AS-C444 Customized Double Cartridge Mechanical Seal

AS-C444 customized double cartridge mechanical seal is made as per drawing. Since it is dual seal，it can suit many applications

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Designing research of O ring seal of mechanical seals

ring seal is one of the sealing forms with the longest application time. Because of its simple structure and low manufacturing cost, it is the most widely used seal form in the industry.

Failure of auxiliary seal is also one of main reasons for failure of mechanical seal, and it is difficult to find this problem through disassembly. Since compensation rings need to have a certain range of freedom of movement, flexible elements are generally used as auxiliary seals，whose main forms include various cross-sectional rubber seals, elastic elements plus polymer seals, welded bellows, piston rings, etc，of which he most commonly used and most effective is O-ring seal. O-ring seal is the most easily formed flexible sealing element. Its sealing principle is to generate contact pressure on the sealing surface through its own elastic deformation. If contact pressure is greater than pressure of the sealed medium, no leakage will occur, and vice versa. That’s how O rings are playing a sealing role.

There is a certain difference between the application of O-ring in mechanical seals and other sealing conditions, that is, not only its own sealing performance but also its impact on performance of mechanical seals must be considered in its design. This article makes a detailed analysis of the friction force generated by O-ring seal during use, points out what needs to be paid attention to in design of O-ring seal in the mechanical seal, and proposes design suggestions.

Friction force

Friction of sealing ring includes adhesive friction and hysteresis friction. When calculating influence of friction of O-ring seal on compensation ring, we use adhesive friction. Adhesive friction consists of two parts, one part is friction caused by compression, which is related to contact length of the mechanical seal; the other part is caused by pressure difference between two sides of the sealing ring, which is related to the projection area of the sealing ring.

The reason why friction force of auxiliary seal ring cannot be ignored, especially balanced mechanical seal, is that load coefficient is close to back pressure coefficient, so end-surface specific pressure mainly depends on spring specific pressure and friction specific pressure. When friction specific pressure is greater than spring specific pressure, end-face specific pressure is negative, and the mechanical sealing surface will be opened while the mechanical seal will fail. Therefore, generally friction specific pressure should not exceed 0.02 MPa.

In addition to friction force, there are many other non-linear factor mutually-associated in sealing ring, such as compression rate, extrusion gap and so on.

compression rate

Compression rate is also one of the most difficult parameters to design. Compression ratio includes initial compression ratio and working compression ratio.

Initial compression rate mainly refers to the compression rate of O-ring seal when it is installed in the groove.This indicator mainly reflects possible installation problems caused by manufacturing tolerances of O-ring seals.Also initial compression rate changes will cause changes in working compression rate.

If initial compression rate is too large, sealing ring for the hole will be twisted in the groove. Thus，state of sealing ring will be unstable during floating process of the compensation ring, and there is a hidden danger of mechanical seal failure.

Initial compression rate will also affect performance of shaft sealing ring. If too large, shaft sealing ring will be overstretched, cross-sectional diameter of sealing ring will be reduced, working compression rate will not meet design requirements, and the mechanical seal will fail;if too small, sealing ring will fail to be loaded into the groove. Therefore, design principle of initial compression rate is: maximum value of inner diameter of the sealing ring is equal to minimum value of bottom diameter of groove on the shaft.

If working compression rate is too large,although sealing is reliable, friction and deformation will increase, and excessive wear and permanent deformation are prone to occur. Especially as auxiliary sealing element of compensation ring, O ring’s working compression rate must be suitable. If it is too large, which will cause the compensation ring to be unable to float, specific pressure of end faces of the mechanical seal will decrease, thus the mechanical seal will fail. However, if too small, the O-ring seal will be unreliable. Therefore,working compression rate is generally selected from 12% to 18%.

cross-sectional diameter

Under same conditions of use and compression rate, frictional force on sealing ring with a smaller cross-sectional diameter is smaller，which will cause less influence on floatability of sealing ring and specific pressure of end faces of mechanical seal.From a processing point of view, smaller cross-sectional diameter means smaller dimensional fluctuations, and it is easier to determine compression rate.

However, from the perspective of mechanical seal reliability, under the same compression rate, larger cross-sectional diameter means higher sealing reliability of itself, but friction force generated is also larger, which will reduce specific pressure of end faces of the mechanical seal and thus educe reliability of the main seal.

Therefore, cross-sectional diameter should be the smallest recommended value, and it is prohibited to select one that is not in the basic size series of O-ring seal.

Extrusion gap

When determining extrusion gap value, it is necessary to consider factors such as fit dimensional tolerance of structural parts and thermal expansion coefficients of materials. Generally, extrusion gap can be determined according to operating temperature range of the equipment and thermal expansion coefficients of different materials. When extrusion gap is so large that the sealing ring is extruded,this problem can be solved by increasing hardness of sealing ring material and/or setting support ring (note the relative relationship between the installation position of the support ring and direction of pressure the sealing ring bears).

Design principles of O-ring grooves

Determine groove depth and tolerance according to the calculated compression rate to ensure unchanged working compression rate.

Determine the groove width according to the cross-sectional area and groove depth of the sealing ring, and ensure that the cross-sectional area of the groove is larger than the cross-sectional area of the sealing ring.

Size of filleted corner of the groove bottom and the groove notch needs to be determined after calculation.Size of filleted corner at the bottom of the groove is mainly related to the cross-sectional area of the groove. It is necessary to ensure that the cross-sectional area of the groove is still larger than the cross-sectional area of the seal ring with the existence of filleted corners. Size of filleted corner of the groove notch affects the extrusion and sealing performance of the sealing ring.

According to the relevant literature: when medium pressure is low, filleted corner radius of the groove notch should be selected as small as possible on the premise that the sealing ring will not be scratched; when medium pressure is high, it is recommended to be 0.2 mm ~ 0.24 mm.

material selection of O-ring seal of mechanical seal

ring seals are generally made of rubber materials, so compatibility of material with chemical and temperature environment of medium, chemical stability of material, and material cost are the first issues to consider.With the extensive development of basic research of materials, new rubber materials suitable for various applications have been developed, and limitations of use of materials have been broken. Nitrile rubber materials have been modified to be used in ethylene glycol solutions;perfluoroether rubber is one of the best rubber materials, basically able to withstand all solutions, and use environment temperature can reach -55 ℃ ～ + 200 ℃,but the price is expensive.Compared with perfluoroether rubber, fluorosilicone rubber has similar chemical and temperature environmental compatibility,but the price drops by an order of magnitude. Therefore, fluorosilicone rubber is the first choice for high-performance O-ring seals. Nitrile and EPDM rubber materials are the main materials used for general-purpose O-rings due to their low prices. However, attention should be paid to the environmental adaptability of the materials when using them.

In short, there are many non-linear and inter-related factors in sealing ring design. It is difficult to express clearly with a certain formula. Therefore,we must pay attention to accumulation of test data instead of being limited to design standards and manuals, and optimize design parameters through experiments to improve performance and longevity of mechanical seal.