API MPMS 2.2F (2004): Standard Test Method for Sediment in Crude Oil by the Centrifuge Method

1. Scope and Application

API MPMS 2.2F (2004), identical to ASTM D4007-02, defines the standard test method for the determination of sediment in crude oil by the centrifuge method. This method is applicable to crude oils with sediment concentrations up to 0.5% by volume and can be extended to higher sediment contents with a note that precision may be adversely affected. The test provides a rapid, field‑ready procedure for quality control, custody transfer verification, and refinery feedstock assessment.

The centrifuge method separates sediment from crude oil by mixing a known volume of sample with toluene, heating, and subjecting the mixture to a centrifugal force that accelerates the settling of insoluble solids and heavy waxy materials. The volume of sediment is read directly in a calibrated cone‑shaped centrifuge tube and reported as a percentage of the original sample volume.

This standard is an integral part of the API Manual of Petroleum Measurement Standards (MPMS), serving as the reference method for sediment measurement in crude oil where water content must be known by separate methods (e.g., API MPMS 2.2A, 2.2B, or 2.2C). It is widely used by producers, transporters, refiners, and third‑party inspection agencies.

2. Technical Requirements and Procedure

2.1 Apparatus and Reagents

The key equipment specified includes:

• Centrifuge: capable of maintaining a relative centrifugal force (RCF) of 500–600 × g (typically 1500–1800 rpm with a 6‑inch radius head). The centrifuge must have a temperature control capability to maintain 60 ± 2 °C.
• Centrifuge tubes: cone‑shaped, 100 mL or 50 mL capacity, conforming to ASTM E 1407, with graduated markings precise to 0.05 mL.
• Solvent: toluene (reagent grade, anhydrous) for dissolving the crude oil matrix without dissolving the sediment.
• Heating bath or oven: capable of maintaining 60 ± 2 °C.
• Shaker or mixing device: to ensure homogeneous dispersion of sediment before centrifuging.

2.2 Sample Preparation

The crude oil sample must be collected and handled in accordance with API MPMS Chapter 8 (Sampling) to ensure representativity. Before testing, the sample is warmed to 60 °C in a water bath and thoroughly mixed to re‑suspend any settled solids.

Procedure outline:

1. Transfer 50 mL (or 100 mL) of the conditioned sample into a clean centrifuge tube.
2. Add 50 mL (or 100 mL) of toluene at 60 °C. Cap and shake vigorously for 10 seconds.
3. Place the tube in the 60 °C bath for 10 minutes, then shake again for 10 seconds.
4. Place the tube in a heated centrifuge (60 °C) and spin for 10 minutes at the specified RCF.
5. Read the volume of sediment in the cone tip to the nearest 0.05 mL.
6. Repeat the centrifugation for 5 minutes and re‑read until two consecutive readings are identical.
7. Report the final volume as percent sediment by volume: % sediment = (mL sediment / initial sample volume) × 100.

2.3 Test Conditions Summary

3. Implementation and Operational Considerations

Successful implementation of API MPMS 2.2F depends on careful control of several operational variables.

3.1 Centrifuge Calibration and Verification

The centrifuge must be calibrated periodically to ensure that the actual RCF matches the set value. A simple calibration involves verifying the rotational speed with a tachometer and calculating the RCF using the radius of the rotor. Temperature control must also be validated at the tip of the rotating tube. Many jurisdictions require a calibration interval of 12 months or after major maintenance.

3.2 Handling Problematic Crudes

Some crude oils present challenges due to:

• High wax content: Wax can co‑precipitate with sediment and raise the apparent sediment volume. In such cases, repeating the centrifugation at 75 °C (if allowed by protocol) or using a different solvent blend may be considered, but any deviation from the standard must be noted in the report.
• Very low sediment (< 0.02%): The method may lack sufficient precision. Refer to ASTM D4007 for guidance on using larger tubes or extended centrifugation.
• Emulsions: If a distinct water layer appears, it should be reported separately; the method is intended for sediment only. Water is determined via Karl Fischer titration (API MPMS 2.2B/2.2C) or distillation (API MPMS 2.2A).

3.3 Recordkeeping and Reporting

Test reports under API MPMS 2.2F should include:

• Sample identification, date, and sampling location.
• Initial sample volume used (e.g., 50 mL or 100 mL).
• Final sediment volume in mL and the calculated percentage.
• Any procedural deviations (e.g., extended centrifugation time, higher temperature).
• Evidence of calibration of the centrifuge (date of last verification).
• Operator identification.

4. Compliance and Quality Assurance

4.1 Conformance with API MPMS and ASTM

API MPMS 2.2F is adopted by contract in many crude oil sales agreements and custody transfer protocols. To claim compliance, the laboratory must follow the exact procedure defined in the standard, including the specified apparatus and conditions. Alternative solvents or centrifugation regimes void the compliance claim unless validated by mutual agreement between parties.

4.2 Precision and Bias

The standard provides precision estimates based on interlaboratory studies:

• Repeatability (same operator, same equipment): 0.1% (absolute) in the range up to 0.5% sediment.
• Reproducibility (different laboratories): 0.3% (absolute) for the same range.

These values apply to fresh, representative samples; aged or degraded samples may show higher variability.

4.3 Quality Control Measures

Recommended quality control steps include:

• Daily verification of centrifuge speed and temperature.
• Use of a control sample (crude with known sediment content) every 20 tests or weekly.
• Participation in proficiency testing programs such as those organized by ASTM or API.
• Operator training and certification per the MPMS Chapter 10 series.

Frequently Asked Questions

References: API MPMS 2.2F (2004) / ASTM D4007-02, API Manual of Petroleum Measurement Standards, Chapter 8 – Sampling, and Chapter 10 – Sediment and Water. This article is issued for technical information purposes and does not replace the official standard.

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