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API TR 2575-2014: A Comprehensive Guide to Sampling and Analysis of Natural Gas Liquids and Condensates
Best practices, technical requirements, and compliance strategies for accurate NGL and condensate measurement
API Technical Report 2575 (first edition, 2014) offers industry-consensus guidance on the sampling and analysis of natural gas liquids (NGLs) and condensates. Developed under the American Petroleum Institute’s Manual of Petroleum Measurement Standards (MPMS) framework, the report addresses the unique challenges that arise when measuring these volatile hydrocarbon mixtures—from sample conditioning at the tap to laboratory analysis. The report consolidates decades of operator experience and provides actionable recommendations for reducing measurement uncertainty in custody transfer, allocation, and quality verification applications.
Scope and Purpose of API TR 2575-2014
The scope of API TR 2575 covers sampling and analysis procedures for both pressurized (single-phase) and atmospheric (two-phase) streams of NGLs and condensates. Applicable products include ethane, propane, butanes, pentanes-plus, and natural gasoline, as well as field condensates with vapor pressures up to 1000 psi at handling temperatures. The report specifically addresses:
- Selection and installation of sampling points
- Sample conditioning (temperature, pressure, mixing) to maintain single-phase conditions
- Sample container types, preparation, leak testing, and transport
- Laboratory analysis methods for composition, density, sulfur, water, and vapor pressure
- Quality assurance and repeatability criteria
API TR 2575 does not supersede mandatory regulatory requirements but is intended as a best-practice reference for operators, engineering firms, and laboratories involved in NGL and condensate measurement. It aligns with and complements GPA 2165, ASTM D3700, and API MPMS Chapter 14.
Important: The report emphasises that improper sampling—especially allowing phase separation—can lead to measurement errors exceeding 2% by volume. Careful adherence to conditioning and handling guidelines is essential for reliable results.
Technical Requirements and Recommendations
Sampling System Design and Sample Conditioning
A key technical requirement is maintaining single-phase flow from the sample tap through the container. The report recommends isokinetic sampling probes and either pressure-reducing or back-pressure regulators to keep the stream above saturation pressure. For streams with a high butane+/pentane+ content, heating jackets or circulation loops are specified to prevent hydrocarbon liquid dropout.
Sample Containers
Only piston cylinders, floating-piston cylinders, or fixed-volume cylinders with proper volume compensation are recommended. Containers must be rated for the maximum operating pressure and fabricated from materials compatible with the fluid (e.g., 316 stainless steel). Prior to use, each container must be checked for leaks, residual moisture, and hydrocarbon residues via hot-solvent cleaning and inert gas purging.
Tip: For samples containing hydrogen sulfide or mercaptans, use disposable sample cylinders or decontaminate immediately after analysis to avoid cross-contamination.
Analytical Procedures
Laboratory analysis shall follow GPA 2177 (gas chromatography) for compositional determination, with carrier gases and columns selected to resolve ethane through decane. Density shall be determined by either a vibrating-tube densitometer calibrated with certified standards or via calculation from extended composition. The table below summarizes recommended analytical methods and expected precision limits:
| Property | Recommended Method | Reference Standard | Repeatability (r) | Reproducibility (R) |
|---|---|---|---|---|
| Composition (C1–C12) | Gas chromatography (single-column or multi-column) | GPA 2177 / ASTM D1945 | 0.3 mole% (major components) | 0.5 mole% |
| Density (liquid at 60 °F) | Vibrating-tube densitometer or hydrometer | ASTM D4052 / D1657 | 0.1 lb/ft³ | 0.2 lb/ft³ |
| Total sulfur | Oxidative microcoulometry or UV fluorescence | ASTM D5623 / D6920 | 5 mg/kg | 10 mg/kg |
| Water content | Karl Fischer coulometric titration | ASTM D4928 | 5 mg/kg | 15 mg/kg |
| Vapor pressure (Reid or true) | Automatic vapor pressure instrument | ASTM D6378 | 0.3 psi | 0.6 psi |
Implementation Highlights for Field and Laboratory
Successful application of API TR 2575 depends on coordination between field operators, sample handlers, and laboratory analysts. Key implementation points include:
- Sample frequency: At least one representative sample per custody-transfer batch or daily for continuous streams; frequency may be increased when composition varies.
- Sample container preparation: Containers must be passivated, leak-tested, and purged with dry nitrogen before each use. A log of cleaning cycles and certifying analyser results should be maintained.
- Hold times and storage: Samples of NGL/condensate should be analysed within 48 hours of collection. If storage is unavoidable, containers must be kept at a temperature below the product’s storage specification (typically <80 °F) and out of direct sunlight.
- Operator training: All personnel involved in sampling must complete a competency course covering phase behaviour, sample conditioning, and container handling. Re‑certification is recommended every three years.
Success indicator: Facilities that implement the full scope of API TR 2575 guidelines (including conditioning equipment, calibrated containers, and trained operators) report a reduction in measurement uncertainty of 0.5–1.5% over non‑compliant systems.
Compliance and Quality Assurance Notes
Although API TR 2575 is a technical report (not a mandatory standard), it is frequently referenced in contracts, sales agreements, and regulatory permits for NGL and condensate custody transfers. To demonstrate compliance, operators should:
- Document all sampling procedures, including equipment specifications, conditioning parameters, and sample chain-of-custody.
- Participate in inter-laboratory proficiency testing programs (e.g., API or GPA round robins) to validate analytical accuracy.
- Perform periodic audits of sampling and laboratory systems against the recommendations of API TR 2575.
- Establish a measurement quality control plan that includes control charts for key properties and regular equipment calibration.
Non-compliance risk: Failure to follow the guidance in API TR 2575 can lead to disputes during custody transfer, unexpected shrinkage, or regulatory penalties where the report is incorporated into permits. For example, using non‑conditioned sampling probes on a high-vapour-pressure condensate stream has been shown to cause errors of 2–3% in measured volumes.
Integration with Other Standards
API TR 2575 works in concert with other industry documents:
- API MPMS Chapter 14 (Natural Gas Fluids Measurement) – Provides the overarching measurement framework; TR 2575 adds sample-specific details.
- GPA 2165 (Standard for NGL Sampling) – Similar scope but API TR 2575 includes additional guidance on condensates and varying fluid properties.
- ISO 3171 (Petroleum Liquids – Automatic Pipeline Sampling) – For pipeline applications, TR 2575 supports the general principles of ISO 3171 with NGL‑specific modifications.
Q: What is the primary purpose of API TR 2575-2014?
A: It provides recommended practices for the sampling and analysis of natural gas liquids (NGLs) and condensates to ensure accurate determination of composition, density, and other quality parameters. The aim is to reduce measurement uncertainty in custody transfer and process control.
A: It provides recommended practices for the sampling and analysis of natural gas liquids (NGLs) and condensates to ensure accurate determination of composition, density, and other quality parameters. The aim is to reduce measurement uncertainty in custody transfer and process control.
Q: Is API TR 2575-2014 a mandatory standard?
A: No, it is a technical report and therefore voluntary. However, it is often incorporated as a requirement in sales contracts and may be cited by regulatory bodies as evidence of good engineering practice in NGL and condensate measurement.
A: No, it is a technical report and therefore voluntary. However, it is often incorporated as a requirement in sales contracts and may be cited by regulatory bodies as evidence of good engineering practice in NGL and condensate measurement.
Q: Which sampling apparatus does the report recommend for high-vapor-pressure NGL streams?
A: The report recommends piston cylinders or floating-piston cylinders that can maintain single-phase conditions. The sampler must include a pressure regulator or back‑pressure control to keep the fluid above its bubble point throughout sample capture.
A: The report recommends piston cylinders or floating-piston cylinders that can maintain single-phase conditions. The sampler must include a pressure regulator or back‑pressure control to keep the fluid above its bubble point throughout sample capture.
Q: How does API TR 2575 relate to GPA 2165?
A: Both documents address NGL sampling. API TR 2575 has a wider scope that explicitly includes condensates and provides more guidance on sample conditioning, container preparation, and laboratory quality assurance. GPA 2165 focuses more narrowly on natural gas liquids sampling hardware and procedures.
A: Both documents address NGL sampling. API TR 2575 has a wider scope that explicitly includes condensates and provides more guidance on sample conditioning, container preparation, and laboratory quality assurance. GPA 2165 focuses more narrowly on natural gas liquids sampling hardware and procedures.
This article is based on API TR 2575:2014 (first edition). All guidance should be confirmed with the latest edition. Published 2026.