FSA Knowledge Series -- Gas Seal (demo)
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Transcript of FSA Knowledge Series -- Gas Seal (demo)
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