API Gravity & Viscosity Blending for Hydrocarbons

The accurate prediction of physical properties for blended hydrocarbon streams is critical for refinery operations, midstream pipeline hydraulics, and crude oil valuation. This fundamental engineering guide details the methodology for calculating the blended API gravity, specific gravity, and kinematic viscosity of multi-stream hydrocarbon mixtures. ## 1. The Relationship Between Density, Specific Gravity, and API Gravity The American Petroleum Institute (API) gravity is a standard measure of how heavy or light a petroleum liquid is compared to water. * If its API gravity is **greater than 10**, it is lighter and floats on water. * If it is **less than 10**, it is heavier and sinks. The mathematical conversion between Specific Gravity (SG) at 60°F and API gravity is defined by **ASTM D287**: $$ SG = \frac{141.5}{API + 131.5} $$ $$ API = \frac{141.5}{SG} - 131.5 $$ ### Global Crude Classifications The U.S. Energy Information Administration (EIA) generally classifies crude oil as follows: * **Condensate**: > 50.0° * **Light Crude**: 31.1° to 50.0° * **Medium Crude**: 22.3° to 31.1° * **Heavy Crude**: 10.0° to 22.3° * **Extra Heavy**: < 10.0° ## 2. Density Blending (Linear) When two or more streams are mixed, density (and thereby Specific Gravity) behaves as an extensive property and blends linearly by **volume** or **mass**. Assuming ideal mixing with no shrinkage, the specific gravity of a blend is given by: $$ SG_{blend} = \frac{\sum (Volume_i \times SG_i)}{\sum Volume_i} $$ ## 3. Viscosity Blending (Non-Linear) Unlike density, kinematic viscosity **does not blend linearly**. A simple weighted average will drastically overestimate the final viscosity of the blend. To solve this, engineers utilize a mathematical transformation known as the **Viscosity Blending Index (VBI)**, derived from the Baird & Refutas equation. The Refutas method follows three steps: **Step 1: Calculate the VBI for each stream** based on its kinematic viscosity ($v_i$) in centistokes (cSt). $$ VBI_i = 10.975 \times \ln(\ln(v_i + 0.8)) + 14.534 $$ **Step 2: Blend the VBI values** linearly using **mass fractions** ($w_i$). $$ VBI_{blend} = \sum (w_i \times VBI_i) $$ **Step 3: Reverse the Refutas equation** to solve for the final blended kinematic viscosity ($v_{blend}$). $$ v_{blend} = \exp\left[\exp\left(\frac{VBI_{blend} - 14.534}{10.975}\right)\right] - 0.8 $$ *Note: While mass fractions are strictly correct, volume fractions are occasionally substituted in field approximations if the specific gravities of the streams are nearly identical.*