1、 Mechanical seal overview

Mechanical seal (end face seal) is a device used to solve the sealing between rotating shaft and engine body. It is a device to prevent fluid leakage, which is composed of at least one pair of fluid pressure at the end face perpendicular to the rotating axis and the elastic force (or magnetic force) of the compensation mechanism, and the auxiliary seal, which is kept in contact with each other and slides relatively. It is commonly used in rotary fluid machinery such as pumps, compressors, reaction agitators, and also used for sealing gear boxes, ship stern shafts, and so on. Therefore, mechanical seal is a universal shaft seal device.

Mechanical seal structures are various, and the most commonly used mechanical seal structure is face seal. The static ring and dynamic ring of the end face seal form a pair of friction pairs, whose function is to prevent medium leakage. It requires that the static ring and the dynamic ring have good wear resistance. The dynamic ring can move flexibly in the axial direction, automatically compensate the wear of the sealing surface, and make it fit well with the static ring; The static ring is floating and plays a buffer role. Therefore, the sealing surface shall have good processing quality to ensure that the sealing pair has good bonding performance. The basic elements of mechanical seal include static ring, dynamic ring, gland, push ring, spring, locating ring, shaft sleeve, dynamic ring seal ring, static ring seal ring, shaft sleeve seal ring, etc.

Elastic elements (springs, bellows) mainly play the role of pre tightening, compensation and buffering. It is required to always maintain sufficient elasticity to overcome the friction of auxiliary seals and transmission parts and inertia of moving rings, so as to ensure good fit of end seal pairs and tracking of moving rings. Materials are required to be corrosion resistant and fatigue resistant.

The auxiliary seal (O-ring, V-ring, U-ring, wedge ring and special-shaped ring, etc.) mainly serves as the seal of stationary ring and dynamic ring, and also plays the role of floating and buffering. It is required that the sealing elements of the static ring can ensure the tightness between the static ring and the gland, the static ring has a certain floatability, and the sealing elements of the dynamic ring can ensure the tightness between the dynamic ring and the shaft or shaft sleeve, and the floatability of the dynamic ring. Materials shall be heat-resistant.

1. Basic structure

1) Face seal friction pair

2) Buffer compensation and pressing mechanism

3) Auxiliary seal (secondary seal flexible element)

4) Transmission mechanism

2. Basic sealing point

1) Main sealing point of end face

2) Auxiliary sealing points of stationary ring and end cover

3) Auxiliary sealing point between moving ring and shaft (shaft sleeve)

4) Connecting sealing point between end cover and pump body

2、 Advantages and disadvantages of mechanical seal

1. Advantages

(1) The structure is reliable, and the leakage can be limited to a small amount. As long as the surface roughness and straightness of the main sealing surface can meet the requirements, and as long as the wear resistance of the material is good, the mechanical seal can achieve a small amount of leakage, or even the leakage can not be seen by the naked eye.

(2) Long life. In mechanical seals, the main wear part is the end face of the seal friction pair. Because the wear amount of the seal end face is not large under normal working conditions, it can be used continuously for 1 to 2 years in general, and for 5 to 10 years in special occasions.

(3) No adjustment is required during operation. Since the mechanical seal relies on spring force and fluid pressure to make the friction pair stick together and automatically keep contact during operation, it is unnecessary to adjust and compress it like ordinary soft packing after assembly.

(4) It is resistant to vibration. The maximum amplitude does not exceed 0.05mm at the speed of 3000r/min, and the PV value increases continuously

(5) Small power loss. The packing seal works on the shaft or shaft sleeve by the packing. The packing seal directly rubs against the shaft. The tighter the packing is, the greater the friction and power consumption will be. The friction of mechanical seal is in the state of half liquid friction, and the friction coefficient is very small. The power loss of mechanical seal is 10~50% of that of packing seal

(6) The bellows seal shaft or shaft sleeve is not worn, and is not sensitive to the vibration of the rotating shaft and the deflection of the shaft to the housing.

(7) Wide application range. When the medium is flammable, explosive, toxic and harmful, mechanical seal can ensure the sealing. It is also suitable for sealing equipment with high temperature, low temperature, high pressure, vacuum and various rotating speeds and corrosive media.

2. Disadvantages

(1) For example, the structure is more complex than the packing seal, the processing precision is required, and certain installation technology is required, especially the installation of dry gas seal. Moreover, the sealing technology has developed rapidly, and the new technologies have brought us new topics for maintenance.

(2) Complex structure and inconvenient disassembly. Compared with other seals, mechanical face seals have many parts, precise requirements and complex structures. Especially, it is difficult to assemble. When disassembling and assembling, the sealing ring should be pulled out from the shaft end, and the machine part (coupling) or all must be disassembled. At present, some improvements have been made to this problem, for example, the split type and Cartridge mechanical seal Etc.

3、 Working principle of mechanical seal

Mechanical seal, also called end face seal, is a pair or pairs of end faces that slide relative to the shaft under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism, relying on the cooperation of the auxiliary seal to keep fit with the other end and slide relative to prevent fluid leakage.

4、 Selection of Common Materials for Mechanical Seals

Clear water, normal temperature: (dynamic) 9Cr18, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (Static) resin impregnated graphite, bronze, phenolic plastic.

River water (with silt), normal temperature: (dynamic) tungsten carbide, (static) tungsten carbide.

Seawater, normal temperature: (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (Static) resin impregnated graphite, tungsten carbide, cermet.

Superheated water 100 ℃: (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (Static) resin impregnated graphite, tungsten carbide, cermet.

Gasoline, lubricating oil, liquid hydrocarbon, normal temperature: (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten, cast iron; (Static) impregnated resin or tin antimony alloy graphite, phenolic plastic.

Gasoline, lubricating oil, liquid hydrocarbon, 100 ℃: (dynamic) tungsten carbide, 1Cr13 surfacing cobalt chromium tungsten; (Static) impregnated with bronze or resin graphite.

Gasoline, lubricating oil, liquid hydrocarbon, containing particles: (dynamic) tungsten carbide; Tungsten carbide (static).

5、 Types and uses of sealing materials

The sealing material shall meet the requirements of sealing function. Due to different sealed media and different working conditions of equipment, sealing materials are required to have different adaptability. The requirements for sealing materials are generally:

1) The material has good compactness and is not easy to leak media.

2) Have appropriate mechanical strength and hardness.

3) Good compressibility and resilience, small permanent deformation.

4) It does not soften or decompose at high temperature, nor harden or embrittle at low temperature.

5) It has good corrosion resistance and can work in acid, alkali, oil and other media for a long time. Its volume and hardness change little and it does not adhere to the metal surface.

6) Small friction coefficient and good wear resistance.

7) It has the flexibility of combining with the sealing surface.

8) Good aging resistance and durability.

9) Convenient processing and manufacturing, cheap price, easy to obtain materials.

6、 Technical essentials of mechanical seal installation and use

1) The radial runout of the equipment shaft shall be ≤ 0.04mm, and the axial runout shall not be greater than 0.1mm;

2) The sealing part of the equipment shall be kept clean during installation, and the sealing parts shall be cleaned. The sealing end face shall be intact to prevent impurities and dust from entering the sealing part;

3) During installation, it is strictly prohibited to bump or knock to avoid damage to the friction pair of mechanical seal and seal failure;

4) During installation, the surface in contact with the seal shall be coated with a layer of clean mechanical oil for smooth installation;

5) When installing the stationary ring gland, the tightening screws must be evenly stressed to ensure the perpendicularity between the end face of the stationary ring and the axis line;

6) After installation, the moving ring can be moved flexibly on the shaft with a certain elasticity by pushing it manually;

7) After installation, rotate the shaft by hand, and the shaft should have no sense of weight;

8) The equipment must be filled with medium before operation to prevent seal failure due to dry friction.

8、 Mechanical seal flushing scheme and characteristics

The purpose of flushing is to prevent the accumulation of impurities, prevent the formation of air bags, maintain and improve lubrication, etc. When the temperature of flushing fluid is low, it also has a cooling effect. The main flushing methods are as follows:

（1） Internal flushing

1. Positive flushing

1) Features: The sealed medium of the working host is introduced into the sealing chamber through the pipeline from the outlet end of the pump.

2) Application: It is used to clean fluid. p1 is slightly greater than p inlet. When the temperature is high or there are impurities, cooler and filter can be set on the pipeline.

2. Backwash

1) Features: The sealed medium of the working host is introduced into the sealing chamber from the outlet end of the pump, and flows back to the pump inlet through the pipeline after flushing.

2) Application: It is used to clean the fluid, and p inlet<p1<p outlet. When the temperature is high or there are impurities, cooler, filter, etc. can be set on the pipeline.

3. Full flush

1) Features: The sealed medium of the working host is introduced into the sealing chamber through the pipeline from the outlet end of the pump, and then flows back to the pump inlet through the pipeline after flushing.

（2） External flushing

Features: clean fluid compatible with the sealed medium in the external system is introduced to the seal chamber for flushing.

Application: The pressure of external flushing solution shall be 0.05 - 0.1 MPa higher than that of the sealed medium, which is applicable to the occasions where the medium is high temperature or solid particles. The flow of flushing fluid shall ensure that it can take away heat, and also meet the flushing needs, so as not to produce erosion on seals. Therefore, it is necessary to control the pressure of the seal chamber and the flow rate of flushing. Generally, the flow rate of cleaning flushing fluid should be less than 5m/s;

The slurry liquid containing particles must be less than 3m/s. In order to reach the above flow rate value, the difference between the pressure of the flushing fluid and the seal chamber should be less than 0.5MPa, generally 0.05-0.1MPa. For double end mechanical seals, 0.1-0.2MP can be taken. The orifice where the flushing fluid enters and exits the seal chamber should be set near the sealing end face and near the moving ring side, In order to prevent the graphite ring from being eroded or deformed due to temperature difference due to uneven cooling, as well as the accumulation and coking of impurities, tangential introduction or multi-point flushing can be used. If necessary, the flushing fluid can be hot water or steam.

9、 Analysis of Typical Failure Causes of Mechanical Seals

（1） Mechanical seal itself

1. The inlay is not in place or flat.

2. The load factor is too large or the end face pressure design is unreasonable.

3. Improper material selection.

4. The sealing surface is uneven.

5. The sealing surface is too wide or too narrow.

（2） Auxiliary system problems

1. The working conditions are complex, but there are no auxiliary facilities such as flushing.

2. The flushing pipe is blocked.

3. The cooling pipe is scaling.

（3） Medium and working conditions

1. The medium is highly corrosive.

2. There are solid particles in the medium.

3. Equipment evacuation.

4. Crystallization of sealing surface.

5. Medium viscosity is too high.

（4） Pump problems

1. The processing accuracy of the shaft is poor, the shaft is in series, the runout, and the installation clearance is too large.

2. The pump vibrates too much after starting.

3. The gland gasket ring is poor.

4. The sealing box is uneven.

5. Mechanical seal installation does not reach the required compression amount.

10、 Common leakage

The proportion of mechanical seal leakage accounts for more than 50% of all pumps under maintenance. The operation of mechanical seal directly affects the normal operation of the pump. The summary and analysis are as follows:

1. Periodic leakage

(1) The axial displacement of the pump rotor is large, the interference between the auxiliary seal and the shaft is large, and the moving ring cannot move flexibly on the shaft. After the pump turns over and the dynamic and static rings are worn, no compensation displacement can be obtained.

Countermeasures: When assembling the mechanical seal, the axial displacement of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate. While ensuring the radial seal, the moving ring should be able to move flexibly on the shaft after assembly (press the moving ring against the spring to spring back freely).

(2) Insufficient lubricating oil on the sealing surface causes dry friction or galling of the sealing face.

Countermeasures: The lubricating oil level in the oil chamber shall be higher than the sealing surface of the dynamic and static rings.

(3) Periodic vibration of rotor. The reason is that the stator is not aligned with the upper and lower end caps, or the impeller is unbalanced with the main shaft, cavitation or bearing damage (wear), which will shorten the seal life and cause leakage.

Countermeasures: The above problems can be corrected according to the maintenance standards.

2. Leakage due to pressure

(1) Mechanical seal leakage caused by high pressure and pressure wave. When the design of spring specific pressure and total specific pressure is too large and the pressure in the seal cavity exceeds 3MPa, the specific pressure of the seal face will be too large, the liquid film will be difficult to form, the seal face will be severely worn, and the heat generation will increase, resulting in thermal deformation of the seal face.

Countermeasures: When assembling the mechanical seal, the spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. Measures should be taken for the mechanical seal under high pressure. In order to make the end face stress reasonable and minimize deformation, materials with high compressive strength such as cemented carbide and ceramics can be used, and cooling lubrication measures can be strengthened.

(2) The mechanical seal leakage pump caused by vacuum operation may cause negative pressure in the seal chamber during startup and shutdown due to the blockage of the pump inlet, gas contained in the pumped medium and other reasons. If there is negative pressure in the seal chamber, it will cause dry friction on the seal end face, and the built-in mechanical seal will cause air (water) leakage, The difference between vacuum seal and positive pressure seal lies in the directionality of the sealing object, and mechanical seal also has its adaptability in a certain direction.

Countermeasures: Double end mechanical seal is adopted, which helps to improve lubrication conditions and sealing performance.

3. Leakage caused by medium

(1) After the mechanical seal of most submersible sewage pumps is disassembled, the auxiliary seals of the static ring and the dynamic ring are inelastic, and some have rotted, resulting in a large number of leakage of the mechanical seal and even shaft grinding. Due to the corrosion of the weak acid and alkali in the high temperature and sewage on the static ring and auxiliary rubber seals of the dynamic ring, the mechanical leakage is too large. The rubber seals of the dynamic and static rings are made of nitrile 40, which is not resistant to high temperature and acid and alkali. When the sewage is acidic and alkaline, it is easy to be corroded.

Countermeasures: For corrosive media, the rubber parts shall be fluororubber with high temperature resistance, weak acid resistance and weak alkali resistance.

(2) Mechanical seal leakage caused by solid particle impurities If solid particles enter the seal end face, they will scratch or accelerate the wear of the seal end face. The accumulation rate of scale and oil stains on the shaft (sleeve) surface exceeds the wear rate of the friction pair, resulting in the moving ring cannot compensate for the wear displacement. The operating life of the hard to hard friction pair is longer than that of the hard to graphite friction pair, Because solid particles will be embedded in the sealing surface of the graphite sealing ring.

Countermeasures: The mechanical seal of tungsten carbide to tungsten carbide friction pair shall be selected at the position where solid particles are easy to enter.

4. Mechanical seal leakage caused by other problems

The design, selection and installation of mechanical seal are not reasonable enough.

(1) The compression amount of the spring 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 amount is too large, the end face specific pressure will be increased. If the friction heat is too much, the sealing surface will be thermally deformed and the end face will be accelerated to wear. If the compression amount is too small, the end face specific pressure of the moving and stationary rings will be insufficient, the sealing cannot be performed.

(2) The end face of the shaft (or shaft sleeve) for installing the dynamic ring seal ring and the end face of the seal gland (or housing) for installing the static ring seal ring shall be chamfered and polished to avoid damaging the dynamic and static ring seal ring during assembly.

11、 Normal operation and maintenance of mechanical seal

1. Preparations and precautions before startup

A. Comprehensively check whether the mechanical seal, accessory devices and pipeline installation are complete and meet the technical requirements.    

B Before starting the mechanical seal, carry out static pressure test to check whether the mechanical seal has leakage. If there are many leaks, find out the causes and try to eliminate them. If it is still invalid, it shall be disassembled, checked and reinstalled. The general hydrostatic test pressure is 2-3 kg/cm2.    

C Turn the gear according to the rotation direction of the pump, and check whether it is light and even. If the turning gear is hard or does not move, check whether the assembly size is wrong and the installation is reasonable.  

2. Installation and shutdown

A Before starting, the sealing chamber shall be kept full of liquid. When transporting solidified medium, the sealing chamber shall be heated by steam to melt the medium. The turning gear must be turned before starting to prevent the soft ring from breaking due to sudden starting.    

B For the mechanical seal using the external sealing oil system of the pump, the oil sealing system shall be started first. Finally stop the oil sealing system after shutdown.    

C. After the hot oil pump is shut down, the cooling water of the sealing oil chamber and the end face seal cannot be stopped immediately. The cooling water can be stopped only when the oil temperature at the end face seal drops below 80 degrees to avoid damaging the sealing parts.

3. Operation

A If there is slight leakage after the pump is started, observe for a period of time. If the leakage does not decrease after 4 hours of continuous operation, stop the pump for inspection.    

B The operating pressure of the pump shall be stable, and the pressure fluctuation shall not be greater than 1 kg/cm2.    

C During the operation of the pump, evacuation shall be avoided to avoid dry friction on the sealing surface and seal damage.

Mechanical seal itself is a precision part with high requirements, which has high requirements for design, machining and assembly quality. When using mechanical seal, various factors of using mechanical seal shall be analyzed to make the mechanical seal suitable for the technical requirements and medium requirements of various pumps and have sufficient lubrication conditions, so as to ensure the long-term and reliable operation of the seal