Dual gas seals have critical design features that allow them to effectively seal pumps and mixers. Key features include the seal face design which uses grooves to control gas flow, dynamic secondary seals to prevent contact between seal faces, and solids exclusion devices to prevent damage. The support system precisely regulates barrier gas pressure and flow. Application considerations for dual gas seals include fluid properties, pump operating conditions, and vessel design. Typical dual gas seal designs incorporate these features for a variety of equipment to provide effective sealing with low emissions.

1. FSA Knowledge Series
Mechanical Seal

2. Agenda – Dual Gas Seals
• Critical Design Features
•
Seal Face Features
•
Secondary Seal Drag
•
Solid Exclusion Devices
•
Support System
• Application Considerations
• Typical Dual Gas Seals

3. Critical Design
Features
Dual Gas Seal

4. 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

5. 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

6. Seal Face Features
H Lift d y n a Dimensional Mapping of
• Hydrodynamic y d roThreem ic L ift E n g in e
Pressure
S p ira l G r o o v e T e c h n o lo g y

7. Seal Face Features
Unidirectional Spiral Groove:
Land
Shallow Tapered
Spiral Groove
Shallow
Annular
Groove
Sealing Dam

8. Seal Face Features
Unidirectional Spiral Groove:
1) Gas enters wide and
deep grooves at OD
2) Gas is compressed
through narrowing
spiral grooves
2
3
3) Gas pressure is equalized
through circumferential
groove
1

9. 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

10. 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
16 µin RMS (0.4 µm)
Low friction coatings
Critical Surface Finish
Dynamic O-ring

11. 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

12. 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

13. 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
Flow Meter
Pressure
Gauge
Pressure
Switch
(With Optional
Switch)
Filter
Check
Valve
Isolation
Valve
Barrier Gas
Supply
To
Mechanical
Seal
Isolation
Valve
Pressure
Regulator

14. Application
Considerations

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Typical Dual Gas Seals

22. Pump Gas Seal Design Features
Simple installation cartridge seal is
100% static tested at the factory
Dynamic O-rings
Most require large bore
seal chambers
Stationary silicon carbide with face pattern
Dead-ended
barrier gas

23. Metal Bellows Gas Seal
Eliminates secondary seal friction:

24. Double Gas Seal for Big Bore
Seal Chambers
BARRIER GAS
SUPPLY
ATMOSPHERE
OUTBOARD
PROCESS
INBOARD
Clockwise Rotation
Counter-Clockwise Rotation

25. 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

26. Mixer Gas Seal Design Features
Designs for Top-Entry
Mixers
Dynamic O-rings
Shaft centered
Through springs to
maintain spring load
Radial clearance
for run-out
Process Side

27. Conclusions
• 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