How Much Do You Really Know About Pump Seals?

Leakage point

There is a wide range of mechanical seals for pumps with different models, but there are five main leakage points:

1) Sealing between sleeve and shafts

2) Sealing between dynamic ring and sleeve

3) Sealing between dynamic and static rings

4) Sealing between the static ring and static ring seat

5) Sealing between the seal end cap and the pump body

Phenomenon: seal cracking sound (end bursting sound) during operation

Possible causes:

Vapourization of the sealing fluid at the sealing interface; enhanced cooling of seal face

Solution:

Increase bypass flush line (if not already present) to enlarge bypass flush line and/or openings in the gland.

Phenomenon: continuous seal drip

Possible causes:

uneven surfaces; blistering of graphite sealing surfaces; thermal deformation of the sealing surface

Solution:

During the installation process, the auxiliary seal is scratched, the O-ring is aged, the auxiliary seal becomes hard and brittle due to compression, and the auxiliary seal becomes soft and sticky due to chemical action.

Phenomenon: spring failure

Possible causes:

Damage to metal accessories, corrosion of the transmission due to corrosive action

Solution:

Check incorrect mounting dimensions;

Check unsuitable materials and seal types to improve flushing of cooling lines;

Check whether there is excessive torque on the gland bolt that causes the gland to deform, and checking that the gland gasket has the right specific pressure.

Check other solid particles between the seal faces, and re-polish the seal faces if needed;

Check cracks at the sealing surface, replace the main and mating seal ring.

Phenomenon: the seal makes a screeching sound during operation

Possible causes:

The insufficient lubricant in the seal.

Solution:

Increase the bypass flush line (if not already there) to enlarge the bypass flush line and/or the opening in the gland.

Materials of the sealing material

1. Rubber asbestos packing

Rubber asbestos packing XS250 (ultimate service temperature 250℃) and rubber asbestos packing XS350 (ultimate service temperature 350℃): the applicable medium is steam, water and alkaline solution.

2. Oil-impregnated asbestos packing

Oil-impregnated asbestos packing YS250 (ultimate use temperature ≤ 250℃), YS350 (ultimate use temperature ≤ 350℃): the outstanding feature of this product is that the packing contains lubricating oil, small coefficient of friction and large compression capacity.

Applicable media: steam, air, industrial water and heavy petroleum products.

3. Polytetrafluoroethylene asbestos packing SMF

Use temperature: -100~+250℃.

Applicable media: weak acid, strong alkali and other corrosive media.

4. Oil-impregnated cotton, hemp packing YMM

Limit operating temperature: ≤120℃.

Applicable medium: river water, tap water, groundwater, seawater, etc.

5. Teflon fiber woven packing

PTFE woven fiber filler is available in six grades.

In the process of use, a small amount of leakage of packing is allowed at the beginning.

After a period of time, it should adjust the packing gland when the filler is in a relatively stable state.

Please do not press too tightly at the beginning to prevent the packing from burning.

6. Carbon fiber woven seal packing

Carbon fiber woven seal packing is a new material with high strength and high modulus.

It has the advantages of high strength, large modulus of elasticity, high-temperature resistance, chemical corrosion resistance, good thermal conductivity, low thermal expansion coefficient and self-lubricating properties etc.

In the initial operation, it should be lightly packed gland, allowing a small amount of leakage of media.

After running for a period of time, it then gradually tighten the packing, so that the packing is into a relatively stable stage.

Carbon PTFE woven packing (FTH-1), if it is found in the operation of leakage, can repeatedly tighten the packing gland.

7. Phenolic fiber woven seal packing

Phenolic fiber weave packing has the advantages of high-temperature resistance, acid corrosion resistance, adsorption, good thermal insulation, small coefficient of friction, wear resistance, no pollution to the human body and the environment and low price.

It is mainly used in packing seal of high-pressure plunger pump, high-pressure water pump, FRP acid pump, methyl ammonia pump, reciprocating pump and oilfield deep well pump.

The performance is shown in the following table.

Applicable media: acid and alkali, organic solvents, engine oil, steam and water etc.

8. Expanded graphite (flexible graphite) materials

Expanded graphite (flexible graphite) material, in addition to the excellent self-lubricating graphite itself, corrosion resistance, high and low temperature resistance, radiation resistance, abrasion resistance, low coefficient of friction and other characteristics, it also has the advantages of flexibility, low density, compression resilience and use a wide range of temperature.

It can be applied to the medium PH value of 0 ~ 14.

In addition to aqua regia, concentrated nitric acid, concentrated sulfuric acid and high temperature dichromate and a few other strong oxidizing media are not recommended for use, other media can be used.

The stuffing ring is cut with a razor blade prior to loading the stuffing box, then there can only be a certain amount of axial twist, set on the shaft and pressed into the stuffing letter, which should be staggered 900 or 1200 per turn of cutout.

A slight leak is allowed at the beginning of the run, and the amount of leakage will decrease over time.

9. Carbon-graphite materials

It has good thermal conductivity, electrical conductivity, chemical stability, thermal shock resistance, self-lubricating and processability.

In the pump industry, it is used for centrifugal pumps, shielding pumps, diesel distribution pumps, submersible pumps, clean water pumps, high temperature hot oil pumps, nuclear secondary pumps, boiler feed pumps, gas pumps, such as end seals, up-ring seals, mechanical seal rings, axial seals, rotary vane and other occasions.

10. Rubber

Natural rubber: it has excellent flexural resistance, good abrasion resistance, cold resistance and insulation and high tear strength.

It has aging resistance, poor weather resistance, oil resistance and solvent resistance.

The rubber pump made of natural rubber material is suitable for working temperature below 78°C, which is suitable for conveying a variety of abrasive slag.

It is suitable for conveying a variety of abrasive slurry, weak acid and weak alkali solution, but not for conveying strong acid, oil, aliphatic and aromatic solution.

Nitrile rubber: it is currently the most widely used oil-resistant rubber, with a series of superior properties, abrasion resistance is 30 ~ 45% higher than natural rubber, high temperature resistance than natural rubber and styrene-butadiene rubber.

However, the weathering resistance of the nitrile rubber is poor.

NBR rubber used in the manufacture of rubber pumps, the maximum operating temperature of 95 ° C.

The rubber pump resistant to a variety of animal and vegetable oils, mineral oil and glycol.

Butyl rubber: butyl rubber lining pump parts are suitable for acid mud work.

The maximum temperature is 100 ℃ and it is resistant to strong acids and alkalis (such as hydrochloric acid, sodium hydroxide), but high concentrations of oxidizing acids (such as concentrated sulfuric acid, concentrated nitric acid) will make butyl rubber degradation.

Fluorine rubber: this rubber has excellent heat resistance, ozone resistance, oil resistance and resistance to various chemicals erosion performance after vulcanization.

However, fluorine rubber in processing and bonding is worse than the general purpose rubber, with higher price.

The rubber pump made of fluoroelastomer is allowed to be used up to 170℃ (in ordinary medium).

11. Ceramic

Corundum ceramics (main component AI2O3):

It generally has a very high hardness, good 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.

In addition to hydrofluoric acid and some other substances containing fluorine and hot or concentrated alkali, ceramics can withstand almost all corrosive media including hot concentrated nitric acid, sulfuric acid, hydrochloric acid, aqua regia, salt solutions and organic solvents, etc.

Silicon nitride ceramics:

Thermal shock resistance is better than alumina ceramics, other properties and alumina ceramics are basically the same.

Silicon carbide ceramic:

it has excellent chemical resistance, high mechanical strength, good abrasion resistance, high temperature resistance, self-lubricating and small coefficient of friction etc., so it can be used as a high-temperature structural materials, mechanical seals for pump and magnetic pump pump shaft etc., which has more obvious advantages than ordinary ceramics.

Silicon carbide is currently an ideal friction material, which is paired with carbon-graphite combination.

Its dry sliding friction coefficient is smaller than alumina, carbide, PV value is higher than alumina and carbide.

Silicon carbide ceramic wear resistance is excellent, in addition to hydrofluoric acid and a few other media, it almost resists all chemical corrosion, including concentrated nitric acid, sulfuric acid, hydrochloric acid and other strong corrosive liquids.

12. Ferrite permanent magnets

Compared with metal permanent magnets, ferrite permanent magnets have the advantages of high coercive force, high insulation, good resistance to external magnetic field interference and low price etc., which can be used as magnets for internal and external rotors of magnetic pumps.

Rare earth cobalt permanent magnets: it have excellent magnetic properties, especially the highest coercive force and magnetic energy product among the existing permanent magnets.

It is suitable for magnetic pump ground permanent magnets, but not widely used due to the high price.

13. Other materials

Pump paint:

It plays the role to protect, decorate and mark.

According to the usage, it can be divided into anti-rust paint, insulation paint and acid paint, etc.

According to the painting level, it can be divided into primer, topcoat and putty etc.

Paper cushion material used for the centrifugal pump:

Felt material is used for pump, which is used for making oil seal, that is to seal the lubricating oil at the friction, prevent water and dust from entering.

It can be divided into fine wool felt, semi-coarse wool felt and coarse wool felt.

Industrial vulcanized rubber sheet (GB5574-85): ordinary rubber sheet, acid and alkali resistant rubber sheet, oil-resistant rubber sheet and heat-resistant rubber sheet.

Pump seal form

The sealing device of the pump is mainly divided into two categories: one is static sealing, the other is dynamic sealing.

Static seals usually include gasket seals, O-ring seals, and thread seals.

Dynamic seal mainly includes soft packing seal, oil seal seal, labyrinth seal, spiral seal, dynamic seal and mechanical seal.

1. Rubber O-ring seal

Rubber O-rings are commonly used in pumps.

Because of its simple shape, it is easy to manufacture and low cost.

No matter how large the overall size of the O-ring is, its cross-section size is very small (only a few millimeters).

Therefore, it is light in weight, less in consumption of materials, easy to install and disassemble.

The more outstanding advantages are that the O-ring has the good sealing ability and a wide application range.

The working pressure of the static seal can reach more than 100MPa and that of the dynamic seal can reach 30MPa.

The suitable temperature is – 60 ~ 200 ℃, which can meet the requirements of various media.

Therefore, it is more and more widely used in pump design.

The O-ring is installed between the groove and the surface to be sealed, and there is a certain amount of compression.

The reaction force generated from this gives the smooth surface and the bottom surface of the groove initial compressive stress, so as to play a sealing role.

When the pressure of the sealed liquid increases, the deformation of the O-ring also increases, so the pressure transferred to the sealing surface also increases, and the sealing effect also increases, which is the reason why O-ring has a good sealing ability.

2. Gasket seal

NO. Name NO. Name NO. Name
1 handle 8 stainless steel shield 15 gaskets
2 barrel cover 9 graphite bearings 16 joint
3 cable guard 10 rotors 17 impeller
4 line pressing gasket 11 drainage outlet 18 pump
5 capacitors 12 gaskets 19 footing
6 barrel 13 plug
7 stator (coil renewal group) 14 strainer

Chemical pump commonly used sealing method, there is gasket sealing method.

The gasket is the basic component of centrifugal pump static seal, which is widely used.

The selection of gasket is mainly determined by the factors such as conveying medium, temperature, pressure and corrosivity of the chemical pump.

Gasket sealing mechanism:

Leakage refers to the phenomenon that the medium flows from the inside to the outside of the limited space, or enters into the interior of the limited space from the outside.

The medium flows through the interface of the internal and external space, that is, the sealing surface leaks.

The root cause of the leakage is that there is a gap on the contact surface, and the pressure difference and concentration difference on both sides of the contact surface is the driving force of the leakage.

Due to the influence of sealing surface form and machining accuracy, it is inevitable that there is a gap on the sealing surface, which will cause the sealing surface not fully coincide and leak.

In order to reduce the leakage, it is necessary to make the contact surface chimeric to the greatest extent, which is to reduce the cross-sectional area of the leakage channel, increase the leakage resistance, and make it greater than the leakage driving force.

When the stress increases enough to cause obvious plastic deformation on the surface, the gap between the sealing surface can be filled and the leakage channel can be blocked.

The purpose of using the gasket is to make use of the characteristics of the gasket material which is easy to produce plastic deformation under the action of compression load, which helps to fill up the small uneven surface of the flange sealing surface to realize the sealing.

Gasket selection

Gasket selection:

Chemical pump seal gasket selection, chemical pump extraction medium has corrosion, volatilization, explosion and other dangerous existence, the sealing performance requirements of the pump body are much higher than other types of pumps.

Chemical pump commonly used sealing method, there is gasket sealing method.

The gasket is the basic component of centrifugal pump static seal, which is widely used.

The selection of gasket is mainly determined by the factors such as conveying medium, temperature, pressure and corrosivity of the chemical pump.

When the chemical pump transport medium, the temperature and pressure is not high, the general selection of non-metallic sealing gasket;

When the medium of the chemical pump is with medium pressure and high temperature, nonmetal and metal composite gasket is selected.

Nonmetal gasket is widely used in pumps, and its materials are generally paper, rubber and polytetrafluoroethylene.

When the temperature is not more than 120 ℃ and the pressure is below 1.0 MPa, green shell paper or die paper gasket is generally used.

If the transmission medium of the chemical pump is oil and the temperature is – 30 ~ 110 ℃, NBR with good aging resistance is generally selected.

When the medium of the chemical pump is – 50 ~ 200 ℃, fluororubber is more suitable.

Because in addition to oil and heat resistance, high mechanical strength is also its main feature.

In chemical pumps, due to the corrosive medium, PTFE is generally used as gasket material.

With the application of the chemical pumps more and more widely, there are more and more kinds of medium to be transported.

Therefore, it is necessary to refer to relevant information or make the correct choices after the experiment when selecting gasket material.

3. Oil seal

Oil seal

Chemical pump oil seal is a kind of self-tightening lip seal, its structure is simple, small size, low cost, convenient maintenance, small resistance torque, which can not only prevent medium leakage, but also prevent the invasion of external dust and other harmful substances and has a certain compensation ability for wear.

But it is not resistant to high pressure, so it is generally used in low-pressure chemical pumps.

Chemical pumps rely on the centrifugal principle to extract liquid materials. Chemical pumps made of special materials are suitable for extracting corrosive liquid from bottles, barrels, cylinders, pools or other containers.

Due to the existence of corrosion, volatilization, explosion and other hazards of the medium extracted from chemical pumps, the sealing performance requirements of the pump are much higher than those of other pump types.

The oil seal of the chemical pump should be installed on the shaft with manufacturing accuracy of h8-h9, the surface roughness of 1.6-0.8 μ m and surface hardening treatment.

The sealing medium should not contain solid particles and impurities, otherwise, it will cause rapid wear of oil seal and shaft and make the seal invalid.

In the selection of this sealing method, it is necessary to fully consider the performance and requirements of the materials transported by the chemical pump, so as to avoid unnecessary leakage and prevent accidents.

The main factors causing oil leakage of oil seal:

The main factor causing oil leakage is that the sealing of the oil seal is not tight.

Even if the seal diameter is too small, it will not contact the seal shaft even if the seal diameter is too small.

Taking S195 diesel engine crankshaft oil seal as an example, this paper analyzes the main reasons for its poor sealing:

  • Poor oil seal manufacturing quality
  • Poor quality of shaft or bearing
  • Improper use and maintenance
  • Improper installation
  • Improper storage and adverse effects caused by environmental pollution

S195 diesel engine crankshaft oil seal

Measures for oil leakage due to loose seal of oil seal:

1) Master and identify the basic knowledge of fake and inferior products, and choose high-quality and standard oil seals.

2) During installation, if the outer surface roughness of the shaft diameter is low or there are rust spots, rust, burrs and other defects, the fine sand cloth or oilstone shall be used to polish and smooth, and clean engine oil or lubricating grease shall be applied on the corresponding position of oil seal lip or shaft diameter.

The outer ring of the oil seal needs to be coated with sealant, the keyway on the shaft needs to be wrapped with hard paper to avoid scratching the oil seal lip.

The oil seal is rotated inward with special tools, and it is not allowed to smash and punch hard to prevent the oil seal from deforming or breaking the spring.

In case of lip flanging, spring falling off and oil seal skew, it must be removed and re-installed.

It should be noted that when the shaft diameter is not worn and the spring force of the oil seal spring is sufficient, please do not tighten the innerspring without authorization.

3) The oil seal used in machinery has poor working conditions, large temperature difference, dust and frequent vibration.

When the force condition of machine parts changes constantly, it is necessary to check, maintain and maintain them frequently.

4) If the shaft diameter and bearing wear are serious, oil seal rubber aging or spring failure should be repaired and replaced in time.

5) The abnormal heating parts or assemblies should be removed in time;

Avoid mechanical overspeed and overload operation to prevent oil seal lip temperature rise, rubber aging, lip early wear, etc.

6) Check the oil level from time to time, if there are too many impurities in the oil and there are alloy powder and metal scrap in the oil, the oil should be replaced thoroughly.

The brand and quality of the oil to be changed should meet the requirements of the season.

It is suggested to add maitrey super sealant & lubricant to the engine oil because it is an excellent gearbox additive and can form an inert material film on the components. It can slow down the oil seal leakage, prolong the service life of the oil seal gear, and reduce the noise of the gearbox.

The use of this super sealing lubricant will not pollute or deteriorate the oil.

4. Thread seal

Thread seal

There are generally two forms of thread seal in chemical pump, one is screw joint gasket seal, the other is screw thread plus filling seal, both of which are used in sealing occasions of small diameter thread connection.

The sealing element of the screw electric diaphragm pump with gasket seal is the gasket, and the thread only plays the role of pressing force.

In addition to the performance of the gasket, the roughness of the sealing surface and the relative geometric position precision with the threaded hole also has a great influence on the sealing effect.

When tightening the thread, the sealing gasket not only bears the compression force, but also bears the torque, which makes the gasket deformed or even damaged.

Therefore, it is generally only applicable to the chemical pump with low pressure when the gasket is non-metallic, the applicable pressure can reach more than 30MPa if the gasket is metal.

The plug used in the chemical pump is another form of thread sealing.

Considering the economy of screw plug manufacturing, the screw thread can not be sealed only by the cooperation of thread, and the sealing gap of the thread is often filled with filler such as raw tape and sealant.

Its bearing capacity depends on the manufacturing accuracy and material of thread, and has no effects with the matching form of plug and threaded hole.

Whether “cone to cone” or “column to cone” is adopted for the threaded hole and plug, the sealing effect is the same, but the use area is different.

5. Labyrinth seal

 

Labyrinth seal

When the design is reasonable, the processing is excellent, the assembly is good, and the rotating speed is high, the labyrinth sealing effect is very good.

However, in practical application, there are many chemical pump leakage, so it is not commonly used in chemical pump seals.

With reasonable design, excellent processing, good assembly and high rotating speed, the labyrinth sealing effect is very good.

However, there are many chemical pump leakage in practical application, so it is not commonly used in chemical pump seals.

The main reasons are as follows:

  • The fit clearance of the sealing pair (such as shaft and bearing gland) is too large, which is inversely proportional to the sealing effect. In some cases, the leakage trend of the chemical pump is also increased due to the rough mating surface and obvious spiral cutting marks.
  • Due to too much lubricating oil injected into the bearing chamber of the chemical pump, the overflow pressure exceeds the sealing resistance.
  • Incorrect installation position of oil window or oil level gauge misleads people’s correct judgment of lubricating oil quantity in oil chamber
  • The increase of oil temperature during operation reduces its viscosity and increases the possibility of chemical pump leakage.
  • The size of oil return tank or oil return hole is too small, or other obstacles prevent the blocked liquid from returning smoothly, resulting in leakage.

The medium extracted from the chemical pump has the risk of corrosion, volatilization and explosion, so the sealing performance of the pump is required to be much higher than that of other pump types.

The use of the labyrinth seal increases the risk of leakage of materials transported by the chemical pump.

Therefore, chemical pumps generally do not use this sealing method.

6. Packing seal

Packing seal

The packing seal of the chemical pump is to put the packing with compressibility and resilience into the stuffing box, and transform the axial compression force of the gland into radial sealing force, so as to play a sealing role.

This sealing method is called packing seal, and the packing is called sealing packing.

The packing seal is widely used in the design of chemical pump because of its simple structure, convenient replacement, low price, wide adaptability to speed, pressure and medium.

Principle of packing seal:

In the machinery industry, packing seal is mainly used as dynamic seal, which is commonly used as shaft seal of centrifugal pump, compressor, vacuum pump and mixer.

After the packing is put into the packing chamber, it is compressed axially by the gland screw.

When there is relative movement between the shaft and the packing, it produces radial force and contacts with the shaft closely due to the plasticity of the filler.

At the same time, the lubricant impregnated in the packing is squeezed out and an oil film is formed between the contact surfaces.

Because the contact state is not particularly uniform, the contact part appears.

The boundary lubrication state is called “bearing effect”;

The contact part and non-contact part form an irregular labyrinth to prevent the leakage of liquid flow, which is called the “labyrinth effect”.

This is the mechanism of the packing seal.

Good sealing is to maintain the “bearing effect” and “labyrinth effect”.

In other words, to maintain good lubrication and proper compression, if the lubrication is poor or the pressure is too tight, the oil film will be interrupted, resulting in dry friction between the packing and the shaft, and finally causing shaft burning and serious wear.

Therefore, we need to adjust the compaction degree of the packing frequently, so that the lubricant in the packing will be lost after running for a period of time, then squeeze out some lubricant, and compensate for the relaxation of compression force caused by the change of packing volume at the same time.

Of course, frequent extruding of the filler will eventually dry up the impregnant, so it is necessary to replace the filler regularly.

In addition, in order to maintain the liquid film and remove the friction heat, it is necessary to make a small amount of leakage at the packing.

Different types

The problems of packing seal in the use of chemical pump:

The shaft seal of the chemical pump is generally used, which has the advantages of wear resistance, heat resistance, good flexibility and high strength.

When packing is used, it has the following disadvantages:

  • The surface of the packing is rough, the friction coefficient is large, and there is a leakage phenomenon. In addition, the lubricant immersed for a long time is easy to lose.
  • The shaft seal of the newly repaired equipment is in good condition at the beginning of the operation, but the leakage will continue to increase after a short time. The work of adjusting the gland and replacing the packing is also becoming more and more frequent, the shaft sleeve has been worn into a vase after less than one cycle of operation. In serious cases, the shaft sleeve will be broken and the packing that cannot be replaced at the back of the water seal ring is rotten and can not play a sealing role.
  • The packing is in direct contact with the shaft and rotates relatively, resulting in wear of the shaft and sleeve, so the sleeve must be replaced regularly or irregularly.
  • In order to make the friction heat between packing and shaft or shaft sleeve dissipate in time, the packing seal must keep a certain amount of leakage, which is not easy to control.
  • The friction between packing and shaft or shaft sleeve reduces the principle power of effective packing seal of motor and consumes electric energy.

chemical pump

Performance and causes of packing seal failure:

According to the principle of packing seal, there are three leakage channels in the sealing cavity:

One is leakage caused by the fluid penetrates through the sealing material;

The second is leakage between packing and packing box;

The third is the surface leakage from the packing and shaft.

The main faults and causes are as follows:

  • The outer surface of the packing is damaged and the outer side of the packing gland leaks because the outer diameter of the packing is too small.
  • When the packing is squeezed into the gap between the shaft and the retaining ring or between the shaft and the gland, it is often due to the excessive or eccentric clearance designed.
  • The leakage of media along the packing gland is due to improper packing assembly or damaged retaining ring.
  • The leakage is too large to be adjusted because the packing is expanded or damaged, the packing is cut too short or the assembly is wrong.

7. Power seal

Power seal

Partial view of K-type dynamic seal with cooling water

1. Impeller 7. Sealing box 13. Spindle

 

19. Spring washer
2. Pump 8. Cooling water nozzle 14. Block acid piece 20. Lock nut L cushion

 

3. Back shroud 9. Water seal ring 15. Top ring 21. Locking nut

 

4. Cross connecting screw 10. K-ring 16. Shaft sleeve gasket 22. Locknut

 

5. Sealing box gasket 11. O-ring 17. shaft sleeve
6. Gasket block of sealing box 12. Sealing box cover 18. Impeller pad

When the chemical pump is in operation, the pressure head generated by the auxiliary impeller balances the high-pressure liquid at the outlet of the main impeller to realize the sealing.

During the shutdown, the auxiliary impeller does not work, so it must be equipped with shutdown sealing device to solve the leakage of the chemical pump.

The seal structure of the auxiliary impeller is simple, reliable and long service life, which can realize no leakage during the operation of the chemical pump.

Therefore, it is often used in the pump conveying the impurity medium in the chemical pump.

There are centrifugal seal, spiral seal and magnetic fluid seal etc., the spiral seal is very promising among them.

The fully enclosed seal has diaphragm type and shielding type, etc.

1) Centrifugal power seal

The premise of the centrifugal power seal:

Centrifugal dynamic seal is to throw liquid media out of the radial direction through centrifugal force and prevent liquid from entering leakage gap to achieve the purpose of sealing.

Centrifugal seal is only suitable for liquid medium, but not for gas medium.

Therefore, if the air tightness is required in the place where the centrifugal seal is used, the combined seal type of centrifugal seal combined with other types of the seal must be accepted.

The most commonly used centrifugal seal is the oil slinger, which is widely used in various transmission devices to seal lubricating oil or other liquids.

The higher the speed of the oil pan, the better the sealing effect;

On the contrary, if the speed is too low or static does not rotate, the oil slinging seal is invalid.

In addition, the oil slinger seal is not limited by high temperature.

Therefore, the seal of the pump shaft and shell sealing cover oil slinger plate is very suitable for high temperature and high-speed, such as heat transfer oil pump.

However, this kind of seal can not be used for high pressure, and it is generally used in the situation where the pressure difference is zero or close to zero.

This is the sealing form of the heat transfer oil pump.

Therefore, the seal of the pump shaft and shell sealing cover oil slinger plate is very suitable for high temperature and high speed, such as heat transfer oil pump,

However, this kind of seal can not be used for high pressure, and it is generally used in the situation where the pressure difference is zero or close to zero. This is the sealing form of the heat transfer oil pump.

The centrifugal oil slinger has the advantages of simple structure, low cost, no friction power consumption, no wear and maintenance, so it is widely used.

Centrifugal sealing structure.

Centrifugal seal is an oil slinging sealing device without an oil pan:

On a smooth shaft, the adhesion of the liquid medium is easy to crawl along the surface of the shaft, so it is easy to leak;

If there are one or two ring grooves on the optical axis, it is difficult for the liquid to cross the sharp mutation interface on the ring groove.

At this time, with the help of the centrifugal force of the rotating shaft, it is easy to shake off the liquid to ensure sealing.

The centrifugal oil slinger is integrated with shaft 1, which blocks the liquid trying to leak, throws the liquid to the circumference of the sealing cover under the action of centrifugal force, and then flows into the oil return hole below for oil return.

A ring groove is arranged at the common place of the sealing cover and the oil slinger, so that the liquid on the sealing cover wall may flow down the ring groove, instead of flowing into the gap between the sealing cover and the shaft.

When designing the centrifugal oil slinging sealing device, the radial clearance and axial clearance between the oil slinger and the sealing cover should be reduced as much as possible, so as to reduce the radial clearance between the sealing cover and the shaft.

At this time, the ring groove of the sealing cover should be large enough, the oil throwing space between the sealing cover and the oil slinger should be large enough, and the oil return channel should be as flowing as possible.

The seal of the oil slinger impeller is equivalent to the arrangement of several pieces on one or both sides of the oil pan, which is used to support the blowing effect produced by the impeller rotation, so that the leaked lubricating oil is thrown to the return hole along with the radial flow, thus reducing the lubricating oil deviation along the shaft.

The blades of the impeller should not be too large or too many.

Because too strong airflow and lubricating oil mix will produce foam, which is unfavorable to oil return, and the blade will consume a lot of power.

Back blade seal and auxiliary impeller seal are often used as shaft seal of is the centrifugal pump.

A number of fixed guide vanes are often arranged inside the seal chamber of the auxiliary impeller, which can stabilize the flow and partially eliminate the pressurization of the smooth surface of the auxiliary impeller to improve the sealing capacity of the impeller.

The characteristic of the centrifugal power seal is that it has no direct contact friction pair and can accept a large sealing gap. Therefore, it can seal the medium containing solid impurities, with small wear, long service life, reasonable design and zero leakage.

However, the ability to overcome the pressure difference is low and the power consumption is large, even up to 1/3 of the useful power of the pump.

In addition, because it is a kind of dynamic seal, so the sealing capacity will be lost immediately after stopping the vehicle.

Therefore, it must be supplemented by a parking seal.

Back blade seal

2) Spiral dynamic seal

The working principle of the screw dynamic seal is equivalent to a screw capacity pump. If the right screw is cut out on the shaft (or the screw groove is engraved on the shell or both), the purpose of the rotation mark of the shaft is clockwise.

Then the friction force between the liquid medium and the shell is the purpose of the reverse time needle mark, and the component force of the friction force F on the spiral line of the right thread is right, so it must be liquid.

The medium pushes the liquid to the right just as the nut loosens along the screw.

With the continuous reduction of the capacity, the pressure head gradually increases, so that the established sealing pressure is balanced with the pressure of the sealed fluid, thus preventing the leakage.

When designing the spiral seal device, special attention should be paid to the purpose of the screw oil displacement mark.

If there is a mistake in the purpose of the target, it can not be sealed.

But on the contrary, the liquid will be driven to the target of leakage, which will greatly increase the leakage.

Obviously, the spiral seal is also a kind of dynamic seal, and the sealing function will be lost in parking or low-speed environment.

So, it is often necessary to use the stop seal, which complicates the device and requires a sufficient axial size.

8. Mechanical seal

Now it is the most widely used seal form in the chemical pump industry because the mechanical seal has the advantages of less leakage and long service life, so the mechanical seal is the main shaft sealing mode in these equipment in the world.

Mechanical seal, also known as end face seal, is defined in the relevant national standards as follows: “a device to prevent fluid leakage by at least one pair of end faces perpendicular to the rotating axis under the action of fluid pressure and the elastic force (or magnetic force) of the compensation mechanism and the coordination of auxiliary seals.

At present, the widely used corrosion-resistant PTFE mechanical seals can effectively prevent the leakage of liquid.

Any form of the seal must prevent the chemical centrifugal pump from idling, once idling, the seal may be burned.

Principle of mechanical seal:

Mechanical seal is also called end face seal.

It is a kind of shaft seal device of rotating machinery, which refers to the device to prevent fluid leakage by at least one pair of end faces perpendicular to the rotating axis under the action of liquid pressure and the elastic force of compensation mechanism and the coordination of auxiliary seal.

Its main function is to change the axial seal which is easy to leak into the face seal which is difficult to leak.

It is widely used in pump, kettle, compressor and other similar equipment of rotating shaft seal.

Mechanical seal is usually composed of moving ring, static ring, pressing element and sealing element.

The moving ring rotates with the pump shaft, and the moving ring and the static ring closely fit together to form a sealing surface to prevent medium leakage.

By the pressure of the liquid in the seal chamber, the end face of the moving ring is pressed on the end face of the static ring, and the proper specific pressure is produced on the end faces of the two rings and a thin liquid film is kept to achieve the purpose of sealing.

The compression element produces pressure, which can keep the end face of the pump close to each other when the pump is not running, so as to ensure that the sealing medium does not leak out and prevent impurities from entering the sealing end face.

In the sealing element, the clearance between the moving ring and the shaft, the clearance between the static ring and the gland, and the elastic element can buffer the vibration and impact of the pump.

Mechanical seal is combined with other parts of the pump in actual operation.

The normal operation of the mechanical seal has a great relationship with its own performance and external conditions.

However, we must first ensure the performance of its own parts, auxiliary sealing device and technical requirements for installation, so that the mechanical seal can play its due role.

The problems existing in the use of the mechanical seal in the chemical pump are as follows:

Mechanical seal parts of the failure of rotating equipment in operation, seal face often appear wear, hot crack, deformation, damage and other conditions, spring used for a long time will also relax, fracture and corrosion.

The auxiliary seal ring will also crack, twist, deformation, fracture and so on.

Failure performance and causes of mechanical seal:

1) Mechanical seal vibration, heating

During the rotating process of the equipment, the end face of the moving and stationary rings will be rough, and the gap between the dynamic and static rings and the sealing cavity is too small, which will cause vibration due to the collision caused by the vibration swing.

Sometimes due to poor corrosion resistance and temperature resistance of the seal face, or insufficient cooling or particle impurities in the end face during installation, vibration and heating of mechanical seal will also be caused.

2) Mechanical seal medium leakage

Leakage during the static pressure test, mechanical seal in installation due to careless, often make the seal end face is bruised, deformed, damaged, unclean, with granular impurities, or due to loose positioning screw, the gland is not compressed, machine equipment accuracy is not enough, so that the sealing surface is not fully fitted, which will cause medium leakage.

If it is shaft sleeve leakage, it is that the shaft sleeve seal ring is not compressed or compressed enough or damaged during assembly.

3) Periodic or paroxysmal leakage

The periodic vibration of the rotor assembly of the mechanical seal will cause leakage if the axial displacement is too large.

4) Frequent leakage of mechanical seal

There are many reasons for the frequent leakage of mechanical seals.

Firstly, the frequent leakage is caused by the defects of the seal face;

The second is the frequent leakage caused by the auxiliary sealing ring;

The third aspect is the leakage caused by spring defects.

Other aspects include leakage caused by rotor vibration, leakage caused by poor quality or looseness of transmission, tightening and thrust parts.

5) Excessive vibration of mechanical seal

The vibration of the mechanical seal is too large, which leads to the loss of the sealing effect.

But the reason for the excessive vibration of the mechanical seal is not only the mechanical seal itself, but also the other parts of the pump.

9. Screw seal

The screw seal is also a form of dynamic seal, which is a spiral groove machined on the rotating shaft or the sleeve of the shaft.

A sealing medium is filled between the shaft and the sleeve.

The rotation of the shaft causes the spiral groove to produce the conveying effect similar to the pump, thus preventing the leakage of the sealing fluid.

Its sealing capacity is related to screw angle, pitch, tooth width, tooth height, action length of tooth and clearance between shaft and sleeve.

Because there is no friction between the seals, the service life is long. However, due to the limitation of structural space, the screw length is generally short, so its sealing ability is also limited.

When the pump is used at reduced speed, its sealing effect will be greatly reduced.

10. Dry gas seal

Dry gas seal, i.e. “dry running gas seal”, is a new type of shaft-end seal which uses slotted seal technology for gas seal, which belongs to non-contact seal.

Principle of dry gas seal:

When the moving ring with a hydrodynamic groove (2.5-10 μ m) is set outside the end face, the hydrodynamic groove pumps the high-pressure isolated gas from the outer diameter side (called the upstream side) into the seal face.

The gas film pressure increases gradually from the outer diameter to the groove diameter, and decreases gradually from the groove diameter to the inner diameter.

Due to the increase of the pressure of the end mask, the opening force is greater than the closing force acting on the sealing ring.

A thin layer of air film (1~3 m) is formed between the friction pairs to make the seal work in a non-contact state.

The formed gas film completely blocked the relatively low pressure sealing medium leakage channel, and realized zero leakage or zero escape of sealing medium.

Selection of chemical pump seal

The chemical pump is often conveying corrosive or toxic volatile media, so its sealing performance is one of the standards to measure the good and bad of the chemical pump.

In the selection of chemical pumps, the following standards should be referred to.

1) There are static seal and dynamic seal.

For static seal, only seal ring and gasket are usually used, and O-ring is the most widely used sealing ring;

For the dynamic seal, rarely used packing seal, which is mainly mechanical seal, and the mechanical seal is divided into single end face and double end face, balanced type and non-equilibrium type.

Among them, the balanced type is more suitable for the sealing of high-pressure medium, which usually refers to the pressure greater than 1.0MPa.

The double end face mechanical seal is mainly used for the medium with high temperature, easy crystallization, viscosity, containing particles and toxic volatilization.

The isolation liquid should be injected into the sealing cavity, and the pressure is generally 0.07 ~ 0.1MPa higher than the medium pressure.

2) Sealing material

Chemical pump static sealing materials usually use fluororubber materials, special cases will use PTFE materials.

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