1. Why Blends Shrink
Liquids are not perfectly additive. When two hydrocarbons of very different molecular size mix, the smaller light molecules occupy the interstitial spaces between the larger heavy molecules rather than simply adding their own volume. The result is a mixture whose actual volume is slightly less than the arithmetic sum of the component volumes — a negative excess volume of mixing. The effect is small in percentage terms but, on pipeline- and terminal-scale volumes, can be hundreds of barrels.
2. The API MPMS 12.3 Correlation
API MPMS Chapter 12.3 gives an empirical correlation (§5.3) fitted to measured blending data, used to generate the standard's shrinkage tables. In customary units:
An SI form (Eq. 3) replaces G with an inverse-density difference. This calculator implements the customary equation directly — the same equation the standard used to build its lookup tables, so it reproduces the tabulated values without interpolation error.
3. What Drives Shrinkage
Gravity difference (G)
Strongest driver
With a G^2.28 exponent, shrinkage rises steeply as the light and heavy gravities diverge — light condensate into heavy crude shrinks far more than two similar crudes.
Light fraction (C)
Peaks mid-range
The C·(100−C)^0.819 form is zero at 0% and 100% (pure components) and largest at intermediate light concentrations.
Scale
Volume × S
A 0.1% shrinkage is trivial per barrel but ~100 bbl on a 100,000-bbl blend.
4. Valid Ranges & Limits
The correlation is fitted within, and tabulated (Table 3) over, the following ranges:
| Variable | Range |
|---|---|
| Light-component concentration (C) | 1% to 99% |
| API gravity difference (G) | 10° to 100° API |
Outside these bounds the correlation is extrapolated and should be used cautiously. The standard also notes the result is the shrinkage at the measured/standard conditions; temperature and pressure effects on the components are handled separately by the Chapter 11/12 volume-correction chain before blending volumes are combined.
5. Worked Example (§5.4.1)
Blend 5,000 bbl of 86.5 °API natural gasoline with 95,000 bbl of 30.7 °API crude:
This calculator reproduces the standard's tabulated/worked result of ≈ 0.0972% and ~97 bbl. The two streams that "should" make 100,000 bbl actually deliver about 99,903 bbl — the 97-barrel difference is exactly what loss-control and custody reconciliation must capture.
6. Standards & References
| Standard | Scope |
|---|---|
| API MPMS Ch. 12.3 (1996, R2011) | Volumetric Shrinkage Resulting From Blending Light Hydrocarbons With Crude Oils — §5.3 equations, Table 3 |
| API MPMS Ch. 12.2 | Calculation of Petroleum Quantities — dynamic measurement |
| API MPMS Ch. 11.1 / ASTM D1250 | Temperature/pressure volume correction (applied to components before blending) |