Tank Blanketing / Padding Gas Fundamentals

1. Tank Blanketing Overview

Tank blanketing (also called padding or inerting) is the practice of supplying an inert gas, typically nitrogen, to the vapor space of a storage tank to maintain a positive pressure above the liquid surface. This prevents air ingress, reduces evaporative losses, protects product quality, and eliminates the formation of flammable vapor-air mixtures in the tank headspace.

Safety

Fire & explosion prevention

Displaces oxygen below the limiting oxygen concentration (LOC), eliminating flammable atmospheres in tank vapor space.

Product quality

Oxidation prevention

Prevents oxygen contact with oxidation-sensitive products such as condensate, solvents, and amine solutions.

Emissions

Reduced VOC losses

Conservation vents with blanketing reduce standing and working losses compared to open atmospheric vents.

When Is Blanketing Required?

Tank blanketing is generally required or recommended for the following situations:

Flammable liquids stored above flash point: Any tank storing liquids at or above their flash point requires inerting to prevent ignitable vapor space conditions. Common examples include condensate, NGL drip, and produced water with dissolved hydrocarbons.
Oxidation-sensitive products: Lean amine (MDEA, DEA), glycol solutions, and specialty chemicals degrade when exposed to oxygen.
Moisture-sensitive materials: Molecular sieve, silica gel, and hygroscopic chemicals require dry nitrogen blanketing.
Regulatory requirements: EPA NSPS Subpart Kb, state air quality regulations, and facility risk assessments may mandate blanketing.
Insurance requirements: FM Global and similar insurers often require blanketing for tanks storing Class I flammable liquids.

Blanket Gas Options

Gas	Purity	Source	Typical Use
Nitrogen (N₂)	95–99.9%	Membrane, PSA, bulk liquid, pipeline	Most common; suitable for all applications
Natural gas	Varies	Plant fuel gas header	Hydrocarbon tanks where N₂ is unavailable; not for inerting
Carbon dioxide (CO₂)	> 99%	Bulk liquid supply	Food-grade tanks, some chemical applications
Combustion gas	85–88% N₂	Inert gas generator	Marine tanks, large storage fields

Design rule: Nitrogen is the preferred blanket gas for midstream applications. Natural gas should only be used as a blanket gas when nitrogen is unavailable, and only for tanks where the vapor space is already hydrocarbon-rich. Natural gas blanketing does not provide oxygen exclusion and does not qualify as inerting.

System Components

A typical tank blanketing system consists of the following components:

Pressure regulator: A pilot-operated or self-operated regulator that admits blanket gas when tank pressure drops below the set point (typically 0.5–2.0 oz/in² gauge).
Conservation vent (pressure/vacuum valve): A combination pressure-vacuum relief device that limits tank pressure and vacuum within the design range.
Emergency vent: A large-capacity vent for fire exposure or equipment failure scenarios, sized per API 2000 Section 4.
Supply piping: Piping from the nitrogen source to the regulator, sized for the maximum demand flow rate.
Flame arrestor: Installed in the vent line to prevent flame propagation into the tank vapor space (required for non-inerted flammable service).
Instrumentation: Pressure gauge, flow meter (optional), oxygen analyzer (for critical inerting applications).

2. Blanket Gas Demand

The blanket gas supply system must provide sufficient flow to replace vapor volume lost during normal tank operations. The total demand is the sum of several independent flow components, each calculated per API 2000.

Demand Components

Total Blanket Gas Demand: Q_total = Q_pump-out + Q_thermal + Q_flash + Q_leakage Where: Q_pump-out = Liquid withdrawal (out-breathing) demand Q_thermal = Thermal breathing (cooling contraction) demand Q_flash = Flash gas from hot liquid filling Q_leakage = Blanket gas lost through fittings, seals, and vent leakage

Liquid Withdrawal (Pump-Out) Demand

When liquid is pumped out of a tank, the vapor space volume increases. Blanket gas must flow in to maintain positive pressure. This is typically the largest demand component.

Pump-Out In-Breathing: Q_pump-out = V_liquid (liquid pump-out rate, ft³/hr) For a cylindrical tank: Q_pump-out = (π/4) × D² × dh/dt Where: D = Tank diameter (ft) dh/dt = Rate of liquid level drop (ft/hr) Example: 500 bbl/hr pump-out = 2,800 ft³/hr = 46.7 ft³/min (SCFM)

Thermal Breathing (In-Breathing)

As ambient temperature drops (typically overnight cooling), the vapor space gas contracts. This creates a vacuum unless blanket gas is supplied. API 2000 provides two methods for calculating thermal in-breathing:

API 2000 Thermal In-Breathing (Simplified): For tanks ≤ 180 ft diameter: Q_thermal = C_i × V_tank^0.7 Where: C_i = In-breathing coefficient from API 2000 Table 4 V_tank = Total tank volume (bbl) Typical thermal in-breathing: Small tanks (< 1,000 bbl): 5–20 SCFH Medium tanks (1,000–10,000 bbl): 20–100 SCFH Large tanks (> 10,000 bbl): 100–500 SCFH

Flash Gas from Hot Liquid Fill

When liquid enters a tank at a temperature higher than the tank liquid temperature, flash gas is generated. This contributes to out-breathing demand and may also affect blanket gas purity:

Flash Gas Rate: Q_flash = F × Q_fill Where: F = Flash ratio (volume of gas per volume of liquid; depends on temperature differential and fluid properties) Q_fill = Liquid fill rate Typical flash ratios: Stabilized condensate at 120°F fill: F ≈ 0.5–2.0 SCF/bbl Hot produced water at 180°F: F ≈ 0.1–0.5 SCF/bbl

Blanket Gas Leakage

All tank systems leak small amounts of blanket gas through roof fittings, gauge hatches, sample valves, and seal gaps. A leakage allowance is typically added to the blanket gas demand:

Tank Condition	Leakage Allowance
New tank, good gaskets and seals	5–10% of total demand
Average condition, maintained fittings	10–20% of total demand
Older tank, worn gaskets, loose fittings	20–50% of total demand
Floating roof (rim seal leakage)	Highly variable; measure field data

Regulator Sizing

The blanket gas regulator must be sized to deliver the maximum instantaneous demand. The regulator C_v is calculated from the flow rate and the available pressure drop:

Regulator C_v (Gas Service): C_v = Q / (22.67 × √(ΔP × (P₁ + P₂) / (T × SG))) Where: Q = Flow rate (SCFH) ΔP = Pressure drop across regulator (psi) P₁ = Inlet pressure (psia) P₂ = Outlet pressure (psia) T = Gas temperature (°R) SG = Gas specific gravity (N₂ = 0.967) Typical regulator set point: 0.5–2.0 oz/in² (0.03–0.125 psig)

Regulator selection: Use a pilot-operated regulator for precise set point control at very low pressures (oz/in² range). Self-operated regulators with diaphragm sensing are adequate for less critical applications. The regulator must have a lockup pressure below the conservation vent set pressure to avoid continuous blanket gas loss through the vent.

3. Breathing Losses

Breathing losses are the evaporative emissions from atmospheric storage tanks caused by temperature changes and liquid level changes. API 2000 provides the methodology for calculating both in-breathing (vacuum) and out-breathing (pressure) demands.

Standing Losses (Thermal Breathing)

Standing losses occur when a tank is idle (no filling or emptying). Diurnal temperature cycles cause the vapor space to expand (out-breathing during daytime heating) and contract (in-breathing during nighttime cooling).

Factor	Effect on Standing Losses
Tank diameter	Larger diameter = proportionally larger vapor space = greater losses
Vapor pressure	Higher VP fluids = more vapor in headspace = greater losses
Solar exposure	Dark-colored tanks or hot climates = greater diurnal temperature swing
Tank color	White/aluminum paint reduces solar heating by 30–50% vs. dark tanks
Insulation	Insulated tanks have greatly reduced thermal breathing
Liquid level	Lower liquid level = more vapor space volume = greater losses

Working Losses (Product Movement)

Working losses occur during liquid filling and emptying operations:

Filling losses (out-breathing): As liquid fills the tank, vapor space is compressed and displaced through the vent. The displaced vapor is saturated with product vapor, causing emissions.
Emptying losses (in-breathing): As liquid is withdrawn, air (or blanket gas) enters to replace the liquid volume. On a subsequent fill cycle, this air-vapor mixture is displaced.

Working Loss (Filling): L_w = n × V_turnover × K_n × K_p × W_v Where: n = Number of turnovers per year V_turnover = Volume displaced per turnover (bbl) K_n = Turnover factor (from API 19.1) K_p = Working loss product factor W_v = Vapor density of stock (lb/ft³)

Emissions Reduction with Blanketing

Nitrogen blanketing combined with conservation vents significantly reduces breathing losses compared to an open vent:

Configuration	Emissions Reduction	Notes
Open vent (no blanketing)	Baseline	Maximum standing and working losses
Conservation vent only (no blanket)	30–60%	Reduces losses by maintaining slight positive pressure
N₂ blanket + conservation vent	85–97%	Virtually eliminates air exchange; standing losses near zero
N₂ blanket + vapor recovery	> 99%	Captures displaced vapors for flare or recovery

Regulatory note: For NSPS Subpart Kb regulated tanks, blanketing with conservation vents may be required as BACT (Best Available Control Technology) or RACT (Reasonably Available Control Technology). Emissions calculations must follow EPA AP-42, Chapter 7.1 methodology using tank-specific parameters.

4. Conservation Vents

Conservation vents (also called pressure/vacuum valves or PV valves) are the primary pressure and vacuum relief devices on blanketed atmospheric storage tanks. They maintain tank pressure within the design range while minimizing blanket gas consumption and emissions.

Set Pressure Selection

Proper set pressure selection requires coordination between the blanket gas regulator and the conservation vent:

Pressure Hierarchy (from low to high): 1. Vacuum set point (in-breathing): -0.5 to -1.0 oz/in² typical 2. Blanket gas regulator set point: +0.5 to +1.0 oz/in² typical 3. Regulator lockup pressure: +1.0 to +1.5 oz/in² 4. Conservation vent pressure set point: +2.0 to +3.5 oz/in² 5. Emergency vent set point: +3.5 to +6.0 oz/in² 6. Tank design pressure: typically +6.0 to +16.0 oz/in² Critical requirement: Regulator lockup < Conservation vent opening Minimum differential: 0.5 oz/in² between lockup and vent opening

API 2000 Vent Sizing

Conservation vents must be sized per API 2000 for the worst-case combination of thermal breathing and product movement:

Sizing Scenario	In-Breathing (Vacuum)	Out-Breathing (Pressure)
Maximum pump-out rate	Yes (primary scenario)	No
Maximum pump-in rate	No	Yes (primary scenario)
Thermal breathing	Yes (additive)	Yes (additive)
Flash gas	No	Yes (if applicable)
Fire exposure	No	Emergency vent only

Conservation Vent Types

Weight-loaded (pallet type): Simple, reliable, low-maintenance design. Pallet weight sets the opening pressure. Most common for midstream atmospheric tanks. Available in sizes from 2 to 24 inches.
Pilot-operated: Uses a small pilot valve to sense tank pressure and actuate the main valve. Provides tighter set pressure control and higher capacity. Used for large tanks or where precise pressure control is critical.
Spring-loaded: Similar to conventional safety valves but designed for very low pressures (oz/in² range). Less common for tank service.
Pipe-away vent: Vents to a closed system (flare header, vapor recovery unit) rather than atmosphere. Requires careful backpressure analysis.

Emergency Vent Sizing

Emergency vents handle abnormal conditions that exceed the capacity of the normal conservation vent. Per API 2000 Section 4, the most common sizing scenario is external fire exposure:

API 2000 Fire Case Venting (Wetted Area Method): Q_fire = 1,107 × A_ws^0.82 Where: Q_fire = Required venting capacity (SCFH of air) A_ws = Wetted surface area exposed to fire (ft²) (shell area up to 30 ft above grade) For blanketed tanks: The emergency vent must handle fire case flow without exceeding the maximum allowable working pressure (MAWP) of the tank.

Flame Arrestors

Flame arrestors prevent flame propagation through the vent into the tank vapor space. They are critical safety devices for tanks storing flammable liquids:

Arrestor Type	Protection Against	Application
End-of-line (detonation)	Atmospheric deflagration, detonation	Open vent pipe termination
In-line deflagration	Deflagration only	Vent piping, moderate length
In-line detonation	Both deflagration and detonation	Long vent piping runs, pipe-away vents
Integrated (vent + arrestor)	Atmospheric deflagration	Conservation vent with built-in arrestor

Maintenance warning: Flame arrestors must be inspected and cleaned regularly. Fouling from dirt, insects, ice, or polymerized product vapor can restrict flow and cause tank implosion under vacuum conditions. API 2210 provides inspection guidance for flame arrestors.

5. Oxygen Exclusion

The primary purpose of tank blanketing in flammable liquid service is to maintain the oxygen concentration in the vapor space below the limiting oxygen concentration (LOC), ensuring the vapor-air-fuel mixture remains outside the flammable envelope.

Limiting Oxygen Concentration (LOC)

The LOC is the minimum oxygen concentration that supports combustion. Below the LOC, the mixture cannot propagate a flame regardless of fuel concentration:

Fuel/Product	LOC with N₂ (vol%)	Recommended Max O₂
Methane	12.1	8–10%
Ethane	11.0	8–9%
Propane	11.5	8–9%
n-Butane	12.1	8–10%
n-Pentane	12.1	8–10%
n-Hexane	11.9	8–10%
Condensate (mixed)	~11–12	6–8%
Methanol	10.0	6–8%
Hydrogen	5.0	2–4%

Safety margin: The maximum allowable oxygen concentration should include a safety margin below the LOC. NFPA 69 recommends operating at no more than 60% of the LOC for systems without continuous oxygen monitoring, or 80% of the LOC with continuous monitoring and automatic shutdown.

Initial Purging (Inerting)

Before placing a tank in blanketed service, the vapor space must be purged to reduce oxygen below the target level. Three purging methods are used:

Method	N₂ Required	Time	Best For
Dilution (sweep) purge	3–5 × vapor space volume	Moderate	Small to medium tanks with good mixing
Displacement (piston) purge	1.0–1.5 × vapor space volume	Longer	Tall tanks with bottom entry and top vent
Pressure-vacuum purge	Least (calculated per cycles)	Longest	Pressure vessels; not practical for atmospheric tanks

Dilution Purge Calculation

Dilution Purge Volume: N₂ volume = V × ln(C_i / C_f) / η_mix Where: V = Vapor space volume (ft³) C_i = Initial oxygen concentration (20.9% for air) C_f = Target final oxygen concentration (%) η_mix = Mixing efficiency (0.7–0.9; 0.8 typical) Example: V = 10,000 ft³, C_f = 6%, η_mix = 0.8 N₂ = 10,000 × ln(20.9/6) / 0.8 = 10,000 × 1.248 / 0.8 = 15,602 ft³

Oxygen Monitoring

For critical inerting applications, continuous oxygen monitoring ensures the blanket system is performing as designed:

Paramagnetic analyzers: High accuracy (0.01% resolution); laboratory or continuous service; requires sample conditioning.
Electrochemical (galvanic) cells: Lower cost, moderate accuracy (0.1% resolution); limited cell life (1–2 years).
Zirconia (ceramic) sensors: In-situ measurement; fast response; high temperature capable; used for combustion and flue gas more than tank blanketing.
Portable analyzers: Used for spot-checking and initial purging verification. Typical accuracy 0.1–1.0%.

Sampling point: Mount oxygen sample points at the highest point in the tank vapor space (top of the roof) where oxygen concentration will be highest due to density stratification. Multiple sample points may be needed for large tanks or tanks with internal obstructions.

6. Worked Example

Size a nitrogen blanketing system for a condensate storage tank at a gas processing plant.

Given: Tank: 5,000 bbl fixed-roof vertical cylindrical tank Diameter: 30 ft Height: 32 ft (shell) Product: Stabilized condensate (API gravity 55, flash point 15°F) Storage temperature: 80°F (above flash point) Maximum pump-out rate: 200 bbl/hr Maximum fill rate: 150 bbl/hr Nitrogen supply pressure: 30 psig Target O₂ concentration: < 8 vol%

Step 1: Liquid Pump-Out Demand

Q_pump-out = 200 bbl/hr × 5.615 ft³/bbl = 1,123 ft³/hr = 18.7 SCFM

Step 2: Thermal In-Breathing

Per API 2000 for a 5,000 bbl tank: Q_thermal ≈ 50 SCFH = 0.83 SCFM (This is small compared to pump-out demand)

Step 3: Flash Gas from Hot Fill

Assuming stabilized condensate at 100°F entering tank at 80°F: Flash ratio F ≈ 1.0 SCF/bbl (small temperature differential) Q_flash = 1.0 × 150 = 150 SCFH = 2.5 SCFM

Step 4: Leakage Allowance

Assume 15% leakage for a tank in good condition: Q_sub-total = 18.7 + 0.83 + 2.5 = 22.0 SCFM Q_leakage = 0.15 × 22.0 = 3.3 SCFM

Step 5: Total Blanket Gas Demand

Q_total = 22.0 + 3.3 = 25.3 SCFM Design flow (with 20% safety factor): 25.3 × 1.20 = 30.4 SCFM Round up: Q_design = 31 SCFM (1,860 SCFH)

Step 6: Set Pressure Selection

Blanket gas regulator set point: 0.5 oz/in² gauge Regulator lockup pressure: 1.0 oz/in² Conservation vent (pressure): 2.0 oz/in² Conservation vent (vacuum): -0.5 oz/in² Emergency vent (pressure): 4.0 oz/in² Tank design pressure: 8.0 oz/in² (0.5 psig)

Step 7: Initial Purge Volume

Vapor space volume (at half-full tank): V_vapor = (π/4) × 30² × 16 = 11,310 ft³ Dilution purge (target 8% O₂, mixing efficiency 0.8): N₂ = 11,310 × ln(20.9/8) / 0.8 N₂ = 11,310 × 0.961 / 0.8 = 13,584 ft³ At 31 SCFM, purge time = 13,584 / 31 = 438 min ≈ 7.3 hours (Or use a temporary high-flow nitrogen supply for faster inerting)

Step 8: Summary

Parameter	Value
Design blanket gas flow	31 SCFM (1,860 SCFH)
Regulator set point	0.5 oz/in²
Conservation vent (pressure/vacuum)	+2.0 / -0.5 oz/in²
Emergency vent set point	4.0 oz/in²
Target oxygen	< 8 vol%
Initial purge volume	~13,600 SCF N₂
Estimated purge time	~7.3 hours at design flow

Design check: The dominant demand is liquid pump-out at 18.7 SCFM. This is a moderate demand and can be supplied by a standard membrane nitrogen generator or a bulk liquid nitrogen installation. The 0.5 oz/in² regulator set point provides good clearance below the 2.0 oz/in² conservation vent setting.

7. Operations & Troubleshooting

Routine Monitoring

Regular monitoring ensures the blanketing system maintains proper protection:

Tank pressure: Monitor continuously or at least daily. Pressure should normally remain between the regulator set point and the conservation vent opening pressure.
Nitrogen consumption: Track daily or weekly consumption. A sudden increase indicates leakage, vent malfunction, or process change.
Oxygen concentration: For critical inerting applications, check at least weekly with a portable analyzer. Continuous monitoring is recommended for high-consequence tanks.
Regulator performance: Verify set point and lockup pressure quarterly. Check for droop (set point shift under flow).
Conservation vent operation: Visual and audible inspection during pump-out and fill operations.

Common Problems and Solutions

Problem	Likely Cause	Solution
High N₂ consumption	Vent leaking, regulator set too high, fitting leaks	Check vent seating, reduce regulator set point, leak test fittings
Tank pressure too low	Regulator undersized, supply pressure low, high pump-out rate	Verify regulator capacity, check supply pressure, reduce pump-out rate
Tank vacuum (roof sucking in)	Vacuum vent stuck, regulator failed, N₂ supply interrupted	Emergency: open thief hatch. Check vacuum vent, restore N₂ supply
Oxygen above target	Air leaking past fittings, vent cycling, initial purge incomplete	Leak test all penetrations, verify vent seating, re-purge if needed
Vent chattering	Regulator lockup too close to vent set point	Increase differential; minimum 0.5 oz/in² between lockup and vent
Flame arrestor plugged	Ice, dirt, polymerized vapor, insects	Clean or replace element; install weather hood
Conservation vent frozen	Water accumulation, cold ambient	Install heater or heat trace on vent; drain water accumulation

Nitrogen Supply Options

Supply Method	Purity	Capacity	Best For
Membrane generator	95–99.5%	5–5,000 SCFM	Continuous demand; remote sites with instrument air
PSA generator	99–99.999%	10–50,000 SCFM	High-purity requirements; larger facilities
Bulk liquid (LOX)	99.998%	Variable	Moderate demand; no power for generation
High-pressure cylinders	99.998%	Low	Temporary, backup, or very small demand
Pipeline supply	95–99.9%	Large	Industrial complexes with nitrogen pipeline

Maintenance Schedule

Task	Frequency
Tank pressure check	Daily (or continuous with alarm)
N₂ consumption tracking	Weekly
Oxygen spot-check (portable analyzer)	Weekly to monthly
Regulator set point verification	Quarterly
Conservation vent inspection and testing	Semi-annually
Flame arrestor inspection and cleaning	Semi-annually (more often in dusty environments)
Emergency vent function test	Annually
Full system leak test	Annually or after any maintenance

Critical safety note: Never enter a blanketed tank without a confined space entry permit, atmospheric testing, and continuous ventilation or supplied air. Nitrogen-enriched atmospheres are an asphyxiation hazard. The absence of oxygen cannot be detected by human senses. Every year, workers are killed by nitrogen asphyxiation in confined spaces.

Tank Blanketing / Padding Gas Design

0.5–2.0 oz/in²

< 2–8 vol%

API 2000

Contents

Use this guide when you need to: