NGL / LPG Volume Correction (CTL) Fundamentals

Propane in a sphere on a hot afternoon can occupy several percent more volume than the same propane at 60 °F — light hydrocarbons expand dramatically with temperature, far beyond what the crude-and-products tables of API MPMS Chapter 11.1 cover. GPA Midstream Standard 8217 (GPA TP-27, API MPMS 11.2.4) is the standard built for them, using a corresponding-states model anchored to twelve reference fluids.

Result

CTL = V₆₀ / Vₜ

Volume to the 60 °F base.

Range

γ₆₀ 0.35–0.69

Ethane through hexanes.

Standard

GPA 8217 / TP-27

= API MPMS 11.2.4.

1. Why Light Hydrocarbons Are Different

The generalized correlation of API MPMS Chapter 11.1 was fit to crude oils, refined products and lubricating oils — fluids denser than about 610 kg/m³ (below roughly 100 °API). Natural gas liquids and LPG (ethane, propane, butane, natural gasoline, Y-grade mixtures) are lighter and far more compressible and expansive. Applying the 11.1 tables to them gives materially wrong volume corrections. GPA 8217 / TP-27 fills exactly that gap with a model valid for the light end.

Magnitude: a propane stream can have a CTL several percent away from unity over normal ambient swings — an order of magnitude more sensitive than crude. Getting the right method is not academic; it is large volume.

2. Scope & Limits

Variable	Range (Table 24E)
Relative density at 60 °F (γ₆₀)	0.3500 to 0.6880
Observed temperature	227.15 to 366.15 K (−50.8 to 199.4 °F)

If the reduced temperature exceeds 1.0 the fluid is supercritical and cannot exist as a liquid — the procedure flags CTL = −1. Below γ₆₀ 0.35 or above 0.688, the fluid falls outside the reference-fluid set (use 11.1 for the heavier end).

3. The Corresponding-States Method

The fluid is characterized solely by its relative density at 60 °F. Two reference fluids from a table of twelve (ethane/ethylene blends through n-heptane) are chosen so one is just denser and one just lighter than the fluid. Each reference fluid has a critical temperature, critical density, critical compressibility, and four saturation-density fitting parameters. Their saturation densities are evaluated at the fluid's own reduced temperature and interpolated to represent the actual fluid — the essence of corresponding states.

Saturation density: ρ_sat = ρc · [ 1 + (k₁·τ^0.35 + k₃·τ² + k₄·τ³) / (1 + k₂·τ^0.65) ], τ = 1 − Tr Scaling factor: h₂ = (Zc₁·ρc₁) / (Zc₂·ρc₂)

4. The Calculation Steps (Table 24E)

1. Round γ₆₀ to 0.0001, TF to 0.1 °F; Tx = (TF + 459.67)/1.8 (K) 2. Check ranges (else CTL = −1) 3. Pick reference fluids 1 (lighter) and 2 (denser) bracketing γ₆₀ 4. δ = (γ₆₀ − γ₁) / (γ₂ − γ₁) 5. Tc = Tc₁ + δ(Tc₂ − Tc₁); Tr,x = Tx/Tc; Tr,60 = 519.67/(1.8·Tc) 6. ρ_sat for both fluids at Tr,60 and at Tr,x 7. X = ρ₆₀,₁ / (1 + δ(ρ₆₀,₁/(h₂ρ₆₀,₂) − 1)) 8. CTL = ρₓ,₁ / [ X·(1 + δ(ρₓ,₁/(h₂ρₓ,₂) − 1)) ] → round 5 dp

5. Worked Example (Example 24/3)

A fluid with relative density 0.4515 at 60 °F, observed at 87.4 °F:

Tx = (87.4 + 459.67)/1.8 = 303.927778 K Ref fluids = EP (65/35) and EP (35/65) δ = (0.4515 − 0.429277)/(0.470381 − 0.429277) = 0.540653 Tc = 343.82887 K Tr,x = 0.883951; Tr,60 = 0.839678; h₂ = 1.103943 CTL = 0.93275

This calculator reproduces the standard's published result of 0.93275 exactly. A volume of 1,000 bbl observed at 87.4 °F is therefore 932.75 bbl at 60 °F — a 6.7% shrinkage that the right method is essential to capture.

6. Standards & References

Standard	Scope
GPA Midstream Standard 8217	= GPA TP-27 = API MPMS Ch. 11.2.4 — temperature correction (CTL) for light hydrocarbons; Tables 23E/24E/53E/54E/59E/60E
API MPMS Ch. 11.1 / ASTM D1250	Crude oils, refined products, lubricating oils (heavier than NGL/LPG)
API MPMS Ch. 11.2.2	Compressibility (CPL) for hydrocarbons

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Frequently Asked Questions

Why can't API MPMS 11.1 be used for NGL/LPG?

Light hydrocarbons are below the density range of Chapter 11.1 and expand far more with temperature; GPA 8217 / TP-27 is fit specifically for them.

How does the corresponding-states method work?

Two reference fluids bracketing the fluid's 60 °F relative density have their saturation densities scaled to the fluid's reduced temperature and interpolated to give CTL.

What if the reduced temperature exceeds 1.0?

The fluid is supercritical and cannot exist as a liquid, so the procedure flags CTL = −1.