Knowledge

I. Failure of Springs or Bellows

During operation, the failure forms of the springs or metal bellows in mechanical seals include permanent deformation, fracture, corrosion, creep or relaxation, etc. Among them, the influencing factors of permanent deformation and fracture failure of metal bellows are the most complex. The loading elastic elements of mechanical seals are mostly cylindrical compression helical springs. Therefore, this section mainly analyzes the failure of cylindrical compression helical springs, and the principle also applies to other springs or metal bellows.

 

1.Permanent deformation

Permanent deformation of springs is one of the main reasons for spring failure. When the permanent deformation of a spring exceeds the allowable range, it will affect the normal operation of the seal. The permanent deformation of a spring, which is the reduction of its free height, will lead to a decrease in the working load when the working height remains constant. The causes of permanent deformation are failure phenomena resulting from unreasonable design and imperfect manufacturing processes of the spring. It is related to the following factors:

① Under given conditions, the main factor affecting the permanent deformation of a spring is the working stress. Under different working load conditions, the permanent deformation of the spring will also be different. Foreign data suggests that the working stress of a spring should not exceed 0.3δb (tensile strength) of its material.

② The permanent deformation of a spring is related to its diameter. Seal designers often pay attention to adjusting the spring diameter to meet the load requirements but seldom consider the impact of the spring diameter on permanent deformation. As a result, they may end up neglecting one aspect for the sake of another. Reducing the spring diameter can decrease the permanent deformation.

③ The smaller the free height of the designed spring, the greater the relative permanent deformation. Tests show that increasing the free height of the spring can reduce its permanent deformation. However, it should also be noted that an excessively large free height may cause bending and instability (for small-diameter springs).

④ The permanent deformation of a spring is related to its pitch. When the free height of the spring remains unchanged, increasing the pitch and reducing the number of working coils will make the spring more prone to permanent deformation.

⑤ The permanent deformation of a spring is related to the material properties, manufacturing processes, and selected heat treatment methods of the spring. For spring manufacturers, it is necessary to strengthen the management of material properties and processing quality. Firstly, they should enhance the quality inspection and proper management of incoming materials and strictly prohibit substandard materials from entering the production site. When choosing the processing and heat treatment processes for springs, they should not only follow general principles but also consider the impact of permanent deformation to improve the quality of mechanical seal springs.

 

In addition to the above factors, the permanent deformation of springs and metal bellows is also related to the usage temperature, which must be within the temperature range specified by the material.

 

2. Fracture

 

Spring fracture is also one of the main forms of spring failure. Depending on the nature of the load, working environment, etc., the fracture forms of springs include fatigue fracture, stress corrosion fracture, and overload fracture, etc.

The causes of fatigue fracture of springs or metal bellows are mostly due to improper design, material defects, poor manufacturing, and harsh working conditions, which lead to the expansion of fatigue cracks. Fatigue cracks often originate in high-stress areas. For example, if the inner surface of a compression spring shows a fracture, it usually extends at a 45° angle to the spring material's axis to the outer surface and then breaks. The fracture of metal bellows often occurs at the bottom of the bellows.

For welded metal bellows, if there are manufacturing defects such as unequal spacing between the plates, there will be larger stresses in some plates, causing these plates to break prematurely. Manufacturing defects refer to uneven plate spacing, unequal wave depth, and inconsistent plate thickness, etc. When installing a static metal bellows mechanical seal, defects may occur due to the inclined connection between the gland and the support point. Such defects can also cause stress in the plates, leading to fracture.

In many cases, when the frequency of the periodic expansion and contraction movement of the welded metal bellows is equal to the natural frequency of the sealing device, resonance may occur, generating large stresses and leading to premature fatigue fracture. Two forms of vibration can occur in the welded metal bellows sealing device: axial vibration and torsional vibration. Axial vibration is caused by the axial movement of the shaft, while torsional vibration is usually caused by the friction force between the friction pairs. The friction force tends to tighten the bellows until it is less than the tightening force inside the bellows. After that, the force is released by itself. This process repeats itself in a cycle. This torsional vibration then transforms into axial vibration. When the weld balls of two adjacent plates collide, the vibration weakens and the amplitude decreases, and this process repeats itself in a cycle.

To prevent resonance, the natural frequency of the seal should be designed to be slightly higher than the main vibration frequency (by changing the material, plate thickness, number of plates, spacing, and installation length), or by using an asymmetric wave shape and using a fork to transmit torque. In addition, various damping methods can also eliminate vibration, such as using a damping plate around the bellows to generate a slight elastic load, ensuring contact with the bellows and reducing vibration before the amplitude forms. The damping plate then dissipates the kinetic energy of the bellows.

Under the combined action of the medium's erosion and material stress, springs and metal bellows may fracture, which is called stress corrosion fracture. Austenitic steel springs are prone to stress corrosion by oxides under alternating stress. For this reason, Hastelloy is recommended.

Springs and bellows working in corrosive media may experience stress corrosion fracture in the stress areas of their cross-sections. Due to the combined action of corrosion and stress, certain weak points of the components are corroded first, forming crack cores. As the load-bearing time increases, the cracks slowly extend subcritically. When the crack reaches a critical size, the elastic element suddenly fractures. Stress corrosion fracture is closely related to the working medium. For example, if the medium contains chlorine, bromine, or fluorine, metal elastic elements are prone to stress corrosion fracture. From a mechanism perspective, stress corrosion fracture is an anodic reaction, while hydrogen embrittlement fracture is mainly a cathodic reaction. In most cases, hydrogen embrittlement fracture of springs occurs when hydrogen atoms penetrate the grain boundaries of the spring material and combine into hydrogen molecules, generating significant stress and resulting in brittle fracture of the spring under low stress loads. Hydrogen embrittlement fracture typically occurs within a bending angle range of 45° to 90°. If a hydrogen-embrittled spring coil is clamped in a vise, and the extended part is clamped with pliers and bent forcefully, the spring can be easily broken into two or three sections. If the fracture is caused by other reasons, it will be found that the spring material still retains sufficient toughness. Brittle fracture of spring materials caused by hydrogen absorption due to chemical reactions in seawater, sulfides, sulfuric acid, sulfates, caustic alkali, liquid ammonia and media containing hydrogen gas.

In addition to the above factors, the fracture and failure of springs or bellows can also be attributed to the following reasons.​​​​​​​

① Heat treatment defects. Improper heat treatment processes can lead to hidden internal defects in the material. For instance, although the required hardness is achieved through heat treatment, the coarse grains of the spring material may cause rapid deformation and eventual fracture during use.

② Tool-induced scratches. During the manufacturing process of springs, especially for hooks in hooked springs, improper manufacturing processes can cause scratches, resulting in stress concentration areas and subsequent hook fractures.

It can be seen that in addition to selecting appropriate materials and determining proper stress values based on the working conditions of the spring during design, adopting suitable processing techniques during manufacturing is also essential to prevent fracture and failure.

 

II. Wear, Breakage or Corrosion of Sealing Drive Components

Drive pins, drive screws, flanges, forks, or even a single large spring can be used to transmit torque and drive the sealing components to rotate. Vibration or installation misalignment, non-coaxiality, etc., can cause wear, bending or damage to the drive components. The fixing screws used in mechanical seals should not be made of hardened materials. When checking for wear, the first step is to inspect the drive connection points. Wear marks can be found on pins, slots, flanges, and forks. The wear of drive pins or drive slots is caused by adhesive-slip action. If the two end faces adhere together for an instant, the rotating ring will not rotate smoothly and will jump during rotation, causing the drive pin to bear a large stress. Frequent start-stop operations or excessive force can also cause the drive pin to break, leading to sudden seal failure. Poor lubrication can also cause adhesive-slip action.

Other reasons for drive pin breakage include: excessive spring force; high medium pressure and the use of unbalanced seals or sealing fluid with poor lubrication performance, resulting in large torque; inclined assembly of drive pins; single force application; only considering the corrosion resistance of the friction pair material during selection, without considering the matching performance; cavitation in the pump, etc.

 

III. Frictional Heat Damage

Abnormal frictional heat damage is also one of the reasons for mechanical seal failure. The shaft (or shaft sleeve), gland, sealing cavity, and sealing components can all be damaged due to abnormal overheating. Frictional heat damage can be identified by friction marks and color changes. As the temperature rises, metals change color. For example, the color of stainless steel: light yellow at about 370°C, blue at about 590°C, and black at about 648°C. In some pumps, the causes of abnormal overheating include: excessive shaft deflection causing friction between the pump throat and the shaft, friction between the gland without positioning guidance and the pump shaft (or shaft sleeve), loose fixing screws rubbing against the sealing cavity, and gland gasket sliding and contacting the rotating ring, etc.

The excessive heat generated by abnormal friction can completely melt PTFE V-rings or carbonize rubber O-rings.

Other causes of abnormal frictional heat generation include: the gland without positioning guidance colliding with the pump shaft (or shaft sleeve), the stationary ring rotating, dirt accumulation in the sealing cavity, and the sealing cavity being out of alignment with the shaft, etc.

 

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