Reliability (Mechanical Seals Failure Modes 1)
Reliability, Failure Modes and Effects Analysis
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| Lubrication failures: Proper functioning of mechanical shaft seals with hard seal face material pairings depends on lubrication by the pumped medium. Dry running and poor lubrication can produce the results as can be seen. | Surface damage due to high temperature on secondary seals |
| Poor Lubrication: Similar to dry running, the frictional heat generated on the seal faces are as a result of poor lubrication and may cause problems. | Severe thermal cracks in the seal face are caused by poor lubrication. Poor lubrication may occur when the viscosity of the pumped medium is very low or if the temperature is well above the boiling point at atmospheric pressure. |
| Noise: When lubrication is poor or totally absent, shaft seals with seal rings made of hard materials tend to generate a loud noise. Depending on the seal design, the hard materials used and the system, the noise can be at a constant level of intensity and frequency or be more random. | When noise is generated from the mechanical shaft seal, some parts of the seal vibrate. This may reduce the life of the seal. Metal bellows seals in particular have a tendency to fatigue on account of vibrations. |
| Contamination failures: The pumped medium is often a mixture of miscible liquids and a solution of solids, in addition to small suspended insoluble particles. The lubricating film in the sealing gap is subjected to large gradients in temperature, pressure and velocity. This increases the risk of precipitation and sedimentation in or near the sealing gap. | Hang-up of a mechanical shaft seal means that the axial movement of the rotating part of the shaft seal is blocked. Hang-up mainly occurs in connection with O-ring-type seals, but is also seen in connection with bellows seals, although the underlying mechanism is different. |
| Opening of the sealing gap: Some suspensions and solutions tend to cause build-up of scattered deposits on the seal faces. As the deposits only cover part of the seal faces, the sealing gap opens correspondingly. The result is a leaking shaft seal. The leakage is small at the beginning, accelerating as more liquid passes the sealing gap. | Deposits on carbon graphite seal face; The settlement accelerates, because the temperature is higher on the surface of the already anchored deposits. |