Overview
ISO 4256:1996 specifies a laboratory method - the LPG method - for determining the gauge vapour pressure of liquefied petroleum gases (LPG). Intended for temperatures approximately 35 °C to 70 °C, the standard describes the required test apparatus, sampling and handling, test procedure (purging, filling, equilibration), measurement corrections and reporting. It replaces the first edition (ISO 4256:1978) and includes normative annexes for equipment and safety.
Key topics and technical requirements
- Scope: Measurement of LPG gauge vapour pressure (absolute vapour pressure minus atmospheric pressure) as an indicator of volatility and storage/handling requirements.
- Test apparatus: Two-chamber assembly (upper and lower chambers) with specific dimensional and volumetric requirements. Upper chamber dimensions and two options for lower chamber volume (33 1/3% or 20% configurations) are specified to meet volumetric ratios required by the method.
- Pressure measurement: Use a Bourdon-type gauge of appropriate range. Readings must be corrected for gauge error by calibration (dead-weight tester or certified secondary gauge).
- Temperature control: Constant-temperature water bath capable of maintaining test temperature within ±0.1 °C for ≤50 °C tests and ±0.3 °C for >50 °C tests. Thermometer ranges and placement are specified.
- Hydrostatic and leak tests: Assembled chambers must be certified to withstand high hydrostatic pressures (manufacturer certification ~7 000 kPa) and must pass a leak test at ~3 500 kPa in a water bath prior to use.
- Procedure highlights: Purge apparatus, fill with sample, invert and shake, immerse in bath, perform timed agitations and readings until two consecutive readings differ by ≤5 kPa. Correct measured values for gauge error and ambient barometric pressure (converted to the standard 101.3 kPa).
- Precision: Repeatability r = 12 kPa; reproducibility R = 19 kPa.
- Safety: The standard includes warnings and an annex on safety precautions for handling flammable LPG.
Applications and users
- Practical applications:
- Determining LPG volatility for design of storage vessels, cylinders and transport containers
- Verifying LPG specifications for safe handling and customer use
- Quality control in LPG production, distribution and import/export testing
- Who uses it:
- Petroleum and LPG laboratories, testing houses and refineries
- Equipment designers and manufacturers (pressure vessels, regulators)
- Regulatory agencies and safety inspectors
- Shipping and storage operators requiring vapour-pressure data for safe operation
Related standards
- ISO 3007:1986 - Petroleum products - Determination of vapour pressure (Reid method)
- ISO 4257:1988 - Liquefied petroleum gases - Method of sampling
- ISO 4256:1978 - First edition (superseded)
Keywords: ISO 4256:1996, LPG vapour pressure, liquefied petroleum gases, gauge vapour pressure, vapour pressure test, pressure gauge, hydrostatic test, LPG method.