Propane, butane and their blends often pass through the same terminal and even the same meter, but a metering system cannot treat them as one fluid. The key differences are vapor pressure, density and how strongly volume changes with temperature. When the blend ratio shifts, all three shift with it, and that directly affects the back pressure needed to keep product liquid in the meter, the conversion between mass and volume, correction to reference temperature, and proving results. This page explains how a metering system should account for those differences.
The general layout of an LPG metering skid, with back-pressure control, vapor eliminator and meter run, is described on the LPG Metering Skids page. Here the focus is on how product differences affect that layout.
LPG is a family of products, not one fluid
What the market calls LPG consists mainly of propane and butanes, with smaller amounts of light olefins such as propylene and butylenes. It can come from natural gas processing or from crude oil refining, so two shipments sold under the same commercial name may have different compositions. Standards for automotive LPG set composition and vapor pressure requirements and include seasonal vapor pressure grades. For a terminal, the conclusion is simple: the metering system has to be designed for every product and blend range it will actually handle, not for a generic "LPG".
Vapor pressure: impact on back pressure and pressure rating
At atmospheric pressure, propane boils at about −42 °C and normal butane at about 0 °C. At the same temperature the gap in vapor pressure is large: at roughly 15 °C, pure propane has a saturation pressure of around 7 bar absolute, while pure normal butane stays below 2 bar. For metering, that has two direct consequences:
· Back-pressure setting: To keep the product liquid in the meter, line pressure must be held sufficiently above the product's vapor pressure at the flowing temperature. Propane-rich product needs a higher pressure level than butane-rich product. A single fixed setting can lead either to unnecessary pressure loss or to flashing when the product changes.
· Pressure rating: The pressure class of the meter run, transmitters and valves is selected for the most volatile product at the highest expected temperature. Using a line designed only for butane in propane service later on calls for the pressure rating to be reassessed.
Butane's higher boiling point is an operating issue in its own right. In cold weather the vapor pressure of butane-rich product drops, which reduces the pressure margin available between the storage vessel and the pump suction. Keeping the product liquid at the pump inlet in that condition is a design input that comes before metering accuracy.
Density and the mass-to-volume conversion
LPG density depends on composition, and LPG standards include methods that calculate density and vapor pressure from composition using factors for each component. As an example, at about 15 °C saturated liquid propane has a density of around 507 kg/m³, and normal butane around 584 kg/m³. The same delivered volume of propane and butane therefore represents clearly different masses, and the difference keeps moving as the blend changes.
Coriolis meters measure mass directly and determine density at the same time; volume is calculated from mass and measured density. With volumetric meters, density has to be measured separately or calculated from composition to arrive at mass. Whichever route is chosen, the source of density data and how it is updated on a product change must be clear. One boundary matters here: quality standards that calculate density from composition are intended for product specifications, not for custody transfer quantity calculation.
Thermal expansion and correction to reference temperature
The density of light hydrocarbons such as propane and butane changes quickly with temperature; for pure propane, a rise of just a few degrees can mean a density drop of more than one percent. Volume-based deliveries therefore use temperature correction methods developed for natural gas liquids and LPG, not the tables used for refined fuels. These methods take the product's density at reference temperature as an input, so measuring butane with a density value that is valid for propane produces a wrong correction factor. Which reference temperature applies depends on national regulations and the contract.
Seasonal blend changes
Raising the propane share in colder months to maintain adequate vapor pressure is common practice. For a terminal, that means handling product with different density and vapor pressure under the same product code over the year. At each seasonal changeover, review:
· Back-pressure setpoint and alarm limits
· Density range and reference density configured in the flow computer or batch controller
· Whether the meter factor is still valid for the new product, and whether proving must be repeated
· Density values used when reconciling storage inventory with metered dispatches
One meter, several products
When propane, butane and blends share a meter run, the common approach is to define a separate recipe for each product in the batch controller or flow computer. The recipe holds the product-specific meter factor, density range, temperature correction parameters and back-pressure setting. On a product change, carry-over of the previous product left in the line should also be considered; with a small line volume the effect may be negligible, with a large one a changeover procedure is needed. Dedicated meter runs per product avoid carry-over and setting changes altogether, at the cost of more investment and space.
Design criteria
Parameter | Why it matters in design | Effect on selection |
Products and blend range | Defines the vapor pressure and density range | Sets pressure class, density measuring range and number of recipes |
Product temperature range | Affects vapor pressure and volume correction | Back-pressure setting and correction method follow from it |
Invoicing basis | Mass or reference volume changes the measurement chain | Coriolis direct mass versus volumetric meter plus density measurement |
Frequency of product changes | Drives setting changes and carry-over risk | Shared line with recipes, or a dedicated line per product |
Seasonal specification | Density and vapor pressure drift through the year | Requires a parameter update and proving plan |
Proving method | Answers whether product-specific meter factors are needed | Prover or master meter connections and proving frequency per product |
Where TLY Enerji contributes
TLY Enerji provides measurement technology selection and engineering support based on process conditions; its product range includes Coriolis mass flow and density meters, pressure and temperature measurement and control valves. Depending on project scope, this can cover instrument selection and supply for propane and butane meter runs, installation and commissioning of flow meters, and PLC and SCADA support for data acquisition and reporting. Product recipes and setpoints are defined together with the operator, based on its product plan.
Data worth sharing for a review
· List of products handled, with typical compositions or sample quality certificates
· Expected blend ratios for summer and winter
· Whether invoicing is by mass or reference volume, and the contractual reference temperature
· Number of products sharing a meter run and how often they change
· Minimum and maximum product temperature and line pressure
· Existing proving facilities and expectations for product-specific proving
Related pages
· LPG metering skids: Keeping product liquid in the meter with back-pressure control and vapor elimination.
· LPG custody transfer metering systems: How the invoicing unit and corrections are set by contract.
· Propane, butane and mixed NGL transfer systems: Managing several products on one transfer infrastructure.
· LPG terminals: What handling a pressurized liquid means for the terminal as a whole.
Questions on propane and butane metering
Can propane and butane be measured with the same meter?
Yes, many terminals use one meter run for several LPG products. Each product gets its own recipe, with meter factor, density range, temperature correction parameters and back-pressure setting chosen for that product. How carry-over is handled at product changes, and whether proving is done per product, should be decided separately.
Can the blend ratio be inferred from density?
Only for a true two-component mix with negligible other components can density give an indication of the ratio. Real LPG also contains isomers, olefins and trace components, and the same density can result from different compositions. Where composition must be known with confidence, sampling and laboratory analysis are needed.
What should change in the metering system when the season changes?
When the propane share changes, density and vapor pressure change too. The back-pressure setting, density data in the flow computer and alarm limits should be reviewed, and the validity of the meter factor for the new product checked. Tying these steps to an operating procedure reduces reconciliation differences during changeover periods.
Can refined-fuel correction tables be used for LPG?
No. Propane and butane lie below the density range of refined fuels and expand far more with temperature, so separate temperature correction methods developed for natural gas liquids and LPG are used. The method and reference temperature to apply should be confirmed against the contract and applicable regulations.