Test Method for Determination of Vapor Pressure of Crude Oil: VPCRx (Expansion Method)
ASTM D6377 test method covers the use of automated vapor pressure instruments to determine the vapor pressure exerted in vacuum of crude oils. This test method is suitable for testing samples that exert a vapor pressure between 25 kPa and 180 kPa at 37.8 °C at vapor-liquid ratios from 4:1 to 0.02:1 (X = 4 to 0.02).
NOTE 1—This test method is suitable for the determination of the vapor pressure of crude oils at temperatures from 0 °C to 100 °C and pressures up to 500 kPa, but the precision and bias statements may not be applicable.
This test method allows the determination of vapor pressure for crude oil samples having pour points above 0 °C.
The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
D6377 determines the vapor pressure of crude oils and crude oil blends by a single expansion method covering a wide range of samples and experimental conditions.
A precise determination of the vapor pressure of crude oils is a valuable diagnostic tool to:
_ Optimize the best route for their refining strategic programs.
_ Assist in determining the hazards associated with their storage and transportation especially by rail.
_ And finally, to bring the final product into specifications to maximize their commercial value.
Eralytics Eravap analyzer by means of its unique design and sophisticated piston-based measurement principle, guarantees precise, and accurate experimental results for dead and live crude oils in 5 minutes….”
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D323 Test Method for Vapor Pressure of Petroleum Products (Reid Method)
D2892 Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column)
D3700 Practice for Obtaining LPG Samples Using a Floating Piston Cylinder
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D5191 Test Method for Vapor Pressure of Petroleum Products (Mini Method)
D5853 Test Method for Pour Point of Crude Oil: VPCRx (Expansion Method)
D6377 Test Method for Determination of Vapor Pressure of Crude Oils
D6708 Practice for Statistical Assessment and Improvement of Expected Agreement Between Two Test Methods that Purport to Measure the Same Property of a Material
Summary of Test Method
The vapor pressure described below faithfully follows the ASTM D6377. Please consult the current version of this standard test method for more details. Employing a measuring chamber with a built-in piston, a sample of known volume is drawn from the sample container into the temperature-controlled chamber at 20 °C or higher.
After sealing the chamber, the volume is expanded by moving the piston until the final volume produces the desired V/L value. The temperature of the measuring chamber is then regulated to the measuring temperature.
After temperature and pressure equilibrium, the measured pressure is recorded as the VPCRX of the sample. The test specimen shall be mixed during the measuring procedure by shaking the measuring chamber to achieve pressure equilibrium in a reasonable time of 5 min to 30 min.
For results related to Test Method D323, the final volume of the measuring chamber shall be five times the test specimen volume and the measuring temperature shall be 37.8 °C.
Significance and Use
Vapor pressure of crude oil at various V/Ls is an important physical property for shipping and storage.
NOTE 2—A V/L ratio of 0.02:1 (X = 0.02) mimics closely the situation of an oil tanker.
Vapor pressure of crude oil is important to crude oil producers and refiners for general handling and initial refinery treatment.
The vapor pressure determined by this test method at a vapor-liquid ratio of 4:1 (VPCR4) of crude oil at 37.8 °C can be related to the vapor pressure value determined on the same material when tested by Test Method D323.
Chilling and air saturation of the sample prior to the vapor pressure measurement is not required.
This test method can be applied in online applications in which an air saturation procedure prior to the measurement cannot be performed.
Set the sample introduction temperature of the measuring chamber between 20 °C and 37.8 °C. For crude oil samples with a pour point higher than 15 °C, set the injection temperature at least 5 °C above the pour point temperature of the sample.
Set the V/L to the desired value X:1 (for test results related to Test Method D323, set the V/L to 4:1).
If the sample is contained in a pressurized floating piston cylinder, mix it vigorously with the mechanical stirrer, otherwise shake the container with the non-pressurized sample, to ensure a homogenous sample.
Follow the manufacturer’s instructions for introducing the test specimen into the measuring chamber. The volume of the specimen shall be such that after the expansion to the final volume the programmed V/L is achieved.
After closing the inlet valve, expand the volume of the measuring chamber to the final volume.
Switch-on the shaker and leave it on during the entire measuring procedure.
Adjust the temperature control to the measuring temperature (for results related to Test Method D323, adjust to a temperature of 37.8 °C). The measuring temperature shall not be lower than at least 10 °C above the pour point temperature of the sample.
Wait for temperature equilibrium between measuring chamber and specimen, and monitor the total vapor pressure every 30 ± 5 s. When three successive readings agree within 0.3 kPa, record this resulting vapor pressure as VPCRX (Tm°C).
Report the results to the nearest 0.1 kPa and specify the
test temperature and vapor-liquid ratio.
VPCRX (Tm°C) = yy.y kPa (1)
X = vapor-liquid ratio, and
Tm = measuring temperature.
The apparatus suitable for this test method employs a small volume, cylindrically shaped measuring chamber with associated equipment to control the chamber temperature within the range from 0 °C to 100 °C. The measuring chamber shall contain a movable piston with a minimum dead volume of less than 1 % of the total volume at the lowest position to allow sample introduction into the measuring chamber and expansion to the desired V/L. A static pressure transducer shall be incorporated in the piston. The measuring chamber shall contain an inlet/outlet valve combination for sample introduction and expulsion. The piston and the valve combination shall be at the same temperature as the measuring chamber to avoid any condensation or excessive evaporation.
The measuring chamber shall be designed to have a total volume of 5 mL to 15 mL and shall maintain a V/L of 4:1 to 0.02:1. The accuracy of the adjusted V/L shall be within 0.01.
NOTE 3—The measuring chambers employed by the instruments used in generating the precision and bias statements were constructed of nickel plated aluminum, stainless steel and brass with a total volume of 5 mL Measuring chambers exceeding a 5 mL capacity and different design can be used, but the precision and bias statement may not be applicable.
The pressure transducer shall have a minimum operational range from 0 kPa to 500 kPa with a minimum resolution of 0.1 kPa and a minimum accuracy of ±0.5 kPa. The pressure measurement system shall include associated electronics and readout devices to display the resulting pressure reading.
Electronic temperature control shall be used to maintain the measuring chamber at the prescribed temperature within ±0.1 °C for the duration of the test.
A platinum resistance thermometer shall be used for measuring the temperature of the measuring chamber. The minimum temperature range of the measuring device shall be from 0 °C to 100 °C with a resolution of 0.1 °C and an accuracy of ±0.1 °C.
The vapor pressure apparatus shall have provisions for rinsing the measuring chamber with the next sample to be tested or with a solvent of low vapor pressure.
The vapor pressure apparatus shall have provisions for shaking the sample during the measuring procedure with a minimum frequency of 1.5 cycles per second.
Vacuum Pump for Calibration, capable of reducing the pressure in the measuring chamber to less than 0.01 kPa absolute.
McLeod Vacuum Gage or Calibrated Electronic Vacuum Measuring Device for Calibration, to cover at least the range of 0.01 kPa to 0.67 kPa. The calibration of the electronic measuring device shall be regularly verified in accordance with Annex A of Test Method D2892.
Pressure Measuring Device for Calibration, capable of measuring local station pressure with an accuracy and a resolution of 0.1 kPa or better, at the same elevation relative to sea level as the apparatus in the laboratory.
NOTE 4—This standard does not give full details of instruments suitable for carrying out this test. Details on the installation, operation and maintenance of each instrument may be found in the manufacturer’s manual.
Quality Assurance and Quality Control
After preparing and calibrating the apparatus according to manufacturer’s instructions and having verified that the instrument is performing properly, use a representative quality control (QC) sample to confirm that the instrument’s performance is in statistical control.
Use a verification fluid as described in Materials and Reagents section, of known volatility as an independent check against the instrument calibration each day the instrument is in use. For pure compounds, multiple test specimens may be taken from the same container over time.
Air saturate the verification fluid at temperatures between 0 °C to 1 °C as described in Test Method D5191. Transfer the verification fluid into the measuring cell using a transfer tube or a syringe. The temperature of the verification fluid shall be at 0 °C to 3 °C during the sample introduction, and the measuring procedure shall be in accordance with Section 12 with a V/L ratio of 4:1 and a measuring temperature of 37.8 °C.
Table 1 provides the accepted reference value (ARV) [Ptot] and uncertainty limits (at least 95 % confidence interval) of reference fluids tested in the 2003 D5191 ILS. As stated before, the Ptot” reference used in Test Method D5191 (and Table 1 of this test method) is equivalent to VPCR4 (37.8 °C) in this test method. This information, combined with the tolerance value recommended by instrument manufacturers, was used to establish the acceptable testing range for the reference fuels to verify instrument performance.
Values obtained within the acceptable testing range intervals in Table 1 indicate that the instrument is performing at the level deemed acceptable by this standard. If values outside the acceptable testing range intervals are obtained, verify the quality of the pure compound(s) and re-check the calibration of the instrument. (Warning—The use of single component verification materials such as those listed in Table 1 will only prove the calibration of the equipment. It will not check the accuracy of the entire test method, including sample handling, because losses due to evaporation will not decrease the sample vapor pressure as happens with losses of light ends in multi component mixtures.
The vapor pressure measurement process can be checked periodically by performing this test method on previously prepared samples from one batch of suitable products. Samples should be stored in an environment suitable for long term storage without sample degradation. Analysis of results from these quality control samples can be carried out using control chart techniques).
NOTE 10—It is recommended that at least one type of verification fluid used in 11.1 have a vapor pressure representative of the crudes regularly tested by the equipment.
Precision and Bias
Precision—The precision of this test method as determined by the statistical examination of interlaboratory test results is as follows:
Repeatability—The difference between successive test results obtained by the same operator with the same apparatus under constant operating conditions on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the values calculated as per the following equations only in one case in twenty:
For samples in pressurized Floating Piston Cylinders:
V/L = 4 and Tm = 37.8°C : repeatability = 2.48 kPa (37.8°C) (2)
V/L = 0.02 and Tm = 37.8°C: repeatability = 5.61 kPa (37.8°C) (3)
For samples in non-pressurized sample containers:
V/L = 4 and Tm = 37.8°C: repeatability = 2.29 kPa (37.8°C) (4)
Reproducibility—The difference between two single and independent results obtained by different operators working in different laboratories on identical material would, in the long run, in the normal and correct operation of the test method, exceed the following values in only one case in twenty:
For samples in pressurized Floating Piston Cylinders:
V/L = 4 and Tm = 37.8°C: reproducibility = 4.26 kPa (37.8°C) (5)
V/L = 0.02 and Tm = 37.8°C: reproducibility = 20.3 kPa (37.8°C) (6)
For samples in non-pressurized sample containers:
V/L = 4 and Tm = 37.8°C: reproducibility = 5.26 kPa (37.8°C) (7)
Bias—Since there is no accepted reference material suitable for determining the bias for the procedures in this test method, a bias cannot be determined.
Relative Bias to Test Method D323—A statistically significant relative bias was observed for crude oil in a limited interlaboratory test program the 2005 ILS6 between VPCR4 (37.8 °C) obtained using this test method and the vapor pressure obtained using Test Method D323.
The precision statements were derived from a 2005 interlaboratory cooperative test program involving 7 laboratories and 6 crude oil samples covering a vapor pressure range between 34 kPa and 117 kPa.
Experimental parameters such as sample types and sampling devices; flows and temperatures should be used in accordance with the manufacturer’s operation manual. The same applies to experimental methods and procedures to keep the system in optimum stability conditions and free of leaks, obstructions and contamination. All calibration steps, formulas and calculation required by D6377 are thoroughly and carefully included in the available methods of the Eravap versions shown below.
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Eravap Crude Oil Vapor Pressure Expansion Method Analyzer
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