Common Sealing Forms And Main Materials Of Chemical Pumps

Leakage point of chemical pump

There are various types and models of mechanical seals used in pumps, but there are mainly five leakage points:

1. Sealing between shaft sleeve and shaft;
2. Sealing between the moving ring and the shaft sleeve;
3. Sealing between dynamic and static rings;
4. Sealing between the static ring and the static ring seat;
5. Sealing between the sealing end cover and the pump body;

1) Phenomenon: During operation, the seal emits a popping sound (end face popping sound)

Possible reason: The sealing liquid vaporizes at the sealing interface, enhancing the cooling of the sealing surface

Solution: Increase the bypass flushing pipeline (if not available), expand the bypass flushing pipeline and/or the openings on the pressure cap

2) Phenomenon: Continuous dripping of the seal. Possible reasons: Uneven surface; Blistering on the graphite sealing surface; Thermal deformation occurs on the sealing surface

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

3) Phenomenon: Spring failure

Possible reasons: Due to corrosion, metal accessories are damaged, and the transmission mechanism is corroded

Solution: Check for incorrect installation dimensions, check if unsuitable materials and sealing types have been used to improve flushing of cooling pipelines, check if excessive torque of the gland bolts has caused deformation of the gland, check if the specific pressure of the gland gasket is appropriate, check for other solid particles between the sealing surfaces, and if necessary, re polish the sealing surfaces. Check for cracks on the sealing surface and replace the main and matching sealing rings.

Phenomenon: During operation, the seal emits a sharp whistling sound

Possible reason: Insufficient amount of lubricant at the sealing point

Solution: Increase the bypass flushing pipeline (if not available) and expand the opening on the bypass flushing pipeline and/or pressure cap.

The main materials of sealing materials for chemical pumps

1. Rubber asbestos packing

Rubber asbestos packing XS250 (maximum operating temperature 250 ℃), rubber asbestos packing XS35 0 (maximum operating temperature 350 ℃).

Applicable media: steam, water, alkaline solution.

2. Oil immersed asbestos packing

Oil immersed asbestos packing YS250 (maximum operating temperature ≤ 250 ℃) and YS350 (maximum operating temperature ≤ 350 ℃): The outstanding features of this product are that the packing contains lubricating oil, has a low friction coefficient, and a large compression amount.

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

3. PTFE asbestos packing SMF:

Operating temperature: -100~+250 ℃, suitable for corrosive media such as weak acids and strong alkalis.

4. Oil immersed cotton, hemp root YMM:

Extreme operating temperature: ≤ 120 ℃, applicable medium: river water, tap water, groundwater, seawater, etc.

5. PTFE fiber woven fillers are available in six different grades.

During use, a small amount of leakage of the packing is allowed. After using it for a period of time and the packing is in a relatively stable state, adjust the packing gland. Do not press too tightly at the beginning to prevent burning out the filler.

6. Carbon fiber woven sealing filler:

Carbon fiber woven sealing filler is a new type of material with high strength and high modulus. It has advantages such as high strength, high elastic modulus, high temperature resistance, chemical corrosion resistance, good thermal conductivity, low coefficient of thermal expansion, and self-lubricating properties.

During initial operation, the packing gland should be lightly tightened to allow for a small amount of leakage of the medium. After running for a period of time, gradually tighten it to allow the packing to enter a relatively stable stage. Carbon fiber polytetrafluoroethylene woven packing (FTH-1), if leakage is found during operation, the packing gland can be tightened multiple times.

7. Phenolic fiber woven sealing filler:

Phenolic fiber woven sealing filler has advantages such as high temperature resistance, acid corrosion resistance, adsorption, good insulation performance, low friction coefficient, wear resistance, no pollution to human body and environment, and low price. Mainly used for packing sealing in high-pressure plunger pumps, high-pressure water pumps, fiberglass acid resistant pumps, ammonia pumps, reciprocating pumps, and deep well pumps in oil fields. The performance is shown in the table below:

Applicable media: acid-base, organic solvents, engine oil, steam, water, etc.

8. Expanded graphite (flexible graphite) material

Expanded graphite (flexible graphite) material not only has the excellent self-lubricating properties, corrosion resistance, high and low temperature resistance, radiation resistance, wear resistance, and low friction coefficient of graphite itself, but also has the advantages of softness, low density, compression resilience, and wide temperature range of use. The applicable medium has a pH value of 0-14. Except for a few strong oxidizing media such as aqua regia, concentrated nitric acid, concentrated sulfuric acid, and high temperature dichromate, which are not recommended for use, all other media can be used. Before installing the packing box, cut a hole in the packing ring with a blade, and then twist it axially only to a certain extent. Place it on the shaft and press it into the packing box. The cut in each circle should be staggered by 900 or 1200. At the beginning of operation, slight leakage is allowed, and as time increases, the amount of leakage will gradually decrease.

9. Carbon graphite materials:

It has good thermal conductivity, conductivity, chemical stability, thermal shock resistance, self-lubricating properties, and processability. In the pump industry, it is used for centrifugal pumps, shielded pumps, diesel distribution pumps, submersible pumps, clean water pumps, high-temperature hot oil pumps, nuclear secondary pumps, boiler feedwater pumps, air pumps and other end face seals, expansion ring seals, mechanical seal rings, axial seals, rotary blades and other occasions.

10. Rubber

Natural rubber: It has excellent flexural flexibility, good wear resistance, cold resistance, and insulation, and high tear strength. Aging resistance, poor weather resistance, oil resistance, and solvent resistance are also poor. The rubber pump made of natural rubber material is suitable for working at temperatures below 78 ℃. Suitable for conveying various abrasive slurries and weak acid and weak alkali solutions, but not suitable for conveying strong acid, oil, aliphatic and aromatic solutions.

Nitrile rubber: It is a widely used oil resistant rubber variety with a series of superior properties. Its wear resistance is 30-45% higher than natural rubber, and its high temperature resistance is better than natural rubber and butadiene styrene rubber. But nitrile rubber has poor weather aging resistance. The rubber pump made of nitrile rubber material has a high operating temperature of 95 ℃. The rubber pump of this type is resistant to various animal and vegetable oils, mineral oils, and ethylene glycol.

Butyl rubber: a lining rubber pump made of butyl rubber material, suitable for acidic mud operation. The high usage temperature is 100 ℃, and it can withstand strong acids and bases (such as hydrochloric acid and sodium hydroxide), but high concentrations of oxidizing strong acids (such as concentrated sulfuric acid and concentrated nitric acid) can degrade butyl rubber.

Fluorine rubber: This type of rubber has excellent properties of heat resistance, ozone resistance, oil resistance, and resistance to various chemical agents after vulcanization. However, fluororubber is inferior in processing and bonding compared to general rubber, and its price is also relatively expensive. The rubber pump made of fluororubber material allows for a high operating temperature of up to 170 ℃ (in ordinary media).

11. Ceramics

Corundum ceramics (mainly composed of AI2O3): generally have extremely high hardness, good wear resistance and chemical stability, and high insulation and melting point. In the pump industry, it is mainly used for ceramic pumps and mechanical sealing rings under special operating conditions. Ceramics can withstand almost all corrosive media, including hot concentrated nitric acid, sulfuric acid, hydrochloric acid, aqua regia, salt solutions, organic solvents, etc., except for hydrofluoric acid and some other substances containing fluorine, as well as hot or concentrated alkaline solutions.

Silicon nitride ceramics: their thermal shock resistance is superior to that of alumina ceramics, and their other properties are basically the same as alumina ceramics.

12. Permanent magnet materials

Ferrite permanent magnet material: Compared with metal permanent magnet materials, it has advantages such as high coercivity, high insulation, good resistance to external magnetic field interference, and low price. It can be used as a magnetic steel for the inner and outer rotors of magnetic pumps.

Rare earth cobalt permanent magnet material: It has excellent magnetic properties, especially high coercivity and magnetic energy product among existing permanent magnet materials, making it suitable for use as permanent magnets on magnetic pumps. However, due to its high cost, this type of material has not been widely used.

13. Paint for other materials used in pumps: for protection, decoration, and marking purposes. According to the effectiveness of use, it is divided into anti rust paint, insulation paint, acid resistant paint, etc; According to the level of application, it is divided into primer, topcoat, putty, etc.

Material of paper pad for centrifugal pump:

Pump felt material: used to manufacture oil seals, which seal the lubricating oil at the friction point without leakage and prevent water and dust from entering. Divided into fine wool felt, semi coarse wool felt, and coarse wool felt.

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

Sealing form of chemical pump

The sealing devices of pumps are mainly divided into two categories: static sealing and dynamic sealing. Static seals usually include gasket seals, O-ring seals, thread seals, and other types. Dynamic seals mainly include soft packing seals, oil seal seals, labyrinth seals, spiral seals, power seals, and mechanical seals.

1. O-ring seal

Rubber O-rings are commonly used in pumps. Due to its simple shape, it is easy to manufacture and has low cost. Regardless of the overall size of the O-ring, its cross-sectional size is very small (only a few millimeters), making it lightweight, less material consumption, simple to use, easy to install and disassemble. The most prominent advantage is that the O-ring has good sealing ability and a wide range of applications. The working pressure of the static seal can reach over 100MPa, and the dynamic seal can also reach 30Mpa. The applicable temperature is -60~200 ℃, which can meet the requirements of various media. Therefore, it is increasingly widely used in pump design.

The O-ring seal is installed between the groove and the sealed surface, with a certain amount of compression. The resulting rebound force provides initial compression stress to the smooth surface and bottom surface of the groove being sealed. So as to play a sealing role. When the pressure of the sealed liquid increases, the deformation of the O-ring also increases, thereby increasing the pressure transmitted to the sealing surface and the sealing effect. This is why O-ring seals have good sealing ability.

Silicon carbide ceramics: With excellent chemical corrosion resistance, high mechanical strength, good wear resistance, high temperature resistance, good self-lubricating performance, and low friction coefficient, they can be used as high-temperature structural materials for mechanical seals of pumps, pump shafts of magnetic pumps, etc. They have more obvious advantages than ordinary ceramics. Silicon carbide is currently an ideal friction material, which is paired with carbon graphite. Its dry sliding friction coefficient is smaller than that of alumina and hard alloys, and its PV value is higher than that of alumina and hard alloys. Silicon carbide ceramics have excellent wear resistance. Except for a few media such as hydrofluoric acid, they can almost withstand the corrosion of all chemicals, including strong corrosive liquids such as concentrated nitric acid, sulfuric acid, and hydrochloric acid.

2. Gasket sealing

Gaskets are the basic components of static seals in centrifugal pumps and have a wide range of applications. The selection of gaskets is mainly determined by factors such as the medium transported by the chemical pump, temperature, pressure, and corrosiveness.

3. Oil seal

It is a self tightening lip seal with a simple structure, small size, low cost, easy maintenance, low resistance torque, which can prevent medium leakage, external dust and other harmful substances from invading, and has a certain compensation ability for wear. However, it is not resistant to high pressure, so it is generally used in low pressure chemical pumps.

4. Thread sealing

There are generally two forms of sealing on chemical pumps, one is threaded connection gasket sealing, and the other is threaded and filled sealing. Both are used in sealing situations with small diameter threaded connections. The sealing component of the threaded electric diaphragm pump connected to the gasket seal is the gasket, and the thread only serves to provide compression force.

5. Labyrinth seal

When the design is reasonable, the processing is excellent, the assembly is intact, and the speed is high, the labyrinth seal effect is very good. However, in practical applications, there are many chemical pump leaks caused by this, so it is not commonly used in chemical pump sealing.

6. Packing seal

It is to place compressible and resilient packing into the packing box, and rely on the axial compression force of the gland to convert it into radial sealing force, thereby playing a sealing role. This sealing method is called packing sealing, and this type of packing is called sealing packing. Due to its simple structure, easy replacement, low cost, adaptability to speed, pressure, and wide range of media, packing seals are widely used in the design of chemical pumps.

7. Power seal

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, thereby achieving sealing. When parking, the auxiliary impeller does not work, so a parking sealing device must be equipped at the same time to solve the possible chemical pump leakage during parking. The sealing structure of the auxiliary impeller is simple, reliable, and has a long service life. It can achieve no leakage during the operation of chemical pumps, so it is often used in pumps transporting media containing impurities.

8. Mechanical seal

The chemical pump industry now uses a wide range of sealing forms. Due to the advantages of low leakage and long service life, mechanical seals are the main shaft sealing method in these equipment in the world today. Mechanical seal, also known as end face seal, is defined in relevant national standards as follows: "A device that prevents fluid leakage by at least one pair of end faces perpendicular to the rotation axis, under the action of fluid pressure, compensation mechanism elasticity (or magnetic force), and the coordination of auxiliary seals to maintain contact and relative sliding.".

9. Spiral seal

Spiral seal is also a form of dynamic seal, which is to process spiral grooves on the rotating shaft or on the housing of the shaft, and fill the sealing medium between the shaft and the housing. The rotation of the shaft causes the spiral groove to produce a conveying effect similar to a pump, thereby preventing the leakage of sealing fluid. The size of its sealing ability is related to the helix angle, pitch, tooth width, tooth height, tooth action length, and the gap between the shaft and the sleeve. Due to the absence of friction between seals, their lifespan is long. However, due to the limitations of structural space, their spiral length is generally short, which limits their sealing ability. When the pump is used at a reduced speed, its sealing effect will be greatly reduced.

10. Dry gas seal

Dry gas seal, also known as dry running gas seal, is a new type of shaft end seal that uses slotted sealing technology for gas sealing, belonging to non-contact sealing.

Dry gas sealing principle: when the dynamic ring with a hydrodynamic pressure groove (2.5~10 µ m) at the outer side of the end face rotates, the hydrodynamic pressure groove pumps the high-pressure isolated gas at the outer diameter side (called the upstream side) into the sealing end face. The gas film pressure gradually increases from the outer diameter to the groove diameter, while the gas film pressure gradually decreases from the groove diameter to the inner diameter. As the end facial mask pressure increases, the opening force formed is greater than the closing force acting on the sealing ring, forming a very thin layer of gas film (1~3 µ m) between friction pairs, so that the seal works in a non-contact state. The formed gas film completely blocks the relatively low-pressure sealing medium leakage channel, achieving zero leakage or zero escape of the sealing medium.