Liquefied petroleum gas (LPG) is propane, butane or a mixture of the two: a gas at atmospheric pressure that can be kept liquid at ambient temperature under moderate pressure. Every loading, metering and safety requirement at an LPG terminal follows from that single property. The product always moves in a closed system, together with its own vapor phase and under pressure, and wherever pressure drops the liquid starts to boil. Terminal design therefore centers on keeping the liquid liquid where it must be, managing the vapor phase in a controlled way, and stopping flow quickly if a release occurs.
Fluid behavior: pressure, temperature and two phases
In a storage tank or a road tanker, liquid LPG and the vapor above it are in equilibrium. The pressure of that equilibrium, the vapor pressure, rises with temperature and depends on the propane-to-butane ratio. Propane-rich mixtures have a higher vapor pressure and butane-rich mixtures a lower one, and density also varies with composition and temperature. Where product changes with the season or the supplier, pump, meter and safety settings may need to be reviewed.
Key engineering topics at LPG terminals
· Flashing and cavitation: When pressure at the pump suction, in a valve or in the meter falls below vapor pressure, the liquid flashes. Pump cavitation and two-phase flow through the meter, with a distorted reading, are the typical results.
· Keeping the meter full of liquid: Legal metrology documents require product in pressurized liquefied gas measuring systems to stay liquid during measurement. In practice this is achieved with back pressure control downstream of the meter and, where needed, a vapor eliminator.
· Vapor return: During loading, vapor from the receiving tank is routed back to the supplying tank through a separate line to balance pressure. For custody measurement, how quantity returned through that line is handled must be defined.
· Mass or volume: LPG is often traded by mass; where volume is the basis, temperature and density correction become decisive.
· Emergency shutoff and interlocks: Remotely operated shutoff valves, breakaway couplings, gas detection, grounding and vehicle position interlocks; which function requires which level of reliability is set by risk assessment.
Bulk LPG terminals versus cylinder filling plants
This page and its subpages cover terminals where LPG moves in bulk between storage tanks, road tankers, pipelines or rail cars. Cylinder filling plants, where individual cylinders are filled by weight and handled on conveyors, are a different process and regulatory field and fall outside the scope of these pages. Road tankers themselves are defined by separate European standards for their pressure vessel and equipment, and terminal-side connections and safety design must be compatible with that vehicle equipment.
How LPG differs from NGL
Natural gas liquids (NGL) are a broader group that includes ethane, propane, butanes and natural gasoline, and their composition can vary widely with the source. LPG corresponds to the propane and butane part of that group, with composition defined within a narrower band. In mixed NGL streams, variability in density and vapor pressure makes measurement design more demanding; that difference is covered on the NGL Terminals page.
Solutions for LPG terminals
· LPG truck loading systems: Closed, controlled tanker loading with liquid and vapor lines.
· LPG truck unloading systems: Pump versus compressor unloading compared.
· LPG metering skids: Keeping the meter in liquid phase with back pressure and vapor elimination.
· LPG custody transfer metering: Commercial unit of measure, density determination and returned vapor.
· LPG loading and unloading skids: What a pre-assembled package covers and its site interfaces.
· Propane and butane metering: How blend ratio affects density and correction.
· LPG vapor return systems: The role and sizing of the vapor balancing line.
· ESD and safety interlocks for LPG terminals: Emergency shutdown, gas detection and loading permissives.
TLY Enerji's scope at LPG terminals
TLY Enerji provides design, product supply and engineering services for LPG truck loading and unloading stations. The scope can include LPG pumps, flow meters, pressure regulators, safety valves, loading and unloading equipment, and control and automation systems. Depending on project needs, support can also extend to site installation, testing and commissioning; the scope of supply is agreed on each project from the technical specification.
LPG terminal questions
What is the most common problem in LPG metering?
The most frequent issue is partial vaporization where pressure approaches the product's vapor pressure, sending two-phase flow through the meter and distorting the measured quantity. Correct pump suction design, maintaining back pressure downstream of the meter and using a vapor eliminator where needed all reduce that risk.
Is a vapor return line mandatory for LPG truck loading?
In closed loading, returning vapor from the receiving tank to the supplying tank balances pressure and eases loading, which is why it is widely used. For custody measuring systems, legal metrology documents set conditions such as measuring and subtracting the returned vapor, or not allowing the connection at all. The arrangement applied depends on project regulations and the contract.
Are LPG terminals and cylinder filling plants designed with the same solutions?
No. A bulk LPG terminal focuses on meters, pumps, loading arms and safety systems that transfer product continuously between tanks and tankers or pipelines. Cylinder filling plants center on filling individual cylinders by weight, handling and checking them. The solutions on this page do not cover cylinder filling plants.