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This Fluid Sealing Association Knowledge Series training presentation introduces gas seals and their particular application considerations. Gas seals are dual seals that achieve zero-emission with low energy consumption. A description is provided on:
• The critical design features of gas seals
• Application considerations
• Typical dual gas seals
Critical Design Features
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
Seal Face Features
Unidirectional Spiral Groove:
Sealing Dam
Shallow Tapered
Shallow Annular Groove
Seal Face Features
Unidirectional Spiral Groove:
2) Gas is compressed through narrowing spiral grooves
1) Gas enters wide and deep grooves at OD
3) Gas pressure is equalized through circumferential 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 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 0.4 µm / 16 µin RMS
▪ Low friction coatings
Face
Seal
Secondary 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 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
Support System
▪ 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
Application Considerations
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 hang-up and impair proper face tracking as well as damage to the sliding surface (aggravated by light spring loads)
Application 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 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 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 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 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
Typical Dual Gas Seals
Pump Gas Seal Design Features
Simple installation cartridge seal is 100% static tested at the factory
Most require large bore seal chambers
Dynamic secondary seals
Barrier gas cavity
Stationary silicon carbide with face pattern
▪ Eliminates secondary seal friction:
Double
Gas Seal for Big Bore Seal Chambers
Process inboard Barrier gas
Clockwise Rotation
Atmosphere outboard
Counter-Clockwise Rotation
Gas Seal for Standard Bore Seal Chambers
Internal Gas barrier pressure regulator
Coaxial plain hydrostatic face
Rotary Silicon Carbide with face pattern
Co-axial Hydrostatic hydrodynamic gas seal with internal barrier gas pressure regulation
Mixer Gas Seal Design Features
Designs for Top-Entry Mixers
Shaft centering spacer
Radial clearance for run-out Through springs to maintain spring load
Conclusions
Gas seal technology has been evolving since the 1960’s and is well established
