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This Fluid Sealing Association Knowledge Series training presentation relates seal leakage to seal face lubrication and power consumption. Included are overviews of:
▪ Leakage, lubrication, and wear of mechanical seals
▪ Forces and pressure profiles that create mechanical seal lubricating films
▪ Leakage Rate-Gap correlations and factors
▪ Power consumption and lubricated regimes of liquid-lubricated mechanical seals
▪ Seal balance, used to improve the duty parameter to promote lubrication
Facts About Leakage and Wear
▪ Seal leakage 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
▪ Some form of contact is necessary for low leakage rates. Non-contacting or “full lift off” seals (hydrostatic or hydrodynamic tend to have visible, sizeably 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
Ac = Hydraulically Closing Area
Ao = Opening Area
P1 = Sealed Pressure
P0 = Atmospheric Pressure
(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
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.
Power Consumption
Power consumption of a liquid lubricated mechanical seal
Heat Generation and Dissipation
Soak from
▪ 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
▪ The key is to maintain the gap profile as parallel as possible, i.e. minimize distortions
from Churning of Rotating Parts
Typically, the required flush flow rate is based upon a max ΔT of 20F
Coefficient of Friction
Lubrication Regimes
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.
Seal Balance
▪ To reduce the axial face contact force which allows to seal high pressures, i.e. up to 3000 psig (200 Barg) with one set of faces
▪ It is the ratio (k) of 2 geometric areas: the closing (Ac) and opening area (Ao)
▪ For unbalanced seals k ≥ 1
▪ For balanced seals k ≤ 1
IDPressure BalanceDiameter Pusher Seal Bellows Seal Effective Balance Diameter
