I. Overview of Mechanical Seals
A mechanical seal (face seal) is a device used to seal the connection between a rotating shaft and the machine body. It consists of at least one pair of end faces perpendicular to the axis of rotation. The fluid pressure, the elastic force (or magnetic force) of a compensating mechanism, and the cooperation of auxiliary seals maintain a tight seal and allow for relative sliding, thus preventing fluid leakage. Mechanical seals are commonly used in rotating fluid machinery such as pumps, compressors, and reaction vessels, as well as in gearboxes and ship stern shafts. Therefore, a mechanical seal is a versatile shaft sealing device.
Mechanical seals come in various structures, with the most common being face seals. In a face seal, the stationary and rotating rings form a friction pair to prevent media leakage. This requires both the stationary and rotating rings to have good wear resistance. The rotating ring must be able to move flexibly axially to automatically compensate for wear on the sealing surface, ensuring a good fit with the stationary ring. The stationary ring must also have a floating property, acting as a buffer. Therefore, the sealing surfaces require high-quality machining to guarantee a good fit between the sealing surfaces. The basic components of a mechanical seal include the stationary ring, rotating ring, gland, push ring, spring, locating ring, shaft sleeve, rotating ring seal, stationary ring seal, and shaft sleeve seal.
Elastic elements (springs, bellows) mainly serve the functions of pre-tightening, compensation, and buffering. They are required to maintain sufficient elasticity to overcome the friction of auxiliary seals and transmission components and the inertia of the rotating ring, ensuring good fit of the end face sealing pair and the following of the rotating ring. The materials are required to be corrosion-resistant and fatigue-resistant.
Auxiliary seals (O-rings, V-rings, U-rings, wedge rings, and other shaped rings, etc.) primarily seal the stationary and rotating rings, while also providing floating and cushioning functions. The stationary ring sealing element must ensure a tight seal between the stationary ring and the gland, and the stationary ring must have a certain degree of floating capability. The rotating ring sealing element must ensure a tight seal between the rotating ring and the shaft or bushing, and the rotating ring must also have some floating capability. Materials must be heat-resistant, etc.
1. Basic Structure
1) End face sealing friction pair
2) Buffering and compression mechanism
3) Auxiliary seal (secondary sealing flexible element)
4) Transmission mechanism
2. Basic sealing points
1) Main sealing point on the end face
2) Auxiliary sealing point between stationary ring and end cap
3) Auxiliary sealing point between the rotating ring and the shaft (shaft sleeve)
4) Sealing point between end cover and pump body
II. Advantages and disadvantages of mechanical seals
1. Advantages
(1) The structure is reliable and the leakage can be limited to a very small amount. As long as the surface roughness and straightness of the main sealing surface can meet the requirements and the material has good wear resistance, the mechanical seal can achieve a very small leakage, or even leakage that is not visible to the naked eye.
(2) Long service life. In mechanical seals, the main wear part is the end face of the sealing friction pair. Because the wear of the sealing end face is not large under normal working conditions, it can generally be used continuously for 1 to 2 years, and in special cases it can be used for 5 to 10 years.
(3) No adjustment is required during operation. Since the mechanical seal relies on spring force and fluid pressure to keep the friction pair in contact, it automatically maintains contact during operation. After assembly, it does not need to be adjusted and tightened like ordinary soft packing.
(4) It has vibration resistance. The maximum amplitude does not exceed 0.05 mm at a rotation speed of 3000 r/min. The PV value is continuously improving.
(5) Low power loss. Packing seals function by the compression of the packing onto the shaft or shaft sleeve. The packing seal rubs directly against the shaft; the tighter the packing is compressed, the greater the friction and the higher the power consumption. Mechanical seals, on the other hand, operate in a semi-fluid friction state with a very low coefficient of friction. The power loss of a mechanical seal is 10-50% of that of a packing seal.
(6) The bellows seal shaft or bushing is not worn and is not sensitive to the oscillation of the rotating shaft and the misalignment of the shaft relative to the housing.
(7) Wide range of applications. Mechanical seals can ensure sealing when the medium is flammable, explosive, toxic or harmful. It is also suitable for sealing equipment with high temperature, low temperature, high pressure, vacuum, various speeds and corrosive media.
2. Disadvantages
(1) The structure is more complex than that of a packing seal, requires higher processing precision, and requires certain installation techniques, especially the installation requirements for dry gas seals are even higher. Moreover, sealing technology is developing rapidly, and new technologies are constantly emerging, bringing new challenges to our maintenance.
(2) Complex structure and inconvenient assembly and disassembly. Compared with other seals, mechanical face seals have a large number of parts, require precision, and have a complex structure. They are particularly difficult to assemble. When disassembling, the sealing ring must be pulled out from the shaft end, and the machine part (coupling) or the entire machine must be disassembled. This problem has been improved to some extent, for example, by using split-type and cartridge-type mechanical seals that are easy to disassemble and assembly and can ensure assembly quality.
III. Working Principle of Mechanical Seals
Mechanical seals, also known as face seals, rely on one or more pairs of end faces that slide relative to each other perpendicular to the shaft. Under the action of fluid pressure and the elasticity (or magnetic force) of the compensation mechanism, they maintain contact with the other end through the cooperation of auxiliary seals and slide relative to each other, thereby preventing fluid leakage.
IV. Selection of Commonly Used Materials for Mechanical Seals
Clear water, room temperature: (dynamic) 9Cr18, 1Cr13 cobalt-chromium-tungsten overlay, cast iron; (static) resin-impregnated graphite, bronze, phenolic plastic.
River water (containing silt), at room temperature: (dynamic) tungsten carbide, (static) tungsten carbide.
Seawater, at room temperature: (dynamic) tungsten carbide, 1Cr13 cobalt-chromium-tungsten surfacing, cast iron; (static) resin-impregnated graphite, tungsten carbide, cermet.
Superheated water at 100 degrees Celsius: (Dynamic) Tungsten carbide, 1Cr13 cobalt-chromium-tungsten surfacing, cast iron; (Static) Resin-impregnated graphite, tungsten carbide, cermet.
Gasoline, lubricating oil, liquid hydrocarbons, at room temperature: (dynamic) tungsten carbide, 1Cr13 cobalt-chromium-tungsten surfacing, cast iron; (static) resin-impregnated or tin-antimony alloy graphite, phenolic plastics.
Gasoline, lubricating oil, liquid hydrocarbons, 100 degrees Celsius: (dynamic) tungsten carbide, 1Cr13 cobalt-chromium-tungsten surfacing; (static) bronze or resin-impregnated graphite.
Gasoline, lubricating oil, liquid hydrocarbons, containing particles: (dynamic) tungsten carbide; (static) tungsten carbide.
V. Types and Applications of Sealing Materials
Sealing materials should meet the requirements of their sealing function. Due to differences in the sealed medium and the operating conditions of the equipment, sealing materials require different adaptability. The general requirements for sealing materials are:
1) The material has good density and is not easy to leak media.
2) It has appropriate mechanical strength and hardness.
3) It has good compressibility and resilience, and small permanent deformation.
4) It does not soften or decompose at high temperatures, nor does it harden or crack at low temperatures.
5) It has good corrosion resistance and can work for a long time in media such as acids, alkalis and oils. Its volume and hardness change little and it does not adhere to the metal surface.
6) Low coefficient of friction and good wear resistance.
7) It has the flexibility to bond with the sealing surface.
8) It has good aging resistance and is durable.
9) It is easy to process and manufacture, inexpensive, and easy to obtain materials.
VI. Key Techniques for Mechanical Seal Installation and Use
1) The radial runout of the equipment shaft should be ≤0.04 mm, and the axial movement should not exceed 0.1 mm;
2) The sealing parts of the equipment should be kept clean during installation. The sealing parts should be cleaned and the sealing end face should be intact to prevent impurities and dust from entering the sealing parts.
3) During installation, impacts and knocks are strictly prohibited to avoid damage to the mechanical seal friction pair and seal failure;
4) Apply a layer of clean machine oil to the surfaces in contact with the seal during installation to ensure smooth installation;
5) When installing the stationary ring cover, the screws must be tightened evenly to ensure that the end face of the stationary ring is perpendicular to the axis.
6) After installation, push the moving ring by hand to make it move flexibly on the shaft and it has a certain degree of elasticity;
7) After installation, rotate the shaft by hand; the shaft should feel neither too heavy nor too light.
8) The equipment must be filled with the medium before operation to prevent dry friction from causing the seal to fail.
VII. Mechanical Seal Flushing Scheme and Features
The purpose of rinsing is to prevent the accumulation of impurities, prevent the formation of air pockets, and maintain and improve lubrication. When the rinsing fluid is at a low temperature, it also has a cooling effect. The main rinsing methods are as follows:
(a) Internal flushing
1. Backflush
1) Features: The sealed medium is introduced into the sealing cavity from the outlet end of the pump through a pipeline using the working host.
2) Application: Used for cleaning fluids, p1 is slightly larger than pin. When the temperature is high or there are impurities, coolers, filters, etc. can be installed on the pipeline.
2. Backwashing
1) Features: The sealed medium of the working host is introduced into the sealing cavity from the outlet end of the pump, and after flushing, it flows back to the pump inlet through the pipeline.
2) Application: Used for cleaning fluids, where p<p1<p<p. When the temperature is high or there are impurities, coolers, filters, etc., can be installed on the pipeline.
3. Full rinse
1) Features: The sealed medium of the working host is introduced into the sealing cavity from the outlet end of the pump through the pipeline, flushed and then flows back to the pump inlet through the pipeline.
(ii) External rinsing
Features: Introduces a clean fluid compatible with the sealed medium from an external system into the sealing cavity for flushing.
Application: The external flushing fluid pressure should be 0.05-0.1 MPa higher than the sealed medium. This is suitable for applications involving high-temperature media or solid particles. The flushing fluid flow rate should ensure heat removal while meeting flushing requirements without causing erosion of the seals. Therefore, the pressure in the sealing cavity and the flushing flow rate must be controlled; generally, the flow rate of the cleaning flushing fluid should be less than 5 m/s.
For slurry liquids containing particles, the flow rate must be less than 3 m/s. To achieve the above flow rate, the pressure difference between the flushing fluid and the sealing cavity should be <0.5 MPa, generally 0.05-0.1 MPa. For double-end mechanical seals, it can be 0.1-0.2 MPa. The orifice positions for the flushing fluid entering and exiting the sealing cavity should be located near the sealing end face, and should be close to the moving ring side. To prevent the graphite ring from being eroded or deformed due to temperature difference caused by uneven cooling, as well as the accumulation of impurities and coking, tangential introduction or multi-point flushing can be used. If necessary, the flushing fluid can be hot water or steam.
VIII. Analysis of Typical Failure Causes of Mechanical Seals
(a) Problems with the mechanical seal itself
1. The inlay is not in place or is uneven.
2. The load factor is too large or the end face specific pressure design is unreasonable.
3. Inappropriate material selection.
4. The sealing surface is uneven.
5. The sealing surface is too wide or too narrow.
(ii) Auxiliary system issues
1. The working conditions are complex, but there are no auxiliary facilities such as washing facilities.
2. The flushing pipe is clogged.
3. Scale buildup on the cooling pipes.
(III) Media and Operating Conditions
1. The medium is highly corrosive.
2. The medium contains solid particles.
3. Evacuate the equipment.
4. Crystallization on the sealing surface.
5. The viscosity of the medium is too high.
(iv) Pump problems
1. Poor machining accuracy of the shaft, shaft misalignment, runout, excessive installation clearance.
2. The pump vibrates too much after it is turned on.
3. The gland gasket is faulty.
4. The sealed box is not level.
5. The mechanical seal was not installed to achieve the required compression.
IX. Common Leakage Phenomena
Mechanical seal leakage accounts for more than 50% of all pump repairs. The proper functioning of the mechanical seal directly affects the normal operation of the water pump. The following is a summary and analysis:
1. Periodic leakage
(1) The pump rotor has a large axial movement, the auxiliary seal has a large interference fit with the shaft, and the dynamic ring cannot move flexibly on the shaft. After the pump reverses and the dynamic and static rings wear, no compensation displacement is obtained.
Countermeasures: When assembling the mechanical seal, the axial movement of the shaft should be less than 0.1mm, and the interference fit between the auxiliary seal and the shaft should be appropriate. While ensuring radial sealing, the rotating ring should be able to move flexibly on the shaft after assembly (the rotating ring should be able to spring back freely when pressed against the spring).
(2) Insufficient lubricating oil on the sealing surface causes dry friction or roughening of the sealing end face.
Solution: The lubricating oil level in the oil chamber should be increased to be higher than the sealing surfaces of the dynamic and static rings.
(3) Rotor periodic vibration. The cause is that the stator is not aligned with the upper and lower end covers or the impeller and main shaft are unbalanced, resulting in cavitation or bearing damage (wear). This will shorten the seal life and cause leakage.
Solution: The above problems can be corrected according to the maintenance standards.
2. Leakage caused by pressure
(1) Mechanical seal leakage caused by high pressure and pressure wave: When the spring specific pressure and total specific pressure are designed too large and the pressure in the sealing cavity exceeds 3MPa, the specific pressure of the sealing end face will be too large, the liquid film will be difficult to form, the sealing end face will be severely worn, the heat generation will increase, and the sealing surface will be thermally deformed.
Countermeasures: When assembling the mechanical seal, the spring compression must be performed according to regulations; excessive or insufficient compression is not permitted. For mechanical seals operating under high pressure, appropriate measures should be taken. To ensure reasonable stress on the end face and minimize deformation, high-pressure-strength materials such as hard alloys and ceramics can be used, along with enhanced cooling and lubrication measures.
(2) Mechanical seal leakage caused by vacuum operation: During the start-up and shutdown of the pump, due to the blockage of the pump inlet and the presence of gas in the pumped medium, negative pressure may occur in the sealing cavity. If there is negative pressure in the sealing cavity, it will cause dry friction on the sealing end face. The internal mechanical seal will leak air (water). The difference between vacuum seal and positive pressure seal lies in the difference in the directionality of the sealing object. Moreover, mechanical seal also has its adaptability in a certain direction.
Solution: Use a double-end mechanical seal, which helps improve lubrication conditions and enhance sealing performance.
3. Leakage caused by the medium
(1) After disassembly, the auxiliary seals of most submersible sewage pumps are inelastic and some have rotted, causing a large amount of leakage and even shaft wear. Due to the corrosive effect of high temperature, weak acid and weak alkali in sewage on the auxiliary rubber seals of stationary and dynamic rings, the mechanical leakage is too large. The rubber sealing ring material of dynamic and stationary rings is nitrile-40, which is not resistant to high temperature and acid and alkali. It is easily corroded when the sewage is acidic or alkaline.
Countermeasures: For corrosive media, fluororubber that is resistant to high temperatures, weak acids, and weak alkalis should be selected for rubber parts.
(2) Mechanical seal leakage caused by solid particles If solid particles enter the sealing end face, they will scratch or accelerate the wear of the sealing end face. The accumulation rate of scale and oil on the shaft (sleeve) surface exceeds the wear rate of the friction pair, causing the dynamic ring to be unable to compensate for wear displacement. The service life of hard-to-hard friction pair is longer than that of hard-to-graphite friction pair because solid particles will be embedded in the sealing surface of the graphite sealing ring.
Countermeasure: Tungsten carbide-to-tungsten carbide friction pairs should be used for mechanical seals in locations where solid particles can easily enter.
4. Mechanical seal leakage caused by other problems
There are still some shortcomings in the design, selection, and installation of mechanical seals.
(1) The spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The error is ±2mm. If the compression is too large, it will increase the end face pressure and generate too much frictional heat, causing thermal deformation of the sealing surface and accelerating end face wear. If the compression is too small, the end face pressure of the dynamic and static rings will be insufficient, and the seal will not be able to be sealed.
(2) The end face of the shaft (or bushing) where the dynamic ring seal is installed and the end face of the sealing cover (or housing) where the static ring seal is installed should be chamfered and polished to avoid damaging the dynamic and static ring seals during assembly.
10. Issues related to the normal operation and maintenance of mechanical seals
1. Preparations and precautions before startup
A comprehensive inspection was conducted to ensure that the mechanical seal, auxiliary devices, and pipeline installations were complete and met technical requirements.
b. Before starting the mechanical seal, a static pressure test should be performed to check for leaks. If there is significant leakage, the cause should be identified and eliminated. If the problem persists, the seal should be disassembled, inspected, and reinstalled. The static pressure test pressure is generally 2-3 kg/cm².
c. Rotate the pump in the correct direction and check if it rotates smoothly and evenly. If rotation is difficult or impossible, check if the assembly dimensions are incorrect and if the installation is proper.
2. Installation and shutdown
Before startup, the sealed cavity should be filled with liquid. When conveying solidified media, the sealed cavity should be heated with steam to melt the media. The engine must be rotated before startup to prevent sudden start-up from causing the soft ring to break.
For mechanical seals utilizing an external oil sealing system, the oil sealing system should be started first. After the pump is stopped, the oil sealing system should be stopped last.
After the hot oil pump stops running, the cooling water for the oil sealing chamber and end face seal should not be stopped immediately. The cooling water should only be stopped when the oil temperature at the end face seal drops below 80 degrees Celsius to avoid damaging the sealing parts.
3. Operation
If a slight leak occurs after pump A is started, it should be observed for a period of time. If the leak does not decrease after 4 hours of continuous operation, the pump should be stopped and inspected.
The operating pressure of pump b should be stable, with pressure fluctuations not exceeding 1 kg/cm².
During operation, pump c should be kept away from cavitation to avoid dry friction on the sealing surface and damage to the seal.
Mechanical seals are precision components with high requirements for design, machining, and assembly quality. When using mechanical seals, it is essential to analyze various factors to ensure they are suitable for the technical requirements of different pumps and the media being used, and that they have adequate lubrication. This is crucial to guaranteeing long-term reliable operation of the seal.
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