FieldEngineersKitEngineering Reference & ASME Code Basis
1. Core Formula & Variable Definitions
AGA Report No. 8 / API MPMS Chapter 14.2 — Compressibility Factor of Natural Gas and Related Hydrocarbon Gases (screening use) · Standing / Sutton pseudo-critical correlations; Hall–Yarborough Z; CNGA method fallback · Wichert–Aziz acid-gas correction (CO₂); Kay mixing rule for N₂ · ISO 12213-2 — molar-composition compression factor (full GERG/AGA-8 Detail out of scope here)
Real-gas density: ρ = P · M / (Z · R · T) = P · SG · Mair / (Z · R · T)
Standing pseudo-criticals (sweet gas): Tpc = 168 + 325·SG − 12.5·SG² [°R], Ppc = 677 + 15·SG − 37.5·SG² [psia]
Reduced properties: Tr = T/Tpc, Pr = P/Ppc · Z = f(Pr, Tr) via Hall–Yarborough (CNGA fallback)
AGA Report No. 8 / API MPMS 14.2 screening · Wichert–Aziz CO₂ · Kay N₂ blend · ISO 12213-2 composition methods out of scope
Screening Z from Standing Ppc/Tpc with Wichert–Aziz CO₂ correction and Kay N₂ blend, then Hall–Yarborough (CNGA fallback). Density uses the real-gas law with M = SG · 28.9625 g/mol. Not a full AGA-8 Detail or GERG-2008 composition equation of state.
- Z (Compressibility factor) — Real-gas Z from Hall–Yarborough on reduced P_r, T_r (ideal gas Z = 1).
- ρ (Real-gas density) — ρ = P M / (Z R T) in kg/m³ or lb/ft³.
- SG (Specific gravity (air = 1)) — Typical lean natural gas 0.55–0.75. M = SG · M_air.
- P / T (Absolute pressure / temperature) — Absolute operating P (bar abs or psia) and T (°C/°F → K/°R).
- P_pc / T_pc (Pseudo-critical pressure / temperature) — Standing correlations; CO₂ via Wichert–Aziz; N₂ via Kay blend.
- P_r / T_r (Reduced pressure / temperature) — P/P_pc and T/T_pc used by the Z correlation.
2. Screening Rules Matching This Calculator
Standing sweet-gas Ppc/Tpc, optional Wichert–Aziz CO₂ and Kay N₂ blend, then Hall–Yarborough Z with CNGA fallback. Real density from the real-gas law. Full AGA-8 Detail / GERG-2008 composition EOS is out of scope.
Lean natural gas M ≈ 17.38 g/mol. Hero ≈ Z = 0.935 · ρ = 22.50 kg/m³ (P_r ≈ 0.65, T_r ≈ 1.50).
CO₂ and N₂ each 0–20 mol%. Combined non-hydrocarbon fraction capped for this screening path.
H₂S not entered in this tool (treated as zero). High acid-gas fractions need composition EOS verification.
For P ≳ 150 bar, supercritical transport, or cryogenic phase envelopes, verify with full molar-composition methods (ISO 12213-2 / AGA-8 Detail).
Quick Reference Lookup Table
| Quantity | Value | Unit | Notes |
|---|---|---|---|
| Z | 0.935 | — | Hall–Yarborough |
| ρ (real) | 22.50 | kg/m³ | 1.404 lb/ft³ |
| ρ (ideal) | 21.03 | kg/m³ | Z = 1 |
| Deviation | +7.0% | — | (ρ − ρ_ideal)/ρ_ideal |
| P_r / T_r | 0.647 / 1.497 | — | Reduced properties |
| P_pc / T_pc | 46.37 / 199.2 | bar / K | 672.5 psia / 358.5 °R |
| M | 17.38 | g/mol | SG × 28.9625 |
| 50 bar · 40 °C · SG 0.65 | Z 0.896 · ρ 40.33 | kg/m³ | High-pressure duty |
| 10 bar · 15 °C · SG 0.55 | Z 0.979 · ρ 6.79 | kg/m³ | Distribution duty |
| 80 bar · 50 °C · SG 0.60 | Z 0.883 · ρ 58.60 | kg/m³ | Transmission duty |
Hero reports Z and real density from Standing Ppc/Tpc + Hall–Yarborough. Default duty (30 bar abs · 25 °C · SG 0.60) asserts Z ≈ 0.935 and ρ ≈ 22.50 kg/m³.
3. Applicability & Fluid Limits
This tool is fluid-property screening for lean-to-moderately rich natural gas. Metallurgy is not computed — apply pipe/valve material selection separately.
| Group / Regime | Range / Condition | Limit / Criterion | Engineering Notes |
|---|---|---|---|
| Lean natural gas (SG ≈ 0.55–0.70) | −50 to 150 °C screening band | Standing Ppc/Tpc valid | Typical transmission / distribution SG. Default SG 0.60 is the worked example. |
| CO₂ / N₂ impurities (≤ 20 mol% each) | Same screening band | Wichert–Aziz · Kay blend | Acid-gas and inert corrections adjust Ppc/Tpc only — not a full multicomponent EOS. |
| High-pressure / supercritical NG (P ≳ 150 bar) | N/A | Out of screening confidence | Verify Z and density with AGA-8 Detail or GERG-2008 before custody transfer or dense-phase design. |
| LNG / cryogenic phase envelopes | Below screening band | Out of scope | Two-phase and cryogenic properties require composition-based flash calculations — not this Z/ρ path. |
Code Applicability & Safety Boundaries
- Screening Z/ρ only — not AGA-8 Detail or ISO 12213-2 certified composition compressibility
- Absolute pressure required (bar abs / psia). Do not enter gauge pressure without adding atmospheric pressure
4. Step-by-Step Worked Example
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Worked example — 30 bar abs · 25 °C · SG 0.60
Lean natural gas at 30 bar absolute and 25 °C with SG = 0.60 (air = 1), no CO₂ or N₂.
Pseudo-criticals (Standing)
Reduced properties
Z and density
5. How to calculate natural gas Z-factor and density
- 1
Enter absolute pressure and temperature
Use bar abs or psia (not gauge) and operating °C or °F in the active unit system.
- 2
Enter specific gravity and impurities
SG relative to air (typical 0.55–0.75). Optionally add CO₂ and N₂ mole % for Wichert–Aziz / Kay adjustments.
- 3
Read Z and real density
Hero shows Z and ρ. Badges summarize SG, P_r, T_r, and ideal-vs-real density deviation.
- 4
Export or carry to related tools
Export CSV/text/PDF for the duty pack, or continue to valve Cv / choked / noise screening with the same pressure context.
6. Frequently Asked Questions & Technical References
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A screening tool that estimates Z from reduced pressure and temperature (Standing P_pc/T_pc + Hall–Yarborough) and converts that into real-gas density ρ = P M / (Z R T).
Standing pseudo-criticals, optional Wichert–Aziz (CO₂) and Kay (N₂), then Hall–Yarborough with a CNGA fallback. It is not a full AGA-8 Detail or GERG-2008 composition EOS.
Ideal density assumes Z = 1. Real density divides by Z, so when Z < 1 the gas is denser than ideal. The default duty shows about +7% denser than ideal.
For custody transfer, P ≳ 150 bar, supercritical transport, LNG envelopes, or rich acid-gas compositions — verify with full molar-composition methods (ISO 12213-2 / AGA-8 Detail).