The mechanical seal is one of the most common types of sealing elements in technology. This type of seals is widely used in pumps, compressors, and chemical production equipment. In some industries, such as the chemical industry, these seals play a leading role.
Fig.1 Mechanical seal
A design feature of the mechanical seal is that tightness is achieved by tightly pressing two parts (rotating and stationary) along the end planes. The friction pair, which acts as the main sealing element, is made of special materials and with high quality processing of the friction surfaces to ensure maximum tightness. As a rule, seals of this type are used to seal rapidly rotating machine shafts, such as the shafts of pumps, compressors, and various chemical apparatus (reactors, mixers, etc.). This is due to the fact that all other types of seals are not as effective and cannot provide a high level of tightness, which is especially important when sealing aggressive or toxic environments.
Reference:
Historically, the shaft-housing type was used to seal assemblies (See Fig. 2).
Types and designs of mechanical seals.
Depending on the operating conditions, different types of mechanical seals are used.
1. Single mechanical seal.
A single mechanical seal is used in equipment operating in chemically neutral and non-toxic liquids, at operating temperatures up to 200°C and pressures up to 20 MPa. They can be external (for abrasive media) and internal (for media with lubricating properties). They can also be equipped with additional cooling devices to increase efficiency.
2. Double mechanical seal (Fig. 4).
The double mechanical seal is used in equipment operating when pumping petroleum products, liquefied gases, media containing abrasive inclusions, as well as those containing harmful and toxic substances, at operating temperatures up to 400°C and pressures up to 30 MPa. Structurally, they are divided into: “back-to-back” (“back-to-back”); “face-to-face” (“face-to-face”); tandem.
They can be additionally equipped with cooling devices, devices for creating “back pressure” (supply of “locking” liquid between the sealing circuits to prevent leakage of the working medium) and devices for “flushing” units (in order to minimize abrasive wear. Additional devices can be autonomous (for example, with an impeller, providing pressure creation or fluid circulation) or external (with piping for connecting external devices).
Fig.4. Back-to-back double mechanical seal.
3. Cartridge (cartridge) type seal (Fig. 5).
One of the most popular types of mechanical seals. Both parts of the seal are made in the form of a single unit (module) manufactured to standard installation dimensions of stuffing box chambers according to API, DIN, ISO, etc. standards. They are produced for specific operating conditions and types of equipment. In addition to one or more sealing circuits, they may include additional devices for heating, cooling, lubrication, creating back pressure, flushing devices, various sensors, etc. Depending on the design and materials used, serial cartridge-type mechanical seals can be used at operating temperatures up to 650 °C and pressure up to 80 MPa.
Rice. 5. Cartridge type seal.
4. Mechanical gas seal (gas-dynamic, dry, etc.)
A typical gas dynamic seal is shown in Fig. 6
Rice. 6 Dry gas-dynamic seal produced by JSC "TREM Engineering"
Used since the mid-80s of the 20th century. The principle of operation is based on the creation of a thin gas layer between the rings of the mechanical seal (a gap of about 2-5 microns), this occurs thanks to special V- or U-shaped pockets, with a thickness comparable to the thickness of the end gap, located on the sliding surface of one of the rings, from the middle of the ring to the outer edge of the ring from the barrier gas side. When the ring rotates, the barrier gas is pumped into the pocket gap, which leads to the formation of a gap, which leads to non-contact gas sliding: this ensures minimal friction losses and wear of the seal. Technical air or nitrogen under pressure greater than the working medium by 5...10% is used as a barrier gas. Ideal for working at low temperatures, with low-temperature boiling liquids, to ensure the cleanliness of the production process (completely eliminates leaks). The disadvantages of this type of seal include complexity and high cost.
5. Magnetic fluid seal
In a magnetic fluid seal, the sealing element is played by a magnetic fluid, which is held in the gap between the shaft and the housing by a permanent magnet. Magnetic fluid seals operate without maintenance and with very little leakage. Because the sealing medium is a liquid, there is virtually no friction between rotating and stationary parts, so the seal does not wear out. Therefore, the service life and overhaul cycles of such seals are usually very long, and the friction torque is very low. MFAs operate stably in ultra-high vacuum, very high temperatures, tens of thousands of rpm and at pressures up to several atmospheres. The most typical application of a magnetic-fluid seal is the sealing of rotation inputs of vacuum processing equipment. MFAs are widely used in biotechnology, pharmaceuticals, and cosmetology. The reliability and high level of tightness of MFAs makes them increasingly popular and attractive for processes with high sterility requirements. The disadvantage of this type of seal is that it cannot be used at high pressure drops.
Reference:
Since most commercially produced mechanical seals are manufactured to standards, seals from different manufacturers can be selected for a specific type of equipment. Examples of interchangeability are given in
In this article we will talk about the most popular mechanical or mechanical seals in modern pumps. The words “face” and “mechanical” in relation to pump seals should be considered synonyms. The first option is more commonly used in domestic literature, the second in Western literature (mechanical seal). You will find out which of them are suitable for water and which for acids. Which of them are not afraid of solid particles, and which can run dry.
Principle of mechanical seals
Figure No. 1 (see below) will help us understand the principle of operation of mechanical seals. It shows a stationary ring (5) in red, which is rigidly attached to the rear wall of the pump housing (7). To ensure that there are no leaks between the stationary ring and the pump housing, an elastomer element (6) is used. Since the ring is motionless, this elastomer does not experience friction and therefore does not wear out. The impeller shaft passes inside the stationary ring, but does not touch it. This is an important point, since if the ring and the shaft were in contact, then no liquid would pass between them, and the ring itself would be a seal. However, gland or lip seals are designed according to this principle. The very idea of a mechanical seal eliminates friction between the shaft and the seal. Friction leads to wear of both the shaft and seal and therefore gland and lip seals are short-lived and require regular inspection and replacement.
Dear site visitors. This article is for reference only. We do not sell mechanical seals.
Since the shaft does not touch the stationary ring, liquid would pass freely between them, if not for the second rotating ring (4), which is mounted on the shaft close to the stationary one. The surface of the movable and fixed rings is called a friction pair. This friction pair is the only rubbing element of the structure. In the gap between the rings, which is less than a micron, a thin film of liquid is formed. It serves to lubricate the surfaces of the friction pair and prevents them from overheating.
To simplify the design, the movable ring should be rigidly fixed to the shaft and the joint between the shaft and the ring should be sealed with elastomer. Then the entire structure would consist of only a pair of rings, one of which is attached to the rear wall of the pump, and the other to the shaft. Unfortunately, this is impossible, because axial displacement of the shaft occurs during pump operation. This would cause the rings to move closer to each other and then move away. Liquid will get into the increased gap between them, even if the gap is only 0.01 mm. The whole sealing principle would go down the drain. An element is required that will ensure continuous and tight contact between the two seal rings. This element will be a spring (10) or a bellows.
Figure 1. Design of a multi-spring bellowsless mechanical seal.
Now it becomes clear why the rotating ring is not rigidly mounted on the shaft. If its movement in the radial plane of the shaft is not so important, then in the axial direction it must regularly move relative to the shaft in order to compensate, thanks to a spring or bellows, for the axial beating of the shaft. In order for the spring (bellows) to act on the rotating seal ring, they must be attached to some element that is rigidly fixed to the shaft and rotates with the shaft. This element can be the seal body or the bellows itself. In Figure No. 1 we see how springs (10) are attached to the seal body, shown in blue, which press the movable ring (4) against the stationary one (5).
To make the movable ring rotate along with the shaft, it is necessary to transmit the torque of the shaft to it. This function can be performed by a central spring or a metal bellows. Our seal uses small peripheral springs (10) that cannot transmit shaft torque. This role is played by the pin (3), which connects the seal body (7) and the rotating ring (4).
All that remains is to add the final touch. To prevent liquid from penetrating between the shaft and the moving ring, an additional elastomer sealing element (2) is used. In this design, due to axial vibrations of the shaft, microdisplacements constantly occur at the junction of the elastomer ring with the shaft, which over time lead to wear of the shaft and sealing element. Seals with bellows do not have this disadvantage. Elastomeric bellows themselves act as such a sealing element; they tightly “grip” both the shaft and the movable ring. The design of such a seal can be seen below in Figure 3. When using metal bellows, an elastomer seal sits between the seal housing and the shaft. The bellows, unlike the spring, is sealed; on one side it fits tightly to the seal body, on the other to the movable ring. Metal bellows seals also eliminate the need for an additional seal between the shaft and the rotating ring, thereby avoiding shaft wear.
Permissible leakage of mechanical seals
As mentioned above, the gap between the rotating and stationary seal rings is less than a micron. In this gap, a thin film of the pumped liquid is formed, which reduces friction. As the gap increases, the thickness of the lubricating film increases, which leads to a decrease in friction and, accordingly, an increase in the service life of the seal. In any case, the presence of a lubricating film between the two seal rings leads to a certain amount of leakage of the working fluid to the outside. Provided that the surface of the friction pair is parallel, a dependence of the volume of leaks on the size of the gap raised to the third power is observed. We will not provide a formula for calculating leaks in this article, but in practice they can range from 0.01 to 30 ml/hour, provided that the seals are in good condition. A larger volume of leaks indicates incorrectly selected or incorrectly installed seals.
The volume of leaks also depends on the following circumstances:
Presence of contamination on the surface of the seals
Seal surface roughness
Presence of radial and axial runout of the impeller shaft
Temperature of the pumped medium
Viscosity of the pumped medium
Shaft rotation speed
Pump housing pressure
Proper seal installation
Spring, bellows and cartridge mechanical seals
Springs are used in mechanical seals to press a rotating seal ring against a stationary one. In a number of designs, the spring also has the function of transmitting torque. The seal may have one central or several peripheral springs. The advantage of central spring seals is that they are cheap and simple. But if the spring breaks, the seal immediately fails. The central spring is powerful enough to be able to transfer torque from the shaft to the seal. It is not protected by the seal housing from exposure to the environment if there are solid impurities in the environment. The seal version with a central spring on the atmosphere side does not have this disadvantage. The design with many peripheral springs fails gradually, which makes it possible to promptly notice a small leak and change the seal. These springs themselves are small, their service life is less than that of the large central spring. They are not capable of transmitting shaft torque to the seal.
Figure 2. Different types of mechanical seals
Bellows seals use a bellows to transmit torque from a shaft to a rotating seal ring. The bellows can be elastomeric or metal. Elastomeric bellows typically use an additional center spring to better seal the faces of a pair of seal rings against each other. It is seals with an elastomeric bellows and a central spring that are the cheapest and most common types of seals for general industrial pumps. They make up the majority of all types of mechanical seals used.
Finally, according to another classification, seals are divided into cartridge and simple (component). Cartridge seals are distinguished by the fact that all elements are combined into a single housing, which makes them much easier to replace. In simple seals, you will have to separately install the rings, spring and bellows, but in cartridge designs it is enough to put the monoblock on the body and secure it with screws and pins.
Double mechanical seals
There are tasks where the pump is required to be completely sealed; even the slightest leaks are unacceptable. In this case, you can use them, but they may have limitations that will still force you to use mechanical seals. For example, pumps with magnetic couplings are extremely resistant to the presence of solid particles in the pumped medium.
To prevent leaks (even small ones) when using mechanical seals, two seals are installed on the shaft at the same time. In this case, between the seals there is a chamber with a barrier liquid. The barrier fluid provides lubrication, flushing and cooling of the seals, and also completely eliminates the chance of the pumped medium getting out. Water, glycerin or other liquids that do not interact with the pumped medium are used as a barrier liquid. There are 2 main options for the arrangement of double seals:
Back to back
Figure 4. Options for placing a double mechanical seal on the shaft. The arrows indicate the direction of the barrier fluid flow.
Let's look at the advantages and disadvantages of each scheme. The "Back to Back" option is a little more common. With this, the pressure of the barrier fluid should be 1-2 bar greater than the pressure of the pumped liquid. This can be achieved using a special vessel, dosing pump or hydraulic booster. This type of seal is good because the gap between the rotating and stationary rings is filled with a barrier fluid, therefore solid particles and dirt present in the pumped medium cannot get there. This dramatically increases the service life of the seal compared to the Tandem scheme.
With the Tandem sealing scheme, the barrier liquid has a pressure lower than the pumped liquid. If the seal depressurizes, the pumped liquid will enter the seal, and not vice versa. This can be significant for a number of applications where foreign liquid cannot be allowed to enter the pressure line. In addition, there is less need to tinker with the barrier fluid pressure control system, which can also be important in a certain situation.
Materials of mechanical (mechanical) seals
By using the right combination of materials for the various seal elements, good seal performance with the specific pumped medium is achieved. It is necessary to speak separately about the materials from which the various sealing elements are made:
Friction pair (fixed and rotating ring)
O-rings or bellows made of elastomers
Other elements (springs, bellows, seal housing, pins, bolts, etc.).
Friction pair materials must have special properties, because they are continuously in close contact with each other and at the same time move relative to each other very quickly (at the speed of rotation of the pump shaft). Their surface must be extremely smooth and their ability to resist wear very high.
Carbon graphite is widely used as a friction pair material. There are many varieties of graphite used in seals. Graphite is the softest seal material. It does not tolerate the presence of solid particles in water, which can destroy its surface and lead to seal failure. In addition to coal, graphite can also be impregnated with resins (for better lubrication) or metals (to reduce the coefficient of friction). These impregnations provide graphite with the lowest coefficient of friction of all materials. If there is a risk of the pump running dry, it is advisable that one of the seal rings be made of graphite. Graphite is also good when working with hot liquids whose lubricating properties are impaired. Metal impregnations reduce the corrosion resistance of graphite and make it impossible to work with food products.
Aluminum oxide (Al2O3), which is also called alumina. Most often used in conjunction with graphite. It is quite hard, but has relatively poor corrosion resistance. The acid resistance of alumina increases as its purity from impurities increases, but pure aluminum oxide is quite expensive, which makes it pointless to use it in seals.
Tungsten carbide (WC) is a very hard material, most resistant to solid particles in water. However, the WC-WC pair has the highest friction coefficient, so it is better to use such a pair at low shaft speeds, or when using additional lubricant.
Silicon carbide (SiC) - has good hardness and thermal conductivity. The material is fragile and the coefficient of friction in it is quite high, higher only for the WC-WC pair. The use of impregnations allows you to reduce this coefficient.
Diamond coating is an ideal coating for the surface of a friction pair. Has the highest hardness and thermal conductivity. It is resistant to corrosion and has a low coefficient of friction. Diamond coating has one, but significant drawback, which determines the rarity of its use - a very high price.
Now let’s look at various combinations of friction pair materials:
Graphite/WC - this pair is good if it is possible to work dry, and also if the temperature of the liquid is high. Hot water has a higher viscosity, as well as an increased evaporation rate in the gap between the surfaces of the friction pair. Because of this, its lubricating properties are reduced. It is the graphite in this pair that provides the low friction coefficient. Depending on the impregnation, graphite imposes restrictions on the use of this pair depending on the aggressiveness of the pumped liquid. The pair does not tolerate solid particles well due to the softness of graphite. For the same reason, any seal with graphite is not suitable for glycol, which can form particulates due to crystallization.
Graphite/SiC - the properties of this pair are close to the previous one, however, rapid wear occurs in hot water.
Graphite/Al2O3 is the cheapest seal pair (often called Carbon/Alumina ceramic). It has limited resistance to corrosion (pH from 5 to 10), wears out most quickly in hot water.
WC - WC - tungsten carbide, used as the material of both rings, tolerates work very poorly without lubrication due to the high coefficient of friction. Failure of the seal during dry operation occurs within a few tens of seconds. But this friction pair has the highest properties when working with solid particles. WC-WC - best suited for working with glycol, which may contain various inhibitors and alkalis, as well as phosphates and silicates. Tungsten carbide best resists possible crystallization of glycol elements due to its hardness.
SiC - SiC - silicon carbide used as the material of both rings has a lower coefficient of friction compared to the previous pair, but quite high compared to graphite. Friction can be reduced by the manufacturer through the use of solid lubricants, in which case the pair has good friction properties. SiC-SiC has the best corrosion resistance properties. The hardness properties of the pair are slightly worse than WC-WC.
Figure 5: Shows comparative seal wear rates (on a scale of 0 to 5) as a function of operating temperature.
Elastomeric Secondary Seal Materials must ensure tightness, must be resistant to the temperature of the pumped liquid, its viscosity and aggressiveness.
NBR or nitrile butadiene rubber (nitrile). An inexpensive material with good properties, which is considered basic for mechanical seals. High wear resistance is combined with resistance to materials such as oils, water, hydrocarbons: oil, gasoline, diesel. fuel. Minimum operating temperature minus 40 degrees Celsius. Maximum operating temperature 100 degrees Celsius for oils, 80 degrees for water. The material does not tolerate abrasive particles in liquids well.
EPDM - ethylene propylene rubber. Compared to NBR, it has better resistance to hot water, acids with a concentration of up to 10%, alkalis, and some alcohols. Completely unsuitable for working with hydrocarbons: gasoline, kerosene, diesel. fuel. The presence of mineral oils and fats in water causes the EPDM rings to swell. The operating temperature range ranges from -40 to +150 degrees Celsius, for some EPDM brands from -50 to +175 degrees. The material tolerates the presence of abrasive particles in liquid well.
Viton (FPM, FKM) - fluorine rubber. Viton is a registered trademark of the American company Du Pont, FPM is the international name for the same material, and FKM is the American name. There is also a Russian designation - FC or SFK. Any of the abbreviations may appear in the catalog of a particular manufacturer. Viton has a wider operating temperature range than previous rubbers and has higher chemical resistance. It is capable of operating at temperatures from -20 to +200 degrees (when used for oils; for water up to +90 degrees). Special brands of frost-resistant fluorine rubber can withstand temperatures down to -50 degrees. Fluorine rubber has excellent chemical resistance to hydrocarbons, alcohols, asphalt and tar, hot water, and concentrated acids. Resistance to alkalis is average, provided that the liquid temperature does not exceed 100 degrees Celsius. Viton does not tolerate ammonia, low molecular weight organic acids (formic, acetic), as well as polar solvents (acetone). Resistance to abrasive particles is moderate.
FFKM - perfluorocarbon rubber. This elastomer has the highest chemical resistance of all elastomers, comparable to Teflon. It can be used at liquid temperatures up to +230 degrees, both for water and oils. FFKM is the most expensive elastomer used in seals, so Viton is usually used instead. Resistance to abrasive particles is moderate.
Other sealing elements
Springs and metal bellows are made of stainless steel or special alloys with increased corrosion resistance. A group of alloys under the general name Hastelloy, which necessarily includes nickel, is popular in use. In addition to it, the composition may include molybdenum, chromium, iron, copper, titanium, manganese and other metals. Hastelloy is significantly more expensive than stainless steel, but its use is necessary, for example, in pumps designed to work with concentrated acids and alkalis.
Other sealing elements (holders, bolts, guides) can be made of metals or hard polymers, depending on the purpose of the pump.
The UDRBROCK company offers rubber end seals for pumps on favorable terms. We guarantee an attentive and individual approach to each client, and also provide warranty service and related services.
For pumps, mechanical and lip seals of the second and third generations are used. Lip seals for pumps are a cuff that is attached to a shaft and are made of various elastomers. The material depends on the composition of the pumped liquid and its temperature.
Mechanical seals are suitable for:
- ERDM (ethylene-propylene rubber) is used for food and alkaline liquids.
- pumping fuel and lubricants – NBR (nitrile rubber).
- acidic liquids – Viton, FRM (fluororubber rubber).
Cuffs are produced in accordance with GOST 8752-79 and differ in type and design. Seals of this type are used either one at a time or in a sequence of several cuffs.
Mechanical seals for pumps consist of two elements - movable and stationary. A fixed elastomer ring is attached inside the end of the housing. The moving part is mounted on the shaft and ensures its tightness. Protective rings made of polyurethane or composite materials are installed between the elements.
The rings provide additional rigidity to the structure and protect the seals from damage. Rubber end seals are durable and virtually leak-free. When pumping toxic, adhesive and flammable liquids, a double seal system is used to ensure complete tightness.
Our works
Features of working with "UDRBROKK"
Our company produces sealing systems using imported high-tech equipment DMH®. If you need to replace the mechanical seals on your pumps, we offer quality products at affordable prices. It is possible to manufacture non-standard sealing products to order.
We produce products with diameters from 3 to 400 mm.
Polite and attentive staff, comfortable prices, quality products are the hallmarks of cooperation with us! To place an order, leave a request on the website or call the office.
| vendor code | d | D(-0/+0.2) | H(+-0.05) | B(+-0.05) | b | ||
|---|---|---|---|---|---|---|---|
| VS82-017 | 17 | 25,4 | 2,85 | 4,2 | 3,5 |
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| VS82-023.40 | 23,4 | 31,8 | 2,85 | 4,2 | 3,5 |
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| VS82-031.30 | 31,7 | 39,7 | 2,85 | 4,2 | 3,5 |
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| VS82-036.10 | 36,1 | 44,5 | 2,85 | 4,2 | 3,5 |
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| VS82-045.40 | 45,4 | 53,8 | 2,85 | 4,2 | 3,5 |
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| VS82-055 | 55 | 63,4 | 2,85 | 4,2 | 3,5 |
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| vendor code | Metric thread | A +0.13 -0.00 | B +0.10 -0.10 | C +0.10 -0.10 | D +0.25 -0.00 | E +0.10 -0.10 | F +0.20 -0.20 | Burst pressure (BAR) | ||
|---|---|---|---|---|---|---|---|---|---|---|
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SC-217 | M10 | 16 | 12,4 | 10,7 | 0,4 | 1,5 | 8,05 | 1350 |
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| SC-222 | M12 | 18 | 14,3 | 12,7 | 0,4 | 1,5 | 9,73 | 1250 |
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| SC-227 | M14 | 22 | 16,4 | 14,7 | 0,4 | 1,5 | 11,38 | 1510 |
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| SC-229 | M16 | 24 | 18,4 | 16,7 | 0,4 | 1,5 | 13,41 | 1400 |
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| SC-232 | M18 | 26 | 20,4 | 18,7 | 0,4 | 1,5 | 14,76 | 1275 |
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| SC-233 | M20 | 28 | 22,5 | 20,7 | 0,4 | 1,5 | 16,76 | 1150 |
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| SC-236 | M22 | 30 | 24,4 | 22,7 | 0,4 | 2 | 18,74 | 1100 |
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| SC-238 | M24 | 32 | 26,4 | 24,7 | 0,4 | 2 | 20,11 | 1050 |
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| SC-240 | M27 | 36 | 29 | 27,2 | 0,4 | 2 | 23,3 | 1130 |
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| SC-242 | M30 | 39 | 33 | 31 | 0,4 | 2 | 25,7 | 860 |
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| vendor code | BSP thread | BOLT thread | A +0.13 -0.00 | B +0.10 -0.10 | C +0.10 -0.10 | D 0.25/ 0.51 |
E +0.15 -0.15 | F +0.20 -0.20 | Burst pressure (BAR) | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
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SC-033 | 1.1/2 | 1.7/8 | 58,6 | 51,39 | 48,44 |
0.25/ 0.51 |
3,38 | 44,86 | 690 |
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| SC-020 | 1/8 | 3/8 | 15,88 | 11,84 | 10,37 |
0.25/ 0.51 |
2,03 | 8,56 | 1500 |
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| SC-021 | 1/4 | 1/2 | 20,57 | 15,21 | 13,74 |
0.25/ 0.51 |
2,03 | 11,45 | 1550 | ||