1. Overview & Applications
Natural gas is odorless, colorless, and tasteless in its natural state. Odorization—the addition of mercaptan-based odorants—provides a distinctive "rotten egg" smell that enables leak detection before gas reaches hazardous concentrations. Odorization is federally mandated for all gas distribution systems serving the public.
Public safety
Leak detection
Enable detection of gas leaks at 1/5 LEL (10,000 PPM) by distinctive odor.
Distribution systems
City gate odorization
Inject odorant at city gate stations serving residential/commercial customers.
Regulatory compliance
49 CFR 192.625
Federal mandate: combustible gas must be readily detectable by odor at 1/5 LEL.
System monitoring
Odor fade testing
Annual testing to verify odor intensity meets regulatory threshold throughout system.
IMAGE: Odorization Safety Margin Diagram
Shows LEL (5%), 1/5 LEL detection threshold (1%), and odor detection margin
Key Concepts
- Odorization: Addition of odorant (mercaptan) to natural gas for leak detectability
- LEL (Lower Explosive Limit): Minimum gas concentration that will ignite (5% methane in air)
- 1/5 LEL: Detection threshold = 1% gas in air (10,000 PPM) per 49 CFR 192.625
- Odorant threshold: Minimum concentration humans can smell (0.2-1.0 PPM for mercaptan)
- Odor fade: Loss of odorant due to adsorption in pipelines, especially new or rehabilitated mains
- Y/Z odorizer: Bypass-type odorant injection system using wick, pump, or drip-feed methods
Why Odorization Matters
| Risk Without Odorization | Consequence | Frequency | Prevention |
|---|---|---|---|
| Undetected indoor leak | Explosion, fatalities, property damage | Rare but catastrophic | Proper odorant dosing + annual testing |
| Outdoor leak (excavation damage) | Gas accumulation in confined space | Common (one call damages) | Detectable odor alerts public/crews |
| Appliance malfunction | Gas buildup in residence | Occasional | Occupants smell gas, evacuate |
| Odor fade in new mains | Delayed leak detection | Predictable in new/rehab pipe | Elevated dosing + fade testing |
| Insufficient odorant dosing | Below detection threshold | Equipment failure or neglect | Calibration + sniff tests + alarms |
Legal mandate: 49 CFR 192.625(a) requires all combustible gas in distribution systems to contain a natural odorant or be odorized so that at a concentration in air of 1/5 the lower explosive limit, the gas is readily detectable by a person with a normal sense of smell. Exemptions: (1) Transmission lines, (2) Offshore gathering, (3) Master meter systems with individual odorization.
Odorization History and Evolution
Historical development of gas odorization in the United States:
- 1937 New London School explosion: 298 deaths from unodorized natural gas leak; led to odorization mandates
- 1940s-1950s: States begin requiring odorization; THT (tetrahydrothiophene) becomes standard odorant
- 1968 Natural Gas Pipeline Safety Act: Federal oversight established; 49 CFR 192 created
- 1970 DOT adopts 1/5 LEL standard: 49 CFR 192.625 codifies detection requirement at 10,000 PPM
- 1990s Y/Z odorizer proliferation: Automated injection systems replace manual wick-feed odorizers
- 2000s Electronic monitoring: Flow computers, remote telemetry, automated calibration verification
2. Regulatory Requirements
49 CFR 192.625 - Odorization of Gas
Federal Regulation: 49 CFR 192.625(a)
"A combustible gas in a distribution line must contain a natural odorant or be
odorized so that at a concentration in air of one-fifth of the lower explosive
limit, the gas is readily detectable by a person with a normal sense of smell."
Key terms defined:
Lower Explosive Limit (LEL) for methane (natural gas):
LEL = 5.0% methane in air = 50,000 PPM
One-fifth LEL:
1/5 LEL = 1.0% methane in air = 10,000 PPM
Detection requirement:
Gas must be "readily detectable" (i.e., distinctly smellable) at 10,000 PPM
This provides 5:1 safety factor between detectable concentration and
explosive concentration.
Human olfactory threshold:
Mercaptan odorants: 0.2-1.0 PPM detection threshold
Required in gas at 1/5 LEL: 10,000 PPM gas concentration
Therefore: Odorant must be detectable at 10,000× dilution in gas
Exemptions from Odorization
49 CFR 192.625(b) and (d) provide limited exemptions:
| Exemption Type | Regulatory Citation | Conditions | Typical Application |
|---|---|---|---|
| Transmission lines | 49 CFR 192.625(b)(1) | Interstate/intrastate transmission, not serving public directly | Cross-country pipelines, not at city gate |
| Offshore gathering | 49 CFR 192.625(b)(2) | Offshore structures and platforms | GOM platforms, offshore Alaska |
| Master meter systems | 49 CFR 192.625(b)(3) | Individual facility odorization at each end-user | Industrial complexes, refineries |
| Process gas (non-combustible use) | 49 CFR 192.625(d) | Used as chemical feedstock, not burned | Petrochemical plants, hydrogen production |
| Temporary blanket gas | Operational necessity | Short-term purging, commissioning | Pipeline commissioning, maintenance |
Odor Testing Requirements
49 CFR 192.625(c) - Testing:
Each operator must conduct periodic sampling of combustible gases to assure
the proper concentration of odorant.
Industry practice (AGA/OPS guidelines):
- City gate stations: Monthly sniff tests
- Distribution system: Annual system-wide testing
- New/rehabilitated mains: Quarterly for first year (odor fade risk)
Sniff test procedure:
1. Collect gas sample in Tedlar bag or glass bottle
2. Dilute sample to 1/5 LEL (10,000 PPM in air)
3. Panel of 3-5 people with normal sense of smell
4. Each person must readily detect odor
5. Document test date, location, concentration, and results
Acceptable result:
≥ 80% of panel detects odor at 10,000 PPM (1/5 LEL)
If < 80%, increase odorant injection rate
Instrumented testing (optional):
Gas chromatography (GC-FID or GC-SCD) measures odorant concentration
Correlate instrumental PPM to odor threshold
Typical: 0.8-1.6 mg/m³ THT at 1/5 LEL
Documentation and Record Keeping
- Odorant injection records: Continuous flow computer logs or daily manual readings
- Calibration records: Annual calibration of odorizer pumps and meters
- Sniff test results: Date, location, concentration, number of panelists, pass/fail
- Odorant inventory: Receipts, storage levels, consumption rates, material safety data sheets (SDS)
- Maintenance logs: Odorizer equipment service, repairs, pump replacement
- Compliance reports: Annual report to state PUC or PHMSA demonstrating compliance
Enforcement: PHMSA (Pipeline and Hazardous Materials Safety Administration) enforces 49 CFR 192. Violations of odorization requirements can result in: (1) Civil penalties up to $200,000 per violation per day, (2) Corrective action orders, (3) Criminal prosecution if negligence causes injury/death. State public utility commissions may impose additional penalties.
State and Local Requirements
Some states have more stringent odorization standards than federal minimums:
| Jurisdiction | Requirement | Notes |
|---|---|---|
| California (CPUC) | 1/5 LEL + quarterly leak surveys + annual fade testing | GO 112-F more stringent than federal |
| Texas (RRC) | 1/5 LEL + monthly sniff tests at city gates | 16 TAC §8.201 |
| New York (PSC) | 1/5 LEL + semi-annual fade testing in new mains | 16 NYCRR Part 255 |
| Pennsylvania (PUC) | 1/5 LEL + quarterly high-consequence area sampling | 52 Pa Code Chapter 59 |
| Most other states | Adopt 49 CFR 192.625 without modification | Federal standard is minimum |
Related Standards and Guidelines
- AGA Operations Conference Proceedings: Industry best practices, odor fade mitigation strategies
- ASTM D5305: Standard test method for determination of odorant concentration by GC
- API RP 1100: Odorization of transmission lines (voluntary, where required by contract)
- NFPA 58: LP-Gas odorization (1.0 lb odorant per 10,000 gal propane minimum)
- CGA G-13: Canadian odorization standard (similar to US 49 CFR 192.625)
3. Odorant Types & Properties
Common Mercaptan Odorants
Primary Odorants (mercaptans = thiols, R-SH):
1. Tetrahydrothiophene (THT, C₄H₈S):
- Most common odorant in US (70-80% market share)
- Chemical formula: Cyclic structure with sulfur
- Molecular weight: 88.17 g/mol
- Density: 0.998 g/cm³ at 20°C (8.33 lb/gal)
- Boiling point: 121°C (250°F)
- Odor threshold: 0.07 PPM in air
- Cost: Moderate (~$5-10/lb, commodity pricing)
2. Tertiary butyl mercaptan (TBM, C₄H₁₀S):
- Alternative to THT, more volatile (lower BP)
- Molecular weight: 90.19 g/mol
- Density: 0.794 g/cm³ at 20°C (6.62 lb/gal)
- Boiling point: 64°C (147°F)
- Odor threshold: 0.01 PPM (more potent than THT)
- Cost: Higher than THT (~$8-15/lb)
3. Dimethyl sulfide (DMS, C₂H₆S):
- Used in Europe, less common in US
- Molecular weight: 62.13 g/mol
- Boiling point: 37°C (99°F) - very volatile
- Odor: "Cabbage-like" rather than "rotten egg"
4. Ethyl mercaptan (ethanethiol, C₂H₆S):
- Historical use (pre-THT era)
- Extremely volatile (BP: 35°C / 95°F)
- Very low odor threshold (0.001 PPM)
- Handling challenges due to volatility
Blends:
Some operators use proprietary blends of THT + TBM + other mercaptans
Benefits: Optimize odor strength, fade resistance, temperature performance
THT Physical Properties (Detailed)
| Property | Value | Units | Significance |
|---|---|---|---|
| Molecular weight | 88.17 | g/mol | Heavier than methane (16.04), less volatile |
| Specific gravity | 0.998 | — | Slightly less dense than water (floats on water) |
| Density at 60°F | 8.33 | lb/gal | Used for injection rate calculations |
| Vapor pressure at 20°C | 1.5 | kPa | Low vapor pressure → minimal evaporation losses |
| Flash point | 7°C (45°F) | — | Flammable liquid; requires proper storage |
| Autoignition temp | 202°C (396°F) | — | Well above typical operating conditions |
| Solubility in water | 3.5 | g/L at 20°C | Slightly soluble; can contaminate condensate |
| Freezing point | -96°C (-141°F) | — | Remains liquid in all field conditions |
Typical Odorant Dosing Rates
Industry Standard Dosing Rates:
Residential/commercial distribution:
0.5-1.5 lb odorant per MMscf gas (typical: 1.0 lb/MMscf)
Calculation basis:
Target: 1.0 mg/m³ THT at 1/5 LEL (10,000 PPM gas in air)
For 1.0 lb THT per MMscf:
1.0 lb/MMscf = 453.6 g / 28,316 m³ = 16.0 mg/m³ in gas stream
When diluted to 1/5 LEL (10,000 PPM = 1% gas in air):
16.0 mg/m³ × 0.01 = 0.16 mg/m³ in air
This exceeds 0.07 PPM (0.26 mg/m³) threshold by factor of ~2× (conservative)
Factors increasing dosing rate:
- Odor fade in new or rehabbed mains: 1.5-2.5 lb/MMscf
- Cold weather (reduced volatility): +20% increase
- High moisture content (odorant adsorption): +10-30%
- Long distribution system (extended residence time): +20-50%
Factors decreasing dosing rate:
- Short distribution runs (< 5 miles): 0.5-0.8 lb/MMscf
- Warm climate (high volatility): -10% reduction
- Dry gas (minimal liquid water): Standard rate OK
Never below 0.5 lb/MMscf without engineering justification and testing
Odor Fade Mechanisms
Odor fade is the loss of odorant through adsorption, absorption, or chemical reaction in the distribution system:
| Fade Mechanism | Cause | Severity | Mitigation |
|---|---|---|---|
| Pipe wall adsorption | New steel, ductile iron, PVC pipe | High (first 6-12 months) | Elevated dosing (2-3× normal) during break-in |
| Pipe coating absorption | Epoxy, polyethylene linings | Moderate (first 3-6 months) | Pre-odorize coated pipe, increased dosing |
| Rust/scale reaction | Oxidized iron in old mains | Moderate (ongoing) | Pigging, increased dosing, main replacement |
| Moisture scrubbing | Free water in low spots | Moderate (seasonal) | Drip pots, line drying, increased dosing |
| Bacterial degradation | Sulfate-reducing bacteria (SRB) | Low (rare) | Biocide treatment, system cleaning |
| Oxidation (high pressure) | O₂ ingress, air contamination | Low (distribution systems) | Eliminate air intrusion, purging |
New main odor fade: Newly installed or rehabilitated mains can adsorb 80-95% of odorant in the first few weeks of operation. Industry practice: Inject 2-3× normal dosing rate for 3-6 months, conduct quarterly sniff tests, gradually reduce to normal dosing as fade diminishes. Document all fade testing and dosing adjustments for regulatory compliance.
Odorant Storage and Handling
- Storage tanks: Stainless steel 316 or carbon steel with epoxy coating; 50-500 gallon capacity typical
- Secondary containment: 110% of tank volume per SPCC regulations (40 CFR 112)
- Ventilation: Outdoor storage preferred; indoor requires mechanical ventilation (explosion-proof)
- Temperature control: Not required (THT stable -140°F to +250°F); avoid direct sunlight to minimize expansion
- Level monitoring: Visual gauge or electronic transmitter with low-level alarm
- Material compatibility: Use Viton or Kalrez seals; Buna-N and neoprene degrade in mercaptans
- Fire protection: Class IB flammable liquid; ABC fire extinguisher, no smoking within 50 feet
4. Y/Z Type Odorizer Systems
Odorizer Types Overview
Three Main Odorizer Configurations:
1. Wick-Feed (historical, rarely used today):
- Canvas wick submerged in odorant tank
- Gas flows over wick, evaporates odorant
- No moving parts (very reliable)
- Poor accuracy (temperature-dependent, no flow feedback)
- Obsolete except for very small systems
2. Bypass (Y-Type) Odorizer:
- Small stream of gas bypasses through odorant vaporizer/bubbler
- Odorant-saturated gas rejoins main flow downstream
- Positive displacement or gear pump injects liquid into bypass
- Flow proportional control: Inject rate ∝ main gas flow
3. Direct Injection (Z-Type) Odorizer:
- Liquid odorant injected directly into gas stream
- Pump (diaphragm, piston, or peristaltic) meters odorant
- Atomizing nozzle ensures mixing
- Most accurate and widely used modern system
Y vs Z comparison:
Y-Type: Simpler, less maintenance, lower capital cost, adequate for small systems
Z-Type: Higher accuracy, flow computer control, better for large/varying flows
Z-Type Direct Injection System
Most common configuration for modern city gate and district regulator stations:
IMAGE: Z-Type Odorizer System Schematic
P&ID showing storage tank, pump, flow computer, check valve, and injection nozzle
Z-Type Odorizer Components:
1. Odorant storage tank (50-500 gallon stainless steel)
2. Positive displacement pump (diaphragm, piston, or peristaltic)
- Flow range: 0.1-10 gallons per day typical
- Materials: 316SS wetted parts, Viton seals
- Drive: Electric motor or pneumatic actuator
3. Flow computer (electronic control module)
- Input: Gas flow rate from turbine or orifice meter
- Output: Pump stroke rate (strokes per minute)
- Set point: lb odorant per MMscf (typically 1.0)
4. Injection nozzle (atomizer)
- Sprays liquid odorant into gas stream
- Provides back-pressure (50-100 psi typical)
- Location: Downstream of meter, in turbulent region
5. Check valve (prevents gas backflow into odorant tank)
6. Level alarm (low-level switch on storage tank)
Operating principle:
- Flow computer reads gas flow rate (scfh or MMscfd)
- Calculates required odorant injection rate (gal/day or lb/hr)
- Adjusts pump stroke rate to deliver target injection
- Pump injects liquid odorant through nozzle into gas stream
- Turbulence ensures rapid mixing and vaporization
Odorant Injection Rate Calculation
Injection Rate Formula:
R_odorant = Q_gas × D_rate
Where:
R_odorant = Odorant injection rate (gal/day or lb/hr)
Q_gas = Gas flow rate (MMscfd or scfh)
D_rate = Dosing rate (lb odorant per MMscf)
Convert dosing rate to volume basis:
D_rate (gal/MMscf) = D_rate (lb/MMscf) / ρ_odorant (lb/gal)
For THT (ρ = 8.33 lb/gal):
D_rate (gal/MMscf) = D_rate (lb/MMscf) / 8.33
Example Calculation:
City gate flow: Q_gas = 50 MMscfd (average)
Target dosing: D_rate = 1.0 lb THT per MMscf
THT density: ρ = 8.33 lb/gal
Step 1: Calculate lb/day odorant required
R_odorant = 50 MMscfd × 1.0 lb/MMscf = 50 lb/day
Step 2: Convert to gal/day
R_odorant = 50 lb/day / 8.33 lb/gal = 6.0 gal/day
Step 3: Calculate pump output (if pump runs continuously)
Pump output = 6.0 gal/day / 24 hr/day = 0.25 gal/hr = 4.2 ml/min
Step 4: Set flow computer dosing rate
Enter: 1.0 lb/MMscf (or 0.12 gal/MMscf) into controller
Controller adjusts pump speed proportionally to gas flow
If gas flow doubles to 100 MMscfd:
R_odorant = 100 × 1.0 = 100 lb/day = 12 gal/day (automatic adjustment)
Pump Sizing and Selection
| Pump Type | Flow Range | Accuracy | Advantages | Disadvantages |
|---|---|---|---|---|
| Diaphragm | 0.1-5 gph | ±2-5% | Reliable, no seals, handles slurries | Pulsating flow, limited accuracy |
| Piston (plunger) | 0.5-20 gph | ±1-2% | High accuracy, high pressure capability | Requires seal replacement, more maintenance |
| Peristaltic | 0.05-2 gph | ±5-10% | No wetted parts, easy tube replacement | Lower accuracy, tube life limited |
| Gear | 1-50 gph | ±1-3% | Smooth flow, high capacity | Expensive, wear on gears, requires filtration |
Calibration Procedures
IMAGE: Odorizer Calibration Test Setup
Shows graduated cylinder, scale, pump connections, and measurement procedure
Annual Odorizer Calibration (Required):
Method 1 - Volumetric Test (field method):
1. Fill graduated cylinder with 100-500 ml odorant
2. Note initial volume (V_initial)
3. Set pump to fixed stroke rate (e.g., 60 strokes/min)
4. Run pump for measured time (e.g., 10 minutes)
5. Note final volume (V_final)
6. Calculate actual flow rate:
Q_actual = (V_initial - V_final) / time
7. Calculate pump calibration factor:
CF = Q_actual / Q_indicated
Where Q_indicated = pump controller display
8. Enter CF into flow computer (if software adjustable)
Or adjust stroke rate to achieve target flow
Acceptance criteria:
CF = 0.95-1.05 (within ±5% of set point)
If outside tolerance, rebuild pump or replace
Method 2 - Gravimetric Test (more accurate):
1. Disconnect odorant supply line from tank
2. Connect line to graduated cylinder on scale
3. Record initial weight (W_initial)
4. Run pump for 30-60 minutes at normal rate
5. Record final weight (W_final)
6. Calculate:
Q_actual (lb/hr) = (W_initial - W_final) / time (hr)
7. Compare to flow computer indicated rate
8. Adjust calibration factor as needed
Calibration frequency:
- New installations: Weekly for first month, monthly for 6 months, then annually
- Existing systems: Annually minimum, more often if high-turndown or critical
Troubleshooting Common Issues
| Problem | Symptom | Likely Cause | Solution |
|---|---|---|---|
| No injection | Odorant level not dropping | Empty tank, pump failure, clogged line | Check level, inspect pump, clean nozzle |
| Low injection rate | Failed sniff test, high tank level | Pump wear, check valve leak, low set point | Recalibrate pump, replace check valve, adjust set point |
| High injection rate | Excessive odor complaints, low tank level | Flow computer error, pump over-stroking | Verify gas flow input, recalibrate pump |
| Erratic injection | Varying odor intensity | Air in pump, clogged suction filter | Prime pump, clean/replace filter |
| Pump cavitation | Noisy pump, poor performance | Low tank level, plugged suction | Refill tank, clear suction line |
| Nozzle clogging | High back pressure, no flow | Particulates, corrosion products | Install suction filter, clean nozzle |
| Seal leakage | Odorant dripping from pump | Worn seals/diaphragm, chemical attack | Replace seals with Viton/Kalrez |
Maintenance schedule: Industry best practice for Z-type odorizers: (1) Weekly visual inspection of pump and tank level, (2) Monthly sniff test at downstream sampling point, (3) Quarterly check of pump calibration, (4) Annual full calibration and service (rebuild pump if needed), (5) Every 2-3 years: Replace pump seals/diaphragm preventatively.
Odorant Consumption and Tank Sizing
Storage Tank Sizing:
Required capacity = Daily consumption × Refill interval × Safety factor
Daily consumption:
C_daily = Q_gas × D_rate / ρ_odorant
Where:
Q_gas = Average gas flow (MMscfd)
D_rate = Dosing rate (lb/MMscf)
ρ_odorant = Odorant density (lb/gal)
Refill interval: 30-90 days typical (depends on access, supplier schedule)
Safety factor: 1.2-1.5 (accounts for flow variations, odor fade periods)
Example:
City gate average flow: 75 MMscfd
Dosing rate: 1.0 lb THT/MMscf
THT density: 8.33 lb/gal
Refill interval: 60 days
Daily consumption:
C_daily = 75 × 1.0 / 8.33 = 9.0 gal/day
60-day consumption:
C_60day = 9.0 × 60 = 540 gallons
With 1.3 safety factor:
Tank required = 540 × 1.3 = 702 gallons
Select: 750-gallon stainless steel tank (next standard size)
Actual refill frequency:
750 gal / 9.0 gal/day = 83 days between refills
Low-level alarm setting:
Set at 15-20% capacity: 0.15 × 750 = 112 gallons
Provides 12-day warning before empty
5. Practical Applications
City Gate Station Odorization
Typical application: Add odorant as gas enters distribution system from transmission line:
IMAGE: City Gate Station Odorization Layout
Shows transmission line, meter run, regulator, odorizer location, and distribution main
City Gate Odorization Design:
System configuration:
1. Transmission line (unodorized) → City gate meter → Pressure regulation
2. Odorizer installed downstream of meter, upstream of distribution
3. Flow computer receives signal from turbine or orifice meter
4. Controller adjusts injection proportional to measured flow
Design parameters:
Flow range: 10-200 MMscfd (20:1 turndown typical)
Operating pressure: 300-600 psig (after regulation)
Dosing rate: 1.0 lb THT/MMscf
Temperature: -20°F to 120°F ambient
Equipment sizing:
Average flow: 100 MMscfd
Peak flow: 180 MMscfd (winter heating demand)
Average consumption: 100 × 1.0 / 8.33 = 12 gal/day
Peak consumption: 180 × 1.0 / 8.33 = 21.6 gal/day
Pump selection:
Capacity: 25 gal/day (116% of peak)
Type: Diaphragm or piston pump
Turndown: 20:1 minimum (0.5-10 gal/hr range)
Tank sizing:
60-day average supply: 12 × 60 = 720 gallons
Select: 1000-gallon tank (provides 83 days average, 46 days at peak)
Redundancy:
Install duplex pump system (one operating, one standby)
Automatic switchover on pump failure
Separate suction lines from common tank
Monitoring:
- Flow computer display (local and SCADA)
- Low tank level alarm (set at 200 gallons = 17 days)
- Pump failure alarm (current sensor or pressure switch)
- Monthly sniff test at downstream sample point (2-5 miles from injection)
District Regulator Station
Smaller systems serving neighborhoods or commercial districts:
District Regulator Odorization:
Typical flows: 1-20 MMscfd
Application: Supplemental odorization or isolated district
Pressure: 60-100 psig (low-pressure distribution)
Dosing rate selection:
If already odorized upstream: 0.5 lb/MMscf (top-up only)
If new mains in district: 1.5 lb/MMscf (odor fade compensation)
If sole odorization point: 1.0 lb/MMscf (standard)
Equipment:
Average flow: 5 MMscfd
Consumption: 5 × 1.0 / 8.33 = 0.6 gal/day
Pump: 0.5-2 gal/day range (peristaltic or small diaphragm)
Tank: 50-100 gallon (83-167 day supply)
Cost-effective approach:
Use pre-packaged odorizer skid (pump + tank + controller)
Typical cost: $10,000-25,000 installed
Alternative: Extend odorization from upstream station (if within 10 miles)
Maintenance:
Quarterly sniff test (higher frequency due to smaller system)
Semi-annual pump calibration
Tank refill 2-3× per year (coordinate with bulk delivery)
Odor Fade Testing Program
Required for new or rehabilitated mains to verify adequate odorant reaches end users:
Odor Fade Testing Procedure (New Mains):
Phase 1 - Pre-commissioning (before gas-in):
1. Flush main with air or nitrogen
2. Pressure test per ASME B31.8
3. Pre-odorize if coating/lining present:
- Circulate odorized nitrogen (2-3× normal dosing) for 24-48 hours
- Allows coating to absorb odorant before gas service
Phase 2 - Commissioning (initial gas-in):
1. Introduce odorized gas at 2-3× normal dosing rate
2. Collect gas samples at:
- Injection point (baseline)
- 25%, 50%, 75%, 100% of main length
3. Sniff test each sample at 1/5 LEL (10,000 PPM)
4. Document results: Pass/fail, odor intensity (1-5 scale)
Phase 3 - Quarterly testing (first year):
1. Maintain elevated dosing (2× normal) for first 3 months
2. Reduce to 1.5× normal for months 4-6
3. Reduce to 1.25× normal for months 7-9
4. Return to 1× normal after 9-12 months (if passing sniff tests)
5. Conduct quarterly sniff tests at multiple points
Acceptance criteria:
- ≥ 80% of panel detects odor at all sample points
- No sample point < 50% of injection point odor intensity
- If any point fails, increase dosing and retest in 30 days
Documentation:
- Sample location (address, GPS coordinates)
- Sample date/time
- Dilution method and final concentration
- Number of panelists and pass/fail votes
- Odorant injection rate during sampling
- Any fade mitigation actions taken
Typical fade behavior:
Weeks 1-4: 50-80% fade (rapid adsorption)
Months 2-3: 20-40% fade (decreasing)
Months 4-6: 10-20% fade (stabilizing)
Months 7-12: < 10% fade (approaching steady-state)
After 12 months: Minimal fade (< 5%), normal dosing sufficient
Emergency Response - Odorizer Failure
Action plan when odorizer fails and gas is being delivered without odorant:
- Immediate actions (within 1 hour):
- Switch to standby pump if duplex system installed
- If no standby, implement emergency odorization (portable injection system)
- Notify gas control and operations management
- Do NOT shut down gas flow (impacts customers)
- Within 4 hours:
- Mobilize repair technician or contractor
- Stage replacement pump/parts
- Conduct downstream sniff tests to assess odor level
- Notify state PUC or PHMSA if failure exceeds 24 hours
- Within 24 hours:
- Restore odorization or implement temporary system
- Conduct system-wide sniff testing (verify adequate odor at extremities)
- Document failure mode, duration without odorization, corrective action
- File incident report per company procedures
- If restoration > 24 hours:
- File written report with regulatory agency (state PUC/PHMSA)
- Implement continuous leak patrol (vehicle-mounted FID surveys)
- Notify emergency responders (fire department) of reduced odor
- Consider restricting new service connections until odorization restored
Odorant Handling Safety
| Hazard | Risk Level | Safety Measure |
|---|---|---|
| Flammability | High | No smoking, no open flames, ABC extinguisher within 25 ft |
| Skin/eye irritation | Moderate | Nitrile gloves, safety glasses, face shield for transfers |
| Respiratory irritation | Moderate | Outdoor storage, ventilation, organic vapor respirator for spills |
| Environmental contamination | Moderate | Secondary containment, spill kit, no storm drain entry |
| Persistent odor | Nuisance | Odor contamination of clothing/tools—handle in well-ventilated area |
Cost Analysis
Odorization Operating Costs:
Annual odorant cost:
Cost_odorant = (Q_gas × 365 days) × D_rate × C_unit
Where:
Q_gas = Average gas flow (MMscfd)
D_rate = Dosing rate (lb/MMscf)
C_unit = Unit cost of odorant ($/lb)
Example:
Q_gas = 100 MMscfd
D_rate = 1.0 lb/MMscf
C_unit = $6/lb (THT bulk pricing)
Cost_odorant = (100 × 365) × 1.0 × $6 = $219,000/year
Additional costs:
- Equipment amortization: $2,000-5,000/year (15-year life)
- Electricity (pump): $100-300/year
- Maintenance (annual service): $1,000-3,000/year
- Sniff testing (labor + lab): $5,000-15,000/year
- Regulatory compliance (documentation): $2,000-5,000/year
Total annual cost: $229,000-242,000 for 100 MMscfd system
Cost per customer:
For 50,000 customers: $229,000 / 50,000 = $4.58/customer/year
Typical residential customer uses 50-100 Mcf/year
Cost per Mcf: $4.58 / 75 Mcf = $0.06/Mcf (~0.5% of typical gas cost)
Conclusion: Odorization is inexpensive insurance for public safety
Best practice summary: (1) Use Z-type direct injection odorizer for city gate stations (most accurate), (2) Dose at 1.0 lb THT/MMscf as baseline (adjust for fade), (3) Install duplex pumps for critical applications (redundancy), (4) Conduct monthly sniff tests and annual pump calibration (compliance), (5) Maintain 60-90 day odorant inventory (supply chain buffer), (6) Document everything (regulatory requirement + liability protection).
Applicable Standards Summary
| Code/Standard | Title | Key Requirement |
|---|---|---|
| 49 CFR 192.625 | Odorization of Gas | Detectable at 1/5 LEL (10,000 PPM), periodic testing required |
| 49 CFR 192.605 | Procedural Manual for Operations | Written procedures for odorization system operation and maintenance |
| ASTM D5305 | Odorant Concentration by GC | Instrumental analysis method for odorant level verification |
| AGA Operations Conference | Odorization Best Practices | Industry guidance on fade testing, dosing rates, troubleshooting |
| NFPA 58 | LP-Gas Code | Minimum 1.0 lb odorant per 10,000 gal propane |
| State PUC regulations | Varies by state | May be more stringent than federal minimums |
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