LPG Vapor Pressure, Propane-Butane Blending & Pump NPSHa Dynamics

Liquefied Petroleum Gas (LPG) is a versatile, clean-burning hydrocarbon fuel composed primarily of commercial Propane ($C_3H_8$), normal Butane ($n-C_4H_{10}$), and isobutane ($i-C_4H_{10}$).

Unlike single-component fluids, LPG is a non-ideal binary mixture stored as a compressed liquid near its bubble point. Designing storage bullets, transfer pumps, and autogas dispensing stations requires rigorous understanding of vapor-liquid equilibrium and thermal expansion.

1. True Vapor Pressure (TVP) & Bubble Point Calculation

Under ASTM D2598 and ISO 8973, the True Vapor Pressure ($P_{vap}$) of an LPG mixture at temperature $T$ is calculated from pure component saturation pressures weighted by liquid mole fractions ($x_i$):

$$P_{bubble}(T) = \sum_{i=1}^{n} x_i \cdot P_{vap,i}(T)$$

Where: * $P_{vap,i}(T)$ is the saturation vapor pressure of pure component $i$ at temperature $T$. * $x_i$ is the liquid mole fraction of component $i$.

Pure component vapor pressures vary dramatically across typical ambient ranges: * At 20°C (68°F): Propane $= 8.36 \text{ barg}$, n-Butane $= 1.08 \text{ barg}$. * At 50°C (122°F): Propane $= 17.52 \text{ barg}$, n-Butane $= 4.93 \text{ barg}$.

Because Propane generates nearly four times the vapor pressure of Butane at summer temperatures, selecting the blend ratio dictates the design pressure (MAWP) of storage tanks under ASME Section VIII.

2. Pumping Boiling LPG: The NPSHa Challenge

Centrifugal and regenerative turbine pumps handling LPG are prone to cavitation. Because liquid LPG is stored in pressurized equilibrium with its own vapor phase ($P_{tank} = P_{vap}$), the vapor pressure cancels the tank pressure in the Net Positive Suction Head equation:

$$\text{NPSHa} = \frac{P_{tank} - P_{vap}}{\rho_{liq} \cdot g} + H_{static} - h_{friction} = H_{static} - h_{friction}$$

(Assuming no nitrogen or fuel gas pad pressure).

This means atmospheric pressure provides zero assistance in feeding the pump. The available suction head is solely the physical liquid height ($H_{static}$) minus suction pipe friction losses ($h_{friction}$).

To avoid cavitation damage: 1. Always maintain a minimum static liquid head of at least $1.5$ to $2.5 \text{ m}$ ($5$ to $8 \text{ ft}$) above the pump suction centerline. 2. Maintain suction piping velocities below $1.0 \text{ m/s}$ ($3.3 \text{ ft/s}$) to minimize friction drop. 3. Install low-NPSHr pumps (e.g., canned motor pumps, multi-stage side-channel pumps).

3. Safe Liquid Filling Limits (NFPA 58 & EN 12493)

Liquid LPG exhibits a volumetric expansion coefficient of approximately $0.003 \text{ K}^{-1}$, meaning its volume expands ten times faster than water under the same temperature rise.

If a tank is filled 100% full of liquid at 15°C, subsequent ambient heating to 45°C causes thermal expansion to hydraulically compress the liquid, generating thousands of bar of hydraulic pressure and rupturing the vessel shell.

To maintain a mandatory safety cushion (at least $3\%$ to $5\%$ vapor space at maximum summer design temperature $T_{max}$), the maximum permissible fill fraction ($V_{fill,\%}$) at filling temperature $T_{fill}$ is defined by:

$$V_{fill,\%} = \frac{\rho_{liq}(T_{max})}{\rho_{liq}(T_{fill})} \cdot 95\%$$

For commercial propane, filling at 15°C with a maximum summer ambient of 45°C limits the maximum safe fill volume to ~86%, which is the thermodynamic origin of the universal "80-85% LPG Tank Filling Rule".