Overview
Mechanical seal refers to a device that prevents fluid leakage by keeping at least one pair of end faces perpendicular to the axis of rotation in contact and sliding relative to each other under the action of fluid pressure and compensation mechanism elasticity (or magnetic force), as well as the cooperation of auxiliary seals.
The elastic loading mechanism and auxiliary seal are mechanical seals of metal bellows, which we call metal bellows seals. In lightweight seals, rubber bellows are also used as auxiliary seals. Rubber bellows have limited elasticity and generally require springs to meet the loading elasticity. Mechanical seals are commonly referred to as "machine seals".
Mechanical seal is a shaft sealing device for rotating machinery. For example, equipment such as centrifugal pumps, centrifuges, reactors, and compressors. Due to the transmission shaft penetrating inside and outside the equipment, there is a circumferential gap between the shaft and the equipment, through which the medium in the equipment leaks outwards. If the pressure inside the equipment is lower than atmospheric pressure, air will leak into the equipment. Therefore, there must be a shaft sealing device to prevent leakage. There are many types of shaft seals, and due to their advantages of low leakage and long service life, mechanical seals are the main shaft seal method in these devices worldwide. Mechanical seal, also known as end face seal, is defined in relevant national standards as follows: "A device that prevents fluid leakage by maintaining contact and relative sliding between at least one pair of end faces perpendicular to the axis of rotation under the action of fluid pressure and compensation mechanism elasticity (or magnetic force), as well as the cooperation of auxiliary seals.

compose
main components
Dynamic ring, static ring, cooling device, and compression spring (depending on the specific equipment).

Auxiliary seals
Sealing rings (including O-ring, X-shaped, U-shaped, wedge-shaped, rectangular flexible graphite, PTFE coated rubber O-ring, etc.).
Elastic compensation mechanism
Spring, push ring.
Spring seat and keys or various screws.

principle
Mechanical seal is a shaft seal device that achieves leakage prevention by maintaining contact and auxiliary sealing through a pair of containerized mechanical seals or several pairs of end faces that slide relative to the axis under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism.
The commonly used mechanical seal structure consists of a stationary ring, a rotating ring, an elastic element spring seat, a set screw, a rotating ring auxiliary sealing ring, and a stationary ring auxiliary sealing ring. The anti rotation pin is fixed on the cover to prevent the stationary ring from rotating.
Rotating rings and stationary rings can often be referred to as compensating rings or non compensating rings based on whether they have axial compensation capability.

Technical Requirements
Sealing ring is the general term for dynamic and static rings, and is the main component that constitutes mechanical seals. The sealing ring largely determines the performance and service life of mechanical seals, therefore, some requirements are put forward for it.

Having sufficient strength and rigidity
Under working conditions such as pressure, temperature, and sliding speed, it should not be damaged, and deformation should be minimized as much as possible to maintain sealing even when working conditions fluctuate. Especially the sealing end face should have sufficient strength and a certain degree of corrosion resistance to ensure that the product has a satisfactory service life.

Has good heat shock resistance
Therefore, it is required that the material has a high thermal conductivity and a small coefficient of linear expansion, so as not to crack when subjected to thermal shock.

Smaller coefficient of friction
The sealing ring matching should have a smaller friction coefficient.

Good self-lubricating properties
If there is short-term dry friction during work, it should not damage the sealing end face. Therefore, the sealing ring should have good self-lubricating properties, and the sealing ring material should also have good wettability with the sealing fluid.

simple structure
The structure of the sealing ring should strive for simplicity and symmetry, with priority given to using a monolithic structure or a combination type (such as an embedded type) sealing ring. It is advisable to avoid using a sealing end face spray coating structure as much as possible.

The sealing ring should be easy to process and manufacture
Installation and maintenance should be convenient and cost-effective.

Pay attention to the issue
During installation
1. Be very careful to avoid installation deviations that may occur during installation
(1) Tightening the gland should be done after aligning the coupling, and the bolts should be evenly supported to prevent the end face of the gland from tilting. Use a feeler gauge to check each point, and the error should not exceed 0.05 millimeters.
(2) Check the fit clearance (i.e. concentricity) between the gland and the outer diameter of the shaft or sleeve, ensuring uniformity around the edges. Use a feeler gauge to check that the allowable deviation at each point is not greater than 0.01 millimeters.
2. The compression amount of the spring should be carried out according to the regulations, and there should be no excessive or insufficient phenomenon. The required error is 2.00 millimeters. Excessive pressure will increase the end face ratio and accelerate end face wear. Being too small can result in insufficient specific pressure and fail to provide a sealing effect.
3. After the installation of the dynamic ring, it should be able to automatically bounce back when pressed against the spring.

Position requirements during installation
It is very important to correctly install the mechanical seal on the machine in order to ensure its stable operation, long service life, and low leakage performance. Precautions to be taken during installation:
1. Confirmation of components
When installing the mechanical seal on the machine, it is necessary to compare it well with the final assembly drawing before installation to confirm whether the parts are ready. At this time, attention should be paid to whether there are any scratches, defects or other abnormal phenomena on the sealing friction pair sealing surface, sealing ring, etc. Also, attention should be paid to whether there are scratches on the shaft or shaft sleeve surface, flange and other components that come into contact with the packing, sealing ring (O-ring), etc. If any abnormal phenomena are found, they must be replaced or repaired before use.
During actual installation, do not bring more parts than necessary to the site. This way, if there are any remaining parts after installation, it indicates that there were missing parts during installation; If there are insufficient parts, it means that unnecessary parts have also been assembled, which serves as a self check during installation.

2. Installation location
The installation essentials vary depending on the type of mechanical seal and the type of machine. I won't go into detail here.


Technical requirements for installation
Mechanical seals are high-precision mechanical components, and their correct installation and operation have a significant impact on their service life. We generally follow the standards set by the Ministry of Petrochemical Industry.
The technical requirements for the installation of pumps with mechanical seals and the coordination of mechanical seals are as follows:
1. Axis curvature: not greater than 0.05 millimeters;
2. Rotor oscillation: not more than 0.06 millimeters near the shaft sleeve at the sealing ring of the dynamic ring;
3. The axial displacement of the shaft is not allowed to exceed ± 0.5 millimeters. If there is a shaft sleeve, it is not allowed for the shaft sleeve to become loose;
4. The alignment error of the coupling: for toothed couplings, it should not exceed 0.08-0.10 millimeters (P2008C), and for elastic couplings, it should not exceed 0.05-0.06 millimeters; (Habitual practice: end jump<0.05, radial jump<0.10)
5. The concentricity tolerance of the sealing fitting joint between the gland (static ring seat) and the shaft centerline is 0.05 millimeters, and the perpendicularity tolerance of the plane in contact with the gasket to the centerline is 0.03-0.05 millimeters. If the requirements are not met, the sealing chamber needs to be processed;
6. The end of the shaft sleeve for installing the dynamic ring seal and the end of the gland (or housing) for installing the static ring seal should be chamfered and smoothed.
1. Be careful when installing and disassembling mechanical seals, and it is strictly prohibited to use hammers and flat shovels to avoid damaging the sealing components. If scaling cannot be removed, it should be cleaned thoroughly before disassembly.
2. If mechanical seals are used at both ends of the pump, take care of each other during assembly and disassembly to prevent neglecting one aspect.
3. For mechanical seals that have been operated, if the cover becomes loose and causes the seal to move, the dynamic and static ring parts must be replaced and should not be re tightened for continued use. Because the original operating trajectory of the friction pair will change after loosening, the sealing performance of the contact surface is easily damaged.

maintenance problems
Preparation work before startup
1. Conduct a comprehensive inspection of the mechanical seal, as well as the installation of auxiliary devices and pipelines, to ensure that they are complete and meet the technical requirements.
2. Perform a static pressure test before starting the mechanical seal to check for any leakage. If there are many leaks, the cause should be investigated and efforts should be made to eliminate them. If it is still ineffective, it should be disassembled, inspected, and reinstalled. The general static pressure test pressure is 2-3 kilograms per square centimeter.
3. Rotate the pump towards the turning wheel and check if it is light and even. If the turning gear is difficult or stationary, the assembly dimensions should be checked for errors and the installation should be reasonable.

Installation and shutdown
1. Before starting, the sealed chamber should be filled with liquid. When transporting solidified media, steam should be used to heat the sealed chamber and melt the media. Before starting, it is necessary to turn the car to prevent sudden starting from causing the soft ring to break.
2. For mechanical seals that utilize the pump's external sealing oil system, the sealing oil system should be started first. Stop the oil sealing system after parking.
3. After the hot oil pump is shut down, the cooling water for the sealing chamber and end face seal cannot be stopped immediately. The cooling water should be stopped only when the oil temperature at the end face seal drops below 80 degrees to avoid damaging the sealing parts.

operation
1. If there is a slight leakage after the pump is started, it should be observed for a period of time. If the leakage does not decrease after running continuously for 4 hours, the pump should be stopped for inspection.
2. The operating pressure of the pump should be stable, with pressure fluctuations not exceeding 1 kilogram per square centimeter.
3. During the operation of the pump, the phenomenon of vacuum should be avoided to prevent dry friction and seal damage on the sealing surface.
3. The sealing condition should be checked regularly. During operation, when the leakage exceeds the standard, the heavy oil should not exceed 5 drops/minute and the light oil should not exceed 10 drops/minute. If there is still no improvement trend within 2-3 days, the pump should be stopped to check the sealing device.

Machine seal malfunction
故障原因
Mechanical seal failures are generally known to people only after abnormal leakage, wear, torque, and other phenomena occur. There are roughly four reasons for the malfunction:
1. The design and selection of mechanical seals are incorrect;
2. Poor quality of mechanical seals;
3. The accuracy of the machine using or installing mechanical seals does not meet the requirements;
4. Machine operation error.
circumstances alter cases

seal failure
1. There are three main reasons for seal failure:
(1) Open the sealing surface
When repairing mechanical seals, 85% of seal failures are not caused by wear and tear, but by leakage before wear and tear.
When the sealing surface is opened, solid particles in the medium enter the sealing surface under the action of liquid pressure. After the sealing surface is closed, these solid particles are embedded in the surface of the soft ring (usually graphite ring), which actually becomes a "grinding wheel" and damages the surface of the hard ring.
Due to the fastening of the dynamic ring or rubber ring on the shaft (shaft sleeve), when the shaft moves in series, the dynamic ring cannot fit in time, causing the sealing surface to open and the sealing surface to close behind, allowing solid particles to enter the sealing surface.
At the same time, there are solid particles between the shaft (sleeve) and the sliding parts, which affect the sliding of the rubber ring or dynamic ring (relative dynamic sealing point, common faults). In addition, the medium will also produce crystals at the friction between the rubber ring and the shaft (sleeve), and there will also be solid substances at the spring, which will cause the sealing surface to open.

(2) Overheating
Due to the heat generated on the sealing surface, the operating temperature of the rubber ring should be lower than the design specifications. The usage temperature of fluororubber and polytetrafluoroethylene is 216 ℃, and the usage temperature of nitrile rubber is 162 ℃. Although they can both withstand high temperatures, there is a risk of further vulcanization of the rubber ring due to the high heat generated by the sealing surface, which ultimately leads to loss of elasticity and leakage. (Cold zone considered brittle) The sealing surfaces may also crystallize due to heat, such as carbon deposition, causing sliding parts to stick and sealing surfaces to condense. Moreover, some polymers may coking due to overheating, and some fluids may lose lubrication or even flash fire due to overheating.
Overheating not only changes the condition of the medium, but also exacerbates its corrosion rate. To cause deformation of metal parts, cracking of alloy surfaces, and certain coating cracks, balanced mechanical seals should be selected in the design to reduce specific pressure and prevent overheating.

(3) Out of tolerance
Correct assembly tolerances are necessary for installing mechanical seals. The shaft (bushing) must have appropriate surface roughness and correct dimensions, but manufacturers rarely provide tolerance data, which is crucial for installation. (Relying on experience and common sense)
The dimensional accuracy and positional tolerance of mechanical seals must meet the requirements of the drawings, and exceeding the tolerance will result in premature failure of the seal.

2. Analysis of Sealing Failure Causes
The sealing surface itself can also provide signs of sealing failure, such as wear marks on transmission parts during vibration. If the marks are not obvious, it is generally caused by improper assembly.
For low-quality graphite rings (dynamic rings), there are more internal pores due to the expansion of gas accumulated inside the graphite during the manufacturing process, which blows out carbon particles. Therefore, during the sealing process, the carbon particles of this low-quality graphite ring are easily detached, causing the sealing surface to stick when the sealing is stopped.
The scratches on the cylindrical surface inside the sealing surface are likely caused by foreign objects entering the sealing surface or improper installation. The annular grooves on the sealing surface are mostly caused by solid particles depositing on the sealing surface.
The cracks in graphite rings (dynamic rings) are caused by the vibration of transmission components, the enlargement of rubber rings, and the internal stress of graphite rings themselves, while coking is caused by high temperature, which is common in high-temperature hot oil media in refineries.
Several strong oxidants such as fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and hydroxides have an erosive effect on graphite, and their corrosive effect intensifies with increasing temperature.
Usually, overheating on the surface of the hard ring (static ring) can cause severe wear on the sealing ring, such as in a vertical pump without cooling. Under high temperature, high pressure, excessive spring compression, and excessive shaft movement, it can cause excessive wear on the sealing surface,
There are four signs to pay attention to when inspecting the surface of the hard ring:
a、 Ceramic ring rupture; b、 Hot cracking; c、 Scratches; d、 The detachment of the coating.
Tight assembly of ceramic rings is the main cause of breakage, and improper assembly is also a common cause.
Due to the different linear expansion rates of the coating material and the substrate material, cracks may appear on the surface of the ring when the temperature rises, especially in Stellite alloy. Among higher grade coating materials, cobalt based tungsten carbide is inferior to nickel based coatings. Cooling the sealing surface can effectively prevent thermal cracking. Solid particles remaining on the sealing surface often damage the surface. For example, sand particles on the grinding wheel can damage the hard ring surface during grinding, causing the sealing surface to open or forming crystals between the sealing surfaces. After re grinding the graphite ring, the grinding material will embed into the surface of the graphite ring. The failure of rubber rings is related to their usage, and high pressure is usually one of the reasons for the failure of the molded O-ring. When it is found that the O-ring has become rectangular or the ring has hardened, the compression amount needs to be adjusted, otherwise it will generate heat. So it is necessary to understand the operating temperature of a synthetic rubber. The swelling of synthetic rubber rings is mostly caused by chemical erosion, and they all have their own characteristics. For example, fluororubber is resistant to high temperatures, while ethylene and propylene O-rings will swell when used in petroleum lubricants. Ozone has an erosive effect on nitrile rubber, so nitrile rubber products should not be installed in electric motors. Therefore, high temperatures and chemical corrosion are usually the main reasons for rubber product hardening and cracking. Cutting and surface scratches on rubber parts during installation are also common causes of seal failure. Old fixing screws, keyways, spline shafts, sharp shaft shoulders, and other signs on the shaft can damage the rubber parts.
Here, the following points need to be added regarding the wear marks on the sealing surface. Checking the wear marks can help analyze the fault.
(1) Wear and tear widening: indicates a serious misalignment of the pump. The reason is:
a、 Bearing damage;
b、 轴振动或轴变形；
c、 Axis bending;
d、 Pump cavitation generates vibration;
e、 The coupling is not aligned;
f、 Severe deformation of the pipe;
g、 密封静环倾斜。
(2) Narrowing of wear marks: The wear marks are narrower than the width of two sealing surfaces, indicating that the sealing surface is deformed due to overpressure, pressure, or temperature.
(3) No scratches:
The sealing surface is not bonded. Check if the compensating mechanisms such as springs are slipping or obstructed.
(4) The sealing surface has no scratches but highlights.
There will be bright spots but no scratches on the warped sealing surface. High pressure, improperly tightened or clamped gland bolts, or rough pump surfaces can all create bright spots. When using a double bolted cover, its stiffness is insufficient and deformation is also a reason for the formation of bright spots.
The occurrence of this symptom indicates that the seal may leak as soon as the car is driven.
(5) The sealing surface has a cut edge:
This is due to the sealing surface being too separated and breaking when closed. Flash evaporation (gasification) is a common reason for the separation of sealing surfaces, especially in hot water systems or when there is condensation in fluids, water expands from liquid to vapor, which can cause the sealing surfaces to separate. (Gasification of cold medium can also cause)
Sealed metal parts such as springs, fixing screws, transmission components, and metal sleeves can all be the root cause of seal failure. Corrosion of springs subjected to alternating stress is their primary issue, as metals rapidly corrode under stress. Stainless steel springs are susceptible to stress corrosion caused by chlorides, and there are many chlorides in the world. Therefore, some foreign departments recommend using Hastelloy steel springs with higher corrosion resistance instead of stainless steel springs. In addition, spring fatigue caused by improper assembly is another reason for failure.
Do not use hardened materials for the fixing screws used in mechanical seals, as heat treatment can reduce the corrosion resistance of the metal, while softer fixing screws without heat treatment can be tightened on the shaft.
Vibration, deviation, and lack of concentricity can cause wear on transmission components. For example, when the sealing surface is stuck during start-up, the transmission components may bend or even be damaged, and the heat generated by friction often exacerbates corrosion.
The wear marks on the outer surface of the metal sleeve may be caused by solid particles entering the sleeve from the sealing side, which interferes with the sealing's follow-up ability. It may also be caused by deviation or lack of concentricity.
Metals need to change color during the temperature rise process, and stainless steel should pay attention to the color at the following temperatures when used.
Light yellow - Temperature range of 700-800 ℉ (approximately 370-432 ℃)
Brown - Temperature range of 900-1000 ℉ (approximately 486-540 ℃)
Blue - Temperature is 1100 ℉ (approximately 590 ℃)
Black - Temperature is 1200 ℉ (approximately 648 ℃)
When the seal fails and does not meet any of the above criteria, maintenance becomes more difficult, but the following leakage situations can be used as a reference:
（1）、泵轴套泄漏
Many shaft sleeves do not protrude from the sealing box, making it difficult to determine the source of the leak. The leakage of the shaft sleeve is usually stable, while the leakage of the sealing surface often increases or decreases. After the sealing surface leaks, the surface becomes uneven, but sometimes it can also break into its original state. (Sometimes don't rush for maintenance, you can observe for a period of time before speaking)
(2) If the sealed area is damp and there is no visible leakage. The centrifugal force generated by the pump during start-up causes the leaked liquid to return to the sealing surface, acting as a barrier. And the liquid leaking from the flange or joint on the pump drips into the packing box.
(3) Thermal expansion can cause the graphite ring embedded in the metal component to loosen, or it may be due to low temperature causing the O-ring to lose elasticity, resulting in leakage.
(4) Fluctuations in flushing pressure can cause seal failure, and the flushing pressure must be higher than the sealing chamber pressure. Using the solenoid valve and delay switch installed in front of the pump can ensure that the residue in the flushing is cleaned before or after the pump is started or stopped. If quenching is used to control temperature, the pressure in the sealing chamber must be maintained.
(5) If a layer of scale settles on the cooling jacket, we can install a graphite liner at the bottom of the sealing chamber and use its thermal barrier effect to solve these problems.
(6) Leakage of heat exchangers is often caused by scale buildup on the cooling surface that hinders heat transfer, resulting in an increase in fluid flow velocity inside the cooler, or the direction of the heat exchanger being reversed.