Knowledge

Through the analysis of failure causes, the technical level of applying mechanical seals can be improved. Structural design improvements are largely derived from fault analysis. To achieve as accurate an analysis of faults as possible, it may sometimes take time and even require the use of specialized testing techniques.

 

I. Principles and Methods of Mechanical Seal Failure Analysis

For every mechanical seal, regardless of the cause of failure, a detailed analysis and study should be conducted, and relevant data should be recorded. After the seal is damaged, the cause of failure should not be limited to the damaged parts. The disassembled mechanical seal should be properly collected, cleaned, and placed separately according to the stationary and rotating parts, with labels attached for inspection and record-keeping.

The inspection procedure is as follows: First, determine the impact of the damaged seal parts on the sealing performance. Then, carefully inspect the wear marks on the sealing rings, transmission parts, loading elastic elements, auxiliary sealing rings, anti-rotation mechanisms, and fastening screws in sequence. A comprehensive inspection should also be carried out on the accessories, such as the gland, shaft sleeve, sealing cavity, and sealing system. In addition, it is necessary to understand the operating conditions of the equipment and the previous failure situations of the seal. Based on this, a comprehensive analysis can be conducted to identify the fundamental cause of the failure.

 

II. Analyzing Fault Causes Based on Wear Traces

Wear traces can reflect the movement and wear conditions of moving parts. Each wear trace can provide useful clues for fault analysis. For instance, if the wear traces of the friction pair are uniform and normal, and the fit between each part is good, it indicates that the machine has good coaxiality. If leakage still occurs at the sealing end face, it may not be caused by the sealing itself. For example, if the wear traces at the end face of a metal bellows mechanical seal are uniform and normal, and the leakage rate is constant, it means that the leakage does not occur between the two end faces but possibly at other locations, such as the static seal where the bellows is fixed.

 

When the wear at the end face is too wide, it indicates that the coaxiality of the machine is very poor. With each rotation of the shaft, the seal undergoes axial displacement and radial oscillation. Clearly, in each rotation, the sealing end face tends to slightly separate and leak. Taking a centrifugal pump as an example, the causes of excessive wear are roughly as follows: misalignment of the coupling, pump shaft bending, pump shaft skew, low shaft precision, excessive pipeline tension, vibration, etc.

 

The causes of vibration also include cavitation, surging, water hammer impact, and unbalanced medium flow. However, the most common causes of vibration are poor alignment of the coupling and poor bearing operation precision.

 

For the vibration caused by hydraulic characteristics, an effective remedial measure is to control the pump's discharge below the design value and reduce the pump's cavitation phenomenon. When the width of the wear marks is less than the width of the narrow ring's surface, it indicates that the seal is under excessive pressure, causing deformation of the sealing surface. In this case, the problem should be solved by adjusting the seal structure and using a seal structure that can withstand high pressure.

 

After a period of operation of the mechanical seal, if there are no wear marks on the friction surface, it indicates that the seal leaked at the beginning of use. The leaking medium was oxidized and deposited near the compensating ring's sealing ring, hindering the compensating ring's compensating displacement. This situation is the cause of the leakage. For high-temperature fluids with high viscosity, if they continuously leak, this situation is more likely to occur.

 

For rubber bellows type seals, if there are no wear marks on the friction surface, it indicates that the sealing surfaces may have been pressed together, and there is no relative rotation between the friction pairs, but the rubber bellows rotates relative to the shaft. In this case, the spring will wear out, and the fixed and rotating parts will also be worn.

 

Sometimes, the rotating ring does not rotate relative to the stationary ring but rotates relative to the static ring cover. In this case, there will be no wear marks on the friction surface. The reason may be that the anti-rotation pin is broken or the hole diameter of the static ring cover is smaller than the outer diameter of the seal and is not installed properly.

 

If there are bright spots but no wear marks on the sealing surface, it indicates that the surface has undergone significant warping deformation. This is caused by excessive fluid pressure, poor stiffness of the sealing ring, and improper installation. For external mechanical seals, if the non-compensating ring is only fixed by two bolts and the static ring cover does not have sufficient thickness, or if the positioning end face is not flat, this phenomenon may also occur.

 

Deep grooves (ring-shaped patterns, similar to dense grooves on a record) appear on the hard ring's surface. The main reason is poor alignment of the pump's coupling or poor followability of the seal. When vibration causes the sealing surfaces to separate, large particles invade between them. If the particles are embedded in the softer carbon-graphite sealing ring's surface, the soft ring grinds the hard surface like a grinding wheel, causing excessive wear of the hard surface. If the separation of the surfaces is caused by vibration, abnormal wear marks will also appear on the transmission pins and other transmission components.

 

For mechanical seals operating in granular media, using hard surfaces for all paired materials is an effective solution to deep grooves on the sealing surface. For example, pairing hard alloys with hard alloys or with silicon carbide is the best choice. Because the particles cannot be embedded in either surface but are ground and pass through between the two surfaces.

 

The wear marks on the outer surface of the metal shaft sleeve may be caused by solid particles entering the sleeve, which interferes with the seal's followability; or it may be caused by shaft deflection and large coaxiality deviation between the shaft and the sealing cavity.

 

III. Damage to End Face Materials Caused by Heat Load

The appearance of notches on one or both end faces indicates that the distance between the two end faces is too large. When the two end faces are forcibly pressed together, notches will occur. Common causes of end face separation include the rapid evaporation of the medium. For example, water, especially in hot water systems or liquids containing condensate water, expands when it evaporates, thus separating the two end faces. Cavitation in the pump and blockage of the seal can also cause notches on the sealing end faces. In this case, it is not caused by vibration or misalignment of the coupling, as this is not sufficient to cause notches on the end faces.

Reducing the temperature of the end faces is a common method to prevent damage to the end faces caused by the rapid evaporation of the medium. At the same time, using materials with good thermal conductivity for the end face pairing is also beneficial, such as pairing nickel-based hard alloys with copper-impregnated graphite. Additionally, using balanced mechanical seals, or using special gland covers to inject cooling fluid from the outside, or directly cooling the seal cavity, are all very effective in reducing the temperature of the sealing end faces.

Failed mechanical seals often have very fine radial cracks on the friction pair end faces, or radial cracks with water bubble marks, or even crazing. This is caused by overheating of the seal, especially ceramic and hard alloy sealing surfaces are prone to such damage. Poor lubricity of the medium, overload, high operating temperature, high linear speed, or improper material pairing, any one of these factors or a combination of several factors can generate excessive frictional heat. If the frictional heat cannot be dissipated in time, thermal cracks will occur. These fine cracks act like cutting edges, cutting the carbon graphite or other mating materials, resulting in excessive wear and high leakage. To solve the problem of seal overheating, in addition to changing the end face balance coefficient and reducing the load, static seals with guide sleeves can be used to force the cooling circulating fluid to the sealing surface, or hydrodynamic grooves can be opened on the sealing end face to solve the problem.

There are many small hot spots and isolated discolored areas on the friction end face, indicating that the seal has deformed and twisted under high pressure and thermal influence. For the thermal deformation of the end face, finite element method calculation and analysis should be used to improve the design of the sealing ring.

Sealing rings with hard materials sprayed on the surface, whether ceramic or hard alloy, may have their surface layers flake off or peel from the base material under thermal load. This phenomenon indicates that the seal has experienced dry friction. To eliminate this phenomenon, first check whether the lubrication and cooling of the seal are sufficient, whether the cooling system is blocked, and whether the operation is proper, and take corresponding countermeasures based on the actual situation.

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