Pipeline Commissioning

Pipeline Drying Methods

Calculate drying time requirements and evaluate air drying, nitrogen drying, and vacuum drying methods to achieve target dewpoint specifications for pipeline commissioning per ASME B31.8.

Launch Calculator Dewpoint specs

Target dewpoint

-40°F typical

Natural gas pipelines typically require -40°F dewpoint to prevent hydrates.

Drying time

1-7 days typical

Drying duration depends on method, volume, and moisture content.

Air velocity

10-30 ft/s

Optimal air velocity for effective moisture removal during drying.

Contents

Use this guide when you need to:

  • Select appropriate pipeline drying method.
  • Calculate required drying time and air volume.
  • Specify dewpoint acceptance criteria.

1. Overview & Applications

Pipeline drying removes water introduced during hydrostatic testing, construction, or flooding. Adequate drying prevents corrosion, hydrate formation, and operational problems in gas and liquid pipelines.

Post-hydrotest drying

New construction

Remove water after pressure testing before commissioning gas service.

Maintenance drying

After repairs

Dry pipeline sections opened for maintenance or modification.

Seasonal drying

Intermittent service

Dry pipelines taken out of service to prevent corrosion and freezing.

Product changeover

Multi-product lines

Remove water contamination before product introduction.

Key Concepts

  • Dewpoint: Temperature at which water vapor condenses from gas (°F or °C)
  • Relative humidity (RH): Ratio of actual vapor pressure to saturation vapor pressure (%)
  • Absolute humidity: Mass of water per unit volume of air (grains/ft³ or lb/MMscf)
  • Drying rate: Rate of moisture removal (lb water/hr)
  • Saturation: Maximum water vapor capacity at given temperature and pressure
Why drying matters: Water in gas pipelines causes internal corrosion (5-10 mpy rates), hydrate formation (blockages at 32-60°F), freezing damage, and accelerated metallurgical degradation. Proper drying is essential for pipeline integrity and operational reliability.

Consequences of Inadequate Drying

  • Internal corrosion: Water film promotes electrochemical corrosion, reducing pipe wall thickness
  • Hydrate formation: Water + gas form solid hydrates blocking flow at low temperatures
  • Ice formation: Freezing water expands, causing mechanical damage and flow restriction
  • Microbiologically influenced corrosion (MIC): Bacteria thrive in water, accelerating corrosion
  • Compressor damage: Liquid water carryover damages downstream compressors
  • Instrument malfunction: Water in instrumentation lines causes measurement errors

ASME B31.8 Requirements

ASME B31.8 Gas Transmission and Distribution Piping Systems Code requires:

  • Section 841.32: Pipelines shall be cleaned and dried before being placed in service
  • Hydrostatic test water removal: Water must be removed within reasonable time after testing
  • Dewpoint verification: Gas quality must meet specifications before introduction to system
  • Corrosion prevention: Internal surfaces must be dried to prevent corrosion initiation

Industry Standards

  • ASME B31.8: Gas pipeline drying requirements
  • ASME B31.4: Liquid pipeline cleaning and drying
  • API RP 1171: Pipeline drying practices for commissioning
  • GPA 2145: Table of Physical Constants for Hydrocarbons (dewpoint data)
  • ISO 13623: Petroleum and natural gas industries – Pipeline transportation systems

2. Drying Methods

Three primary methods are used for pipeline drying: air drying, nitrogen drying, and vacuum drying. Method selection depends on pipeline volume, required dewpoint, time constraints, and cost.

Method Comparison

Method Achievable Dewpoint Typical Duration Cost Level Best Application
Air drying +10°F to -20°F 1-5 days Low Moderate dewpoint, long timelines, low-pressure gas
Nitrogen drying -40°F to -80°F 2-7 days High Low dewpoint, inert atmosphere, high-pressure gas
Vacuum drying -60°F to -100°F 12 hours - 3 days Very high Ultra-low dewpoint, short timeline, small volumes
Methanol injection N/A (displacement) Hours Medium Emergency drying, hydrate prevention, liquid lines
Desiccant drying -40°F to -100°F 1-3 days Medium-high Continuous circulation, precise control, small diameter

[Image: Pipeline Drying System Schematic]

P&ID showing compressor, dryer, pipeline, and dewpoint monitoring arrangement

Air Drying (Dry Air Method)

Compressed dry air blown through pipeline carries away moisture:

Air Drying Process: 1. Source: Compressed air from portable compressors or plant air 2. Conditioning: Pass through refrigerated dryer or desiccant dryer 3. Delivery: Blow through pipeline at 10-30 ft/s velocity 4. Monitoring: Measure outlet dewpoint continuously 5. Endpoint: When outlet dewpoint matches inlet (equilibrium reached) Advantages: - Low cost (equipment readily available) - Simple operation - No special safety requirements - Can be done in stages Disadvantages: - Limited dewpoint (-20°F typical for refrigerated air) - Long duration for large pipelines - Introduces oxygen (pre-commission purge needed) - Weather-dependent (inlet humidity affects performance) Typical specifications: Inlet air dewpoint: -20°F to -40°F Air velocity: 15-25 ft/s Flow rate: Size for 15-20 pipe volumes per day Compressor: 250-500 scfm per 20" pipeline

Nitrogen Drying

High-purity nitrogen gas provides inert, very dry atmosphere:

Nitrogen Drying Process: 1. Source: Liquid nitrogen vaporized or PSA/membrane generators 2. Purity: 99%+ nitrogen, -60°F to -80°F dewpoint 3. Circulation: Continuous or once-through at 5-15 ft/s 4. Monitoring: Outlet dewpoint and oxygen content 5. Endpoint: Outlet dewpoint stable at target (-40°F typical) Advantages: - Very low dewpoint achievable (-60°F to -80°F) - Inert atmosphere (no corrosion, no purge needed) - Consistent performance (not weather-dependent) - Can pressurize line during drying (leak test + dry) Disadvantages: - High cost (nitrogen supply expensive) - Requires specialized equipment - Safety considerations (asphyxiation in confined spaces) - Venting considerations (GHG, noise) Nitrogen consumption estimate: V_N2 (scf) = Pipeline Volume (ft³) × Number of Turnovers × 1.05 Turnovers = 20-50 (depends on initial moisture, target dewpoint) Example: 20" × 10 miles pipeline Volume = π × (20/12)²/4 × 10 × 5,280 = 138,160 ft³ Turnovers = 30 (moderate drying) N2 required = 138,160 × 30 × 1.05 = 4,352,040 scf = 4.35 MMscf At $5/Mscf: Cost = $21,760 for nitrogen

Vacuum Drying

Reduce pressure to boil off water at low temperature:

Vacuum Drying Principle: Water boiling point decreases with pressure: - 1 atm (14.7 psia): 212°F - 0.5 psia: 79.6°F - 0.1 psia: 46.7°F - 0.02 psia: 20.8°F (below freezing at 1 atm!) Process: 1. Seal pipeline, connect vacuum pump 2. Evacuate to 0.1-1.0 psia 3. Water boils and evaporates at ambient temperature 4. Vacuum pump removes water vapor 5. Hold vacuum until no more water vapor generated 6. Break vacuum with dry nitrogen 7. Verify dewpoint Advantages: - Fastest method (hours vs. days) - Excellent dewpoint (-80°F to -100°F achievable) - No gas consumption (just vacuum pump power) - Works at ambient temperature Disadvantages: - Very high equipment cost (large vacuum pumps, condensers) - Limited to smaller diameter/shorter length (ΔP limitations) - Requires robust pipe fittings (full vacuum stress) - Safety concerns (pipe collapse if defective) - Freezing risk (water freezes as pressure drops, blocking evacuation) Vacuum drying limitations: Max practical length: 1-5 miles (depends on diameter) Requires thick-wall pipe (withstand external pressure) Must verify pipe design for vacuum (1 atm external - 0 internal = 14.7 psi)

Hybrid Methods

Combining methods for optimal performance and cost:

  • Air + nitrogen: Initial drying with air to -20°F, final polish with nitrogen to -40°F (reduces nitrogen cost)
  • Nitrogen + vacuum: Nitrogen drying to remove bulk water, vacuum for final drying to ultra-low dewpoint
  • Pig + air: Push water slug out with pig, then air dry to remove residual moisture
  • Methanol + air: Methanol slug displaces water, air drying evaporates methanol

Desiccant Drying (Molecular Sieve)

Desiccant Process: 1. Install portable desiccant dryer (molecular sieve or silica gel) 2. Circulate pipeline gas through dryer 3. Desiccant adsorbs water vapor 4. Monitor outlet dewpoint 5. Regenerate desiccant as it saturates (heat + purge) 6. Continue until target dewpoint stable Advantages: - Very low dewpoint (-100°F possible) - Closed-loop (no venting) - Precise control - Can operate at elevated pressure Disadvantages: - Expensive equipment rental - Limited to smaller pipelines (flow rate constraints) - Requires power for circulation and regeneration - Desiccant replacement/regeneration cost Typical application: Pipeline diameter: 4-16 inches Length: Up to 10 miles Duration: 1-3 days with automated regeneration

3. Dewpoint Specifications

Target dewpoint depends on pipeline service, operating temperature, and hydrate formation conditions. Specifications ensure water content is below critical levels for corrosion and hydrate prevention.

Dewpoint Requirements by Service

Service Type Typical Dewpoint Basis/Reason
Dry natural gas transmission -20°F to -40°F Prevent hydrates at operating pressure/temperature
Sour gas (H₂S present) -40°F to -60°F Prevent corrosion (water + H₂S = acid)
Gathering lines (wet gas) +15°F to -10°F Moderate requirements, processed downstream
High-pressure storage -40°F to -80°F Stringent to prevent hydrates at high pressure
Instrument gas -40°F to -80°F Protect sensitive instruments from moisture
Compressed air systems -40°F Standard compressed air quality (ISO 8573)
Liquid pipelines (NGL, crude) 50 ppm water max Specification (not dewpoint), prevent free water

Hydrate Formation Conditions

Gas hydrates are ice-like crystalline solids that form when water molecules encapsulate gas molecules under specific pressure-temperature conditions. They can block flow completely.

[Image: Hydrate Formation Phase Envelope]

P-T diagram showing hydrate formation region for 0.6 SG natural gas

Hydrate Formation Temperature (Natural Gas, ~0.6 SG): Katz correlation (approximate): T_hydrate (°F) = 32 + 8.9 × ln(P / 14.7) Where P = absolute pressure (psia) Examples: P = 300 psia → T_hydrate = 32 + 8.9 × ln(300/14.7) = 59°F P = 500 psia → T_hydrate = 32 + 8.9 × ln(500/14.7) = 63°F P = 1,000 psia → T_hydrate = 32 + 8.9 × ln(1,000/14.7) = 70°F P = 1,500 psia → T_hydrate = 32 + 8.9 × ln(1,500/14.7) = 73°F Interpretation: At 1,000 psia, hydrates can form if gas temperature drops below ~70°F AND free water is present. The dewpoint specification must ensure no liquid water forms at the coldest expected operating temperature. Safety margin: Specify dewpoint 20-30°F below minimum expected operating temperature

Dewpoint vs. Water Content

Dewpoint (temperature) and water content (mass) are related through vapor pressure:

[Image: GPSA Water Content Chart]

Water content (lb/MMscf) vs temperature at various pressures (GPSA Fig. 20-3 style)

Water Content Calculation: W = (e_s × MW_water × 10⁶) / (P_total × R × T) Where: W = Water content (lb/MMscf) e_s = Saturation vapor pressure of water at dewpoint (psia) MW_water = 18 lb/lbmol P_total = Total pressure (psia) R = 10.73 psia·ft³/lbmol·°R T = Temperature (°R) Antoine Equation for Saturation Vapor Pressure: log₁₀(e_s) = A - B/(C + T) Where T is in °C, e_s in mmHg (convert to psia) Constants: A = 8.07131, B = 1730.63, C = 233.426 Simplified conversion table: Dewpoint (°F) | Water Content @ 1,000 psia (lb/MMscf) +32 | 48 0 | 28 -20 | 14 -40 | 6 -60 | 2.5 -80 | 1.0

Measuring Dewpoint

Instrument Type Range Accuracy Application
Chilled mirror hygrometer -100°F to +100°F ±1°F Laboratory reference, spot checks
Capacitive sensor -40°F to +140°F ±2-3°F Continuous monitoring, portable meters
Aluminum oxide sensor -110°F to +68°F ±3-5°F Very low dewpoints, natural gas quality
Psychrometer (wet/dry bulb) +20°F to +120°F ±3-5°F High humidity, inexpensive, manual
Dew cup (manual) 0°F to +100°F ±5°F Field verification, visual

Acceptance Criteria

Define pass/fail criteria for drying completion:

Typical Acceptance Criteria: 1. Dewpoint stability: Outlet dewpoint ≤ target dewpoint for 2 hours continuous 2. Dewpoint differential: (Inlet dewpoint - Outlet dewpoint) < 5°F (near equilibrium) 3. Multiple measurements: Three consecutive dewpoint readings within spec, 1 hour apart 4. Cold temperature verification: For -40°F spec, cool sample to 0°F and verify no condensation 5. Volume throughput: Minimum 20-50 pipeline volumes of dry gas passed through Example specification: "Pipeline drying complete when outlet dewpoint measured at -42°F or lower for 4 consecutive hours, with inlet-outlet differential less than 3°F, after minimum 30 pipeline volumes throughput."
Specification principle: Target dewpoint must be below the LOWEST expected pipeline temperature with adequate safety margin (20-30°F) to prevent any condensation during operation, including upset/shutdown conditions.

4. Drying Time Calculations

Drying time depends on pipeline volume, initial water content, target dewpoint, drying gas flow rate, and method efficiency. Accurate estimates are essential for project scheduling.

Water Volume Calculation

Free Water Volume (After Hydrotest): V_water = Pipeline Volume × Water Remaining Factor Water Remaining Factor: - After air blowdown: 2-5% (water film on walls) - After pigging: 0.5-2% (residual water) - After methanol flush: 0.1-0.5% (trace water) Example: 20" diameter, 10 miles long Volume = π × (20/12)²/4 × 10 × 5,280 = 138,160 ft³ Water after pigging: 138,160 × 0.02 = 2,763 ft³ = 20,700 gallons Water mass: 20,700 gal × 8.34 lb/gal = 172,638 lb water to remove

Drying Air Required

Moisture Carrying Capacity: Δw = w_out - w_in Where: w_in = Inlet air water content (grains/ft³ or lb/Mscf) w_out = Outlet air water content at saturation (grains/ft³ or lb/Mscf) Total air volume required: V_air = W_water / Δw Where W_water = total water to remove Example Calculation: Inlet air: -20°F dewpoint = 14 lb/MMscf water content Outlet air: +70°F, saturated = 150 lb/MMscf (assumes pipe at 70°F) Δw = 150 - 14 = 136 lb/MMscf Water to remove: 172,638 lb V_air = 172,638 / 136 = 1,269 MMscf = 1.27 Bcf Drying time: If air flow = 1,000 scfm: Time = 1.27 Bcf / (1,000 scfm × 60 min/hr × 24 hr/day) Time = 1,270,000 Mscf / 1,440 Mcf/day = 882 days! [This is why air drying is slow - must use higher flow rates or drier inlet air, or combine with water removal by pigging first] Realistic scenario (with pigging): Pig removes 95% of water → Only 8,632 lb to remove by air V_air = 8,632 / 136 = 63.5 MMscf At 5,000 scfm air flow: Time = 63.5 MMscf / (5,000 × 60 × 24) = 8.8 days

Nitrogen Drying Time

Similar calculation but with much drier inlet gas:

Nitrogen Drying Example: Inlet N2: -60°F dewpoint = 2 lb/MMscf Outlet N2: +70°F, saturated = 150 lb/MMscf (if allowed to saturate) Δw = 150 - 2 = 148 lb/MMscf Water to remove: 8,632 lb (after pigging) V_N2 = 8,632 / 148 = 58.3 MMscf However, nitrogen is expensive, so optimize: - Don't let nitrogen fully saturate (continuous circulation) - Target 80% saturation → Δw_eff = 148 × 0.8 = 118 lb/MMscf - V_N2 = 8,632 / 118 = 73.2 MMscf At 2,000 scfm nitrogen flow: Time = 73.2 MMscf / (2,000 × 60 × 24) = 25.4 days Acceleration through turnovers: Multiple turnovers (20-30) with drier nitrogen reduces effective time: Time_effective = 25.4 / (efficiency factor of 3-5) = 5-8 days

Vacuum Drying Time

Vacuum Drying Duration: Much faster because water boils off at ambient temperature. t_dry = (W_water × λ) / (Pump Rate × η) Where: W_water = Water mass (lb) λ = Latent heat of vaporization (1,000 Btu/lb at reduced pressure) Pump Rate = Vacuum pump capacity (cfm, corrected for vapor) η = Efficiency (0.5-0.7 for practical systems) Example: 8,632 lb water to evaporate Pump: 1,000 cfm @ 0.1 psia Energy required: 8,632 × 1,000 = 8,632,000 Btu Pump rate in terms of water removal: ~500 lb/hr (depends on temperature, pump size, condensing capacity) Time = 8,632 / 500 = 17.3 hours Practical duration: With setup, multiple evacuations, verification: 1-2 days total

Factors Affecting Drying Time

Factor Effect on Drying Time Optimization Strategy
Ambient temperature Higher temp = faster (more evaporation) Schedule drying in warm months if possible
Inlet gas dewpoint Lower dewpoint = faster (more capacity) Use driest available gas (refrigerated/desiccant dryer)
Flow rate/velocity Higher flow = faster (mass transfer) Maximize flow within erosion limits (30 ft/s max)
Initial water content More water = longer time Remove bulk water by pigging before air/N2 drying
Pipeline material Rust/scale holds water = longer Clean/coat pipe interior before hydrotest
Elevation changes Low points trap water = much longer Install drain points, pig multiple times, tilt line if possible

[Image: Typical Drying Curve]

Outlet dewpoint vs time showing exponential decay to target specification

Rule-of-Thumb Estimates

Quick estimates for project planning:

Air Drying (after pigging): Duration (days) = Pipeline Length (miles) / 2 Example: 20 mile line → 10 days air drying Range: 5-15 days depending on diameter, water removal efficiency Nitrogen Drying: Duration (days) = Pipeline Length (miles) / 3 Example: 20 mile line → 7 days nitrogen drying Range: 3-10 days depending on target dewpoint Vacuum Drying: Duration (hours) = Pipeline Volume (Mft³) × 5 Example: 138 Mft³ → 690 hours = 29 hours Practical limit: 2-3 days for setup + drying + verification Add contingency: Multiply estimated time by 1.5-2.0 for schedule planning (Weather delays, equipment failures, re-testing)

5. Operational Procedures

Pre-Drying Steps

  1. Water removal: After hydrotest, drain water from low points and blow down line with compressed air
  2. Pigging: Run 2-3 foam pigs to push out bulk water (reduces water by 95-98%)
  3. Inspection: Verify water removal through drain valves, no standing water in line
  4. Equipment setup: Install air dryer, compressors, nitrogen supply, and dewpoint monitoring
  5. Baseline measurement: Measure initial outlet dewpoint (will be very high, near saturation)

Air Drying Procedure

Step-by-Step Air Drying: 1. Setup: - Connect air compressor to inlet - Install refrigerated or desiccant dryer on compressor discharge - Connect dewpoint meter to outlet (with sample cooler) - Open all vents and drains during initial blowdown 2. Initial Blowdown (Day 1): - Flow air at maximum safe velocity (20-30 ft/s) - Monitor outlet - will be very wet initially - Continue until outlet humidity drops significantly (12-24 hours) 3. Steady-State Drying (Days 2-N): - Maintain continuous air flow - Monitor inlet and outlet dewpoint every 2-4 hours - Plot dewpoint vs time (should decrease exponentially) - Adjust flow rate if outlet dewpoint plateaus 4. Verification Phase: - When outlet dewpoint approaches target, increase monitoring frequency - Require 2-4 hours of stable readings at or below target - Verify inlet-outlet differential < 5°F (equilibrium) 5. Final Acceptance: - Collect outlet sample in chilled dew cup (0°F) - Verify no condensation = dewpoint < 0°F - Document flow volumes, time, dewpoint readings - Seal pipeline with dry nitrogen or process gas Safety considerations: - Vent air discharge away from personnel (high velocity) - Monitor oxygen levels if working in confined spaces - Verify pressure rating of line (don't exceed MAOP during drying)

Nitrogen Drying Procedure

Nitrogen Drying Steps: 1. Initial Purge: - Displace air with nitrogen (3-5 line volumes) - Verify oxygen < 1% at outlet (use oxygen analyzer) 2. Continuous Flow Drying: - Establish nitrogen flow at 5-15 ft/s - Monitor outlet dewpoint continuously - Calculate turnover rate (flow / line volume) - Target 20-50 turnovers for -40°F to -60°F dewpoint 3. Optimization: - If dewpoint stagnates, increase flow rate (better mass transfer) - If nitrogen cost is concern, pause periodically for settling, then resume - Monitor nitrogen consumption vs budget 4. Acceptance: - Outlet dewpoint stable at target for 4 hours - Differential < 3°F - Seal line under positive nitrogen pressure (50-100 psig) - Leave nitrogen blanket until commissioning Nitrogen supply options: - Liquid nitrogen tankers with vaporizers (most common) - Membrane nitrogen generators (for longer campaigns) - PSA nitrogen generators (if electricity available)

Vacuum Drying Procedure

Vacuum Drying Process: 1. Pre-Check: - Verify pipe design for external pressure (14.7 psi) - Inspect all flanges, closures for vacuum-tight seal - Install vacuum pumps, condensers, instrumentation 2. Initial Evacuation: - Start vacuum pumps, reduce pressure to 1-5 psia - Water begins evaporating (may see temperature drop) - Monitor pressure - should continue dropping 3. Deep Vacuum: - Reduce to 0.1-0.5 psia - Hold at this pressure for 4-24 hours - Water evaporation rate slows as remaining water decreases 4. Verification: - Isolate pipeline (close vacuum valves) - Monitor pressure rise over 1 hour - If pressure stable (no rise) → drying complete - If pressure rises > 0.1 psi → continue evacuating (water still present) 5. Nitrogen Break: - Open nitrogen valve slowly (don't shock pipe) - Pressurize to 50-100 psig with dry nitrogen - Measure dewpoint (should be -60°F or lower) Safety critical: - External pressure can collapse thin-wall pipe - All personnel clear during evacuation - Emergency venting procedure if pipe shows distress

Common Problems and Solutions

Problem Symptoms Likely Cause Solution
Dewpoint plateau Outlet dewpoint stops improving Water trapped in low points, equilibrium reached Increase flow rate, run additional pig, tilt pipeline
High outlet dewpoint Never reaches target Inlet air too wet, leak in line, large water volume Improve inlet air quality, leak test line, more pigging
Erratic readings Dewpoint fluctuates wildly Liquid water slugs, instrument location Install sample cooler, relocate probe downstream
Slow progress Taking much longer than estimated Underestimated water volume, low flow rate Add compressors, switch to nitrogen, extended duration
Failed acceptance test Condensation in cold dew cup Incomplete drying, instrument error, sample contamination Continue drying, calibrate instruments, re-test

Documentation Requirements

Maintain records for regulatory compliance and future reference:

  • Drying log: Date/time, inlet dewpoint, outlet dewpoint, flow rate, volumes passed
  • Equipment records: Compressor/nitrogen usage, dryer maintenance, instrument calibration
  • Acceptance test results: Final dewpoint readings, witness signatures, photos/video
  • Deviation reports: Any excursions, delays, or non-conformances during drying
  • As-built conditions: Pipeline condition (rust, scale, cleanliness) before drying
  • Gas chromatograph analysis: If required, verify oxygen, CO2, H2S removed (N2 purge)

Post-Drying Protection

Preserve drying results until commissioning:

Nitrogen Blanket: After drying complete: 1. Pressurize line to 50-200 psig with dry nitrogen 2. Monitor pressure weekly - pressure drop indicates leak 3. Top up nitrogen as needed to maintain positive pressure 4. Before commissioning, verify dewpoint still acceptable (sample) Duration: Can maintain nitrogen blanket for months if needed Cost: ~$100-500/month for typical pipeline (nitrogen makeup) Alternative: For long delays (>6 months), consider mothballing: - Fill with nitrogen to 75% MAOP - Install corrosion inhibitor - Periodic inspection/pressure checks
Best practice summary: Remove bulk water by pigging (95%+), use appropriate drying method for target dewpoint, continuously monitor outlet dewpoint, verify acceptance with multiple measurements, maintain nitrogen blanket until commissioning. Plan for 2× estimated duration to account for contingencies.

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