Mechanical shaft seals for pumps - Grundfos

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No or insufficient flow Poor venting Vibration [1] OMV AG, Pumps in Process Engineering 2002 This temperature increase is often 10 to 20 K. The temperature specification of a shaft seal is given for the actual temperature of the seal. This must be taken into account by the system designer. See fig. 5.20. Exceeding the maximum temperature of the seal may cause damage to elastomeric parts. Another typical failure due to elevated temperatures is poor lubrication resulting in a higher wear rate. See lubrication failures, page 77. No flow occurs when the pump contains pumped medium and the pump is running against closed valve. The heat generated by friction in the shaft seal and the heat generated on account of turbulence around the impellers result in a high temperature increase in the entire pump. The increased temperature can damage the elastomeric parts of the shaft seal in particular. In addition to the temperature increase, the risk of dry-running is also increased, when there is no or insufficient flow through the pump and the seal chamber. Under certain conditions, it is possible to collect air around the shaft seal, depending on the design of the shaft seal chamber, the operating conditions and the pumped medium. Poor venting resulting in continuous or periodic dry running has disastrous consequences to the life of most mechanical shaft seals. Mechanical vibrations result in higher forces on each part of a mechanical shaft seal. The result is extended wear on all parts, chipped seal rings and possibly opening of the sealing gap. Vibrations can be generated from friction between seal faces if the operating conditions are exceeded. Otherwise, vibrations are often associated with worn bearings or special liquid flow conditions. The width of the wear track on the seat will be extended in case of radial vibrations of the shaft occur. See fig. 5.15 Summary The main causes of failures on mechanical shaft seals used in pumps are described. It is difficult to state causes of shaft seal failures exactly, even with knowledge of the pump system. A detailed failure analysis is needed to reduce future failures on shaft seals. 87
88 7. Shaft seal failure analysis The previously described failures have been collected into a failure analysis scheme. See pages 89 - 90. The purpose of the scheme is to assist in troubleshooting and give input to possible improvements of seal usage and manage shaft seal parameters involved. We shall refer to these main shaft seal components: • complete shaft seal • seal rings • elastomers • metal parts • shaft/sleeve. The first column contains the main component. The second column shows the result of the inspection. The figures indicate possible causes of failure. See the “Key to failure analysis scheme”, page 91.
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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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Page 33 and 34: 34 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 83: 86 Failure of mechanical shaft seal
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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