1. Overview
Drip pots (drips, condensate traps) collect liquids that condense from gas streams in pipelines. Periodic blowdown removes accumulated liquids to prevent operational problems.
Why Drips Are Needed
Liquid slugging
Equipment Protection
Prevents compressor damage and meter errors from liquid carry-through.
Hydrate prevention
Flow Assurance
Reduces hydrate formation risk at low temperatures.
Corrosion control
Integrity
Eliminates water pooling that causes internal corrosion.
Common Drip Locations
| Location | Purpose | Typical Size |
|---|---|---|
| Pipeline low points | Collect condensate from grade changes | 10-30 bbl |
| Compressor suction | Protect compressor from liquid slugs | 20-50 bbl |
| Meter station inlet | Ensure dry gas for accurate metering | 10-20 bbl |
| Regulator upstream | Prevent liquid carryover through valve | 10-30 bbl |
Key principle: Drip pots must be sized for the maximum expected liquid accumulation between blowdowns, plus a safety margin for upset conditions.
2. Drip Pot Sizing
Drip sizing is based on condensate rate, desired blowdown frequency, and a safety factor for upsets.
Sizing Formula
Required Drip Volume:
V_drip = Q_condensate × t_blowdown × SF
Where:
V_drip = Drip pot volume (bbl)
Q_condensate = Condensate accumulation rate (bbl/day)
t_blowdown = Days between blowdowns
SF = Safety factor (1.5-2.0 typical)
Example:
Condensate rate: 2 bbl/day
Blowdown frequency: Weekly (7 days)
Safety factor: 1.5
V_drip = 2 × 7 × 1.5 = 21 bbl → Use 24" × 30 ft (≈25 bbl)
Vertical Drip Dimensions
| Pipe Diameter | Volume per ft | 10 bbl Length | 20 bbl Length |
|---|---|---|---|
| 12" | 0.14 bbl/ft | 71 ft | 143 ft |
| 16" | 0.25 bbl/ft | 40 ft | 80 ft |
| 20" | 0.39 bbl/ft | 26 ft | 52 ft |
| 24" | 0.56 bbl/ft | 18 ft | 36 ft |
Condensate Sources
- Retrograde condensation: Pressure drop through regulators causes temperature drop (JT cooling) and liquid dropout
- Water condensation: Cooling below water dewpoint as gas travels through buried pipeline
- Carryover: Liquid entrainment from upstream separators during slug events
Design tip: Oversize drips by 50-100%. They are inexpensive and provide flexibility for extended blowdown intervals during bad weather or higher-than-expected condensate rates.
3. Gas Loss Calculation
Gas is vented during blowdown operations. Accurate loss estimation is required for emissions reporting (EPA Subpart W) and production accounting.
Engineering Method
Gas Volume from Real Gas Law:
Q_scf = (P × V) / (Z × T) × (T_std / P_std)
Where:
P = Operating pressure (psia)
V = Drip pot gas volume (ft³)
Z = Compressibility factor (from Hall-Yarborough or similar)
T = Operating temperature (°R)
T_std = 519.67°R (60°F standard)
P_std = 14.696 psia
Example:
Operating: 500 psig (514.7 psia), 60°F (519.67°R)
Drip volume: 10 bbl = 56.15 ft³
Gas SG: 0.65, Z ≈ 0.88
Q_scf = (514.7 × 56.15) / (0.88 × 519.67) × (519.67 / 14.696)
Q_scf = 63.22 × 35.36 = 2,236 scf = 2.24 MCF per blowdown
Field Estimation Method
When detailed operating data isn't available, use emission factors based on blow counts:
| Component | Base Factor | Basis |
|---|---|---|
| Wet blow | 1.0 MCF/blow | Gas displaced by liquid during initial blows (500 psig reference) |
| Dry blow | 5.0 MCF/blow | Gas vented after liquid cleared (higher volume per blow) |
| Dissolved gas | 1.05 scf/gal | Gas released from condensate at 100 psig (scales with P^0.5) |
Pressure Adjustment
Pressure Scaling for Field Factors:
Emission Factor = Base Factor × (P_actual + 14.7) / (P_reference + 14.7)
Example at 800 psig:
Wet blow factor = 1.0 × (814.7 / 514.7) = 1.58 MCF/blow
Dry blow factor = 5.0 × (814.7 / 514.7) = 7.92 MCF/blow
Higher pressure = more gas per blowdown = higher emissions
Emissions Conversion
Methane Emissions:
CH₄ (lbs) = Gas Loss (MCF) × 1000 scf/MCF × 0.85 (methane fraction) × 0.0424 lb/scf
CO₂ Equivalent:
CO₂e (metric tons) = CH₄ (lbs) × 28 (GWP) / 2204.62
Example:
Total gas loss: 10 MCF
CH₄ = 10 × 1000 × 0.85 × 0.0424 = 360 lbs
CO₂e = 360 × 28 / 2204.62 = 4.57 mt CO₂e
Reporting note: Per EPA 40 CFR 98 Subpart W, blowdown emissions from pipeline drips must be reported if facility exceeds 25,000 mt CO₂e/year threshold.
4. Blowdown Procedures
Safe blowdown procedures protect personnel and minimize environmental impact.
Pre-Blowdown Checklist
- Area clear: No personnel within 300 ft of discharge point
- Wind check: Discharge direction favorable (downwind from operator)
- Ignition sources: No vehicles, equipment, or smoking nearby
- Valve inspection: Exercise blowdown valve to verify operation
- PPE: FRC, safety glasses, chemical-resistant gloves
Blowdown Steps
Manual Blowdown Procedure:
1. Position 50+ ft from discharge, upwind
2. Slowly crack valve open (1/4 turn)
- Initial discharge is high-pressure gas
- Listen for pressure equalization
3. Gradually open as pressure decreases
- Watch for liquid discharge (sound/appearance changes)
4. Full open when liquid appears
- Continue until steady gas flow (no slugs)
- Duration: typically 2-5 minutes
5. Close slowly when clear gas observed
- Avoid water hammer from rapid closure
6. Record: date, time, estimated volume, observations
Blowdown Frequency Guidelines
| Condensate Rate | Frequency |
|---|---|
| < 0.5 bbl/day | Monthly |
| 0.5-2 bbl/day | Weekly |
| 2-5 bbl/day | Every 2-3 days |
| > 5 bbl/day | Daily or automatic |
Minimize dry blows: Dry blows contribute 5× more gas loss than wet blows. Blow until liquid stops, then close promptly. Excessive "clearing" wastes gas.
5. Safety & Compliance
Primary Hazards
| Hazard | Control |
|---|---|
| High-pressure gas release | Stand clear, use extended handles, hearing protection |
| Flammable atmosphere | Clear ignition sources, verify wind direction |
| H₂S (sour gas) | Personal monitors, buddy system, SCBA available, upwind position |
| Liquid hydrocarbon contact | FRC, chemical gloves, eye protection |
| JT freezing | Insulated gloves, slow valve opening |
H₂S Considerations
For sour gas service (H₂S > 100 ppm in gas):
- Two-person minimum (buddy system)
- Personal H₂S monitors with audible alarms
- SCBA equipment available on-site
- Wind indicator visible (sock or ribbon)
- Increase exclusion zone to 500+ ft
- Consider closed-loop blowdown to recovery tank
Environmental Compliance
EPA Subpart W Requirements:
Facilities must report blowdown emissions if:
- Total facility emissions exceed 25,000 mt CO₂e/year
- Blowdowns counted toward Source Category W.5 (pipeline venting)
Recommended practices:
- Document all blowdown events (date, time, volume, duration)
- Track annual totals by drip location
- Consider vapor recovery for high-volume operations
- Implement automatic drips to minimize per-event losses
Complacency kills: Drip blowdown is routine but hazardous. Three leading causes of incidents: (1) failure to check wind direction → H₂S exposure, (2) ignition source not cleared → fire, (3) valve failure from poor maintenance → uncontrolled release.
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