NGL Product Specifications Fundamentals

1. Overview of NGL Products

Natural gas liquids (NGLs) are the hydrocarbon components heavier than methane that are extracted from natural gas during processing. These components exist as gases in the reservoir and wellhead gas stream but condense into liquids at moderate pressures and ambient temperatures. NGL extraction is performed at gas processing plants using cryogenic turboexpander processes, lean oil absorption, or refrigeration-based recovery systems, and the resulting mixed NGL stream is subsequently separated into individual specification products through fractionation.

The NGL product hierarchy follows carbon number, with each component having distinct physical properties, market applications, and commercial specifications. Understanding this hierarchy is fundamental to gas processing plant design, fractionation train configuration, and product marketing.

NGL Product Hierarchy

Y-Grade (Mixed NGL) vs. Specification Products

NGL products are marketed in two fundamentally different forms, each with distinct specification requirements and economic considerations:

• Y-grade (raw mix or mixed NGL): The unfractionated NGL stream as produced by the gas plant, containing a mixture of ethane through natural gasoline in proportions determined by the inlet gas composition and plant recovery efficiency. Y-grade is transported by pipeline to centralized fractionation facilities (such as those at Mont Belvieu, Texas or Conway, Kansas) where it is separated into individual specification products. Pipeline specifications for Y-grade focus on contaminant limits rather than precise composition
• Specification products: Individual NGL components that have been separated by fractionation and meet specific purity, vapor pressure, and contaminant requirements defined by GPA 2140 or individual product purchase contracts. These products command higher prices than Y-grade because the fractionation value has been added

Market Destinations

Each NGL product serves distinct end-use markets with different demand drivers and pricing mechanisms:

• Petrochemical feedstock: Ethane is the primary feedstock for ethylene production in steam crackers. Propane serves as an alternative cracker feed and is used for propylene production via propane dehydrogenation (PDH). Isobutane feeds alkylation units in refineries to produce high-octane gasoline blendstocks
• Fuel markets: Propane (LPG) is widely used for residential and commercial heating, agricultural crop drying, engine fuel, and industrial process heat. Butane is used as a portable fuel and in lighter fluid applications
• Gasoline blending: Normal butane is blended into gasoline to adjust vapor pressure, particularly during winter months when higher RVP is permitted. Natural gasoline (C₅+) is blended directly into the gasoline pool as a high-value octane contributor and diluent for heavy crude oil transport

Product Value Chain Economics

The economics of NGL extraction and fractionation depend on the spread between the value of NGLs as individual products and their heating value as components of the natural gas stream. This spread—commonly called the NGL margin or frac spread—determines whether it is profitable to extract NGLs from the gas or leave them in the residue gas stream for sale as pipeline-quality natural gas. When NGL product prices are high relative to natural gas, deep NGL recovery is favored. When natural gas prices rise relative to NGL values, ethane rejection (leaving ethane in the gas stream) becomes economically attractive.

2. GPA 2140 — Commercial Specifications

GPA Standard 2140, titled "Liquefied Petroleum Gas Specifications and Test Methods," is the primary industry standard governing the quality requirements for commercial NGL products in North America. Published by the Gas Processors Association, this standard defines minimum purity levels, maximum contaminant concentrations, and required test methods for each NGL product grade. Compliance with GPA 2140 is typically a contractual requirement for product sales and pipeline deliveries, and the standard serves as the baseline for most commercial NGL purchase agreements.

The standard covers multiple product designations, each tailored to specific market applications and end-use requirements. The most commonly referenced grades are HD-5 propane, commercial propane, commercial butane, ethane-propane (E-P) mix, and natural gasoline.

HD-5 Propane Specifications

HD-5 (Heavy Duty – 5% maximum propylene) is the predominant propane grade sold in North America for engine fuel, residential heating, and internal combustion applications. The "HD-5" designation indicates that propylene content is limited to a maximum of 5 liquid volume percent, which is critical for engine fuel applications where propylene can cause gum deposits and valve damage. Key specification parameters include:

The HD-5 propylene limit is the most distinguishing feature of this grade. Propane produced from gas processing plants (as opposed to refinery sources) typically contains negligible propylene because natural gas contains no olefins. Refinery-sourced propane, however, often contains significant propylene from catalytic cracking operations, requiring careful blending or olefin removal to meet HD-5 specifications.

Commercial Propane

Commercial propane is a broader grade that permits higher olefin (propylene) content than HD-5, making it suitable for applications where propylene is acceptable or even desirable (such as petrochemical feedstock). This grade is commonly used in commercial and industrial heating applications where engine fuel quality is not required.

Commercial Butane

Commercial butane encompasses both normal butane and isobutane, or mixtures thereof. The primary quality parameters focus on vapor pressure (which controls storage tank design pressure), composition limits, and contaminant levels. Butane is used extensively for gasoline blending (seasonal RVP adjustment), petrochemical feedstock, and portable fuel applications.

Ethane-Propane (E-P) Mix

E-P mix is a blended product containing both ethane and propane, typically produced when full fractionation into separate ethane and propane streams is not economically justified or when pipeline infrastructure supports mixed product delivery to petrochemical facilities. E-P mix specifications focus on methane content limits (which affect storage and handling) and the ethane/propane ratio (which determines petrochemical cracking yields).

Natural Gasoline (C₅+)

Natural gasoline is the heaviest NGL fraction, consisting primarily of pentanes and heavier hydrocarbons (C₅ through approximately C₈). It is a high-value product used for gasoline blending, crude oil diluent, and petrochemical feedstock. Natural gasoline specifications focus on Reid vapor pressure (RVP), composition, color (indicating product quality and absence of degradation), and the doctor test for mercaptan sulfur.

Natural gasoline RVP grades are designated numerically (e.g., 10 RVP, 12 RVP, 26 RVP, 34 RVP), with the number representing the maximum allowable Reid vapor pressure in psi. Lower RVP grades command premium prices for summer gasoline blending when refinery RVP limits are tight. Higher RVP grades (26–34 psi) contain more butane and are typical for winter blending or pipeline diluent service.

3. Product Quality Testing

Product quality testing is the foundation of NGL specification compliance. Each test method measures a specific physical or chemical property of the NGL product, and the results determine whether the product meets the applicable GPA 2140 grade requirements. Testing is performed both in the laboratory (grab samples analyzed periodically) and through online analyzers (continuous monitoring for real-time process control). A robust quality testing program ensures that off-spec product is detected before it enters storage, pipelines, or customer facilities.

Key Test Methods

Vapor Pressure — ASTM D1267

Vapor pressure is the single most important specification parameter for LPG products because it directly determines storage vessel design pressure, product purity, and safe handling requirements. ASTM D1267 measures the gauge vapor pressure of liquefied petroleum gases at 100°F using a pressure-temperature apparatus. The sample is loaded into a test cylinder at a 4:1 liquid-to-vapor ratio, heated to 100°F in a water bath, and the equilibrium pressure is recorded. Results are reported in psig at 100°F. High vapor pressure indicates excessive light-end contamination (methane or ethane in propane, propane in butane), while low vapor pressure may indicate heavy-end contamination.

Weathering Test — GPA 2140

The weathering test measures the temperature at which 95% of an LPG sample evaporates under atmospheric pressure. This test serves as a practical indicator of product purity: a propane sample with excessive butane contamination will fail to fully evaporate at the specified temperature because the heavier butane fraction has a higher boiling point. For commercial propane, the 95% evaporation temperature must be at or below −37°F; for commercial butane, it must be at or below 36°F. The weathering test is performed by slowly evaporating a 100 mL sample in a graduated cylinder and recording the temperature when 95 mL has evaporated.

Copper Strip Corrosion — ASTM D1838

The copper strip corrosion test evaluates the corrosiveness of LPG products to copper and copper alloys, which are commonly used in valves, fittings, and instrumentation. A polished copper strip is immersed in a pressurized sample at 100°F for one hour, then compared to ASTM D130 color standards. The rating scale runs from 1a (slight tarnish, barely detectable) to 4c (dark gray/black corrosion). GPA 2140 requires a maximum rating of No. 1 (encompassing 1a and 1b), indicating negligible corrosion. Failure of the copper strip test typically indicates the presence of reactive sulfur compounds such as hydrogen sulfide, elemental sulfur, or mercaptans.

Total Sulfur — ASTM D2784

Total sulfur content is measured by the lamp combustion method (ASTM D2784), in which the LPG sample is burned in a glass lamp and the combustion gases are absorbed in a hydrogen peroxide solution. The resulting sulfuric acid is titrated to determine total sulfur content, reported in parts per million by weight (ppm) or grains per 100 cubic feet. Total sulfur limits protect downstream equipment from corrosion, prevent catalyst poisoning in petrochemical processes, and ensure compliance with emissions regulations when the product is burned as fuel.

Moisture Content — ASTM D1142

Water content in NGL products is determined by the Karl Fischer titration method (ASTM D1142) or by visual inspection for free water. Moisture is a critical contaminant because water in LPG systems can form hydrates at low temperatures (blocking valves and instruments), promote internal corrosion of carbon steel equipment, and freeze in regulators and orifices during pressure reduction. GPA 2140 requires that NGL products be free of liquid water. Most commercial contracts specify a maximum water content of 10–20 lb/MMscf equivalent or require dew point control below the minimum expected storage temperature.

Residue on Evaporation — ASTM D2158

The residue test measures the amount of non-volatile material remaining after a 100 mL sample of LPG is evaporated under controlled conditions. The residue consists of heavy hydrocarbons, oil contamination, dissolved polymeric material, and other involatile substances that can cause deposits in regulators, burner tips, and engine intake systems. GPA 2140 specifies a maximum residue of 0.05 mL per 100 mL of sample. Residue failures are commonly caused by absorption oil carryover from lean oil plants, compressor lubricant contamination, or pipeline scraper wax.

Testing Frequency

Online Analyzers vs. Laboratory Analysis

Modern NGL processing facilities use a combination of online continuous analyzers and periodic laboratory sampling to ensure product quality. Online analyzers provide real-time feedback for process control, enabling operators to detect composition upsets within minutes and take corrective action before significant volumes of off-spec product are produced. Laboratory analysis provides definitive results for custody transfer documentation, specification compliance certification, and periodic validation of online analyzer accuracy.

The most important online analyzer in NGL quality control is the gas chromatograph operating per GPA 2261. This analyzer continuously measures the composition of NGL streams, enabling real-time calculation of vapor pressure, heating value, specific gravity, and compliance with composition limits. Online chromatographs are typically installed on product run-down lines from fractionation columns and at pipeline custody transfer points.

Specification Compliance Documentation

Product quality records must be maintained for each batch or delivery to demonstrate specification compliance. Required documentation typically includes chromatographic analysis reports showing full composition, test results for each GPA 2140 parameter, batch or tank identification, sample time and date, analyst identification, and any deviations from specification with corrective actions taken. These records serve as the basis for custody transfer invoicing and are subject to audit by purchasers, pipeline operators, and regulatory agencies.

4. Pipeline Specifications for Mixed NGL

Mixed NGL (Y-grade or raw make) transported by pipeline from gas processing plants to centralized fractionation facilities must meet specifications that are fundamentally different from those for finished specification products. Pipeline NGL specifications focus on contaminant control (methane, CO₂, H₂S, water), vapor pressure limits for safe pipeline transport, and temperature constraints rather than on product purity or composition ratio. The composition of Y-grade NGL varies widely depending on the inlet gas source, plant recovery targets, and seasonal operating conditions, so pipeline specifications must accommodate this variability.

Typical Y-Grade Pipeline Specifications

Vapor Pressure Requirements

Pipeline vapor pressure limits ensure that the NGL remains in the liquid phase throughout the pipeline system, including at high-elevation points, pump suction locations, and during transient operations. The maximum allowable vapor pressure is typically specified at the pipeline inlet temperature and is set to provide a minimum subcooling margin (typically 10–25 psi below the pipeline operating pressure at the highest elevation point). Methane is the primary component controlling Y-grade vapor pressure; even small increases in methane content (0.5–1.0 mol%) can significantly elevate vapor pressure and cause two-phase flow conditions in the pipeline.

Vapor pressure exceedances are among the most common specification violations in Y-grade NGL pipeline operations. Causes include upset conditions in the demethanizer or deethanizer (allowing methane breakthrough), cold ambient temperatures affecting column performance, and transient operations during plant startup or rate changes.

Temperature Limits

NGL pipelines typically specify both maximum and minimum delivery temperatures. Maximum temperature limits (usually 100–120°F) prevent excessive vapor pressure that could cause pump cavitation or pipeline overpressure. Minimum temperature limits (typically 35–40°F) prevent potential hydrate formation if any residual water is present and ensure the product does not approach its bubble point at pipeline operating pressure. Some pipelines also specify a maximum temperature rate-of-change to prevent thermal shock to pipeline coatings and expansion joint systems.

Custody Transfer Measurement

NGL custody transfer (the point at which product ownership changes hands) requires accurate measurement of both quantity and quality. The primary measurement standard for NGL composition at custody transfer points is GPA 2261, which specifies the gas chromatographic method for analysis of natural gas and similar gaseous mixtures. Online chromatographs operating per GPA 2261 are installed at custody transfer meter stations to provide continuous composition data for:

• Volume measurement: Turbine meters or Coriolis meters measure the total liquid volume, which is temperature- and pressure-corrected using GPA 2145 physical property tables
• Component valuation: The chromatograph provides the molar composition (ethane, propane, isobutane, normal butane, pentanes+), which is used to calculate the individual component volumes and their respective market values for settlement
• Specification compliance: The chromatograph data confirms that contaminant limits (methane, CO₂, H₂S) are met at the custody transfer point
• Heating value: The composition data enables calculation of the gross heating value per GPA 2145, which may be used for pricing in Btu-based purchase contracts

Pipeline Quality NGL vs. Specification Products

It is important to distinguish between pipeline-quality Y-grade NGL and fractionated specification products. Pipeline Y-grade is a bulk commodity whose value depends on its overall composition (the relative amounts of each NGL component), not on meeting specific product purity requirements. The fractionation facility that receives Y-grade NGL separates it into individual products that must then meet GPA 2140 specifications. The pipeline specification ensures that the Y-grade can be safely transported and efficiently fractionated, while the product specification ensures that the fractionated products meet commercial quality requirements for their intended end use.

5. Off-Spec Product Handling

Off-specification NGL product—product that fails to meet one or more GPA 2140 quality parameters—is an inevitable occurrence in gas processing and fractionation operations. Off-spec events can result from process upsets, equipment malfunctions, feed composition changes, or operational errors. The key to minimizing the commercial impact of off-spec production is early detection (through continuous online monitoring), rapid response to divert product away from specification storage, and effective reprocessing or reblending strategies to bring the product back into compliance.

Common Off-Spec Conditions

Reprocessing Options

Off-spec NGL products can typically be brought back into specification through one or more of the following reprocessing strategies:

• Reblending: The most common and least expensive approach. Off-spec product is blended with on-spec product in calculated ratios to dilute the contaminant below specification limits. Blending calculations are based on simple material balance: the blend ratio required depends on the off-spec contaminant concentration and the available on-spec product contaminant level. This approach requires sufficient on-spec inventory to achieve the desired blend ratio without consuming excessive good product
• Rerunning through fractionation: Off-spec product can be reprocessed through the fractionation column that originally produced it (or a different column in the train). For example, propane with excessive C₄+ content is returned to the depropanizer for refractionation. This approach is effective but consumes column capacity and energy that would otherwise be used for new production
• Supplemental treating: For specific contaminants such as H₂S, CO₂, or mercaptans, the off-spec product can be routed through a supplemental treating system (caustic wash, molecular sieve, or activated carbon bed) to remove the offending contaminant without full refractionation
• Drying: Off-spec product with excessive moisture can be reprocessed through a dehydration system (molecular sieve dryer or glycol contactor) to reduce water content below specification limits

Product Downgrading

When reprocessing is not practical or economically justified, off-spec product may be downgraded to a lower-value grade that accommodates its quality characteristics. Common downgrading pathways include:

• HD-5 propane to commercial propane: If the propylene content exceeds the HD-5 limit of 5% but all other parameters are within specification, the product can be sold as commercial propane at a lower price
• Specification butane to natural gasoline: Butane with excessive C₅+ content (failing the weathering test) may be reclassified and sold as a component of the natural gasoline pool
• Specification product to Y-grade: Individual products that cannot meet any specification grade may be returned to the Y-grade NGL system for redelivery to the fractionation facility

Commercial Implications

Off-spec production has significant commercial consequences beyond the direct cost of reprocessing or downgrading. Pipeline penalties for delivering off-spec Y-grade can include product rejection (requiring the shipper to remove the product from the pipeline at their expense), quality bank adjustments (financial penalties assessed based on the degree of specification exceedance), and in severe cases, pipeline access suspension. Customer contracts for specification products typically include quality guarantees with price adjustments or rejection provisions for off-spec deliveries. The cumulative impact of repeated off-spec events can include loss of preferred shipper status, contractual penalties, and damage to commercial relationships.

Quality Control Best Practices

Effective quality control programs incorporate the following practices to minimize off-spec production and its commercial impact:

• Continuous online monitoring: Install GPA 2261 chromatographs and online sulfur analyzers on all product run-down lines with alarm set points at 90% of specification limits to provide early warning of approaching off-spec conditions
• Automated diversion: Configure control systems to automatically divert product from specification storage to off-spec tanks or recycle when analyzer readings exceed pre-set limits, preventing contamination of on-spec inventory
• Specification trending: Maintain control charts of key quality parameters over time to identify gradual trends (increasing sulfur, rising C₅+ in propane) before they reach specification limits. Statistical process control methods enable identification of drift patterns that predict future off-spec events
• Root cause analysis: Conduct formal root cause analysis of every off-spec event to identify process, equipment, or operational issues that can be corrected to prevent recurrence. Document findings and corrective actions in the facility management of change system
• Adequate off-spec storage: Provide sufficient off-spec tankage to contain product during upset events without forcing product into specification tanks. Industry practice recommends off-spec storage capacity equal to 12–24 hours of maximum product run-down rate
• Calibration and maintenance: Maintain a rigorous calibration schedule for online analyzers (typically monthly using certified reference standards per GPA 2261) and perform preventive maintenance on sampling systems, chromatograph carrier gas supplies, and detector systems to ensure analyzer reliability

References

1. GPA Standard 2140 — Liquefied Petroleum Gas Specifications and Test Methods
2. GPA Standard 2145 — Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry
3. GPA Standard 2261 — Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography
4. ASTM D1267 — Standard Test Method for Gage Vapor Pressure of Liquefied Petroleum (LP) Gases
5. ASTM D1838 — Standard Test Method for Copper Strip Corrosion by Liquefied Petroleum (LP) Gases
6. ASTM D2784 — Standard Test Method for Sulfur in Liquefied Petroleum Gases
7. ASTM D1142 — Standard Test Method for Water Vapor Content of Gaseous Fuels
8. ASTM D2158 — Standard Test Method for Residues in Liquefied Petroleum (LP) Gases
9. GPSA — Chapters 1, 2, and 16