ISO 91:2017

Overview

ISO 91:2017 - Petroleum and related products - Temperature and pressure volume correction factors (petroleum measurement tables) and standard reference conditions - defines how to convert measured hydrocarbon volumes at ambient conditions to standard reference conditions for transactional and custody-transfer purposes. The standard covers temperature volume correction factors, compressibility factors for volumes measured under pressure, and the specification of standard reference conditions (see Annex B).

Keywords: ISO 91:2017, temperature volume correction factors, petroleum measurement tables, compressibility factors, standard reference conditions, custody transfer.

Key topics and technical requirements

• Purpose: Convert observed liquid hydrocarbon volumes to equivalent volumes at agreed reference temperature and pressure for accounting and trade.
• Volume correction factors: Guidance and sources for temperature and pressure correction factors (petroleum measurement tables) to adjust thermal expansion and compressibility.
• Compressibility factors: Correction of volumes measured under pressure to the equilibrium (saturation) pressure corresponding to the measured temperature.
• Commodity groups: Recognizes three primary groups - crude oil, refined products, and lubricating oils - and a special application category for experimentally derived coefficients.
• Defining limits (examples from the standard):
  • Temperature: approximately −50 °C to +150 °C (−58 °F to 302 °F)
  • Pressure: 0 to 1 500 psig (≈ 0 to 103.4 bar)
  • Density ranges for crude and products (typical): ~611 to 1 164 kg/m3 (at standard base temps)
• Reference implementation: For custody transfer, ISO 91:2017 requires reference to API MPMS Chapter 11.1-2004 (Adjunct to ASTM D1250-04 / IP 200/04) including Addendum 1‑2007, which provides the implementation procedures and CTPL (correction factor for temperature and pressure of the liquid).

Practical applications and users

ISO 91:2017 is used wherever accurate, consistent volumetric measurement of liquid hydrocarbons is required for trade, accounting or regulatory purposes:

• Custody transfer and commercial trading of crude oil, refined fuels, NGL and LPG
• Tank gauging and inventory management at terminals and storage facilities
• Pipeline and metering systems where volume at standard conditions must be reported
• Measurement and metrology engineers, terminal operators, trading houses, fiscal metering teams, and regulators who need standardized correction methods
• Software developers and system integrators implementing measurement and conversion algorithms compliant with industry standards

Related standards

• API MPMS Chapter 11.1 (2004) / Adjunct to ASTM D1250-04 / IP 200/04 (Addendum 1-2007) - primary reference implementation
• API MPMS Chapters 11.2.x and 11.5 - compressibility and density intraconversions
• API MPMS Chapter 11.2.4 / GPA TP-27 - NGL and LPG temperature corrections
• ISO 13443 - standard reference conditions for natural gas (excluded from ISO 91:2017)

ISO 91:2017 consolidates international best practice for temperature and pressure volume correction factors, ensuring consistent and auditable volumetric conversions used across the petroleum industry.