Overview
EN ISO 4256:1998 (identical to ISO 4256:1996) defines the LPG method for determination of gauge vapour pressure of liquefied petroleum gases (LPG). The method is intended for measuring LPG vapour pressures at elevated temperatures (approximately 35 °C to 70 °C) and provides the test apparatus, procedural steps, calibration/correction requirements and safety notes needed for reliable results. This standard is used to support safe design and handling of LPG storage vessels, transport containers and consumer equipment.
Key topics and technical requirements
- Scope and purpose: Measurement of gauge vapour pressure (absolute minus atmospheric pressure) of LPG products-an indirect indicator of the most volatile constituents and lowest temperatures for vaporization.
- Test temperature range: Approximately 35 °C to 70 °C, with bath stability requirements: ±0.1 °C for ≤50 °C and ±0.3 °C for >50 °C.
- Apparatus: Two-chamber vapour pressure assembly (upper and lower chambers) with a Bourdon-type pressure gauge. The standard specifies chamber geometry/volume ratios (upper chamber dimensions and lower-chamber volume ratios), bleeder-valve assembly, straight-through and inlet valves, and a constant-temperature bath.
- Gauge requirements and calibration: Gauge selection by expected LPG vapour pressure; gauge error must be determined and corrected-either by calibration against a dead-weight tester or comparison with a certified gauge.
- Preparation and testing procedure: Cleaning, purging, sample introduction, inversion and vigorous shaking, immersion in temperature-controlled bath, repeated agitation and observation until two consecutive readings differ by ≤5 kPa.
- Integrity checks: Hydrostatic strength test (manufacturer certification to withstand ~7 000 kPa) and leak test (immersion at 3 500 kPa) before service.
- Safety and annexes: Contains warnings about hazardous materials and detailed safety precautions (annex B). Annex A covers thermometer/temperature measurement requirements.
Practical applications and users
- Who uses it: LPG producers, independent testing laboratories, storage and tank designers, cylinder manufacturers, transport operators, regulatory bodies and equipment calibration labs.
- Why it matters: Accurate LPG vapour pressure data are essential for:
- Selecting properly rated storage vessels and pressure relief devices
- Determining safe operating limits for handling and transport
- Product specification, quality control and regulatory compliance
- Sizing of vapor recovery and dispensing systems
Related standards
- ISO 3007 - Petroleum products - Determination of vapour pressure - Reid method (related apparatus components)
- ISO 4257 - Liquefied petroleum gases - Method of sampling
- Note: EN ISO 4256:1998 is the CEN adoption of ISO 4256:1996; consult local national adoptions and applicable regulations for implementation details.
Keywords: EN ISO 4256, ISO 4256, LPG vapour pressure, liquefied petroleum gas, gauge vapour pressure, vapour pressure testing, LPG testing method, pressure gauge calibration.
