Special mechanical seal refers to the mechanical seal that is different from ordinary mechanical seal in terms of medium, pressure, speed, temperature, structure and use. It is also called high parameter mechanical seal in China. The mechanical seal is divided into ordinary and special types, which is convenient to describe the structure, working conditions and use requirements of mechanical seal. Therefore, some seal manufacturers list normal seal products, High load mechanical seals are also listed; Some manufacturers refer to mechanical seals other than product samples as special mechanical seals; Some manufacturers list or mark the mechanical seals with high temperature, high pressure and high speed respectively to distinguish them from ordinary mechanical seal products; Some manufacturers distinguish ordinary mechanical seals from special mechanical seals by using temperature. For example, UK Crane Company regards the temperature of - 20~+230 ℃ as the normal product, and mechanical seals beyond the above temperature range are considered as special mechanical seals
The special mechanical seals listed in this chapter only refer to the mechanical seals for kettle, high temperature, high pressure, high speed, low temperature, vacuum and strong corrosive media and those with abrasive media. The mechanical seals used in aerospace, nuclear power and military departments are not included in this chapter.
Mechanical seal for kettle
The mechanical seal for kettle refers to the seal of agitator (mixer). According to the use of agitators in imported devices, all large kettle agitators use mechanical seal, which accounts for 63.4% of the total agitators, while most agitators without mechanical seal are small agitators with small power and in stacks and tanks
In recent years, mechanical seal has been widely used in agitators. First, because of the large swing of agitator shaft, some agitator shafts are 7~8m long. Although there is a bottom support device, the swing is still large, and the packing seal can no longer meet the requirements; Second, the use conditions are becoming more and more stringent, and the safety requirements are also becoming higher and higher. The call to reduce leakage and prevent environmental pollution has prompted the design and production departments to adopt mechanical seals; Third, the structure of the kettle mechanical seal has been continuously improved and its supporting auxiliary devices have been widely used, making the sealing effect better, thus saving maintenance costs. Some people have made statistics, although the net purchase cost of the mechanical seal is higher than that of the packing seal, after installation and use, the maintenance cost of the mechanical seal only accounts for 1/20 of the packing seal maintenance cost, and the total cost after five years, Mechanical seal is half lower than packing seal
1、 Difference between kettle mechanical seal and pump mechanical seal
1. Most of the mechanical seals for kettles are vertical, a few are inclined, and very few are horizontal. In general, the dead weight of rotating parts is not considered when calculating the specific pressure of mechanical seal end faces
2. Most of the shaft diameters of the mechanical seals for pumps are below 100mm, while the shaft diameters of the mechanical seals for kettles are also more than 100mm. The weight of the mechanical seals for pumps generally ranges from several hundred grams to several kilograms, while the weight of a set of mechanical seals for kettles sometimes ranges from several hundred kilograms to more than one thousand kilograms. Therefore, the lifting device required for installation and maintenance shall be considered in the design
3. The sealing medium of mechanical seal for kettle is mostly gas phase. Therefore, the specific pressure of end face of kettle seal is higher than that of pump seal
4. The cauldron seals are mostly double ended and balanced structures. In addition, split structure and easy inspection and disassembly structure are also used for the convenience of installation and maintenance
2、 Mechanical seal for top mounted kettle
The top mounted kettle seal is widely used, accounting for more than 95% of the imported mechanical seal for axe. For example, the RD13, RD23, RD200ST of EKATO Company in West Germany and the 2K07T of Tanken Seiko in Japan are top mounted seals, which are used in PTA, polyethylene and polypropylene plants
The typical structure of the mechanical seal for the top mounted kettle is shown in Figure 10-1, which is the structural diagram of the RD200ST kettle seal of EKATO Company. This seal is a seal structure with water-cooled sleeves on both ends, and the applicable operating temperature is 130~180 ℃. If the operating temperature is below 130 ℃, the water-cooled jacket can be used instead of the upper cooling fan. When the operating temperature is 300~400 ℃, The structure with flange cooler at the bottom must be adopted, where the flange cooler acts as a thermal resistance barrier. When installing the cooling water pipeline, it should be noted that the height between the cooling water outlet pipeline and the funnel should be at least 200mm, and the water temperature at the outlet should be lower than 40~50 ℃.
For sealing liquid storage tank and pressure balancing device for cauldron seal, see Section II Pressure Device in Chapter VII
3、 Mechanical seal for bottom mounted kettle
The mechanical seal for the agitator driven at the bottom of the reaction bus is a special structure. Because the upper part of the seal must contact with the product, the working conditions are more stringent
Requirements for mechanical seal for bottom mounted kettle:
1. The solid medium deposited at the bottom of the stirring kettle shall not contact with the mechanical seal in structure;
2. When the seal fails, the mechanical seal can be removed for maintenance without removing the liquid in the kettle. It is better to replace it quickly and replace the mechanical seal under the pressure of the reactor. It is also required that the mechanical seal can be removed and reinstalled without removing the heavy transmission mechanism; 3. The structure has a small size stub shaft and a low height, with a mounting arm
Bottom mounted seal is adopted for the 3rd and 4th reaction kettles and powder tower agitators of polypropylene plant. Figure 10-4 shows Φ 260 Structural diagram of bottom mounted mechanical seal. In order to prevent the product from entering the seal chamber, there is a pot bottom shaped seal cover 34 at the lower part of the flange connecting the agitator paddle, as well as two bushings 35, 36 and thrust plate 33, so that large particles are thrown out by centrifugal force, and small particles are difficult to enter the seal chamber. For the upper part (below the seal cover) contacting the materials, some use friction pair seals; Some use lip seals (axial and transverse lip seals); Some are sealed with bushings (as shown in Figure 10-4). The surface of the seal cover, the upper part of the shaft, and the surface of the connecting flange should be ground and polished to prevent materials from sticking to the surface
4、 Shutdown sealing device
When the kettle seal is defective and the mechanical seal needs to be replaced under pressure, the shutdown seal device can be used (Figure 10-5).
This device can seal the shaft as long as the external pressure p is applied at the shaft seal during shutdown, but the shaft cannot rotate again at this time. It is especially suitable for sealing containers for aseptic and fermentation operations, but not for occasions where products are easy to solidify and thicken.
5、 Several parameters related to mechanical seal for kettle
1. Service life
The service life of mechanical seal for kettle is generally more than half a year. In fact, the service life is related to the rotational speed of mixing shaft, seal size, friction pair materials and other factors. Figure 10-6 shows the service life test results of EKATO for seals of different sizes at different rotational speeds
2. Scope of application
The application scope of kettle seal is related to the use conditions and the auxiliary devices. Each sealing company has its own description of the application scope of the product. Figure 10-7 shows the application scope of EKATO RD023 and RD200ST seals under the conditions of sealing liquid storage tank and pressure balance device
High temperature mechanical seal
When the working temperature of the mechanical seal exceeds 80 ℃, it will be used as a high-temperature mechanical seal (some seal companies take the working temperature>90 ℃ as the boundary). At high temperatures, the main problems of the seal include the following: liquid vaporization of the seal end face, thermal stress and thermal deformation of the friction pair, assembly and insert parts may be loosened due to improper matching, and the auxiliary seal ring may be aging, cracking Bonding and elasticity disappear, spring fatigue and strength decrease, and corrosion and wear of materials accelerate
1、 Requirements for high-temperature mechanical seals
1. Select high temperature resistant sealing material
Under high temperature, the temperature resistance of materials is particularly important. Reasonable selection of sealing materials can make mechanical seals of general structure still suitable for high temperature working conditions, simplify procedures and save costs. Table 10-1 lists the service limit temperatures of different sealing materials
2. Adopt metal bellows type mechanical seal or seal structure combining metal bellows and ordinary mechanical seal
From the point of view of heat resistance, the auxiliary seal ring is the weakest link on the mechanical seal. When the temperature exceeds 80 ℃, ordinary rubber is no longer applicable, and when the temperature exceeds 230~250 ℃, polytetrafluoroethylene cannot be used. In addition, the spring will be blocked due to the deposition of pyrolysates of high-temperature and flammable media. Therefore, when the temperature exceeds 220 ℃, the metal bellows type mechanical seal should be used, Or adopt the sealing structure combining metal bellows and common mechanical seal
3. Take cooling measures
In order to make the seal operate reliably under high temperature, the mechanical seal shall be cooled as far as possible to avoid working under dry friction. The temperature in the seal gap must be kept below the vaporization temperature of the seal liquid
The cooling measure is to control the temperature of the seal. The common temperature controls include flushing, blocking and strengthening the liquid circulation (see Section I Temperature Control Device in Chapter VII for details). At high temperature, the following measures can also be taken:
(1) Diphy1 (mixture of diphenyl and diphenyl oxide) is used as the blocking liquid, which is a cold and pressureless heat exchanger (the blocking liquid is transported by pump efficiency spiral);
(2) Nitrogen and argon can also be used at ultrahigh temperature or without proper liquid to prevent oxidation caused by air
4. Problems with inlaid friction pairs
Sealing ring inlay is to make full use of the characteristics of end face materials and matrix materials to improve the use value of materials. Generally, hot inlay is used for high-temperature seals. However, due to the difference in thermal expansion coefficient between end face wear resistant materials and matrix materials, friction rings will become loose and fall off when used at high temperatures. Therefore, special attention should be paid to the temperature used for the friction ring with inlaid structure. The reason for the looseness and falling off of the hot inlaid structure is not only related to the service temperature, but also related to the size of the shaft diameter. When the base of the friction pair ring is stainless steel, high nickel alloy, and titanium alloy, the relationship between the allowable service temperature of the hot inlaid carbide and the different shaft diameters
5. Allowable clearance between friction pair ring and shaft
Under high temperature conditions, due to the difference in thermal expansion coefficient between the friction pair ring and the shaft, the gap between the friction pair ring and the shaft tends to zero, which results in the disappearance of the floating property of the friction pair ring and leads to seal failure. Therefore, the clearance between the friction pair ring and the shaft must be reasonably and correctly selected in the design.
2、 Example of high temperature mechanical seal
1. HSHF+MFL seal of Burgmann
The circulating oil pump GA-102AB of the aromatics unit has a power of 1650kW, a rotating speed of 6748r/min, the transmission medium is gasoline, the operating temperature is 262~284 degrees, the inlet pressure is 0.87MPa, and the outlet pressure is~175MPa. The mechanical seal adopts the hybrid structure of Burgmann HSHF+MFL, which is suitable for sealing under high temperature, high pressure and high-speed working conditions. Here, HSHF is a single end, hydrodynamic spiral structure with pump effect MFL is a single end metal bellows structure Both HSHF and MFL are spring stationary to meet the requirements of high-speed operation. The seal is structurally arranged in series and installed at the suction end and discharge end of the oil pump. Because HSHF has a pump efficiency screw, it is related to the rotation direction of the shaft. The pump efficiency sleeve (12A) at the suction end is a right screw, and the pump efficiency shell (25A) is a left screw; The pump efficiency sleeve (12B) at the discharge end is a left screw, and the pump efficiency housing (25B) is a right screw. During installation, it should be assembled correctly according to the requirements in the drawing,
2. Y4D metal bellows seal
Y4D metal bellows seal is used by Eagle Company for sealing at low pressure (below 0.5MPa) and temperature of - 200~+400 ℃. The corrugated pipe is made of Inconel 718 material. The oxidation resistance of this alloy can reach 980 ℃, and the yield strength can reach 1426N/mm2 at a low temperature of - 196 ℃. Therefore, Inconel 718 is suitable for both high and low temperature conditions. Y4D metal corrugated pipe mechanical seal is used for all high-temperature vacuum residue, KSK oil, light and heavy gas oil seals in Eureka unit
3. Type 59B seal
Although the operating temperature range of 59B type seal is - 50~+230 ℃, if the pumped medium passes through the cooler through the conveying device and then enters into the seal, 59B type seal can still be used in situations where the medium temperature is higher than 230 ℃. For example, the medium temperature of the kettle liquid pump GA-116AB of the aromatics unit is 290 ℃, which means the 59B type seal with external cooling is used
4. Introduction to 605 series bellows mechanical seal of Shanghai Water Pump Plant
Shanghai Pump Factory has introduced the technology of American EG&G SEAL Company and has produced many series of seals, of which 605 series are welded bellows mechanical seals. Its models, specifications and performance are shown in Table 10-2
High pressure mechanical seal
It is generally believed that when the working pressure of the mechanical seal exceeds 4~5MPa, it is regarded as high pressure. Some seal companies regard the working pressure as high pressure when it reaches 3MPa; Some companies measure PV value; Some companies consider pressure and temperature together
It is generally believed that under high pressure, mechanical and thermal deformation will harm the function and reliability of mechanical seals. The higher the pressure, the greater the harm. Therefore, the design of seals used for extremely high pressure is improved than that of standard seals. Not only is there a strict ratio between the opening force and the closing force, but also the entire seal structure is required to withstand high pressure without deformation. In addition, It is also necessary to maintain a stable liquid film to ensure sealing performance
1、 Requirements for high-pressure mechanical seals
1. Increase the cross section of sealing parts
The cross section of rotating parts should be increased, and the stationary ring should also have a larger cross section. Do not have steps or grooves on the outer diameter of the stationary ring. However, pay attention to that with the increase of the width of the sealing surface b, the thermal deformation and wear will also increase. Also, increase the thickness of the gland to prevent deformation and withstand pressure. Use heavy load springs, and use pins with larger diameters to adapt to larger starting torque
2. Improve the machining accuracy of sealing parts
(1) The hole surface of the building with stationary ring at the bottom of the gland shall be kept high straightness, because under high pressure, any higher point on the surface will be transmitted to the stationary ring to cause leakage;
(2) Improve the machining accuracy of the inner and outer diameter and width of the sealing surface, because under high pressure, the accuracy of the inner and outer diameter and width of the seal has a significant impact on the change of the pressure. The roughness of the seal end surface after grinding and polishing is Ra0.05~0.025 (▽ 12~▽ 13), and the flatness must be limited to one light band. In addition, the perpendicularity of the friction end surface and the accuracy of the shaft should also be improved accordingly
Pay attention to the erosion corrosion and wear caused by different media under high pressure. Figure 10-12 (a) and (b) show the relationship between pressure and seal size when the media of different friction pair materials are hydrocarbon, water and aqueous solution at different rotating speeds
5. Hydrodynamic mechanical seal is adopted
Hydrodynamic (also known as hydrodynamic, hot fluid dynamic) mechanical seal is a new sealing surface structure developed to consider the stability of the lubricating film in the sealing gap under the conditions of high pressure and friction heat. Figure 10-13 (a) and (b) are the layout drawings of hot fluid dynamic mechanical seal groove. This structure has many similar forms (see Figure 4-8), Its thermohydrodynamic effect is formed on the seal. When the seal ring rotates, the groove can make the liquid strongly cool the sealing surface far away from it. During this cooling, a hydrodynamic wedge and a high-pressure zone equal to the number of grooves are formed on the initial surface of the seal ring. As a result of tangential flow and pressure drop, a comet shaped lubrication wedge is formed behind each groove (Figure 10-14). Therefore, with the increase of load and sliding speed on the sealing surface, the friction coefficient decreases (Figure 10-15)
2、 Introduction to HSHF High Pressure Mechanical Seal of Burgmann Company
The hydraulic turbine GAT-101A of the feeding pump of the aromatics unit has a power of 968kW, a rotating speed of 6748r/min, the transmission medium is hydrocarbon, the operating temperature is 43~39 degrees, the inlet pressure is 15.33MPa, and the outlet pressure is 1.9Mpa. The mechanical seal is the HSHF type high-pressure seal of Burgmann Company. Its approved product is a single end, fluid dynamic pressure spiral structure with pump efficiency. The original operating parameters are P=5 (14) MPa, t=300'C, V<50 m/s, Since the single end sealing structure can not meet the use requirements, the double end structure is adopted. One moving ring is used for two stationary rings. The structure is compact. It is a cylindrical structure, which is easy to install (Figure 10-16)
The seal at the inlet end of the hydraulic turbine is HSHF D3/95, the pump efficiency sleeve (6A) is a right screw, and the pump efficiency housing (16A) is a left screw; The seal used for the outlet end is HSHF D4/95 specification, the pump efficiency sleeve (6B) is a left screw, and the pump efficiency housing (16B) is a right screw. Therefore, the two sets of seals are related to the direction of rotation. The inlet and outlet ends cannot be installed incorrectly, and the pump efficiency sleeves cannot be interchanged.
High speed mechanical seal
When the linear speed exceeds 25~30m/s, it can be regarded as high speed. Some seal companies regard the sliding surface speed as high as 33m/s, which can be regarded as high speed mechanical seal. " ZBJ22-001-88 of China stipulates that when the average linear speed of the seal face is 25~100m/s, it is a high-speed mechanical seal, and when the average linear speed of the seal face is greater than 100m/S, it is an ultra-high speed mechanical seal
At high speed, due to the influence of centrifugal force, vibration and imbalance will be caused. High speed, like high pressure, will produce thermal deformation, thermal stress cracks, wear and sediment, which will cause leakage loss. This loss will increase strongly with the increase of sliding speed
1、 Requirements for high-speed mechanical seals
1. Reduce the weight (mass) of rotating parts and improve the dynamic balance
At high speed, the influence of centrifugal force makes the spring twist and deviate from the center of pin hole, so the spring must be stationary. This reduces the weight of the rotating parts and the resulting imbalance.
2. Remove friction heat in time
Increase the flow of cooling medium, the high-speed pump is 10~20L/min, and the flushing volume of some high-speed seals is 50~100L/min.
When oil is used as blocking fluid, low viscosity oil should be selected to minimize gas solubility and coking. If the friction pair is working at high temperature, do everything possible to ensure that the sealing fluid will not leave residue when evaporating, so use polyethylene glycol as a synthetic medium as the blocking fluid,
3. Timely check the wear of sliding end face
When the end face wear reaches 0.005mm, the end face shall be ground, and the end face roughness and flatness shall be rechecked.
2、 9AB high-speed pump mechanical seal.
The diameter of high-speed mechanical seal used for compressor is relatively large, while the size of mechanical seal used for high-speed pump is generally about 2 inches. The 9AB seal of John Crane Packing Company in the United States is the most typical mechanical seal used for high-speed pump [Figure 10-17 (a), (b)]. This seal is commonly used on the gearbox and pump body seal of Sundstrand high-speed pump in the United States
In order to save space and obtain a high weight to power ratio, no matter the pump structure and mechanical seal structure are very compact, general petrochemical companies have introduced this high-speed pump. At present, 51 units are used in relevant units of Yangzi Petrochemical Company (see Table 3-41), and nearly 200 units are used in the system of China Petrochemical Corporation. In the figure, 60C is used for gear box seal, and 60A or 60A+60B is used for pump impeller end seal
Low temperature mechanical seal
General low temperature mechanical seal refers to the seal whose working temperature is between 0~- 50 ℃. The low temperature of API-610 means that the temperature is lower than - 29 ℃. In China, the temperature of the seal chamber is usually lower than - 20 ℃
In fact, the low-temperature range is wide, which can be divided into:
Low temperature: - 20~- 50 ℃
Medium and low temperature: - 50~- 100 ℃
Extremely low temperature: - 100~- 196 ℃
Ultra low temperature:<- 196 ℃
The pumped cryogenic medium mainly refers to liquefied gases and volatile liquids, such as liquefied gases such as liquid nitrogen, liquid oxygen, methane, ethane, ethylene, etc.
The physical properties of nitrogen, oxygen, air and other liquefied gases are shown in Table 10-3
The physical properties of ethylene, propylene, methane, ethane and propane are shown in Figure 10-18 (a), (b), (c) and (d·
1、 Requirements for cryogenic mechanical seals
1. Material requirements
With the decrease of temperature, the tensile strength and hardness of metal materials increase while the toughness decreases
Plastic and rubber are more sensitive to low temperature than metal materials. Generally, plastic and rubber can only be used at temperatures above - 50 ℃. For the cryogenic pump, the data [13] introduces that the service temperature limit of neoprene rubber is - 60 ℃, that of neoprene rubber is - 65 ℃, that of silicone rubber is - 80 ℃, that of fluorinated rubber is - 45 ℃, and that of polytetrafluoroethylene is - 79 ℃. It is believed that the use limit of mechanical seals using these materials as fillers is - 70 ℃. If the medium temperature is below - 70 ℃, bellows type mechanical seal or double face mechanical seal shall be used
Data [6] describes that polytetrafluoroethylene can still maintain a little flexibility at the boiling point of helium (- 268.9 ℃), which is suitable for low-temperature materials. However, polytetrafluoroethylene has a large expansion coefficient, so when used at - 50~- 100 ℃, tetrafluoroethylene is often filled with graphite or glass fiber instead of pure tetrafluoroethylene. In addition, at low temperatures, the dynamic ring often cannot float after the V-ring is installed on the shaft sleeve. Therefore, pay attention to the assembly dimensions of the V-ring.
2. Structural requirements
Under normal low temperature of 0~- 50 ℃, single end mechanical seal is often used, which has unbalanced or balanced structure, but most of them use balanced structure. This is because the temperature of the liquid seal end surface rises after friction, and the pressure of the liquid film rises accordingly, pushing the seal surface away, causing increased leakage. Therefore, the end face specific pressure at low temperature should be higher.
When the liquid temperature is lower than - 70 ℃, bellows seal or double face seal shall be used. However, when double face seal is used, the temperature of shaft seal part shall be designed as 0 ℃. At this time, it is used as gas seal, and the seal ring can be ordinary O-ring or V-shaped seal ring.
The sealing structure is suitable for the pump body structure. The commonly used cryogenic pump adopts a vertical structure
3. Auxiliary device
(1) The amount of flushing shall be increased as much as possible, the cooling measures shall be strengthened, or the pressure in the sealing chamber shall be increased to control the gasification phenomenon caused by the rise of liquid temperature. The flushing pipeline installed from the pump outlet end shall be absolutely avoided to flush the sealing end face, because at low temperature, the liquid in the flushing pipeline will be vaporized. Therefore, the counter current flushing method is generally adopted
(2) To prevent the edge of single end seal from icing due to the contact of moisture in the atmosphere with the seal during the pump shutdown, methanol and other icing agents can be used to dissolve it, and the blocking part can also be protected by nitrogen dry gas
In a word, both full cooling and cold insulation measures should be taken.
(3) The exhaust pipeline shall be installed at the position higher than the sealing surface, so that the generated gas can also escape from the exhaust pipeline.
For the auxiliary devices of the cryogenic pump, see the auxiliary system of the cryogenic pump in Section I of Chapter VII.
2、 Mechanical seal of low-temperature methane pump
The cryogenic pump is usually a vertical structure. The sealing chamber is located at the upper part of the pump. The sealing structure is a double end balanced mechanical seal. The structure and layout of cryogenic pump are shown in Figure 10-19.
1. Structural features of cryogenic pump
(1) Cylindrical structure, no need to worry about deformation at low temperature
(2) The first stage impeller is located at the bottom and adapts to the change of impeller stage by increasing or decreasing the length of the cylindrical cylinder (only the second stage is listed in Figure 10-19, and there are more than ten stages actually).
(3) The liquid flows into the impeller from top to bottom, and the suction end exhausts to play an effective role as a gas separator.
(4) The vertical structure has smooth exhaust, and the gas will not be trapped in the middle of the pump·
(5) No shaft bending problem, the shaft seal can be far away from the pump body, reducing the impact of heat. The motor mounting frame is of wind shielding structure, so the temperature rise of the motor has little impact on the pump.
(6) The pump casing is buried underground and easy to keep cold
Therefore, the cryogenic pump used for liquefied petroleum gas is usually of the above structure
2. Mechanical seal of low-temperature methane pump
The steam pressure of methane liquefied gas at - 99 ℃ is close to 3MPa. The outlet pressure of the bottom pump of the demethanizer of the ethylene plant at - 53 ℃ is approximately 3MPa. The double end tandem mechanical seal is adopted. It is BB81-K7+BB81B-S7 of Bira, with dimensions of 45 x 39 and 50 x 45
Corrosion resistant mechanical seal
In corrosive medium, the parts of mechanical seal are strongly eroded by the medium, and the friction surface is corroded and worn at the same time, This phenomenon is called "abrasion", and its corrosion rate is about 10~50 times that without friction. Some structural materials have excellent corrosion resistance, but poor wear resistance; while other structural materials have good wear resistance, but poor corrosion resistance. Therefore, friction pairs should be selected to be both corrosion and wear resistant, and auxiliary seal rings, elastomers and structural parts should also have good corrosion resistance
When designing corrosion resistant mechanical seals, the following measures are generally taken:
1. Structural isolation measures
The external structure is adopted. After proper selection, the external seal can ensure that there is no contact between metal parts and process medium, and small parts such as springs and pins can not be corroded by process medium. There are two types of external mechanical seals: ordinary mechanical seal and bellows type mechanical seal. The bellows type seal structure is better (Figure 10-21). If the corrosion resistance of the seal material is allowed, In fact, most of the mechanical seals of the process pump of the imported device are internal seals. Double end seal structure can be used to transport dangerous media, and isolation liquid can be used for protection. In addition, measures with a spring protective layer can also be used. For example, the spring is covered with a plastic hose or sprayed with a layer of anti-corrosion coating, Figure 10-22 shows the sealing structure of spring with protective sleeve
2. Corrosion resistant materials shall be selected
The most fundamental way out is to select corrosion resistant materials. Most of the mechanical seals used in imported petrochemical plants have solved the corrosion problem in terms of materials. Therefore, the selection table of seal models corresponding to different media is available in the samples or manuals of large foreign seal companies
3. Right 3. Type 154. The structure is shown in Figure 10-23, 10-24, 10-25, 10-26
The leaked corrosive or toxic media shall be collected, safely discharged or discharged after treatment. For example, some are equipped with recovery liquid trays, and some are equipped with discharge pipelines, which are indicated on the drawing to be discharged to a safe area
Most of the mechanical seals of Yingjiao imported by Yangzi Petrochemical Company are corrosion resistant seals, such as the 59U, 59B and 10T types of British Crane, BB8 type of Japanese Bira and Y4D type of Eagle. Type, TF type of Burgmann Company in West Germany and Concertina Walls type seal of Wernert Company
Mechanical seals for corrosion resistant pumps in China include 151, 152 and 15
Mechanical seal containing abrasive medium and high viscosity medium
The sealing medium generally has impurities, so it needs to be treated. Even the treated medium will produce crystallization, solidification and reactant precipitation in the process. Sometimes the sealing medium itself is a variety of suspension slurry, and the particles in these media play an abrasive role in sealing
High viscosity medium usually refers to medium with viscosity greater than 0.7 Pa · s (700cP)
1、 Mechanical seal containing abrasive medium
In actual production, the sealing problem of liquid solid two-phase or gas solid two-phase media is often encountered. The enemy of mechanical seal is the solution containing solid particles
Abrasive media include the following situations:
1. The material to be treated is slurry;
2. There are impurities mixed in the machine body and pipeline;
3. Due to temperature change, there is crystallization precipitation or solidification;
4. Dissolved matter precipitates in the sealing end face;
5. Solid substances are produced due to chemical reaction;
6. Metal substances precipitate and adhere to the seal face due to electrochemical action
When ordinary single end mechanical seal encounters abrasive medium, the following faults occur:
1. The seal face was worn early;
2. The moving ring cannot act because of the blocked clearance and the increased friction force of the seal ring;
3. The sealing ring cannot play a buffer role;
4. The spring is blocked by impurities and cannot act;
5. The shaft cannot rotate;
6. The whole seal fails
The following measures shall be taken for seals containing abrasive media: 1. Strengthen flushing and cooling measures to prevent impurities from clogging in the sealing cavity. For example, single end seals shall be lubricated with liquid, and double end seals shall be added with blocking liquid or blocking liquid and auxiliary injection liquid;
2. Friction pair materials shall be hard to hard matching, and commonly used hard alloy to hard alloy (or ceramic);
3. Use large springs as soon as possible to reduce the chance of particle blockage;
4. If bellows seal is used instead of ordinary mechanical seal, it is recommended to use rubber bellows and welded metal coated bellows or synthetic rubber wrapped bellows, because ordinary metal bellows have poor ability to deal with mortar fluid and are not suitable
5. The impurity filtering auxiliary device is adopted, and the elephant liquid separator has good separation effect and simple structure (see Section 3, Impurity Filtering Device, Chapter 7)
2、 Mechanical seal for non viscous medium
Generally, the liquid with a specific gravity of more than 0.65 has a lubricating property, which is sufficient for mixing most seals. If the specific gravity of the liquid is less than 0.65, a balanced seal is required
The viscosity, like the specific gravity, represents the lubricating property of the liquid. The liquid with high dry degree often has good lubricating properties. When the liquid viscosity is lower than 0.7Pa · S, the standard design is used, and no modification is required
For the liquid with viscosity of 0.7~1.6Pa · S, a reliable driving design of a heavy load pressure bearing device is required for the seal, which is to compensate for the increased shear force on the sealing surface under the viscous environment. In addition, the surrounding environment control should be strengthened, and the sealing surface should be properly lubricated. When the high viscosity medium is picked up, there is no liquid film between the sealing surfaces, Single face seal with external flushing, single face seal with external lubrication or double face seal with external sealing fluid must be used
The viscosity of process medium of each imported unit of Yangzi Petrochemical Company at the service temperature does not exceed 0.7 Pa · S, except that the viscosity of fuel oil is between 0.1 and 0.4 Pa · s, and the viscosity of other medium rarely exceeds 0.1 Pa · S
Vacuum mechanical seal
Vacuum mechanical seal is widely used in metallurgy, petrochemical, medicine, food, electronics, military and other fields
Vacuum sealing refers to the use of sealing technology in a device with a certain degree of vacuum to prevent the atmosphere from penetrating and maintain a certain degree of vacuum
The pressure unit used in vacuum technology is Torr, which is equivalent to 1/760 standard atmospheric pressure or 1 mm high mercury column (133.322Pa).
