Mechanical shaft seals for pumps - Grundfos

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Wear mechanical properties, including decrease in wear resistance. When the metallic binder phase disappears, the surface becomes matted. The selective corrosion of the binder phase may induce stresses, leading to cracks in the seal rings. See fig. 5.9. On other surfaces of the seal rings, heavy erosion can occur where the binder phase is corroded. See fig. 5.10. In stainless steel pumps and pipe systems, tungsten carbide with cobalt binder corrodes in tap water. Fig. 5.10: Corrosion of the binder phase followed by severe erosion on a tungsten carbide seal ring with cobalt binder phase In ceramic materials such as aluminium oxide, the process of a corrosion attack often dissolves or oxidises the glass phase, resulting in a decrease in wear resistance. When the glass phase has disappeared from the surface of the seal ring, the porosity increases. This affects the mechanical strength of the seal ring. The strength can be reduced with a few percentages or it can totally crumble, depending on the material grade and pumped medium. Corrosion seldom occurs on the metal parts of the seal in stainless steel pumps. The reason is that seal metal parts are commonly made of a higher grade of stainless steel than other pump metal parts. Fig. 5.11: Normal wear on surface of tungsten carbide seal ring Because the thickness of the lubricating film is of the same order of magnitude as the surface roughness, the seal faces will wear to some extent. This normal wear on well-performing seals will be so small that the seal will be able to survive for many years. See fig. 5.11. In special cases, wear can cause problems, but often seals work perfectly with severe wear up to 0.5–1 mm, even with heavily grooved seal faces, as long as the axial flexibility of the seal ring is maintained. 83
84 Failure of mechanical shaft seals Wear, continued 5. Installation failures Deep seal face grooves close to the pumped medium side indicate that hard particles from the pumped medium have entered the sealing gap. See fig. 5.12. Deep grooves close to the atmospheric side indicate that hard precipitates from the pumped medium have been created where the lubricating film evaporates. See fig. 5.13. Some mechanical seal failures come from wrong mounting and handling. Examples can be shaft misalignment, seats not mounted perpendicular to the shaft, axially moving shaft and wrong assembly length, etc. Misalignment Fig. 5.12: Deep seal face grooves close to the pumped medium side Fig. 5.14: Well-aligned shaft seal Run-out The position and width of wear tracks on the seat indicate various problem areas. If the width of a wear track on the seat is the mirror of the sliding face of the opposite seal ring, the shaft seal seems to be well aligned with no run-out of the shaft. See fig. 5.14. If the sliding face of the seat is broader than the rotating sliding face, a wider track on the seat all way around indicates a high run-out of the shaft. See fig. 5.15. Tilted This can also be seen if, for some reason, there is an unbalance of the rotating mass. Run-out Centre Fig. 5.13: Grooves on seal faces at the evaporation zone Fig. 5.15: Shaft seal with radial run-out An uneven depth of the wear track around the seat indicates a tilted/crooked mounting of the seat. See fig. 5.16. Centre
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Mechanical shaft seals for pumps
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Contents Preface 5 Chapter 1. Intro
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Chapter 1 1. Types of shaft seals 2
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Fig. 1.2: Braided stuffing box pack
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O-ring, stationary O-ring, rotating
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The rotating part The rotating part
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Closing force The parts of the seal
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In the examples above, where the ar
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Non-parallel seal faces System In p
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Atmosphere A B C D Vapour pressure
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1993 Rubber bellows seal introduced
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26 Mechanical shaft seal types and
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28 Mechanical shaft seal types and
Page 29 and 30: 30 Mechanical shaft seal types and
Page 43 and 44: Chapter 3 Materials 1. Seal face ma
Page 45 and 46: The metals used for metal-impregnat
Page 47 and 48: Silicon carbide (SiC) 100 µm Fig.
Page 49 and 50: 2. Seal face material pairings Carb
Page 51 and 52: WC against WC A shaft seal with WC
Page 53 and 54: 3. Testing of shaft seals Various t
Page 55 and 56: [ °C ] 250 200 150 Organic film bi
Page 57 and 58: Chemical adhesion of surfaces All s
Page 59 and 60: 5. Materials of other shaft seal pa
Page 61 and 62: 64 Tribology The science of frictio
Page 63 and 64: 66 Tribology Duty parameter The mec
Page 65 and 66: Surface film 68 3.5 Tribology 3.0 2
Page 67 and 68: 70 Tribology The lapped surface has
Page 69 and 70: 72 2.0 1.5 Tribology 1.0 0.5 Dry ru
Page 71 and 72: 74 Tribology Summary This section d
Page 73 and 74: 76 Failure of mechanical shaft seal
Page 79: 82 Failure of mechanical shaft seal
Page 85 and 86: 88 7. Shaft seal failure analysis T
Page 87 and 88: s of shaft seal failure Incorrect i
Page 89 and 90: Chapter 6 Standards and approvals 1
Page 91 and 92: Example of designation for a mechan
Page 93 and 94: 2. Approvals Specific approvals of
Page 95 and 96: The guideline on in-place cleanabil
Page 97: Summary Different applications requ
Page 100 and 101: contact points 65 contact surface 8
Page 102 and 103: M magnetic-drive system 40 malfunct
Page 104 and 105: spring force 13, 15 springs and bel
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Mechanical shaft seals for pumps - Grundfos

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