are unstable, the pressure in the sealed chamber will often change, resulting in mechanical seals. Periodic leaks. For the mechanical seal leakage caused by these reasons, the axial movement of the pump shaft should be strictly controlled during installation, and the bearing with poor clearance should be replaced in time. The circular runout of the shaft should be controlled within the standard and the new shaft should be replaced if necessary.
3.6 There are two main types of leakage due to leakage pressure caused by pressure:
(1) The mechanical seal leakage caused by high pressure and pressure wave is because the spring specific pressure and total specific pressure design are too large and the pressure in the sealed chamber exceeds 3 MPa, so that the sealing end face is overpressured, the liquid film is difficult to form, and the sealing end face is seriously worn. The heat is increased to cause thermal deformation of the sealing end face. Treatment: When assembling the mechanical seal, the amount of spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. Mechanical seals under high pressure conditions should be taken. In order to minimize the deformation of the end face, a material with high compressive strength such as cemented carbide or ceramics may be used, and lubrication measures for cooling may be enhanced, and a deformable transmission method such as a key or a pin may be selected.
(2) Mechanical seal leakage caused by vacuum operation During the start and stop process, due to the blockage of the pump inlet, the pumping medium contains gas, etc., it may cause negative pressure in the seal cavity, causing dry friction on the seal face, built-in machinery Water leakage (gas) phenomenon occurs in the seal. The difference between the vacuum seal and the positive pressure seal lies in the directional difference of the seal object, and the mechanical seal also has adaptability in a certain direction. Treatment: Double mechanical seals are used to improve lubrication and improve sealing performance.
4 Conclusion
Leakage of mechanical seals is a common problem in pump operation. The solution should be checked from the beginning of the mechanical seal selection, not just the repair. Mechanical seal is a kind of high-demand component. It has requirements for assembly and daily maintenance during use. Therefore, problems should be comprehensively analyzed to affect the various factors affecting the mechanical seal, to find its root cause, and to take effective solutions. Good daily maintenance and maintenance to ensure safe operation of the pump.
About the author: Xiong Yulu, director of workshop equipment, is engaged in equipment maintenance and management, and is familiar with the maintenance of power equipment.
Pumps are indispensable for pharmaceutical companies to transport liquid materials. The leakage of mechanical seals has been plaguing maintenance personnel for many years. Of course, the causes of mechanical seal leakage are various and cannot be generalized. However, in order to accurately judge and quickly find the cause, and to formulate a reasonable maintenance plan, the structure, working principle and factors affecting the normal operation of the mechanical seal must be made. Have some understanding, so that you can get twice the result with half the effort. Combining the experience of pump maintenance accumulated over the years, the reasons for the leakage of mechanical seals are summarized, and the maintenance methods for different phenomena are summarized.