Worked Example: Pipeline Tariff Gas Quality Specification Compliance & Treating Diagnostics

Interstate pipeline operators enforce strict tariff limits on transported natural gas. Off-specification gas introduces severe operational risks: internal pipeline corrosion from carbonic acid ($H_2CO_3$), lethal toxicity from hydrogen sulfide ($H_2S$), pipeline capacity derating from inert dilution ($N_2, CO_2$), and explosive or unstable burner flames from improper Wobbe Index.

When a producer delivers gas that violates any tariff threshold, pipeline operators have the statutory right to immediately shut in the interconnect valve or impose steep penalty tariffs. This worked example demonstrates how to evaluate gas quality against FERC (Federal Energy Regulatory Commission) generic pipeline standards and formulate corrective treating remedies.

1. Problem Statement & Pipeline Interconnect Data

A natural gas processing facility delivers treated gas to an interstate pipeline interconnect custody meter. Laboratory gas chromatography and on-line analyzer telemetry report the following stream properties:

Stream Composition:

• Methane ($\text{CH}_4$): $90.00\text{ mol}\%$
• Ethane ($\text{C}_2\text{H}_6$): $5.00\text{ mol}\%$
• Carbon Dioxide ($\text{CO}_2$): $2.50\text{ mol}\%$
• Nitrogen ($\text{N}_2$): $2.50\text{ mol}\%$

On-Line Quality Telemetry:

• Hydrogen Sulfide ($H_2S$): $3.50\text{ ppmv}$ ($4.97\text{ mg/Sm}^3$)
• Total Sulfur: $10.0\text{ mg/Sm}^3$
• Water Vapor Content: $5.50\text{ lb/MMSCF}$ ($88.1\text{ mg/Sm}^3$)
• Oxygen ($O_2$): $0.00\text{ mol}\%$ (not detected)
• Delivery Pressure: $65.0\text{ barg}$ ($957\text{ psia}$)
• Delivery Temperature: $20.0^\circ\text{C}$ ($68.0^\circ\text{F}$)

2. Step 1: Calculate Physical Properties per ISO 6976

Using the ISO 6976 / GPA 2172 engine at reference conditions ($15^\circ\text{C}, 101.325\text{ kPa}$):

A. Molar Mass:

$$M_{mix} = (0.90 \times 16.043) + (0.05 \times 30.070) + (0.025 \times 44.010) + (0.025 \times 28.013) = \mathbf{17.742\text{ g/mol}}$$

B. Real Relative Density:

$$d_{ideal} = \frac{17.742}{28.9626} = 0.6126 \implies d_{real} = \mathbf{0.6137}$$

C. Gross Heating Value ($HHV$):

$$HHV_{real} = \mathbf{37.322\text{ MJ/Sm}^3} \quad (1002.2\text{ BTU/scf})$$

D. Gross Wobbe Index ($W_G$):

$$W_G = \frac{HHV_{real}}{\sqrt{d_{real}}} = \frac{37.322}{\sqrt{0.6137}} = \frac{37.322}{0.7834} = \mathbf{47.642\text{ MJ/Sm}^3} \quad (1279.3\text{ BTU/scf})$$

3. Step 2: Tariff Compliance Audit vs FERC Standard

We evaluate the actual parameters against the standard FERC Interstate Pipeline Tariff limits:

Overall Tariff Verdict:

• Total Checks: 9
• Pass Count: 6
• Fail Count: 3
• Overall Status: ❌ REJECTED (SHUT-IN HAZARD)

4. Step 3: Root Cause Analysis & Engineering Remedy

Three interlinked parameters caused the rejection: 1. $\text{CO}_2$ exceeds $2.0\text{ mol}\%$ ($2.50\%$). 2. Total Inerts exceed $4.0\text{ mol}\%$ ($5.00\%$). 3. Wobbe Index is below $48.80\text{ MJ/Sm}^3$ ($47.642\text{ MJ/Sm}^3$).

Notice the root cause: The excess $2.50\text{ mol}\%$ $\text{CO}_2$ acts as a thermal ballast (inert dilution) that depresses the gross heating value and increases relative density, driving the Wobbe Index down below the lower tariff boundary.

Corrective Process Modifications:

graph TD
    A[Raw Gas: CO2=2.5%, Inerts=5.0%, Wobbe=47.64] --> B[Amine Sweetening Unit Trim]
    B --> C[Increase MDEA Circulation Rate +15%]
    C --> D[Treated Gas: CO2=1.2%, Inerts=3.7%]
    D --> E[Re-evaluate Wobbe Index]
    E --> F[New Wobbe: 49.12 MJ/Sm3]
    F --> G[Full FERC Tariff Compliance: 9/9 PASS]

Quantitative Remediation Plan:

1. Increase Amine Contactor Performance:
2. By increasing lean amine circulation rate or boosting amine regeneration reboiler steam, reduce treated gas $\text{CO}_2$ from $2.50\text{ mol}\%$ down to $1.20\text{ mol}\%$.
3. Resulting Gas Composition:
4. Methane: $91.20\text{ mol}\%$
5. Ethane: $5.07\text{ mol}\%$
6. Nitrogen: $2.53\text{ mol}\%$
7. Carbon Dioxide: $1.20\text{ mol}\%$
8. Total Inerts: $3.73\text{ mol}\%$ ($\le 4.0\text{ mol}\%$ $\to$ PASS)
9. Resulting Wobbe Index:
10. Recalculated $HHV_{real} = 37.85\text{ MJ/Sm}^3$
11. Recalculated $d_{real} = 0.594$
12. Recalculated $W_G = \frac{37.85}{\sqrt{0.594}} = \mathbf{49.12\text{ MJ/Sm}^3}$ ($\ge 48.80\text{ MJ/Sm}^3$ $\to$ PASS)

5. Key Takeaways for Pipeline Operations

1. Inert Penalties: Inerts ($\text{CO}_2, \text{N}_2$) do not burn; paying pipeline transportation tariffs on inerts is paying to compress and pump dead weight.
2. Corrosion Mechanism: At $65\text{ barg}$, a $\text{CO}_2$ partial pressure of $2.5\% \times 66\text{ bar} = 1.65\text{ bar}$ creates extreme mild carbon steel corrosion rates ($>2\text{ mm/yr}$) if any free water condenses.
3. Automated Interlock: Facilities must equip custody analyzers with high-$\text{CO}_2$ and low-Wobbe alarm interlocks to divert off-spec gas to flare or recycle before it crosses custody boundaries.