Petroleum Refining & Midstream Blending
Crude Oil API Gravity & Pipeline Blend Formula
Convert API Gravity to 60°F Specific Gravity SG = 141.5 / (API + 131.5), compute Exact Volumetric Blended Specific Gravity, and recover True Blended API Gravity without linear-API averaging error.
Never take a volume-weighted average of API degrees directly!
American Petroleum Institute Gravity at 60°F is defined inversely from Specific Gravity relative to water (SG_60): API = (141.5 / SG_60) - 131.5 (so pure water SG_60 = 1.0000 equals 10.00° API, heavy bitumen < 10° API sinks in water, and light West Texas Intermediate ~40° API floats)! Because mass and volume conserve ADDITIVELY in Specific Gravity (assuming ideal zero-shrinkage mixing)—SG_blend = (V_1 * SG_1 + V_2 * SG_2) / (V_1 + V_2)—averaging API degrees directly by volume is mathematically wrong: harmonic convexity guarantees True Blended API = 141.5 / SG_blend - 131.5 is ALWAYS strictly LOWER than the naive volume-weighted API average whenever API_1 != API_2!
Verify on Heavy_Crude_API_1 = 20.0° API (B2 -> SG_1 = 141.5 / 151.5 = 0.9340 in E2), Heavy_Volume V_1 = 50,000 bbl (B3), Light_Sweet_API_2 = 40.0° API (B4 -> SG_2 = 141.5 / 171.5 = 0.8251), and Light_Volume V_2 = 50,000 bbl (50/50 volumetric blend, B5): Blended Specific Gravity is 0.50 * 0.93399 + 0.50 * 0.82507 = 0.8795 (E3); True Blended API Gravity is 141.5 / 0.87953 - 131.5 = 29.38° API (E4); whereas Naive Volume-Weighted API falsely reports 30.00° API—a -0.62° API quality penalty (E5) that can breach a pipeline 30.0° API tariff specification!
| Midstream Crude Blending Metric (50k bbl 20° API + 50k bbl 40° API) | Formula | Verified Output |
|---|---|---|
| Stream 1 Specific Gravity SG_1 | 141.5 / (API_1 + 131.5) | 0.9340 (vs SG_2 = 0.8251) |
| Blended Specific Gravity SG_blend | (V_1*SG_1 + V_2*SG_2) / (V_1 + V_2) | 0.8795 |
| True Blended API Gravity (°API) | 141.5 / SG_blend - 131.5 | 29.38° API (Below 30° Spec!) |
| Harmonic Blend Bias vs Naive Avg | True_API_Blend - Naive_Vol_Avg_API | -0.62° API |
Exact Specific-Gravity-Routed Blended API Formula
Validate API_1 > -131.5 (physically API >= 0 for commercial bitumens/crudes) and V_1 + V_2 > 0!
=LET(ap_1, B2, vl_1, B3, ap_2, B4, vl_2, B5, IF(OR(ap_1 < 0, vl_1 < 0, ap_2 < 0, vl_2 < 0, vl_1 + vl_2 <= 0), "Invalid Crude Blend", LET(sg_1, 141.5 / (ap_1 + 131.5), sg_2, 141.5 / (ap_2 + 131.5), sg_b, (vl_1 * sg_1 + vl_2 * sg_2) / (vl_1 + vl_2), ROUND(141.5 / sg_b - 131.5, 2))))Linear-API Averaging Error Audit Formula
Test blending Naphtha Condensate (API_2 = 65.0°) 50/50 with Heavy 15.0° Bitumen: Naive average says 40.00° API, while True Blended API is only 36.36° API (-3.64° error)!
=IF(ISTEXT(E4), E4, ROUND(E4 - (B3 * B2 + B5 * B4) / (B3 + B5), 2))Try the example
Use the original values or change them to test your understanding.