ASTM D323-20 Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method)

The standard “ASTM D323-20 Standard test method for vapor pressure of petroleum products (Reid method)” published by the American Society for Testing and Materials (ASTM) describes the following test methods for determining the vapor pressure of gasoline, volatile crude oil and other volatile petroleum products:

• Procedure A applies to gasoline and other petroleum products having a vapor pressure less than 180 kPa (26 psi).
• Procedure B may also be applicable to other materials, but only gasoline was included in the interlaboratory testing program to determine the sensitivity of this test method.
• Procedure C is for materials having a vapor pressure greater than 180 kPa (26 psi).
• Procedure D is for aviation gasoline with a vapor pressure of approximately 50 kPa (7 psi).

Since the external atmospheric pressure is initially balanced by the atmospheric pressure in the vapor chamber, the Reid vapor pressure is the absolute pressure at 37,8 degrees in kilopascals. The Reid vapor pressure differs from the actual vapor pressure of the sample due to the presence of water vapor and air in the confined space and some minor sample evaporation.

This test method is not applicable to fuels containing oxygenated compounds other than liquefied petroleum gases or methyl t-butyl ether. For determining the vapor pressure of liquefied petroleum gases, the test methods described in ASTM D1267-18 or ASTM D6897-16 standards are applicable. For determining the vapor pressure of gasoline-oxygenate mixtures, the ASTM D4953-20 standard is applicable. The sensitivity of crude oil has not been determined for many years. ASTM D6377-20 standard describes a test method for determining the vapor pressure of crude oil. IP 481 is a test method for determining the vapor pressure of crude oil saturated with air.

In applying the Reid method, the liquid chamber of the vapor pressure apparatus is filled with the cooled sample and connected to a vapor chamber heated to 37,8°C in a bath. The combined apparatus is immersed in a 37,8°C bath until a constant pressure is observed. The appropriately corrected reading is reported as the Reid vapor pressure.

All four procedures described use liquid and vapor chambers of the same internal volume. Procedure B uses a semiautomatic apparatus that is submerged in a horizontal bath and rotated as it reaches equilibrium. A Bourdon gauge or pressure transducer may be used in this procedure. Procedure C uses a liquid chamber with two valve openings. Procedure D requires more stringent limits on the ratio of the liquid and vapor chambers.

Vapor pressure is an important physical property of volatile liquids. This test method is used to determine the vapor pressure at 0 degrees Celsius for petroleum products and crude oils with initial boiling points above 37,8 degrees Celsius. Vapor pressure is critical for both automotive and aviation gasolines and affects starting, warming up, and vapor lock tendency at high operating temperatures or high altitudes. Maximum vapor pressure limits for gasoline are legally required in some areas as a measure of air pollution control.

The vapor pressure of crude oil is important to the crude oil producer and refiner for overall handling and initial refinery processing. Vapor pressure is also used as an indirect measure of the evaporation rate of volatile petroleum solvents.

If the procedure is not followed carefully, large errors in vapor pressure measurements can occur. The following list highlights the importance of strictly adhering to the precautions given in the procedure:

• Checking the pressure gauge: To ensure greater accuracy of the results, after each test all indicators are checked against a pressure gauge. The gauge is read with the gauge in a vertical position and after a light tap.
• Leak check: All apparatus is checked for both liquid and vapor leaks before and during each test.
• Sampling: Since the initial sampling and processing of samples will greatly affect the final results, utmost precautions are taken to prevent losses due to evaporation and minor changes in composition. In no case should any part of the Reid apparatus be used as a sample container before the test has been carried out.
• Cleaning the instrument: The pressure gauge, liquid chamber and vapor chamber are thoroughly cleaned and it is ensured that no residue remains in the sample.
• Assembling the device: The requirements in this standard are carefully followed.
• Shake the device: Shake the device vigorously according to the instructions to ensure balance.

The extreme sensitivity of vapor pressure measurements to evaporative losses and resulting compositional changes necessitates the utmost care and attention in handling samples. These principles apply to all samples for vapor pressure determination except those specifically excluded from samples having vapor pressures above 180 kPa (26 psi).

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