FSA Knowledge SeriesMechanical Seal

Agenda – Dual Gas Seals• Critical Design Features

• Seal Face Features• Secondary Seal Drag• Solid Exclusion Devices• Support System

• Application Considerations• Typical Dual Gas Seals

Critical DesignFeatures

Dual Gas Seal

Critical Design Features• Seal Face Features

− Film thickness and stiffness− Low speed lift-off− Gas consumption− Gas flow patterns within the sealing interface

• Secondary seal drag

− Avoid seal face hang-up • Solids exclusion devices

− Exclude solids from seal face gap and dynamic secondary seals

• Support System

• Barrier gas supply, regulation and monitoring

Seal Face Features

• Barrier Gas Consumption– Minimal leakage past inboard seal faces results in low gas

flow into process stream– Majority of barrier gas consumption is past outboard seal

faces to atmosphere

Seal Face Features

• Hydrodynamic Lift Three Dimensional Mapping of Pressure

Hydrodynamic Lift EngineSpiral Groove Technology

Seal Face FeaturesSeal Face Features

Shallow TaperedSpiral Groove

Land

ShallowAnnularGroove Sealing Dam

Unidirectional Spiral Groove:

Seal Face FeaturesSeal Face Features

1) Gas enters wide and deep grooves at OD

2) Gas is compressed through narrowing spiral grooves

3) Gas pressure is equalized through circumferential groove

12

3

Unidirectional Spiral Groove:

Secondary Seal Drag

• Low film stiffness requires light spring loads to avoid face contact

• Light spring loads can’t overcome dynamic secondary seal drag

• Low drag dynamic secondary seal designs are required

• Bellows designs eliminate need for dynamic secondary seals

Secondary Seal DragSecondary Seal Drag

•Secondary seal squeeze

− O-rings - cavity design, chemical swell, thermal expansion

− Spring energized PTFE seals - design parameters, spring design

•Sleeve Surface Finish

− Target surface finish of 16 µin RMS (0.4 µm)

− Low friction coatings

Critical Surface FinishCritical Surface Finish Dynamic O-ringDynamic O-ring

Secondary Seal DragSecondary Seal Drag

• Lubrication− Compatibility of O-ring lubricant with elastomer

compound• Elastomer compound

− Compatibility with process fluid. Absorption can result in swell, changes in physical properties & unpredictable performance

− Surface finish / Low friction coating − Hardness to achieve sealing with low squeeze.

Target hardness is 75 or lower (Durometer Shore A)

− Resistance to compression set− Curing system effect on chemical compatibility

Solids Exclusion DevicesSolids Exclusion Devices

• Exclusion device objectives:− Prevent solids from collecting at dynamic

secondary seal− Prevent solids from entering seal face gap

• Exclusion techniques:− Create physical restriction to keep solids

out− Generate fluid flow patterns to keep solids

out

• Purpose of panel is to regulate, control, and monitor flow of barrier gas to the seal

• Many systems are unitized on a panel as shown in the figure

• Care must be taken not to switch pressure off during stand-by

• Additional optional equipment:

• Pressure amplifier• Accumulator• Instrumentation

Support SystemSupport System

Barrier Gas Supply

ToMechanical

Seal

IsolationValve

Pressure Regulator

CheckValve

Pressure Switch

Flow Meter (With Optional

Switch)Pressure Gauge

Filter

IsolationValve

Application Considerations

Fluids with Suspended Solids:• Solids between seal faces can clog hydrodynamic

micro-features (grooves)• Centrifugal forces push solids between seal faces

in back to back configurations• Solids between seal faces can result in 3 body

abrasion of seal faces• Solids at dynamic secondary seal can cause O-

ring hang-up and impair proper face tracking as well as damage to the sliding surface (aggravated by light spring loads)

Application ConsiderationsApplication Considerations

Fluids with Dissolved Solids:• Solids come out of solution in seal chamber due to:

• different environmental conditions• drying effect of gas leakage

• Similar to problems attributed to fluids with solids• Migration of fluid between seal faces during static

conditions may leave damaging residue

Application ConsiderationsApplication Considerations

Reverse Rotation of Pumps:• At shutdown, gravity may allow static head in

discharge line to reverse flow through pump• Reverse flow causes impeller and pump shaft to

reverse rotate• Especially an issue with vertical pumps• Reverse rotation of unidirectional gas seal faces can

cause damage to faces

Application ConsiderationsApplication Considerations

Batch Operations:• Start and stop procedures may involve momentary

slow speed operation• Especially an issue with variable frequency drives• Duration of slow speed operation and frequency of

starts and stops is critical• Repeated slow speed operation can cause

cumulative damage to faces

Application ConsiderationsApplication Considerations

Stand-by Pumps:• Small static barrier gas leakage into the pump casing

can accumulate over time• Proper venting of pump is necessary before start-up

Low Flow / Low Suction Head Pumps:• Centrifugal pumps can tolerate 1-2% of entrained gas• Barrier gas leakage expands in low pressure suction• Proportion of gas present must be evaluated at the

lowest pressure point

Application ConsiderationsApplication Considerations

Small Mixer Vessels:• Barrier gas leakage can accumulate over

time, and increase pressure in mixer vessel• Increased pressures will affect seal face

hydrostatic load support• Increased pressures may exceed vessel

rating or affect reactions in vessel

Application ConsiderationsApplication Considerations

Typical Dual Gas Seals

Pump Gas Seal Design FeaturesPump Gas Seal Design Features

Most require large boreseal chambers

Most require large boreseal chambers

Simple installation cartridge seal is100% static tested at the factory

Simple installation cartridge seal is100% static tested at the factory

Dead-ended barrier gas

Dead-ended barrier gas

Dynamic O-ringsDynamic O-rings

Stationary silicon carbide with face patternStationary silicon carbide with face pattern

Metal Bellows Gas Seal

Eliminates secondary seal friction:

Counter-Clockwise Rotation Clockwise Rotation

ATMOSPHEREOUTBOARD

ATMOSPHEREOUTBOARD

PROCESSINBOARD

PROCESSINBOARD

Double Gas Seal for Big Bore Seal Chambers

BARRIER GASSUPPLY

BARRIER GASSUPPLY

Co-axial Hydrostatic hydrodynamic gas seal with internal barrier gas pressure regulation

Gas Seal for Standard Bore Seal Chambers

Rotary Silicon Carbide with face pattern

Coaxial plain hydrostatic face

Internal Gas barrier pressure regulator

Mixer Gas Seal Design FeaturesMixer Gas Seal Design Features

Shaft centeredShaft centeredDynamic O-ringsDynamic O-rings

Process SideProcess Side

Designs for Top-EntryMixers

Designs for Top-EntryMixers

Radial clearancefor run-out

Radial clearancefor run-out

Through springs tomaintain spring load

Through springs tomaintain spring load

• Gas seal technology has been evolving since the 1960’s and is well established

• Dual gas seals offer many benefits including:

− zero product emissions

− tolerance of off-design pump operation

− significantly reduced energy consumption

• Critical design features for gas seals include:

− seal face topography

− dynamic secondary seal design

− solids exclusion devices

ConclusionsConclusions