I peak point
There are a variety of mechanical seals for pumps of various designs, but there are five common leak points:
- The seal between the sleeve and the shafts
- The seal between the dynamic ring and the sleeve
- The seal between the dynamic and static rings
- The seal between the static ring and the static ring seat
- The seal between the seal cover and the pump body
Phenomenon: Popping or popping sound during operation
Possible Causes:
- Vaporization of sealing fluid at the sealing interface
- Improved seal face cooling
Solution:
- Increase the size of the bypass discharge line (if not already present) or enlarge the openings in the gland.
Phenomenon: Continuous Sealing Drip
Possible Causes:
- Irregular surfaces
- Bubbles on graphite sealing surfaces
- Thermal deformation of the sealing surface
Solution:
- During installation, check the auxiliary seal for scratches, aging of the O-ring, hardness and brittleness due to compression, and softness and stickiness due to chemical action.
Phenomenon: Spring Failure
Possible Causes:
- Damage to metal components
- Transmission corrosion
Solution:
- Check for incorrect mounting dimensions
- Ensure appropriate materials and seal types to improve cooling line flushing
- Check for excessive torque on the gland bolt, deformation of the gland and adequate pressure on the gland joint
- Check for the presence of solid particles between the seal faces and consider repolishing if necessary
- Inspect the sealing surface for cracks and replace the main and mating seal ring if necessary.
Phenomenon: Squeaky sound during operation
Possible cause:
- Insufficient lubrication in the seal
Solution:
- Increase the size of the bypass discharge line (if not already present) or enlarge the openings in the gland.
Sealing material materials
1. Asbestos rubber packaging
XS250 Asbestos Rubber Gasket (with a maximum operating temperature of 250℃) and XS350 Asbestos Rubber Gasket (with a maximum operating temperature of 350℃) can be used with steam, water and alkaline solutions.
2. Oil-impregnated asbestos packaging
Oil Impregnated Asbestos Gaskets YS250 (with a maximum operating temperature of 250°C) and YS350 (with a maximum operating temperature of 350°C) have the unique characteristic of being lubricated with oil, resulting in a low coefficient of friction and high compression capacity.
These gaskets are suitable for use with steam, air, industrial water and heavy petroleum products.
3. SMF polytetrafluoroethylene asbestos packaging
Operating temperature range: -100°C to 250°C
Suitable for use with: weak acids, strong alkalis and other corrosive media.
4. Oil-impregnated cotton, YMM hemp packaging
Maximum operating temperature: 120 ℃
Suitable for use with: river water, tap water, groundwater, sea water, etc.
5. Teflon fiber woven packaging
PTFE woven fiber fill is available in six different grades.
During use, a small amount of gasket leakage is acceptable at first.
After a certain amount of time has passed, it is recommended to adjust the gasket when the filling reaches a stable state.
It is important not to apply excessive pressure at the beginning so as not to damage the gasket.
6. Carbon fiber woven seal packaging
Carbon fiber woven seal gasket is a state-of-the-art material with exceptional strength and modulus.
It has several advantages, including high strength, large modulus of elasticity, heat resistance, chemical resistance, excellent thermal conductivity, low coefficient of thermal expansion and self-lubricating properties, among others.
When starting the operation, it is recommended to lightly seal the gland, allowing a small amount of medium to leak out.
After running for a certain period, gradually tighten the gasket to bring it to a stable state.
If you encounter leakage during operation of the carbon PTFE woven gasket (FTH-1), you can repeatedly tighten the gasket.
7. Phenolic fiber woven seal packaging
Phenolic fiber woven packaging has several advantages, including heat resistance, acid corrosion resistance, adsorption capacity, excellent thermal insulation, low coefficient of friction, wear resistance, non-toxicity to humans and the environment, and an affordable price.
It is mainly used as gasket seal for high pressure plunger pumps, high pressure water pumps, FRP acid pumps, methyl ammonia pumps, reciprocating pumps and deep well oilfield pumps.
The performance of phenolic fiber woven gasket can be seen in the following table.
It is suitable for a variety of media, including acids, alkalis, organic solvents, engine oil, steam and water.
8. Expanded graphite materials (flexible graphite)
Expanded graphite (also known as flexible graphite) is a material with many exceptional qualities, including self-lubricating properties, corrosion resistance, resistance to extreme temperatures, radiation resistance, abrasion resistance and a low coefficient of friction. Furthermore, it has the advantage of being flexible, light and able to maintain its shape even under compression, making it suitable for use in a wide range of temperatures.
Expanded graphite can be used with media that have a PH value ranging from 0 to 14. With the exception of aqua regia, concentrated nitric acid, concentrated sulfuric acid, and high temperature dichromate (among a few other strong oxidizing media), it can be used with most other media.
When installing the sealing ring, it must be cut with a razor blade before being loaded into the stuffing box. There must be a limited amount of axial twist and it must be placed on the shaft and pressed into the stuffing box, with the cutouts staggered every 900 or 1200 degrees per turn.
At the beginning of operation, a small amount of leakage is allowed, and the amount of leakage should decrease over time.
9. Carbon-graphite materials
This material has excellent thermal conductivity, electrical conductivity, chemical stability, thermal shock resistance, self-lubricating properties and is easy to process.
In the pump industry, it is used for various applications such as end seals, top ring seals, mechanical seal rings, thrust seals and rotary vane seals in centrifugal pumps, shield pumps, diesel distribution pumps, submersible pumps, water pumps, high temperature hot oil pumps, nuclear secondary pumps, boiler feed pumps and gas pumps.
10. Rubber
Natural rubber: It is known for its excellent flexural strength, good abrasion resistance, cold resistance, insulation and high tear strength. However, it has poor weather resistance, aging resistance, oil resistance and solvent resistance. Natural rubber pumps are suitable for working temperatures below 78°C and are ideal for conveying various abrasive slurries, weak acid and weak alkaline solutions. However, they are not suitable for transporting strong acids, oils, aliphatic and aromatic solutions.
Nitrile Rubber: Nitrile rubber is the most widely used oil-resistant rubber, featuring superior properties such as greater abrasion resistance (30-45% higher than natural rubber) and better resistance to high temperatures compared to natural rubber and styrene-butadiene rubber. However, its weather resistance is poor. Nitrile rubber pumps can withstand temperatures of up to 95°C and are resistant to animal and vegetable oils, mineral oil and glycol.
Butyl Rubber: Pump parts lined with butyl rubber are suitable for use with acid mud. The maximum temperature it can withstand is 100°C and it is resistant to strong acids and alkalis (such as hydrochloric acid and sodium hydroxide). However, high concentrations of oxidizing acids (such as concentrated sulfuric acid and concentrated nitric acid) will cause degradation of butyl rubber.
Fluorine Rubber: Fluorine rubber is known for its excellent heat resistance, ozone resistance, oil resistance and resistance to various chemical erosions after vulcanization. However, it is worse at processing and gluing compared to general purpose rubber and is more expensive. Fluoroelastomer pumps can be used up to 170°C in common media.
11. Ceramics
Corundum ceramic (mainly composed of AI2O3):
It has high hardness, excellent abrasion resistance, chemical stability, high insulation and high melting point.
In the pump industry, it is mainly used in special working conditions, such as ceramic pumps and mechanical seal rings.
Ceramics can withstand almost all corrosive media, including hot concentrated nitric acid, sulfuric acid, hydrochloric acid, aqua regia, saline solutions and organic solvents, with the exception of hydrofluoric acid and some other fluorine-containing substances and hot or concentrated alkalis.
Silicon Nitride Ceramics:
It has better thermal shock resistance compared to alumina ceramics, and its other properties are similar to those of alumina ceramics.
Silicon Carbide Ceramics:
It has excellent chemical resistance, high mechanical strength, good abrasion resistance, high temperature resistance, self-lubrication and a small coefficient of friction, making it suitable as a high-temperature structural material, mechanical seal for pumps and magnetic pump shafts, offering advantages more significant than common ceramics.
Silicon carbide is currently an ideal friction material, often combined with carbon-graphite.
Its dry sliding friction coefficient is lower than that of alumina and carbide, and its PV value is higher than that of alumina and carbide.
Silicon carbide ceramics have excellent wear resistance and can resist almost all chemical corrosion, except hydrofluoric acid and some other media, including concentrated nitric acid, sulfuric acid, hydrochloric acid and other strongly corrosive liquids.
12. Ferrite Permanent Magnets
Ferrite permanent magnets have several advantages over metal permanent magnets, such as high coercive force, good insulation, resistance to external magnetic field interference, and low price. These characteristics make ferrite permanent magnets ideal for use as magnets in internal and external rotors of magnetic pumps.
Rare Earth Cobalt Permanent Magnets are known for their exceptional magnetic properties, particularly their greater coercive force and magnetic energy product compared to other types of permanent magnets.
Although these magnets are suitable for use as permanent grounding magnets in magnetic pumps, they are not as widely used due to their high cost.
13. Other materials
Pump painting:
It serves for protection, decoration and marking purposes. It can be classified into several categories based on its use, such as anti-rust paint, insulating paint and acid paint, etc. It can also be categorized based on paint level such as primer, topcoat and putty etc. .
Centrifugal Pump Paper Pad Material:
Felt material is used in pumps as an oil seal to prevent water and dust from entering. Helps seal the lubricating oil at the friction point. This material can be divided into fine wool felt, semi-thick wool felt and thick wool felt.
Industrial Vulcanized Rubber Sheet (GB5574-85):
This material includes various types of rubber sheets, such as ordinary rubber sheets, acid and alkali resistant rubber sheets, oil resistant rubber sheets and heat resistant rubber sheets.
Pump sealing way
The pump sealing device is mainly classified into two types: static sealing and dynamic sealing.
Static sealing typically encompasses gasket seals, O-ring seals, and thread seals.
Dynamic seal mainly includes soft gasket seals, oil seals, labyrinth seals, spiral seals, dynamic seals and mechanical seals.
1. Rubber seal ring
Rubber O-rings are a popular choice for use in pumps due to their simple shape and low manufacturing cost. Despite the O-ring's overall size, its cross-section is small, typically just a few millimeters, making it lightweight and efficient in terms of material consumption. These features also make installation and disassembly easier.
One of the most notable advantages of O-rings is their excellent sealing ability and wide range of applications. Static seal can withstand working pressures exceeding 100MPa, while dynamic seal can withstand up to 30MPa. Furthermore, the suitable temperature range for O-rings is -60 to 200℃, making them suitable for a variety of media.
As a result, O-rings are becoming increasingly popular in pump design. They are installed between the groove and the surface to be sealed and are subjected to a certain compression, which generates a reaction force that applies an initial compressive stress to the smooth surface and the bottom of the groove, thus providing a seal.
When the pressure of the sealed liquid increases, the O-ring undergoes greater deformation, which transfers greater pressure to the sealing surface, increasing the sealing effect. This is why O-rings are known for their good sealing ability.
2. Joint sealing
| NO. | Name | NO. | Name | NO. | Name |
| 1 | to lead | 8 | stainless steel shield | 15 | joints |
| two | barrel lid | 9 | graphite bearings | 16 | articulation |
| 3 | cable protector | 10 | rotors | 17 | impeller |
| 4 | line pressing joint | 11 | drain outlet | 18 | pump |
| 5 | capacitors | 12 | joints | 19 | base |
| 6 | barrel | 13 | plug | ||
| 7 | stator (coil renewal group) | 14 | filter |
A commonly used sealing method for chemical pumps is the gasket sealing method. The gasket is a critical component of static sealing in centrifugal pumps and is widely used. Gasket selection is mainly based on factors such as transported medium, temperature, pressure and corrosivity of the chemical pump.
Joint sealing mechanism:
Leakage refers to the flow of a medium from the inside to the outside of a confined space or from the outside into the confined space. This occurs through the interface of the internal and external space, i.e. the leaky sealing surface.
The cause of leakage is the presence of a gap in the contact surface, driven by differences in pressure and concentration on both sides of the surface. Improper sealing surface shape and machining precision can result in gaps that lead to leaks.
To reduce leakage, it is important to maximize the coupling of the contact surface, reducing the cross-sectional area of the leakage channel and increasing the leakage resistance to be greater than the driving force.
When the stress increases to the point of causing significant plastic deformation of the surface, the gap in the sealing surface may be filled, blocking the leak channel.
A gasket is used to take advantage of the material's ability to undergo plastic deformation under compressive load, which helps fill small irregularities in the flange sealing surface to achieve a seal.
Gasket selection:
Pump chemical seals and gaskets play a critical role in ensuring pump safety. The presence of corrosive, volatile and potentially explosive materials in the pump extraction medium requires the sealing performance of the pump casing to be significantly higher compared to other types of pumps.
Gasket sealing is a commonly used method in chemical pumps. The gasket is a key component of the static seal of the centrifugal pump and is widely used.
Gasket selection is mainly based on factors such as chemical pump transport medium, temperature, pressure and corrosivity.
For chemical pumps with low temperature and low pressure conveying medium, non-metallic sealing gaskets are generally used. When the medium has medium pressure and high temperature, non-metallic and metal composite gaskets are chosen.
Non-metallic gaskets, made from materials such as paper, rubber and polytetrafluoroethylene (PTFE), are widely used in pumps. For temperatures not exceeding 120°C and pressures below 1.0 MPa, green paper or matrix paper gaskets are normally used. For oil transmission media with temperatures between -30°C and 110°C, NBR with good aging resistance is generally selected. Fluorinated rubber is a suitable choice for chemical pump media with temperatures between -50°C and 200°C due to its oil and heat resistance as well as its high mechanical strength.
In chemical pumps, due to the corrosive nature of the medium, PTFE is often used as a sealing material. As chemical pumps are becoming increasingly widespread and are being used to transport an increasing variety of media, it is important to consult relevant information or perform experiments to make the correct gasket material selection.
3. Oil Seal
Chemical pump oil seal is a self-tightening lip seal that is characterized by its simple structure, compact size, low cost, ease of maintenance, low resistance torque and the ability to prevent the entry of medium leaks, dust and other harmful substances. . In addition, it has a certain level of wear compensation.
However, it is not designed for high pressure applications and is typically used in low pressure chemical pumps.
Chemical pumps work using the centrifugal principle to transfer liquid materials. These pumps, made from specialized materials, are ideal for transferring corrosive liquids from containers such as bottles, barrels, tanks or swimming pools.
Due to the inherent dangers posed by the medium being transferred, such as corrosion, volatility and explosions, sealing requirements for chemical pumps are significantly higher compared to other types of pumps.
To ensure proper sealing, the chemical pump oil seal must be installed on the shaft with a manufacturing accuracy of H8-H9 and a surface roughness of 1.6-0.8μm, with a surface hardening treatment applied.
It is important to ensure that the sealing medium is free of solid particles and impurities as this can cause rapid wear of the seal and shaft, rendering the seal ineffective.
When selecting this sealing method, it is important to consider the performance and requirements of the materials being transferred by the chemical pump to avoid leaks and possible accidents.
The main factors that cause oil leakage from the oil seal:
Oil leakage is usually caused by poor oil seal sealing. If the diameter of the seal is too small, it may not contact the shaft, causing leakage.
The following are the main factors that cause poor sealing of the S195 diesel engine crankshaft oil seal:
- Poor manufacturing quality of oil seals
- Poor shaft or bearing quality
- Improper use and maintenance
- Improper installation
- Improper storage and environmental pollution
It is important to address these factors to ensure proper sealing and prevent oil leaks.
Measures for oil leakage due to loose oil seal seal:
(1) Familiarize yourself with the basic principles of identifying counterfeit and low-quality products and choosing standard and high-quality oil seals.
(2) During installation, if the shaft diameter has low roughness on the outer surface or there are rust stains, burrs or other defects, use fine sandpaper or oiled stone to polish and smooth. Apply clean engine oil or lubricating grease to the corresponding position of the oil seal lip or shaft diameter.
Coat the outer ring of the oil seal with sealant and wrap the key on the shaft with stiff paper to avoid scratching the edge of the oil seal. Use special tools to turn the oil seal inward and do not use force to avoid deformation or damage to the spring.
If there is edge flanging, spring sagging, or oil seal drooping, remove and reinstall it. Please note that if the shaft diameter is not worn and the spring force of the oil seal is sufficient, do not tighten the internal spring without authorization.
(3) Seals used in machines often face poor working conditions, large temperature fluctuations, dust and frequent vibrations. When the strength conditions of machine parts change frequently, it is important to check, maintain and repair them regularly.
(4) If the shaft diameter and bearing wear are severe, repair or replace the oil seal rubber or spring as soon as possible.
(5) Remove any parts that are abnormally heating and avoid mechanical overspeed and overload to prevent edge temperature rise, rubber aging and premature edge wear.
(6) Regularly check the oil level, and if there are many impurities or metal debris in the oil, replace it completely. Choose a brand and quality of oil that meets seasonal requirements.
Consider adding Maitrey Super Sealant & Lubricant to your engine oil as it is an excellent gearbox additive that forms a film of inert material on components. This can delay oil seal leakage, extend the service life of the oil seal gear, and reduce gearbox noise. This super-sealing lubricant does not pollute or deteriorate the oil.
4. Thread seal
There are two common forms of thread seals on chemical pumps: screw gasket seal and screw thread plus filler seal. Both forms are used to seal small diameter threaded connections.
The gasket acts as a sealing element in a screw electric diaphragm pump with a gasket seal, while the thread provides only pressing force.
The roughness of the sealing surface and the accuracy of its relative geometric position with the threaded hole also greatly impact the sealing effect in addition to the gasket performance.
When tightening the thread, the joint is subjected to compression force and torque, which can cause deformation or damage. As a result, gasket seals are only suitable for chemical pumps with low pressure. If the gasket is made of metal, it can withstand pressures greater than 30MPa.
Another way to seal threads in chemical pumps is to use a plug. To keep the plug manufacturing cost low, the screw thread alone is not sufficient for sealing, and the thread gap is usually filled with a filler such as raw tape or sealant.
The load capacity of the plug depends on the precision of its manufacture and the thread material, and is not affected by the corresponding shape of the plug and the threaded hole.
Whether “cone to cone” or “column to cone” is used for the threaded hole and plug, the sealing effect is the same, but the areas of use are different.
5. Labyrinth seal
When the design is solid, the processing is first-rate, the assembly is of high quality, and the rotation speed is high, the labyrinth sealing effect is very effective.
However, in real-world applications, chemical pump leaks are common, which is why labyrinth seals are not widely used in chemical pumps.
Reasons for this include:
- The adjustment gap between sealing components (such as shaft and bearing gland) is too large, which reduces the sealing effect. In some cases, rough surfaces and visible spiral cut marks can also increase the likelihood of chemical pump leaks.
- Excessive use of lubricating oil in the bearing chamber can lead to overflow pressure that exceeds the sealing strength.
- Incorrect installation of the oil window or oil level gauge may lead to incorrect assessments of the amount of lubricating oil in the oil chamber.
- An increase in oil temperature during operation can reduce its viscosity and increase the risk of chemical leaks into the pump.
- A small oil return tank or return hole, or other obstructions, may prevent the liquid from flowing smoothly, causing leaks.
The medium transported by the chemical pump presents a risk of corrosion, volatilization and explosion, which is why the sealing performance of the pump must be significantly higher compared to other types of pumps.
However, the use of labyrinth seals increases the likelihood of material leaks in chemical pumps.
As a result, labyrinth seals are generally not used in chemical pumps.
6. Packaging seal
Gasket sealing of a chemical pump involves inserting a compressible, resilient gasket into the stuffing box. The axial compression force exerted by the gland is then transformed into a radial sealing force, providing a sealing effect.
This sealing method is called gasket sealing, and the gasket material is called sealing gasket.
Packing seal is a popular choice in chemical pump design due to its simple structure, ease of replacement, low cost and versatility in adapting to different speeds, pressures and media.
Packaging seal principle:
In the machinery industry, packing seals are mainly used as dynamic seals and are commonly found in centrifugal pumps, compressors, vacuum pumps and mixers as shaft seals. The gasket is placed in the gasket chamber and compressed axially by the gasket screw. When there is relative movement between the shaft and the gasket, a radial force is generated and the gasket comes into close contact with the shaft due to the plasticity of the filling. This also causes the lubricant inside the gasket to be expelled, forming an oil film between the mating surfaces.
However, due to the non-uniform contact state, some parts of the gasket will contact the shaft, while others will not. This limit state of lubrication is known as “rolling effect”. The contact and non-contact parts create an irregular labyrinth, which prevents liquid flow from leaking, known as the “maze effect”.
A good seal is obtained by maintaining both the “rolling effect” and the “labyrinth effect”. Poor lubrication or excessive pressure can cause the oil film to rupture, causing dry friction between the gasket and the shaft, eventually resulting in shaft damage and wear.
To avoid this, the degree of packing compression must be adjusted frequently to ensure adequate lubrication and compression. Over time, the lubricant within the gasket can be lost, so some lubricant must be extracted to compensate for the relaxation of compression force caused by the change in gasket volume. However, frequent extrusion of the filler may eventually cause the impregnant to dry out, so the filler must be replaced regularly.
Finally, to maintain the liquid film and remove frictional heat, a small amount of leakage must be allowed into the gasket.
The problems of packaging sealing in the use of chemical pump:
The chemical pump is commonly equipped with a shaft seal, which has advantages such as wear resistance, heat resistance, good flexibility and high strength.
However, the use of packaging also has some disadvantages:
The rough surface of the gasket leads to a high coefficient of friction and increases the likelihood of leaks. Furthermore, lubricant used for a long time may run out.
Initially, the shaft seal of newly repaired equipment works well, but after a short period of operation, leaks begin to occur more frequently. The need for gland adjustment and gasket replacement becomes more frequent, and the shaft sleeve can be worn into a vase-like shape after just one cycle of operation. In severe cases, the shaft sleeve may even break and the water seal ring may not be able to perform its sealing function due to the rotten and irreplaceable gasket.
Constant friction between the rotating gasket and the shaft or shaft sleeve leads to wear, necessitating regular or irregular replacement of the sleeve.
To ensure that the frictional heat between the gasket and the shaft or shaft sleeve dissipates in a timely manner, a certain amount of leakage must be maintained, which may be difficult to control.
Additionally, friction between the gasket and the shaft or shaft sleeve reduces the effectiveness of the gasket seal, which affects engine power and increases energy consumption.
Gasket seal performance and causes of failure:
According to the principle of gasket sealing, there are three sources of leakage in the sealing cavity:
- Fluid penetrating through sealing material.
- Leak between gasket and stuffing box.
- Surface leakage of the gasket and shaft.
The main failures and their causes are as follows:
- The outer surface of the gasket is damaged, causing leakage on the outside of the gasket, which can occur if the outer diameter of the gasket is too small.
- Excessive or eccentric play during the design process can cause the gasket to be compressed in the gap between the shaft and retaining ring or between the shaft and gland.
- Improper gasket assembly or a damaged retaining ring can cause media to leak along the gasket.
- If the gasket is expanded or damaged, cut too short, or assembled incorrectly, the leak may be too large to adjust.
7. Electrical seal
Partial view of K-type dynamic seal with cooling water
| 1. Impeller | 7. Stuffing box | 13. Spindle | 19. Pressure washer |
|---|---|---|---|
| 2. Bomb | 8. Cooling water nozzle | 14. Acid Block | 20. Locknut pad L |
| 3. Back coverage | 9. Water Seal Ring | 15. Top Ring | 21. Lock nut |
| 4. Cross connection screw | 10. K Ring | 16. Shaft sleeve gasket | 22. Locknut |
| 5. Stuffing box gasket | 11. Sealing ring | 17. shaft sleeve | |
| 6. Stuffing box seal block | 12. Stuffing box cover | 18. Impeller pad |
When the chemical pump is in operation, the pressure generated by the auxiliary impeller balances the high-pressure liquid at the pump outlet, ensuring a proper seal.
During shutdown, the auxiliary impeller stops working, therefore it must be equipped with shutdown sealing device to prevent chemical leakage.
The auxiliary impeller has a simple and reliable sealing structure with a long service life, ensuring that there is no leakage during pump operation.
As a result, it is often used in pumps that transport impure media in the chemical industry.
There are several types of seals, including centrifugal seals, spiral seals, and magnetic fluid seals. The spiral seal is particularly promising.
The fully closed seal can be diaphragm type or shield type, among others.
1) Centrifugal power seal
The Principle of Centrifugal Seals:
A centrifugal dynamic seal works by expelling the liquid medium in a radial direction through centrifugal force, thereby preventing liquid from entering the leak opening to achieve a sealing effect. This type of seal is only suitable for liquid media and not for gaseous media.
Therefore, if tightness is required when applying a centrifugal seal, a combination of centrifugal seals and other seal types must be used.
The most commonly used centrifugal seal is the oil baffle, which is widely used in various transmission devices to seal lubricating oil or other liquids. The higher the crankcase speed, the better the seal performance. On the other hand, if the speed is too low or there is no rotation, the oil seal becomes ineffective.
Additionally, the oil baffle seal is not limited by high temperatures, making it a suitable option for high-temperature, high-speed applications such as a heat transfer oil pump. However, it cannot be used in high pressure applications and is typically used in situations with zero or near-zero pressure difference.
The centrifugal oil launcher has the advantages of simple structure, low cost, no frictional energy consumption, no wear and low maintenance, making it a widely used option.
The structure of centrifugal seals:
A centrifugal seal is an oil baffle sealing device without a crankcase. In a smooth shaft, the adhesion of the liquid medium facilitates its leakage along the surface of the shaft. However, if there are one or two annular grooves on the shaft, it becomes difficult for the liquid to cross the sharp interface in the annular groove. With the help of the centrifugal force of the rotating shaft, it is easy to shake the liquid and ensure sealing.
The centrifugal oil thrower is integrated into the shaft, which blocks the liquid trying to leak out, and throws the liquid to the circumference of the sealing cap under the action of centrifugal force. The liquid then flows to the oil return hole below for oil return.
A ring groove is located at the junction of the seal cover and oil deflector, allowing liquid in the seal cover wall to flow through the ring groove instead of entering the space between the seal cover and the shaft.
When designing a centrifugal oil baffle sealing device, it is important to reduce the radial clearance and axial clearance between the oil baffle and the sealing cover as much as possible, in order to reduce the radial clearance between the sealing cover and the shaft . The seal cover ring groove must be large enough, the oil throwing space between the seal cover and the oil baffle must be large enough, and the oil return channel must be as smooth as possible.
The oil deflector impeller seal is equivalent to several pieces arranged on one or both sides of the crankcase, which support the blowing effect produced by the rotation of the impeller. This throws the leaked lubricating oil into the return port along with the radial flow, thus reducing the lubricating oil diversion along the shaft.
The size of the impeller blades should not be excessive and there should not be too many of them. This is because strong airflow combined with a lubricating oil mixture can cause foaming, which impairs oil return and increases energy consumption.
Back blade seals and impeller auxiliary seals are often used as shaft seals in centrifugal pumps.
To stabilize flow and increase sealing capacity, a set of fixed guide vanes is often placed within the sealing chamber of the auxiliary impeller. This helps reduce pressure on the smooth surface of the auxiliary impeller.
One of the benefits of centrifugal electric seals is that they do not have direct friction contact and can accommodate a wide sealing gap. This makes them suitable for sealing media containing solid impurities and feature low wear, long service life and a reliable zero-leakage design.
However, they have limited ability to handle pressure differences and consume a significant amount of energy, sometimes up to a third of the pump's useful power.
Furthermore, being a dynamic seal, the sealing capacity is lost as soon as the pump stops and must therefore be supplemented by a parking seal.
2) Spiral dynamic seal
The working principle of a dynamic screw seal is similar to that of a screw pump. If a screw thread is cut into the shaft (or the screw slot is engraved into the housing, or both), the shaft rotation will be clockwise.
The friction between the liquid medium and the shell produces a counterclockwise force, and the component of this frictional force F along the right-hand thread is to the right, causing the liquid to be pushed to the right, just like a nut moves along a bolt.
As the volume decreases, the head increases, balancing the established sealing pressure with the pressure of the fluid to be sealed, thus preventing leaks.
When designing a threaded sealing device, it is important to pay attention to the direction of displacement of the screw oil. If there is an error in this regard, the seal will not work properly and leaks may occur.
Please note that the threaded seal is a type of dynamic seal and its sealing function may be lost when the device is at rest or operating at low speeds. In these cases, a limiting seal may be required, which adds complexity to the device and requires adequate axial clearance.
8. Mechanical seal
The mechanical seal, also known as the end seal, is currently the most widely used form of seal in the chemical pump industry due to its low leakage and long service life. It is considered the main shaft sealing method for this type of equipment worldwide.
According to relevant national standards, a mechanical seal is defined as a device that prevents fluid leakage through at least one pair of end faces perpendicular to the axis of rotation, depending on the fluid pressure and the elastic (or magnetic) force of the seal. compensation. mechanism, in coordination with auxiliary seals.
Widely used corrosion-resistant PTFE mechanical seals are effective in preventing fluid leakage.
It is important to note that any form of seal must prevent the chemical centrifugal pump from idling, as idling can cause seal failure.
Mechanical seal principle:
Mechanical seals, also known as end face seals, are shaft sealing devices used in rotating machines to prevent fluid leaks. They work by using a pair of end faces perpendicular to the axis of rotation, along with liquid pressure and elastic force from a compensating mechanism, to create an airtight seal.
Mechanical seals are commonly used in pumps, boilers, compressors, and other similar rotary shaft equipment. They are composed of a movable ring, a static ring, a pressure element and a sealing element.
The movable ring rotates with the pump shaft and fits perfectly with the static ring to form a sealing surface, which prevents the medium from escaping. Liquid pressure in the sealing chamber presses the end face of the moving ring against the end face of the static ring, creating a thin film of liquid and a specific pressure suitable for achieving a seal.
The compression element generates pressure, keeping the pump end together when the pump is not running and preventing leaks and impurities from entering. The sealing element includes an elastic element to dampen pump vibrations and impacts, as well as gaps between the moving ring and the shaft and between the static ring and the gland.
Mechanical seals are integrated with other pump parts during operation. The performance of the mechanical seal depends on its own components, the auxiliary sealing device and the technical installation requirements. To ensure the proper functioning of the mechanical seal, it is important to first meet these requirements.
The problems that exist when using the mechanical seal in the chemical pump are the following:
Mechanical seals in rotating equipment can fail for a variety of reasons, including wear on the seal faces, hot cracking, deformation and damage. Over time, springs can also become relaxed, fractured, and corroded.
Additionally, auxiliary seal rings may experience cracks, twists, deformations and fractures.
Failure performance and causes of mechanical seal:
- Mechanical seal vibration and heating: During operation, the roughness of the end faces of the moving and stationary rings and the small gap between the dynamic and static rings and the sealing cavity can cause vibration and collision. In some cases, poor corrosion resistance and temperature resistance of the seal faces, insufficient cooling or particle impurities on the end faces during installation can also cause vibration and heating.
- Mechanical Seal Medium Leakage: Leakage during static pressure testing, careless installation, bruising, deformation, damage, dirt, granular impurities, loose positioning screws, an uncompressed gland, or insufficient equipment precision can cause medium leakage. Shaft sleeve leakage may occur if the shaft sleeve sealing ring is not sufficiently compressed, damaged during assembly, or compressed.
- Periodic or Paroxysmal Leakage: Periodic vibration of the mechanical seal rotor assembly can cause leakage if there is excessive axial displacement.
- Frequent leakage of mechanical seal: Frequent leakage of mechanical seals can occur due to various reasons, including defects in the seal face, auxiliary seal ring and springs. Other causes may include rotor vibration, poor quality or looseness in the transmission and clamping parts.
- Excessive vibration of the mechanical seal: Excessive vibration of the mechanical seal may result in loss of seal effectiveness. However, the cause of excessive vibration is not just limited to the mechanical seal itself, but can also be attributed to other parts of the pump.
9. Screw seal
A threaded seal is a type of dynamic seal created by machining a spiral groove into the rotating shaft or sleeve surrounding the shaft. A sealing medium is filled between the shaft and the sleeve to prevent fluid leakage.
As the shaft rotates, the spiral groove creates a pump-like conveying effect, which helps keep the fluid sealing. The sealing ability of the threaded seal is influenced by factors such as screw angle, pitch, tooth width, tooth height, tooth action length and clearance between shaft and sleeve.
One of the advantages of threaded seals is their long service life, as there is no friction between the seals. However, sealing ability is limited due to the short bolt length, which is often limited by structural space limitations. Furthermore, when the pump is operated at a reduced speed, the sealing effect of the threaded seal is greatly reduced.
10. Dry gas seal
Dry gas seal, also known as “dry running gas seal”, is a new type of shaft end seal technology that uses slotted seal technology for gas sealing and is considered a non-contact seal.
Dry gas seal principle:
When a movable ring with a hydrodynamic groove (ranging from 2.5 to 10 micrometers) is positioned on the outer edge of the end face, the hydrodynamic groove creates a flow that pumps isolated high-pressure gas from the outer diameter (also known as the upstream side). ) on the sealing surface.
The pressure of the gas film increases progressively from the outer diameter toward the groove diameter and gradually decreases from the groove diameter toward the inner diameter.
As a result of the increased pressure in the end mask, the opening force is stronger than the closing force applied to the sealing ring.
A thin layer of air (1-3 millimeters) is created between the friction surfaces, allowing the seal to operate without contact.
This formed gas film effectively blocks the leakage of the relatively low pressure sealing medium, achieving zero leakage or escape of the sealing medium.
Chemical Pump Seal Selection
The chemical pump is often used to transport corrosive or toxic volatile substances, making its sealing performance a crucial factor in determining the quality of the pump.
When selecting a chemical pump, the following standards should be considered.
1) There are static seal and dynamic seal.
For static seals, only O-rings and gaskets are typically used, with O-rings being the most commonly used sealing rings.
For dynamic seals, gasket seals are rarely used and are mainly replaced by mechanical seals, which can be divided into single end face, double end face, balanced and non-balanced types.
The balanced type is best suited for sealing high pressure media, typically defined as pressures greater than 1.0 MPa.
Double-sided mechanical seals are mainly used for media with high temperature, tendency to crystallize, high viscosity and presence of particles or toxic volatilization.
An insulating liquid must be introduced into the sealing cavity, with a pressure generally 0.07 to 0.1 MPa higher than the average pressure.
2) Sealing material
For static sealing of chemical pumps, fluororubber materials are commonly used. In special cases, PTFE materials can be used.
