Seal Fundamentals - Basic Operating Principles

Seal Fundamentals Basic Operating Principles FSA Knowledge Series

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Abstract This Fluid Sealing Association Knowledge Series training presentation introduces the basic operation and construction of mechanical seals. This section includes:

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▪

Essential elements of a mechanical seal

▪

Effective forces in a mechanical seal

▪

Leakage of a liquid lubricated mechanical seal

▪

Power Consumption of a liquid lubricated mechanical seal

▪

Lubrication regimes of liquid lubricated mechanical seals

▪

Seal Balance

Essential Elements of a Mechanical Seal Seal faces: one rotating with the shaft and one stationary in the pump casing, cover or flange.

Secondary seals: one to seal the rotating face to the shaft and one to seal the stationary face to the pump cover or flange.

Metal parts: to transmit torque and to provide an axial mechanical force to load the faces Gland Plate Seal Chamber Drive Collar

Stationary O-Ring

Stationary Face

Drive Set Screws Rotating Face Drive Pin 4

Spring

Rotating O-Ring

Essential Requirements of a Mechanical Seal Essential requirements for proper operation of a mechanical seal

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▪

Seal faces must be flat and polished

▪

Seal faces must be installed perpendicular to the shaft

▪

Spring force must be sufficient to maintain contact of the faces

Roughn ess 10 μm Pre cision Turned

5 μm Gr ound

1 μm Lap ped

0.1 μm Polished

Per cen tag e Bea ring Area 4 %

12 %

40 %

95 %

Degree of roughness in relation to the percentage bearing area

The Fluid in the Pump and Seal Area Mechanical Seal Cooling

Sealed Medium

Lubrication

Typical Pump Cross-section

Compromise of Leakage and Wear

Key point: the fluid contacts the seal faces and other parts in wide open areas, in very small gaps and at the exit of the seal faces. Pressure and temperature of the fluid will depend on its location and determine its respective state, i.e. liquid, gaseous, solid or a mixture.

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Leakage and Wear A few facts about the leakage (and wear) behavior of contacting mechanical seals

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▪

It is essential for proper lubrication and low wear of the faces

▪

Normal leak rates range between immeasurably small to steady drips or temporary to even small streams. Some seals leak some of the time, some seals never leak (measurably), and some leak all the time. Leakage patterns can be constant, progressive or erratic in nature.

▪

It can be in liquid, gaseous and/or solid state

▪

Successful contacting seals tend to have very low wear rates and low leakage rates

Leakage and Wear

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▪

Some form of contact is necessary for low leakage rates. Non-contacting or “full lift off” seals (hydrostatic or hydrodynamic tend to have visible and sizably larger leakage rates).

▪

The large majority of mechanical seals never wear out and are removed from service for some other reason.

▪

Seal failures occur for a wide range of reasons. Some failures occur as an interaction with the tribology of the interface.

Effective Forces in a Mechanical Seal ▪

Axial and radial forces

▪

Closing and opening forces

▪

Hydro static and hydrodynamic forces

P1

Ao Ac

h

ΔP

ΔP

ΔP

(2)

(3)

(4)

(5)

Ao

Ac FSpring

P0

(1)

FSpring P1

Ac = Hydraulically Closing Area Ao = Opening Area P1 = Sealed Pressure P0 = Atmospheric Pressure 9

(1) Spring and Hydraulic Closing Forces (2) Lubricating Film Thickness (3) Pressure Curve in Static Condition (4) Linear Pressure Drop (5) Pressure Curve with Hydrodynamic Components

Leakage Leakage of a liquid lubricated mechanical seal 𝝅 𝒉𝟑𝚫𝑷 𝑸= 𝒓 𝟔 𝝁 𝒍𝒏 𝟎 𝒓𝒊

Key point: leakage rate Q strongly depends on the gap height h

Leakage creates the lubricating fluid film that prevents a mechanical seal from overheating. In order to maintain an acceptable leakage rate (Q), a stable gap/fluid film is required.

The gap height is determined by several factors: materials, manufacturing quality, lubrication regime, face distortions The leak rate of a contacting seal is also influenced by other pump related factors such as run outs and vibration levels.

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Power Consumption Power consumption of a liquid lubricated mechanical seal ▪

Face friction, churning and soak in heat

▪

Flush to dissipate the heat in order to control the gap temperature

▪

Coefficient of friction can swing considerably during operational transients

▪

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The key is to maintain the gap profile as parallel as possible, i.e. minimize distortions

Heat Generation and Dissipation Heat Soak from Hot Product

Flush Flow

Heat from Churning of Rotating Parts

Face Heat Generation

Typically, the required flush flow rate is based upon a max ΔT of 20F

𝑭𝒏𝒆𝒕 = Net Contact Force 𝒇𝒄 = Coefficient of Friction 𝒗𝒈 = Sliding Velocity H = Frictional Heat

𝑯 = 𝑭𝒏𝒆𝒕 · 𝒇𝒄· 𝒗𝒈

Lubrication Regimes F

F

F

V

V

V

1.00 Mixed

Solid Friction

Hydrodynamic Lubrication

Friction Coefficient

0.50 0.30 0.20

𝑮=

0.10

(𝑮𝒂𝒑 𝑽𝒊𝒔𝒄𝒐𝒔𝒊𝒕𝒚)(𝑺𝒍𝒊𝒅𝒊𝒏𝒈 𝑽𝒆𝒍𝒐𝒄𝒊𝒕𝒚) (𝑭𝒐𝒓𝒄𝒆 𝑩𝒂𝒍𝒂𝒏𝒄𝒆 𝑹𝒂𝒕𝒊𝒐)(𝑷𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝑫𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒕𝒊𝒂𝒍)

0.05 0.03 0.02 0.01 1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

Seal Duty Parameter

The duty parameter is a tool to evaluate the severity of an application and the expected leakage behavior of a mechanical seal. It is a dimensionless quantity for the tribological characterization of the operating conditions of a mechanical seal.

Shaft Diameter

50 mm (2.000”) 12

Balance Ratio

Sealed Pressure

Shaft Speed

1.1

0.5 MPag (72 psig)

1500 RPM 3000 RPM

0.7

Sealed Fluid

Dynamic Viscosity

Duty Parameter

Theoretical Leakage

Water

0.47 cP

1.5E-8

0.4 ml/hr

ISO VG 46 Oil

17.2 cP

1.5E-6

11.3 ml/hr

Seal Balance Balancing a mechanical seal reduces the axial face contact force which allows higher pressure to be sealed.

▪ Seal balance is defined as the ratio (k) of 2 geometric areas; the hydraulic closing (Ac) and opening contact area (Ao) ▪ ▪

For unbalanced seals, k ≥ 1 For balanced seals, k < 1

k>1 Ao

Sealed Pressure

Ac

Atmosphere OD Pressure Balance Diameter

ID Pressure Balance Diameter OD Pressure Balance Diameter

Double Balanced Seal 13

Unbalanced Seal Effective Balance Diameter

Bellows Seal Balance Diameter

Ao Ac

k<1 OD Pressure Balance Diameter

Balanced Seal

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