Mechanical shaft seals for pumps - Grundfos

More documents

Recommendations

Info

In the examples above, where the areas of the sliding faces and the spring force are equal, the closing force is reduced from 224 N to 195 N by reducing the balancing ratio from k = 1.17 to k = 0.98. A smaller closing force gives less wear on the sliding faces because improved lubrication is obtained. The result is also a higher leakage rate. Leakage The lubricating film formed in the sealing gap during pump operation results in the escape of some of the pumped medium to the atmospheric side. If the mechanical seal works well and no liquid appears, the lubricating film has evaporated due to heat and pressure decrease in the sealing gap. Therefore, no liquid seeps out of the seal. Fig. 1.18: Seal with excessive leakage Note that evaporation of water can take place at temperatures below 100 °C, unless the surrounding atmosphere is saturated with vapour. Think of how you can dry your clothes outside on a clothes line. The leakage rate of a mechanical shaft seal depends of a number of factors such as: • surface roughness of seal faces • flatness of seal faces • vibration and stability of pump • speed of rotation • shaft diameter • temperature, viscosity and type of pumped medium • pump pressure • seal and pump assembly. 17
18 Introduction Calculation of leakage rate The leakage rate of a liquid-lubricated mechanical shaft seal with parallel seal faces through the sealing gap can be calculated by means of this approximate formula: Formula 2: Q = π x R m x h 3 x ∆p 6 x η x b R = m 4 (22 + 17) = 9.75 mm b = 2 (22 – 17) Where Q = leakage rate per unit of time R = average radius of the sliding face m h = gap height between the sliding faces (thickness of the lubricating film) Δp = differential pressure to be sealed h = dynamic viscosity of the pumped medium b = radial extension of the sealing gap (sliding face width). = 2.5 mm The leakage rate, Q, is then linear to the radius, R , sliding face width, b, and pressure difference, Δp. m The gap height, h, however, is extremely important. Note that twice the height causes eight times as much leakage, with all other conditions remaining the same. b = 2 (22 – 17) It seems as if the leakage decreases = 2.5 mmwhen viscosity, h, increases. But when viscosity increases, the lubricating film and thus the sealing gap increases, which may result in an increase in the leakage rate. The increase in sealing gap height with an increase in viscosity is not linear. This makes it difficult to predict whether or not an increase in viscosity results in a higher or lower leakage rate. Q = π x 9.75 x 10-3 m x (0.2 x 10-6 m) 3 x 1 x 106 N/m2 = 1.63 x 10-11 m3 /s = 0.06 ml/h 6 x 0.001 N x s/m2 x 2.5 x 10-3 The roughness and flatness of the two sliding faces maffect the height of the sealing gap and thus the Q leakage. = The hydrodynamic pressure increases with the speed. This can cause an increase of the gap height and thus the leakage rate. π x R x h m 3 x ∆p 6 x η x b A gap height between the sliding faces of 0.2 micron is typical for a mechanical shaft seal running in water. Consequently, the seal faces have to be very smooth and flat. The calculation example below applies to a Grundfos type H seal running in water at 20 °C at a Q = pressure of 10 bar. A sealing gap of 0.2 mm is assumed. π x R x h m 3 x ∆p 6 x η x R = m 4 b (22 + 17) = 9.75 mm Δp = 10 bar = 1 MPa = 1 x 106 N/m2 D = 22 mm o D = 17 mm i Viscosity = 1 cst = 0.001 N x s/m2 h = 0.0002 mm = 0.2 x 10-6 Q = m Thus, π x R x h m 3 x ∆p 6 x η x b R = m 4 (22 + 17) b = 2 = 9.75 mm and (22 – 17) = 2.5 mm b = 2 (22 – 17) = 2.5 mm Using formula 2, the leakage rate, Q, is as follows: b = 2 (22 – 17) = 2.5 mm b = 2 (22 – 17) Q = = 2.5 mm π x 9.75 x 10-3 m x (0.2 x 10-6 m) 3 x 1 x 106 N/m2 = 1.63 x 10-11 m3 /s = 0.06 ml/h 6 x 0.001 N x s/m2 x 2.5 x 10-3 m If the roughness of the seal faces is higher, resulting in a sealing gap of 0.3 micron, the leakage rate is 0.2 ml/h. b = 2 (22 – 17) = 2.5 mm R = m 4 (22 + 17) = 9.75 mm b = 2 (22 – 17) = 2.5 mm Q = π x 9.75 x 10-3 m x (0.2 x 10-6 m) 3 x 1 x 106 N/m2 = 1.63 x 10-11 m3 /s 6 x 0.001 N x s/m2 x 2.5 x 10-3 m
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 18 and 19: Non-parallel seal faces System In p
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
Page 73 and 74:
76 Failure of mechanical shaft seal
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
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
show all

Mechanical shaft seals for pumps - Grundfos

Create successful ePaper yourself

Delete template?

Save as template?