When natural gas liquids (NGL) change hands, the invoice depends on more than the tonnage or volume delivered: it also depends on how much ethane, propane, butanes and heavier material the batch contained, since each component can carry its own price. NGL custody transfer measurement therefore rests on two pillars: a verifiable measurement of total quantity, most often as mass, and a representative composition that splits that mass into components. The sales contract sets which components are priced separately and whether they are reported as mass or as equivalent liquid volume.
Meter technology and skid hardware are covered on the NGL metering skids page; general custody governance at loading racks has its own page. Here the focus is on what is specific to NGL: the quantity calculation, sample representativeness and verification.
Commercial basis: mass and composition together
Because NGL composition can shift with gas plant operation, inlet gas and season, its density is not constant, and two batches of identical volume can differ in mass and commercial value. Total quantity is therefore usually established as mass. Whether that mass is measured directly with a Coriolis meter or inferred from volume and density is compared on the NGL metering skids page. For custody transfer, the real task is determining which components make up that mass in a way both parties accept.
From measured mass to component quantities
In a typical NGL custody transfer arrangement, the measured quantity becomes a commercial result through the following sequence:
1. The meter and flow computer record total mass for the batch or period, along with the pressure and temperature that confirm the product stayed fully liquid in the meter.
2. A flow-proportional composite sample is collected over the same period and kept single-phase under pressure on its way to the laboratory.
3. The sample is analyzed by gas chromatography to establish mole or mass fractions of each component.
4. Total mass is multiplied by the mass fractions to obtain the mass of each component.
5. Where the contract requires it, component masses are converted to equivalent liquid volumes at defined reference conditions.
6. Results, together with measurement and analysis records, are carried into the custody ticket or period statement.
An error in any link flows straight into the invoice. An unrepresentative sample distorts the ethane or propane share even when total mass is right; a drifting meter shifts every component quantity by the same proportion even when the analysis is right.
Getting a representative sample
The sample is often the weakest link in NGL custody measurement. Light ends such as methane and ethane escape easily when pressure drops or a vapor space forms in the sample container. Open or partially open methods cause a small, systematic loss that is acceptable for routine specification testing but not when product value is calculated component by component. Constant-pressure methods such as floating piston cylinders, which keep the sample pressurized and single-phase, are preferred wherever component accuracy drives value.
· Sample point: Located close to the meter where the stream is well mixed and single-phase, with the probe drawing from a representative part of the flow cross-section.
· Flow proportionality: Grabs are triggered by throughput rather than by time, so composition changes within a batch are weighted correctly.
· Pressure maintenance: Sample lines, cylinders and fittings are designed to keep the product above its bubble point pressure.
· Chain of custody: Labeling, sealing, transport and laboratory conditioning of cylinders follow a written procedure, and the retention period for referee samples is defined in the contract.
Online analyzers can follow composition more closely, but the parties should agree in advance which result is the invoicing basis, how the analyzer is verified and which result prevails if the two disagree.
Verification and reconciliation
Confidence in a custody system comes from the ability to verify it regularly in the field, not from the accuracy figure on a meter datasheet. Legal metrology sets separate error limits for the meter and for the complete measuring system, and the system limit is always wider. For NGL service, verification typically covers:
· Proving the meter under flow against a prover or master meter, ideally close to operating pressure and temperature, and recording the meter factor
· Separate verification of the densitometer where mass is inferred, and monitoring of zero and density readings on Coriolis meters
· Calibration of the chromatograph with reference blends and a component-by-component view of analysis uncertainty
· An audit trail of flow computer parameter changes, alarms and events
· Periodic comparison of delivering and receiving party figures with independent data such as tank inventory or weighbridge results
Vapor return and quantities outside the meter
If a vapor return line is used during truck loading, the vapor flowing back from the tanker to storage is still product. Legal metrology recommendations for pressurized liquefied gas measuring systems either restrict gas-phase connections between tanks or require the returned gas to be measured and subtracted from the delivered quantity. Whether returned vapor is metered, and how it is accounted for if it is not, should be settled at the start of any NGL custody design.
Design inputs that shape the system
Parameter | Why it matters | Effect on selection |
Invoicing basis | Total mass, component mass and component equivalent volume need different calculation chains | Defines flow computer calculations, analysis frequency and report format |
Composition range and variability | Density and vapor pressure change with composition | Drives the mass measurement method, density measurement and analysis approach |
Ethane and light-end content | Light ends raise vapor pressure and are prone to loss during sampling | Sets required back pressure, sampling method and cylinder type |
Mode of delivery | Truck batches, pipeline and continuous flow call for different sampling and proving layouts | Batch or period accounting, prover connections and record structure |
Legal metrology status | If the measurement is under statutory control, the system must be assessed as a whole | Accuracy class, sealing and documentation requirements |
Field verification | A system that cannot be verified on site is hard to defend commercially | Prover or master meter connections, need for a spare meter run |
Data and audit trail | Records decide disputes | Flow computer, SCADA interface and archiving requirements |
Where TLY Enerji contributes
On NGL custody projects, TLY Enerji provides engineering support in reviewing the process and contractual requirements at the delivery point and selecting a suitable measurement technology. Depending on project scope, this can include supply of Coriolis meters and pressure and temperature instruments, integration of measurement data into PLC and SCADA systems, site installation, testing, commissioning and follow-up technical support. Which sampling and analysis equipment falls within the scope of supply, and how responsibilities are split, is agreed through the technical specification.
Information we need to assess your delivery point
· Invoicing basis in the sales contract and the components priced separately
· Typical and extreme composition analyses, plus any seasonal trend
· Mode of delivery, batch size and frequency
· Pressure and temperature range at the meter, minimum and maximum flow
· Existing or planned sampling and laboratory arrangements
· Legal metrology status and the counterparty's verification expectations
· Whether a vapor return line is used and how the contract treats returned vapor
Related pages
· NGL metering skids: Skid hardware and meter selection for mass, volume and component measurement.
· LPG custody transfer metering: Units of sale and corrections for LPG, which has a narrower composition range.
· Custody transfer truck loading skids: Fluid-independent governance of custody measurement at loading racks.
· Quality measurement and analysis systems: Core concepts of composition measurement by chromatography.
· Propane, butane and mixed NGL transfer: Handling several products on shared infrastructure with product-specific settings.
Frequently asked questions
Is NGL sold by mass or by volume?
That depends on the contract and the applicable regulations. Because density is not constant in a variable mix, total quantity is commonly measured as mass. Components can then be priced on a mass basis or converted to equivalent liquid volumes component by component. The contract should state the method clearly, including the reference conditions used for any volume conversion.
What is component equivalent liquid volume, and why is it used?
Some contracts price components by volume rather than mass. In that case the mass of each component is converted, using that component's density at defined reference conditions, into the liquid volume it would occupy as a pure product. This gives a component-by-component quantity that is comparable across batches and independent of the measured volume of the blend. The densities and reference conditions used for the conversion should be written into the contract.
How does loss of light ends in a sample affect the invoice?
If methane and ethane escape from the sample container, the analysis will show a heavier mix than was actually delivered. Even with correct total mass, the ethane share comes out low and the propane and butane shares come out high. Where components carry different prices, that bias goes straight into the invoice amount. Pressurized, single-phase sampling methods exist to reduce this risk.
How is returned vapor handled commercially?
With a vapor return line, part of the product flows back from the tanker to storage as vapor. Legal metrology recommendations either restrict that gas-phase connection or require the returned gas to be measured and subtracted. The parties need to agree whether returned vapor is metered, calculated by a defined method or disregarded, because that decision directly shapes the metering layout.