LPG Vapor Pressure & Blend Sizer
Calculate True Vapor Pressure (TVP), density, pump NPSHa cavitation safety, and NFPA 58 safe liquid filling limits for Propane/Butane LPG blends.
LPG Blend Selection
Operating Storage Conditions
Pump Suction Line (NPSHa Check)
Storage Tank Filling Safety (NFPA 58)
About This Tool
The LPG True Vapor Pressure & Propane-Butane Blend Sizer is a dedicated design utility for midstream terminals, refinery gas plants, LPG bottling stations, and fuel distribution networks. Liquid LPG is a multi-component mixture of Propane, n-Butane, and isobutane whose thermodynamic equilibrium behavior changes drastically with temperature and composition.
This tool evaluates True Vapor Pressure (ASTM D1267 / ASTM D2598), liquid density at operating and standard reference conditions (ISO 8973), pump NPSHa cavitation vulnerability, and mandatory tank overfill prevention thresholds (NFPA 58 / EN 12493).
True Vapor Pressure (TVP) & Storage Tank MAWP
In pressurized storage bullets and transport road tankers, the vessel design pressure (MAWP) must be selected based on the maximum vapor pressure generated by the blend at summer ambient temperatures (typically 45°C to 55°C / 115°F to 130°F).
Because pure Propane exhibits a much higher vapor pressure (~17.5 bar at 50°C) compared to Butane (~4.9 bar at 50°C), optimizing the Propane-to-Butane ratio directly impacts vessel shell thickness, relief valve set pressures, and capital equipment costs.
Pump NPSHa & Cavitation in Boiling Liquid Service
Liquid LPG stored in pressurized bullets is at its bubble point (boiling liquid in equilibrium with vapor). Consequently, the vapor pressure inside the vessel exactly equals the surface pressure:
NPSHa = H_static - h_friction + (P_pad / ρg)
Because the tank pressure and vapor pressure cancel each other out, available net positive suction head is purely provided by static liquid elevation above the pump centerline minus suction piping friction loss. If static head is insufficient, boiling liquid flashes into vapor within the impeller eyes, resulting in severe cavitation and seal failure.
Safe Liquid Filling Density (The 80% Outage Rule)
Liquid LPG possesses an exceptionally large thermal expansion coefficient (expanding roughly ten times faster than liquid water). If an operator fills a storage vessel 100% full of liquid at 15°C, and summer solar radiation warms the vessel to 45°C, the expanding liquid has no vapor space to occupy. Because liquids are virtually incompressible, hydraulic pressure escalates catastrophically within seconds, causing violent vessel rupture.
Standards (NFPA 58, EN 12493) mandate that tanks be filled only to a maximum density that guarantees at least 3% to 5% vapor cushion remains at peak design temperature.
Frequently Asked Questions
In standard water pumping from atmospheric tanks, atmospheric pressure (10.3 m of liquid column) provides substantial head pushing fluid into the pump. With boiling LPG, however, the liquid is at its boiling point, so its vapor pressure cancels out the tank pressure. The only driving force into the pump is physical gravity elevation (static head). A minimum static liquid level of 2.0 to 3.0 meters above the pump suction nozzle is universally recommended for LPG service.
In cold winter climates, commercial Autogas typically increases Propane content (e.g. 60% to 70% Propane) to ensure sufficient vapor pressure for vehicle engine ignition and fuel vaporization at sub-zero temperatures. In hot summer conditions, Butane content is increased (e.g. 60% to 70% Butane) to prevent excessive tank pressures under direct sun exposure.
RVP (ASTM D323) is an empirical laboratory test measured in a bomb with a fixed 4:1 air-to-liquid vapor space ratio at 37.8°C (100°F). TVP (ASTM D1267 / ASTM D2598) is the actual thermodynamic bubble point pressure exerted by the liquid with zero vapor space at operating temperature. TVP is the actual physical pressure used for mechanical vessel design and pump sizing.
Input / Output Units
Specific to the active calculator.