What methods are used for pipeline drying?

Five primary methods: air drying (lowest cost, 5-20 days), nitrogen drying (very low dewpoint, 2-7 days), vacuum drying (fastest, 1-3 days, smaller pipes), methanol slug treatment (gathering lines/hydrate prevention), and desiccant/molecular-sieve drying (precise control). Most projects pre-pig to remove 95-99 percent of bulk water before any drying method.

What dewpoint is required for natural gas pipelines?

Typical specifications: -20F for gathering lines (under 500 psia), -20F to -40F for transmission (500-1000 psia), -40F for high-pressure service, and -60F to -80F for arctic and sour gas service. Dewpoint should be specified 20-30F below the lowest expected operating temperature, and at the operating pressure (since dewpoint rises with compression).

What standards govern pipeline drying?

ASME B31.8 Section 841.32 (gas transmission/distribution), ASME B31.4 (liquid pipelines), API RP 1171 (commissioning practices), and GPA 2145 (water content data). Per ASME B31.8, pipelines must be cleaned and dried before being placed in service, with dewpoint verification before introducing product gas.

How is drying time calculated?

Drying time depends on water mass to remove, drying gas moisture-carrying capacity (delta-w), and gas flow rate. Time = water mass / (delta-w * flow). Pre-pigging (removes 95% of water) reduces drying time dramatically. Rule of thumb: air drying ~ length(mi)/2 days, nitrogen drying ~ length(mi)/3 days, vacuum drying 1-3 days regardless of length. Add 50-100 percent contingency.

What is dewatering pigging?

Mechanical removal of bulk water from a pipeline by running pigs (medium-density foam, high-density foam, or bi-directional squeegee). Pigging removes 90-99 percent of liquid water before the drying step. A typical sequence runs 2-3 pigs (foam then squeegee) to minimize residual water that the drying method must evaporate.

How is dewpoint measured in the field?

Common instruments: chilled mirror hygrometer (lab reference, plus or minus 1F), capacitive sensor (portable field, plus or minus 2-3F), aluminum oxide sensor (online monitoring at very low dewpoints, plus or minus 3-5F). Sample at three points: inlet (verify dry supply), outlet (primary, slowest to dry), and low points (water accumulates). Acceptance requires outlet dewpoint at or below target for 24 hours continuous.

Pipeline Drying Fundamentals — Methods & Calculations

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.

Pipeline drying cross-section showing dewatering pig pushing bulk water, residual water film on pipe walls, dry gas flow direction, and the three stages: before drying, drying zone with evaporation, and dried section — Pipeline drying process: Pigging removes 95-99% of bulk water, drying removes remaining film and trapped pockets.

Key Numbers Example

Example: A 10-mile, 12" pipeline holds ~7,400 bbl of water after hydrostatic testing. Pigging removes 95-99%, leaving ~75-370 bbl as film water and trapped pockets that must be removed by drying.

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/4 × (20/12)² × 10 × 5,280 = 115,193 ft³ Turnovers = 30 (moderate drying) N2 required = 115,193 × 30 × 1.05 = 3,628,580 scf = 3.63 MMscf At $5/Mscf: Cost = $18,143 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

Methanol Slug Treatment

Principle: Methanol absorbs water (0.7 lb H₂O per lb MeOH) Process: 1. Calculate water to remove from GPSA charts 2. Size methanol slug (typically 20-40% of pipe volume) 3. Push slug through with dry gas or nitrogen 4. Collect methanol/water mixture at downstream end Best for: Gathering lines, hydrate prevention, smaller-diameter lines Caution: Toxic/flammable - full PPE required

Dewatering Pigs (Pre-Drying Mechanical Removal)

Run before any drying method to remove 90-99% of bulk water:

Pig Type	Removal	Application
Medium-density foam	70-90%	Initial pass
High-density foam	85-95%	Second pass
Bi-directional + squeegee	95-99%	Final dewatering

Best practice: Run 2-3 pigs in sequence before drying to minimize residual water. Dewatering pigging is the most cost-effective first step regardless of which drying method follows.

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

Industry Standard Dewpoints (Quick Reference)

Service	Dewpoint Spec
Gathering (<500 psia)	-20°F @ operating P
Transmission (500-1000 psia)	-20°F to -40°F @ 1000 psia
High-pressure (>1000 psia)	-40°F @ operating P
Arctic service	-60°F to -80°F
Sales gas (contract)	7 lb/MMscf @ 14.7 psia

Pressure Effect on Dewpoint

Important: Dewpoint rises with pressure compression. Gas at -20°F dewpoint @ 500 psia → Compressed to 1,000 psia → New dewpoint ≈ -10°F (rises ~10°F per 2x pressure) Best practice: Specify dewpoint at MAOP "Dewpoint ≤ -20°F at 1,200 psig"

Dewpoint vs. water content: Dewpoint is intuitive for operations (relates directly to hydrate risk). Water content (lb/MMscf) is used in contracts. Always specify pressure basis.

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/4 × (20/12)² × 10 × 5,280 = 115,193 ft³ Water after pigging: 115,193 × 0.02 = 2,304 ft³ = 17,233 gallons Water mass: 17,233 gal × 8.34 lb/gal = 143,731 lb water to remove

Residual Water Estimation (Film Method)

After pigging (95-99% removed): Film method: M = π × D × L × t × ρ Where t = 0.001-0.005" film thickness Example - 10 mi × 12" pipe: M = π × 1.0 ft × 52,800 ft × 0.000167 ft × 62.4 lb/ft³ M = 1,730 lb ≈ 200 gallons residual water (after pigging)

Water Content of Natural Gas (GPSA)

Water content of natural gas vs temperature chart from GPSA showing logarithmic plot with curves for 100, 250, 500, 1000, and 2000 psia pressures, with sales gas spec line at 7 lb/MMscf and arctic/standard transmission spec reference lines — Water content of natural gas vs temperature at various pressures. Source: GPSA.

Temp (°F)	100 psia	500 psia	1000 psia
-40	2.0	0.6	0.35
-20	5.5	1.4	0.8
40	34	7.8	4.3
80	111	26	14

Water content in lb/MMscf. Source: GPSA.

Methanol Slug Requirements

M_MeOH = (W_vapor + W_residual) / (0.7 × P) Where: W_vapor = Vapor-phase water = (W_initial - W_target) × V W_residual = Residual free water after pigging (typically 1-2% of pipe volume) 0.7 = Methanol absorption capacity (lb H₂O/lb MeOH) P = Methanol purity (fraction, e.g. 1.0 for 100%) Example: 10 mi × 12" at 500 psia, 80°F → -20°F dewpoint W_initial = 26 lb/MMscf, W_target = 1.4 lb/MMscf V = 0.041 MMscf Vapor water = (26 - 1.4) × 0.041 = 1.0 lb Residual water (1.5% of 310,000 gal × 8.34 lb/gal) ≈ 38,800 lb Total water = 1.0 + 38,800 ≈ 38,800 lb Methanol = 38,800 / 0.7 = 55,400 lb = 8,360 gal Note: Residual free water dominates methanol requirements. Vapor-phase water is negligible in comparison for post-hydrotest drying.

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: 143,731 lb V_air = 143,731 / 136 = 1,057 MMscf = 1.06 Bcf Drying time: If air flow = 1,000 scfm: Time = 1.06 Bcf / (1,000 scfm × 60 min/hr × 24 hr/day) Time = 1,057,000 Mscf / 1,440 Mcf/day = 734 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 7,187 lb to remove by air V_air = 7,187 / 136 = 52.8 MMscf At 5,000 scfm air flow: Time = 52.8 MMscf / (5,000 × 60 × 24) = 7.3 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: 7,187 lb (after pigging) V_N2 = 7,187 / 148 = 48.6 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 = 7,187 / 118 = 60.9 MMscf At 2,000 scfm nitrogen flow: Time = 60.9 MMscf / (2,000 × 60 × 24) = 21.1 days Acceleration through turnovers: Multiple turnovers (20-30) with drier nitrogen reduces effective time: Time_effective = 21.1 / (efficiency factor of 3-5) = 4-7 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: 7,187 lb water to evaporate Pump: 1,000 cfm @ 0.1 psia Energy required: 7,187 × 1,000 = 7,187,000 Btu Pump rate in terms of water removal: ~500 lb/hr (depends on temperature, pump size, condensing capacity) Time = 7,187 / 500 = 14.4 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

Water removal: After hydrotest, drain water from low points and blow down line with compressed air
Pigging: Run 2-3 foam pigs to push out bulk water (reduces water by 95-98%)
Inspection: Verify water removal through drain valves, no standing water in line
Equipment setup: Install air dryer, compressors, nitrogen supply, and dewpoint monitoring
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

Dewpoint Measurement (Field Instruments)

Method	Accuracy	Application
Chilled mirror	±0.5-2°F	Lab standard, most accurate
Capacitance sensor	±3-5°F	Portable field use
Aluminum oxide	±2-5°F	Online monitors

Monitoring Procedure (Sample Points & Acceptance)

Typical pipeline air drying curve for 10-mile 12-inch pipeline showing outlet dewpoint temperature declining from +75 deg F to -40 deg F over 120 hours, with four phases: bulk water removal, film evaporation, deep drying, and verification hold — Typical pipeline air drying curve showing the four drying phases and acceptance criteria.

Sample points: 1. Inlet - verify dry gas supply 2. Outlet - primary measurement (slowest to dry) 3. Low points - water accumulates here Acceptance criteria (24-hour stability): • Outlet dewpoint ≤ target for 24 hrs continuous • Multiple sample points within ±5°F • Dewpoint stable (not rising) Sample system: 1/4" SS tubing, fast-loop flow, coalescing filter, heat trace in cold weather

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.

Pipeline Drying Methods: Commissioning Engineering Guide

-40°F typical

1-7 days typical

10-30 ft/s

Contents

Use this guide when you need to: