Seal Fundamentals - Seal Face Topography

Seal Fundamentals Seal Face Topography FSA Knowledge Series

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Abstract (Seal Face Topography) This Fluid Sealing Association Knowledge Series training presentation reviews seal face topographical features used on mechanical seals to reduce or eliminate contact between the faces. A description is provided on:​

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Basic Principles of Load Support

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Types of seal face features Liquid lubricated seal face features

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Gas lubricated seal face features

Basic Principles of Load Support 4

Basic Principles Seal face topographical features are used to create a load support force between the faces to partially or completely offset closing forces.

There are two primary load support mechanisms found in seal face features: ▪ Hydrostatic load support ▪ Hydrodynamic load support Most features create a combination of hydrostatic and hydrodynamic load support.

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Hydrostatic load support Hydrostatic load support is created by manipulating the pressure profile across the face.

The amount of load support created is independent of rotation of the faces.

Several methods used to control pressure profile: ▪ Controlled taper.

▪ Deep hydropads.

▪ Shallow grooves.

Deep Hydropad Face design 6

Hydrostatic load support OD Pressure OD Pressure With convergent taper, high pressure extends further across the faces creating a non-linear drop. This increases the force separating the faces.

With plain seal faces that are flat, pressure drops linearly from the high pressure at the OD to the low pressure at the ID.

Convergent Taper Faces

Flat Parallel Faces (cross section view)

OD Pressure

OD Pressure Shallow grooves act much like a convergent taper, creating a similar non-linear pressure profile and separation force.

Hydropads allow full pressure across the faces in the areas of the deep cutouts, increasing the separation force

Deep Hydropad Face 7

Shallow Groove Face

Hydrodynamic load support

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Hydrodynamics mechanism in seals is the same as the mechanism found in journal bearings

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Creates lift through relative motion of surfaces

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Key component in seals is a varying gap between two flat surfaces, setting up a “squeeze film”

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Fluid pressure increases as it is drawn from a wider gap to a smaller gap

Hydrodynamic load support Hydrodynamics can be modeled by the Reynolds equation, shown derived for incompressible fluids:

d  h3 dp  dh     = 6 U  dx   dx  dx x direction h (min gap)

 (fluid viscosity)

U (sliding velocity)

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p (pressure)

Hydrodynamic load support In basic translation, Reynolds equation says load support (pressure generation) is: ▪ Proportional to sliding speed (U) ▪ Proportional to viscosity (μ)

▪ Proportional to slope of converging gap (dh/dx) ▪ Inversely proportional to the cube of the minimum gap (h3)

d  h3 dp  dh     = 6 U  dx   dx  dx

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Hydrodynamic load support ▪

Film stiffness is key to stability of a hydrodynamic seal

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Cube relationship of gap to pressure means pressure dramatically increases as faces get close and drops off as they separate Normal Gap

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Close Gap

Types of Seal Face Features 12

Types of Seal Face Features Many variations in surface features are used to create load support ▪ Deep grooves / hydropads

Bidirectional Designs

▪ Shallow grooves ▪ Surface textures

Features can be applied to liquid lubricated or gas lubricated seals ▪ Design is different between the two due to differences between compressible gases and incompressible liquids.

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Unidirectional Designs

Liquid Lubricated Feature Designs Deep groove hydropad features may be used to create hydrostatic lift or introduce more lubrication between faces

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Some hydrodynamics exist, but are limited due to groove depth being much larger than the gap between the faces

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Typically machined in features with depths of 1.3 mm / 0.050” or more

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Typically, bidirectional

Liquid Lubricated Feature Designs Shallow groove (relative to the fluid film gap) features may be used to fully control contact and lift between liquid lubricated seal faces

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Balance between hydrostatic and hydrodynamic load support can be varied based on groove geometry

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Hydrodynamics can be very strong due to incompressible nature of liquids

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Designs are most commonly bidirectional and less aggressive than features for gas due to the strong hydrodynamics

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Unidirectional features can create a strong pumping effect, which can be useful in certain application situations

Analysis showing pressure generated by a bidirectional pattern in water

Liquid Lubricated Feature Applications Many applications can benefit from the use of liquid lubricated features:

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Poor lubricating liquids

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High rotational speeds

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High pressures

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Flashing / vaporizing liquids

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Heat sensitive liquids

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High differential pressure dual seals

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Applications where heat / energy reduction is a concern

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Many others – consult your seal supplier

Gas Lubricated Feature Designs Shallow groove features may be used to create hydrostatic and hydrodynamic lift on gas lubricated faces

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Balance between hydrostatic and hydrodynamic load support can be varied based on groove geometry.

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Designs need to be more aggressive than comparable liquid features with larger surface area and deeper grooves to create sufficient lift

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Unidirectional features are more effective in lower speed applications

Analysis showing pressure generated by a unidirectional pattern in gas

Gas Lubricated Feature Applications Gas lubricated features find usage in many seal designs:

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Dual gas seals for pumps

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Compressor seals

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Dry running back-up seals

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Fan / blower seals

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Mixer seals

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Steam Turbine seals

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Many others – consult your seal supplier

Lift-Off Seal Considerations Lift off seals leak, especially hydrostatic designs for slow speed applications (e.g., mixers) Leakage is dependent on seal size, pressure, and shaft speed

Some system pressure is required to assist in creating a flow across the faces and to set up hydrostatics Lift off seals can be sensitive to upset conditions, such as:

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Loss of pressure or pressure reversal

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Reverse rotation (for uni-directional designs)

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Vibration

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Shaft movement

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Contamination (e.g., solids, polymerization, …)

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