Sour Gas Safety

H2S Concentration Conversions

Convert H2S concentrations between ppm, mol%, mg/m³, and grains/100 scf. Understand toxicity levels, exposure limits, and NACE MR0175 material requirements for safe sour gas operations.

Launch Calculator Toxicity levels

Sweet gas threshold

< 4 ppm

Gas is considered "sweet" when H2S < 4 ppm (0.0004 mol%) or 0.25 grains/100 scf.

Sour gas definition

≥ 4 ppm H2S

Requires NACE MR0175 materials, H2S monitoring, and special safety protocols.

OSHA PEL (ceiling)

20 ppm / 15 min

Maximum allowable exposure: 20 ppm for 15 minutes, 10 ppm 8-hr TWA.

Contents

Use this guide when you need to:

  • Convert H2S between common concentration units.
  • Determine sour gas classification criteria.
  • Select appropriate materials for sour service.

1. Overview & Unit Systems

Hydrogen sulfide (H2S) is a highly toxic, flammable gas commonly encountered in oil and gas production. Accurate concentration conversions are critical for safety, regulatory compliance, and materials selection.

ppm (parts per million)

Volume basis

Most common for gas analysis and safety monitoring (ppmv by volume).

mol% (mole percent)

Molar basis

Used in process design, phase equilibrium, and gas composition reports.

mg/m³ (milligrams per cubic meter)

Mass concentration

OSHA/NIOSH exposure limits specified as mass per volume at standard conditions.

grains/100 scf

Traditional oil field unit

Legacy unit still used in pipeline gas specs and sulfur removal equipment.

Key Properties of H2S

Property Value Units
Molecular weight 34.08 g/mol
Boiling point -76°F (-60°C) at 1 atm
Specific gravity (air = 1) 1.176 dimensionless
Lower explosive limit (LEL) 4.3% vol% in air
Upper explosive limit (UEL) 45.5% vol% in air
Odor threshold 0.01–0.3 ppm rotten egg smell
Olfactory fatigue threshold 100–150 ppm sense of smell paralyzed
Olfactory fatigue danger: At concentrations above 100-150 ppm, H2S paralyzes the sense of smell. Workers may believe the gas has dissipated when in fact concentrations are increasing to lethal levels. Always rely on electronic H2S monitors, not smell.
H2S molecular structure ball-and-stick model showing central yellow sulfur atom bonded to two white hydrogen atoms at 92.1 degree bond angle, with S-H bond length of 1.34 Angstroms, lone electron pairs visible on sulfur, and molecular weight of 34.08 g/mol
Hydrogen sulfide (H₂S) molecular structure showing the bent geometry with 92.1° bond angle. The molecule's high density (heavier than air) and toxicity make proper monitoring essential.

Standard Conditions for Conversions

  • Temperature: 60°F (15.6°C) or 25°C depending on standard used
  • Pressure: 14.696 psia (101.325 kPa) - standard atmosphere
  • Molar volume at STP: 379.5 scf/lbmol (22.414 L/mol at 0°C, 1 atm)
  • Molar volume at 60°F: 379.49 scf/lbmol (US oil/gas standard)

2. Conversion Equations

All conversions assume ideal gas behavior, which is accurate for H2S at atmospheric pressure and ambient temperature.

ppm to mol%

Volume Basis (Identical): mol% = ppm / 10,000 ppm = mol% × 10,000 Example: 1,000 ppm H2S = 1000 / 10,000 = 0.1 mol% 0.5 mol% H2S = 0.5 × 10,000 = 5,000 ppm Note: ppmv (parts per million by volume) equals mol% × 10,000 because both are volume/volume ratios (Avogadro's law: equal volumes contain equal moles at same P/T).

ppm to mg/m³

Mass Concentration at Standard Conditions: mg/m³ = (ppm × MW_H2S) / 24.45 Where: MW_H2S = 34.08 g/mol 24.45 = molar volume at 25°C, 1 atm (L/mol) Therefore: mg/m³ = ppm × 34.08 / 24.45 mg/m³ = ppm × 1.394 And reverse: ppm = mg/m³ / 1.394 ppm = mg/m³ × 0.717 Example: 10 ppm H2S = 10 × 1.394 = 13.94 mg/m³ 15 mg/m³ = 15 × 0.717 = 10.76 ppm At 60°F (15.6°C) instead of 25°C: mg/m³ = ppm × 1.416 (slightly higher density at lower temp)

ppm to grains/100 scf

GPSA Standard Formula (Oil Field Unit): grains/100 scf = ppm × (MW_H2S / V_m) × (7000 / 10,000) Where: V_m = 379.49 scf/lb-mol at 60°F, 14.696 psia (GPSA std) 7000 grains = 1 lb MW_H2S = 34.08 lb/lb-mol grains/100 scf = ppm × (34.08 / 379.49) × 0.7 grains/100 scf = ppm × 0.06286 ppm = grains/100 scf / 0.06286 ppm = grains/100 scf × 15.91 Verification (GPSA Pipeline Spec): 4 ppm × 0.06286 = 0.251 grains/100 scf ✓ 0.25 grains/100 scf × 15.91 = 3.98 ppm ≈ 4 ppm ✓ This matches the GPSA specification: H₂S < 0.25 grains/100 scf (about 4 ppmv)

mol% to grains/100 scf

Direct Conversion: Since mol% = ppm / 10,000: grains/100 scf = mol% × 10,000 × 0.06286 grains/100 scf = mol% × 628.6 mol% = grains/100 scf / 628.6 Example: 0.1 mol% H₂S = 0.1 × 628.6 = 62.86 grains/100 scf 1 grain/100 scf = 1 / 628.6 = 0.00159 mol% = 15.9 ppm

Conversion Table (GPSA Standard Conditions)

ppm mol% mg/m³ (60°F) grains/100 scf lb/MMscf Classification
4 0.0004 5.8 0.25 0.36 Sour gas / Pipeline spec
10 0.001 14.4 0.63 0.90 OSHA 8-hr TWA limit
20 0.002 28.8 1.26 1.80 OSHA ceiling (15 min)
100 0.01 144 6.29 8.98 NIOSH IDLH
500 0.05 720 31.4 44.9 Knockdown level
1,000 0.1 1,439 62.9 89.8 Typical amine feed
5,000 0.5 7,195 314 449 High sour gas plant feed
10,000 1.0 14,390 629 898 Very sour (Permian Basin)
50,000 5.0 71,950 3,143 4,490 Extreme sour (acid gas)

Table values calculated using GPSA formulas: grains/100scf = ppm × 0.06286; lb/MMscf = ppm × 0.0898; mg/m³ = ppm × 1.439 at 60°F

Temperature Correction for mg/m³

When converting at non-standard temperatures:

Temperature Correction: mg/m³ (at T) = mg/m³ (at 25°C) × (298.15 / T_K) Where T_K is temperature in Kelvin Example at 60°F (288.7 K): mg/m³ (60°F) = mg/m³ (25°C) × (298.15 / 288.7) mg/m³ (60°F) = mg/m³ (25°C) × 1.033 10 ppm at 60°F = 10 × 1.416 = 14.16 mg/m³ (vs 13.94 at 25°C)

3. Toxicity Levels & Exposure Limits

H2S is immediately dangerous to life and health (IDLH) at concentrations as low as 100 ppm. Understanding exposure limits and physiological effects is critical for safety.

OSHA Regulatory Limits (29 CFR 1910.1000)

Limit Type Concentration Duration Regulation
PEL (8-hr TWA) 10 ppm (14 mg/m³) 8-hour time-weighted average 1910.1000 Table Z-2
Ceiling limit 20 ppm (28 mg/m³) Maximum 15 minutes Never exceed
Action level 5 ppm 8-hour TWA Triggers monitoring requirements

NIOSH Exposure Limits

Limit Type Concentration Definition
REL (10-min ceiling) 10 ppm (15 mg/m³) Recommended exposure limit
IDLH 100 ppm (140 mg/m³) Immediately dangerous to life/health
SCBA required ≥ 100 ppm Self-contained breathing apparatus mandatory

Physiological Effects by Concentration

Concentration (ppm) Duration Physiological Effect
0.01–0.3 - Odor threshold (rotten eggs); detectable by smell
10 8 hours OSHA permissible exposure limit; mild eye irritation
20–50 1+ hours Eye irritation, headache, nausea, sore throat
50–100 30–60 min Severe eye/respiratory irritation, coughing, loss of smell
100–200 2–15 min Olfactory fatigue (can't smell H2S), severe respiratory distress
200–500 Minutes Pulmonary edema, unconsciousness, possible death
500–700 Minutes Collapse, respiratory arrest, high fatality risk
700+ Seconds Immediate collapse, respiratory paralysis, death ("knockdown")
The "knockdown" effect: At concentrations above 500–700 ppm, H2S causes immediate collapse and respiratory paralysis. Victims have no warning and collapse within seconds. This makes rescue extremely dangerous without proper SCBA equipment.
H2S concentration levels and physiological effects chart on logarithmic scale from 0.01 to 1000 ppm, showing color-coded danger zones: green at odor threshold 0.01-0.3 ppm, yellow at OSHA PEL 10 ppm, orange at ceiling 20 ppm, red at NIOSH IDLH 100 ppm, dark red at olfactory fatigue 100-200 ppm where you cannot smell H2S, and lethal knockdown zone above 500 ppm
H₂S toxicity chart showing concentration levels and physiological effects. Critical warning: Above 100 ppm, olfactory fatigue prevents detection by smell—always use electronic monitors.

H2S Monitoring Requirements

  • Confined spaces: Continuous monitoring required when H2S may be present (> 4 ppm possible)
  • Personal monitors: Workers in sour gas areas must wear calibrated personal H2S detectors
  • Alarm setpoints: Low alarm 10 ppm, high alarm 15–20 ppm, evacuation 20–25 ppm
  • Calibration: Daily bump test, monthly full calibration with certified gas
  • Fixed monitors: Area monitors at potential leak points (wellheads, compressors, amine units)
  • Wind socks: Visual wind direction indicators for evacuation planning

Emergency Response Procedures

  • Evacuation: Move upwind or crosswind immediately when alarm sounds (H2S is heavier than air)
  • SCBA required: Only trained personnel with SCBA may enter areas > 100 ppm
  • Victim rescue: Never attempt rescue without SCBA and backup team
  • First aid: Move victim to fresh air immediately; administer oxygen if trained; CPR if not breathing
  • Medical attention: All H2S exposures > 20 ppm require medical evaluation (delayed pulmonary edema risk)

4. Sour Gas Classification & Pipeline Specifications

The presence of H2S triggers specific regulatory requirements, material selection criteria, and operational procedures.

Sour Gas Definition

Industry Standard Threshold: Sour gas: H2S ≥ 4 ppm (0.0004 mol%, 0.0051 grains/100 scf) Sweet gas: H2S < 4 ppm The 4 ppm threshold triggers: - NACE MR0175/ISO 15156 material requirements - H2S safety training for all personnel - Continuous H2S monitoring - Special emergency response planning - Additional regulatory compliance (state-dependent)

Total Sulfur Specifications

Pipeline gas specifications often limit both H2S and total sulfur content:

Component Typical Limit Basis
H2S (hydrogen sulfide) 0.25 grains/100 scf (195 ppm) Corrosion, toxicity
Total sulfur 5 grains/100 scf (20 ppm S) SO2 emissions when burned
Mercaptans (RSH) Included in total sulfur Odor, corrosion
Carbonyl sulfide (COS) Included in total sulfur LNG processing issues

Common Pipeline Specifications

Pipeline System H2S Limit Total Sulfur Limit
Interstate transmission 0.25–0.5 grains/100 scf 5 grains/100 scf
Distribution (utility) 0.25 grains/100 scf 5 grains/100 scf
LNG plants < 4 ppm < 1 ppm total S
Gas processing plants Varies (sweet outlet) 2–5 grains/100 scf outlet

Conversion Example: Pipeline Spec Verification

Problem: Gas analysis shows 0.15 grains/100 scf H2S and 3.2 grains/100 scf total sulfur. Pipeline spec is 0.25 grains H2S max, 5 grains total S max. Does gas meet spec? Solution: H2S check: 0.15 grains/100 scf < 0.25 grains/100 scf ✓ PASS Total sulfur check: 3.2 grains/100 scf < 5 grains/100 scf ✓ PASS Convert H2S to ppm for safety assessment: 0.15 grains/100 scf × 779.4 ppm/(grain/100 scf) = 117 ppm This is sour gas (≥ 4 ppm), so NACE materials required even though pipeline spec is met. Key insight: Pipeline spec compliance doesn't exempt from sour gas materials requirements if H2S ≥ 4 ppm.

Regulatory Drivers for H2S Limits

  • 29 CFR 1910.1000: OSHA workplace exposure limits (10 ppm TWA, 20 ppm ceiling)
  • 49 CFR Part 192: Federal pipeline safety standards (state-specific sour gas rules)
  • EPA Clean Air Act: SO₂ emissions limits from gas combustion (drives total sulfur specs)
  • State regulations: Texas RRC, California DOGGR have specific sour gas well requirements
  • NACE MR0175/ISO 15156: Material selection for sour service (≥ 4 ppm H2S)
Safety vs. quality specifications: Pipeline quality specs (e.g., 0.25 grains/100 scf ≈ 4 ppm) set limits for corrosion and end-use product quality, while OSHA limits (10 ppm TWA) address worker exposure. Both requirements must be met independently.
Simplified process flow diagram of amine gas treating unit for H2S removal showing sour gas entering contactor/absorber tower, sweet gas exiting top, rich amine flowing through flash tank and lean/rich heat exchanger to stripper/regenerator with reboiler, lean amine returning through aerial cooler and pump, and acid gas (H2S plus CO2) going to SRU or flare
Amine gas treating unit process flow diagram. Sour gas contacts lean amine in the absorber; the rich amine is regenerated in the stripper, releasing H₂S to sulfur recovery.

5. NACE MR0175/ISO 15156 Material Requirements

NACE MR0175 (now ISO 15156) specifies material requirements for equipment used in H2S-containing environments. Compliance is mandatory for sour gas service (H2S ≥ 4 ppm).

Sour Service Classification

NACE MR0175 Applicability Criteria: Sour service materials required when BOTH conditions met: 1. H2S partial pressure ≥ 0.05 psia (50 ppb in gas phase), AND 2. Water is present (liquid or condensing conditions) Note: Industry practice typically uses 4 ppm H2S as threshold regardless of water presence due to safety and liability concerns. H2S partial pressure calculation: P_H2S = P_total × y_H2S Where: P_total = Total system pressure (psia) y_H2S = Mole fraction of H2S Example: At 1000 psia with 100 ppm H2S: P_H2S = 1000 × (100 / 1,000,000) = 0.1 psia This exceeds 0.05 psia threshold → sour service materials required

Carbon Steel Requirements (NACE MR0175 Part 2)

Material Type Hardness Limit Additional Requirements
Carbon & low-alloy steels ≤ 22 HRC (237 HBW) All product forms
Welds (base metal ≤ 22 HRC) ≤ 22 HRC in HAZ PWHT often required
Cold-worked components ≤ 22 HRC after cold work Stress relief may be needed
Fasteners (bolts/studs) ≤ 22 HRC Special sour service bolting required

Stainless Steel Requirements (NACE MR0175 Part 3)

Alloy Type Hardness Limit Chloride Consideration
Austenitic (e.g., 316, 316L) ≤ 22 HRC (237 HBW) SCC risk if Cl⁻ > 50 ppm
Duplex (e.g., 2205, 2507) ≤ 30 HRC (varies by grade) Good H2S and Cl⁻ resistance
Super austenitic (6Mo) ≤ 22 HRC Excellent corrosion resistance
Martensitic (410, 13Cr) ≤ 22 HRC Limited H2S resistance

Corrosion-Resistant Alloys (CRAs) for Severe Sour Service

Alloy H2S Service Limit Applications
Alloy 625 (UNS N06625) No limit (per MR0175) Valves, tubing, high H2S wells
Alloy 825 (UNS N08825) No limit Heat exchangers, piping
Duplex 2205 (UNS S32205) < 25% H2S (environment-dependent) Piping, pressure vessels
Super duplex 2507 Moderate H2S + high Cl⁻ Offshore, high chloride environments

Sulfide Stress Cracking (SSC) Mechanisms

SSC is the primary failure mode in sour service. It occurs when three conditions coincide:

  • Susceptible material: High-strength steel (hardness > 22 HRC)
  • H2S environment: Wet H2S present (even trace amounts with water)
  • Tensile stress: Applied or residual stress (e.g., from welding, cold work)
Sulfide Stress Cracking (SSC) triangle diagram showing three conditions required for failure: susceptible material with high hardness greater than 22 HRC at top, H2S plus water wet sour environment at bottom left, and tensile stress from applied or residual sources at bottom right, with SSC Failure in center indicating all three must be present
The SSC (Sulfide Stress Cracking) triangle: all three conditions—susceptible material, H₂S+water, and tensile stress—must be present for cracking to occur. Remove any one to prevent failure.

SSC Prevention Strategies

Material Selection Hierarchy for Sour Service: 1. Carbon steel (≤ 22 HRC) with proper heat treatment - Most economical for mild sour service - Requires PWHT for welds to control HAZ hardness - Maximum hardness 22 HRC (237 HBW, ~100 ksi yield) 2. Low-alloy steel (e.g., 1¼Cr, 2¼Cr) if higher strength needed - Must still meet ≤ 22 HRC hardness limit - PWHT mandatory 3. Stainless steels for corrosion resistance - 316L common, but beware chloride SCC - Duplex preferred for combined H2S + Cl⁻ 4. Nickel-base alloys (625, 825) for severe service - High H2S partial pressure (> 10 psi) - High temperature (> 300°F) - Combined H2S + CO2 + Cl⁻

Example: Material Selection for Sour Gas Valve

Given conditions: - H2S: 500 ppm (0.05 mol%) - Pressure: 1200 psig (1215 psia) - Temperature: 100°F - Water present: Yes (wet gas) Calculate H2S partial pressure: P_H2S = 1215 psia × (500 / 1,000,000) P_H2S = 0.61 psia This exceeds 0.05 psia NACE threshold → sour service materials required. Material options: Option 1: Carbon steel valve body (ASTM A216 WCB) - Hardness ≤ 22 HRC verified by mill cert - Trim: 316 SST (hardness ≤ 22 HRC) - All welds PWHT to ≤ 22 HRC - Cost: Baseline Option 2: 316 SST body and trim - Hardness ≤ 22 HRC - Risk: Potential chloride SCC if formation water has Cl⁻ - Cost: 3× carbon steel Option 3: Duplex 2205 body and trim - Better H2S + Cl⁻ resistance than 316 - Hardness verification required - Cost: 4× carbon steel Recommendation: Option 1 (carbon steel) is adequate for P_H2S = 0.61 psia at 100°F. Option 2 or 3 only needed if chlorides present or temperature exceeds 180°F.

Post-Weld Heat Treatment (PWHT) Requirements

  • Purpose: Reduce HAZ (heat-affected zone) hardness to ≤ 22 HRC and relieve residual stresses
  • When required: All carbon/low-alloy steel welds in sour service unless qualified by testing
  • Temperature: 1100–1200°F (593–649°C) for carbon steel
  • Hold time: 1 hour per inch of thickness (minimum 1 hour)
  • Verification: Hardness survey of weld and HAZ after PWHT (≤ 22 HRC required)
The 22 HRC limit: This hardness limit (approximately 100 ksi yield strength) is the most critical NACE MR0175 requirement. Materials harder than 22 HRC are extremely susceptible to SSC in sour service and can fail catastrophically without warning. Always verify hardness with certified test reports.

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