Mechanical shaft seals for pumps - Grundfos

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Non-parallel seal faces System In practice, pressure the seal faces become distorted due to temperature and pressure gradients. The most typical deformation is a tapered seal face. Pump Atmospheric pressure Pump pressure Atmospheric pressure pressure For non-parallel seal faces, the hydrostatic pressure no longer decreases linearly from the pump side to the atmospheric side. In this situation A formula B 2 is no longer C Dvalid for calculating the leakage rate. Converging Pump sealing gap pressure When the sealing gap opens towards the pumped medium, as shown in fig. 1.19, the hydrostatic pressure increases. This is called a converging sealing gap. It appears as the blue curve in fig. 1.21. Diverging sealing gap Pump Atmospheric pressure When pressure the sealing gap opens towards the atmospheric side, as shown in fig. 1.20, the hydrostatic pressure decreases. This is a called a diverging sealing gap. It appears as the orange curve in fig. 1.21. The pressure distribution in the sealing gap is obtained by adding the hydrostatic pressure and the hydrodynamic pressure. This is shown Atmosphere in fig. 1.22. Note the similarity with fig. 1.14 a, page 14. Pump pressure Atmosphere Fig. 1.21: Hydrostatic pressure distribution for different sealing gap geometries Pump pressure A B C D A B C D A B C D A B C D Atmospheric pressure pressure Atmospheric pressure Pump pressure Pump pressure A B C D A B C D Fig. 1.19: Converging sealing gap Fig. 1.20: Diverging sealing gap Atmospheric Atmosphere A B C D A B C D Fig. 1.22: Hydrostatic and hydrodynamic pressure distribution for different 12 sealing gap geometries ute pressure [bar] 10 8 6 Water Pumped medium pressure Absolute pressure [bar] Parallel 12 10 Converging Diverging Vapour 8 6 4 2 0 80 Water Pumped medi 90 100 110 120 19
20 Introduction A Pump medium pressure Pressure Liquid pressure Vapour pressure Atmospheric pressure Stationary seat B Rotating seal ring Entry into sealing gap Start of evaporation C Exit to atmosphere Fig. 1.23: Pressure distribution in a sealing gap with hot water D Distance Deposits and wear tracks When the lubricating film in the sealing gap evaporates, dissolved solids are left deposited on the seal faces. If the thickness of deposits exceeds the necessary thickness of the lubricating film, the seal starts to leak. In case of hard deposits, wear tracks can develop in one of the seal rings, see fig. 1.24a. In case of soft and sticky deposits, a build-up can cause the seal faces to separate, see fig. 1.24b. Evaporation The absence or inadequate formation of lubricating film frequently causes damage to the seal faces. Evaporation of the pumped medium in the sealing gap occurs where the pressure is below the vapour pressure of the pumped medium. The frictional heat in the seal faces increases the temperature of the medium resulting in an increase of the vapour pressure. This moves the start of evaporation point to the pumped medium side. See fig. 1.23. For seals in cold water, the lubricating film extends through the entire sealing gap. For a wellfunctioning seal, the only leakage escaping on the atmospheric side is vapour. The evaporation will occur even in cold water due to leakages through the very narrow sealing gap, i.e. 0.0002 mm. A partial lack of lubricating film often occurs in the sliding seal faces towards the atmospheric side when pumping water above 100 °C. This is due to evaporation of the lubricating film. Stationary seat Rotating seal ring Fig. 1.24a: Development of wear tracks due to hard deposits Stationary seat Rotating seal ring Fig. 1.24b: Deposits build-up on seal faces
Page 1 and 2: Mechanical shaft seals for pumps
Page 3 and 4: Contents Preface 5 Chapter 1. Intro
Page 6 and 7: Chapter 1 1. Types of shaft seals 2
Page 8 and 9: Fig. 1.2: Braided stuffing box pack
Page 10 and 11: O-ring, stationary O-ring, rotating
Page 12 and 13: The rotating part The rotating part
Page 14 and 15: Closing force The parts of the seal
Page 16 and 17: In the examples above, where the ar
Page 20 and 21: Atmosphere A B C D Vapour pressure
Page 22 and 23: 1993 Rubber bellows seal introduced
Page 25 and 26: 26 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
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76 Failure of mechanical shaft seal
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88 7. Shaft seal failure analysis T
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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
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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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