ISO 13758:1996

Overview

ISO 13758:1996 specifies the valve freeze method for assessing the dryness of liquefied petroleum gases (LPG) consisting predominantly of propane and/or propene. The procedure is a functional pass/fail test used to determine whether dissolved or free water in LPG could cause freezing and malfunction of pressure-reducing regulators in domestic, industrial and automotive systems.

The test measures the freeze-off time (or valve freeze time): the elapsed time from the start of flow until the test valve ceases to pass liquid under specified conditions.

Key Topics

• Principle: A liquid-phase aliquot cools the test valve by vaporization while flowing through a wide-open position. After cooling, the valve is set to a small pre-set orifice and the time to freeze-off is recorded.
• Apparatus: A calibrated LPG test valve (with purge and test positions), a stopwatch accurate to at least 0.2 s, a wipe cloth, and-if needed-a pressurized sample container (minimum capacity 11.5 L).
• Sampling sensitivity: Moisture measurements are sensitive to sampling errors; testing is recommended at the LPG supply source when possible. If sampling is required, use methods referenced in ISO 4257.
• Procedure highlights:
  • Connect the test valve to the liquid line or a sampled container using clean, dry metallic fittings.
  • Ensure inlet pressure does not exceed 700 kPa above the vapour pressure of the product; use a regulator upstream if necessary.
  • Purge for 15 s in the wide-open position and establish a uniform frost before starting the timed test.
  • Disregard the first run, then repeat until three consecutive freeze-off times agree within ±2 s.
  • If three consecutive runs exceed 180 s, discontinue and report as "over 180 s".
• Result expression: Report the average of three qualified runs to the nearest 1 s. For pass/fail use a typical criterion of 60 s ("pass" if freeze-off time > 60 s; "fail" if ≤ 60 s).

Applications

• Preventive quality control for LPG supplied to domestic, industrial, and automotive pressure-reducing systems.
• Functional screening to detect water-related freezing risk that can cause regulator blockage or malfunction.
• Useful for suppliers and testing laboratories as a specification-level pass/fail check.

Related Standards

• ISO 4257 - Method of sampling LPG (referenced normative standard for sample collection).

Practical notes: the test does not quantify moisture in mg/kg directly; results indicate functional performance risk. Users must follow safety practices and account for possible antifreeze agents that can affect freeze times. Test reports should reference ISO 13758, product identification, results, any deviations from the procedure, and the test date.