For natural gas liquids (NGL) with variable composition, the quantity a metering skid should be built around is usually mass. Volume changes noticeably with temperature and pressure, and correcting it requires knowing the properties of the product itself; mass does not depend on those conditions. Component quantities, meaning how much of the delivered mix is ethane, propane or butane, are not a separate measurement but a calculated result obtained by combining measured mass with the compositional analysis of a representative sample. A well-designed NGL metering skid links these three levels.
Why a liquefied gas must be kept liquid in the meter is explained on the LPG metering skids page, and the contractual and verification side of component-based settlement on the NGL custody transfer page. This page concentrates on the hardware and measurement architecture decisions for the skid.
Volume, mass and components: comparing the three measured quantities
Measured quantity | Strength for NGL | Limitation and extra data required |
Volume at operating conditions | Measured directly with positive displacement, turbine or ultrasonic meters | Conversion to base conditions needs NGL-specific temperature correction and a pressure correction based on bubble point pressure; crude and refined product tables do not cover NGL |
Mass | Independent of temperature and pressure; accounts for variable density directly | Requires direct Coriolis measurement or combining volume with flowing density; the meter must stay in the liquid phase |
Component quantities | Gives each component of the mix separately; feeds fractionation and sales calculations | Requires a representative pressurized sample and chromatographic analysis in addition to measured mass |
The international industry standard for NGL mass measurement describes two routes for single-phase liquids within a defined density range: direct mass measurement with a Coriolis meter, and inferred mass measurement, where volume at flowing conditions is multiplied by density at the same conditions. The same framework describes converting measured mass into equivalent liquid volumes of the components using the compositional analysis. Even for parties that want volume-based reporting, mass becomes the starting point of the calculation.
Choosing between direct and inferred mass measurement
Because a Coriolis meter measures mass flow, density and temperature in one device, it is widely used for variable-composition product; the density signal can also serve fill-limit calculations, product identification and two-phase detection. Inferred mass is considered for large-diameter pipeline applications or plants with an installed base of volumetric meters. In that case the online densitometer is placed close to the meter so that it sees the same pressure and temperature, and a verification routine is defined for the densitometer as well.
Either way, the accuracy on the device datasheet does not describe the uncertainty of the whole skid. Adequate back pressure, temperature measurement, zero stability, installation effects and the calculation chain all add to the total system uncertainty.
Back-pressure setting: a composition range, not a single product
A pressure-holding valve downstream of the meter keeps the product from flashing inside the meter. On an NGL skid, the setting is chosen so that it stays above the bubble point pressure of the lightest mix expected, at the highest product temperature. Since NGL vapor pressure is generally not measured and the composition is not always known, the industry also uses correlations that estimate vapor pressure from normally measured data. The same vapor pressure information is needed to calculate the pressure correction factor in volumetric measurement.
Integrating sampling and analysis hardware on the skid
If component quantities are to be calculated, a sampling system is added to the skid. The sample point must sit where the flow is well mixed and the liquid phase is maintained, and the sample must be held in a pressurized cylinder. A flow-proportional composite sample weights the changing composition over the batch in the same way as the total mass. Online analysis may also be considered, but unlike the gas chromatography used for natural gas quality, NGL analysis requires controlled vaporization of a liquid sample and a method that also covers the heavier components. The concept of chromatography itself is explained on the natural gas quality measurement page.
Skid components: standard scope and project-specific additions
Component | Function on an NGL skid | Standard / option |
Strainer and vapor separation element | Protects the meter from solids and from vapor carried over, for example at the end of unloading | Standard; type depends on application |
Mass meter, or volumetric meter with densitometer | Produces mass and density data | Standard; technology depends on the measurement approach |
Pressure and temperature transmitters | Monitoring the liquid phase, corrections and diagnostics | Standard |
Back-pressure (pressure-holding) valve | Prevents flashing in the meter | Standard |
Flow computer or batch controller | Mass, component and equivalent volume calculations; records | Standard; calculation scope per project |
Sampling system and pressurized cylinders | Representative sample for component calculations | Needed for component-based reporting |
Online analyzer connection | More frequent composition monitoring | Option |
Prover or master meter connections | In-situ meter verification | Usually needed for custody transfer |
How the skid changes with its purpose
· Process and allocation metering: For allocation between fields or the input-output balance of a fractionation unit; mass and density are usually enough and composition can come from periodic analysis.
· Truck loading and unloading metering: Batch operation, end-of-unloading vapor handling and fill-limit calculation take priority.
· Sales metering: Verification connections, sealing, audit trail and component-based calculation may become mandatory; these requirements are detailed on the NGL custody transfer page.
Safety and compliance approach
The skid carries a flammable liquid under pressure, so pressure rating, materials and seals are selected for the lightest expected mix and the highest temperature. Hazardous area classification determines the protection method of electrical equipment. An explosion-protection or legal metrology approval for a single meter does not by itself demonstrate compliance of the whole skid: whoever combines the equipment must also assess additional risks arising from the combination.
How TLY Enerji supports NGL metering
TLY Enerji can provide engineering support on NGL metering projects for choosing the measured quantity and meter technology, supplying Coriolis meters, pressure and temperature instruments, control valves and process analyzers, system integration, field installation, testing and commissioning, and data acquisition through PLC and SCADA. The scope is agreed through the project's technical specification.
Data required to design a metering skid
· Purpose of the skid: allocation, operational tracking, truck loading/unloading or sales metering
· Quantity to be reported: mass, volume at base conditions or component quantities
· Composition and density range, including heavy-end content
· Minimum, normal and maximum flow; line pressure and temperature
· Existing sampling and laboratory capabilities
· Preferred verification method: prover, master meter or external calibration
· Which standards and contract terms the flow computer calculations must follow
· Control system interface, hazardous area classification and layout constraints
Related pages
· NGL custody transfer metering systems: The calculation and verification path from mass and composition to the invoice.
· LPG metering skids: Baseline explanation of keeping liquefied gas liquid for propane and butane.
· Natural gas quality measurement and analysis systems: Conceptual basis of gas chromatography and compositional analysis.
· Propane, butane and mixed NGL transfer: Managing different products and recipes on the same meter run.
Questions about NGL metering skids
Is volumetric metering simply wrong for NGL?
No. Volumetric meters can be used for NGL, but conversion to base conditions requires temperature correction methods specific to NGL and LPG and a pressure correction based on vapor pressure. As composition changes, the inputs to those corrections change too. That is why plants with volumetric meters usually add online density measurement, calculate mass and report on a mass basis.
Can the Coriolis density reading replace compositional analysis?
No. Density is an average property of the mix, and two mixes of different composition can have the same density. The density signal is valuable for spotting product changes, calculating fill quantities and detecting two-phase flow. The quantity of each individual component, however, can only be determined by chromatographic analysis of a representative sample.
What should the back-pressure valve be set to?
There is no single number. The setting depends on the bubble point pressure of the highest vapor pressure mix expected through the skid at its highest operating temperature, plus the pressure losses between meter and valve. Without a clear composition range and temperature limits in the project data, the setting cannot be determined with confidence, which is why the composition range is one of the first design inputs requested.
Does every NGL metering skid need a sampling system?
Not necessarily. For process metering that tracks only total mass or density, periodic laboratory analysis can be enough. If component-based reporting, sales calculations or fractionator feed control are required, a flow-proportional sampling system and pressurized sample containers should be planned as part of the skid.
Where does the densitometer go in inferred mass metering?
The densitometer should see pressure and temperature conditions as close as possible to those at the meter, so it sits near the meter in a section where the liquid phase is maintained. Verification of the densitometer and consistency between its temperature measurement and the meter's must also be defined in the design, because the mass result comes from combining both measurements.