Dual Gas Seals

Dual Gas Seals FSA Knowledge Series

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Abstract 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:​

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The critical design features of gas seals

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​Application considerations

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Typical dual gas seals

Critical Design Features Dual Gas Seal 4

Critical Design Features Seal Face Features ▪

Film thickness and stiffness

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Low speed lift-off

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Gas consumption

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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 5

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Barrier gas supply, regulation and monitoring

Seal Face Features Barrier Gas Consumption

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Minimal leakage past inboard seal faces results in low gas flow into process stream

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Majority of barrier gas consumption is past outboard seal faces to atmosphere

Seal Face Features Hydrodynamic Lift Three-Dimensional Mapping of Pressure

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Seal Face Features Unidirectional Spiral Groove: Land

Shallow Tapered Spiral Groove

Shallow Annular Groove 8

Sealing Dam

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 9

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Secondary Seal Drag

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Low film stiffness requires light spring loads to avoid face contact

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Light spring loads can’t overcome dynamic secondary seal drag

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Low drag dynamic secondary seal designs are required

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Bellows designs eliminate need for dynamic secondary seals

Secondary Seal Drag Secondary seal squeeze ▪

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

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Spring energized PTFE seals - design parameters, spring design

Critical Surface Finish

Sleeve Surface Finish ▪

Target surface finish of 0.4 µm / 16 µin RMS

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Low friction coatings Seal Face

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Dynamic O-ring

Secondary Seal Drag Lubrication ▪

Compatibility of O-ring lubricant with elastomer compound

Elastomer compound

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

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Surface finish / Low friction coating

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Hardness to achieve sealing with low squeeze. Target hardness is 75 or lower (Durometer Shore A)

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Resistance to compression set

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Curing system effect on chemical compatibility

Solids Exclusion Devices Exclusion device objectives: ▪

Prevent solids from collecting at dynamic secondary seal

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Prevent solids from entering seal face gap

Exclusion techniques:

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Create physical restriction to keep solids out

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Generate fluid flow patterns to keep solids out

Support System Flow Meter

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

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

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

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Additional optional equipment:

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Pressure amplifier

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Accumulator

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Instrumentation

Pressure Gauge

Check Valve

Filter

Isolation Valve

Barrier Gas Supply 14

Pressure Switch

(With Optional Switch)

To Mechanical Seal

Isolation Valve

Pressure Regulator

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

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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

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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

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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

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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

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Typical Dual Gas Seals 22

Pump Gas Seal Design Features Simple installation cartridge seal is 100% static tested at the factory

Dynamic secondary seals

Most require large bore seal chambers

Stationary silicon carbide with face pattern 23

Barrier gas cavity

Metal Bellows Gas Seal ▪

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Eliminates secondary seal friction:

Double Gas Seal for Big Bore Seal Chambers Barrier gas supply

Atmosphere outboard Process inboard

Clockwise Rotation 25

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

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

Mixer Gas Seal Design Features Atmosphere

Designs for Top-Entry Mixers

Dynamic secondary seals Shaft centering spacer

Through springs to maintain spring load

Radial clearance for run-out

Process Side

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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:

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Seal face topography

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Dynamic secondary seal design

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Solids exclusion devices

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