Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Scope and Application
“content”: “
Scope and Application
API MPMS 6.5 (1991, Reaffirmed 2012) specifies requirements for metering systems used in the loading and unloading of marine vessels, including tankers and barges. It is part of the American Petroleum Institute’s Manual of Petroleum Measurement Standards (API MPMS) and addresses custody transfer measurement of liquid hydrocarbons such as crude oil, refined petroleum products, and similar liquid cargoes. The standard applies to both shore-based and shipboard metering systems that are employed when the quantity transferred is determined by flow measurement rather than tank gauging.
The primary scope includes:
- Metering systems designed for custody transfer of liquid hydrocarbons during marine loading and unloading operations.
- Requirements for measurement accuracy, equipment selection, installation, operation, and maintenance.
- Guidelines for meter proving to ensure traceability and reliability of quantity determination.
- Considerations for ancillary equipment such as strainers, air eliminators, temperature and pressure transmitters, and sample connections.
While the standard focuses on liquid measurement, it does not cover cargoes like LNG, LPG, or other gases; those are addressed by separate API MPMS chapters. The 1991 edition with reaffirmation in 2012 confirms that the technical provisions remain current and are widely referenced in contracts and regulatory requirements for marine custody transfer.
Technical Requirements
Meter Selection and Accuracy
API MPMS 6.5 requires that meters used for marine custody transfer achieve a system accuracy of ±0.25% or better under normal operating conditions. Acceptable primary meter types include positive displacement, turbine, Coriolis mass flow, and ultrasonic meters, provided they meet performance criteria. The choice depends on fluid properties, viscosity, flow rate range, and installation environment. The table below summarizes typical accuracy expectations and flow rate ranges for common meter types used in marine applications.
| Meter Type | Typical Accuracy (±% of reading) | Turndown Ratio | Typical Flow Range (m³/h) | Notes |
|---|---|---|---|---|
| Positive Displacement | 0.1 – 0.2 | 10:1 – 20:1 | 5 – 5,000 | Suitable for high-viscosity fluids; robust for marine duties. |
| Turbine | 0.15 – 0.25 | 10:1 – 15:1 | 10 – 10,000 | Requires fully developed flow profile; sensitive to viscosity changes. |
| Coriolis (Mass) | 0.05 – 0.15 | 20:1 – 100:1 | 1 – 2,500 | Direct mass measurement; excellent accuracy; limited pipe sizes for high flow. |
| Ultrasonic (Transit-Time) | 0.15 – 0.30 | 10:1 – 30:1 | 100 – 20,000 | Non-intrusive; ideal for large diameter pipes (12"/300 mm and above). |
Meter Proving and Calibration
The standard mandates periodic meter proving using either a pipe prover (ball or bidirectional), a master meter, or a small volume prover. The proving frequency depends on the service type (fiscal vs. operational) and the stability of meter performance. For marine loading terminals, proving is typically performed at the beginning of cargo transfer operations or according to a fixed schedule. The prover system must be designed, installed, and calibrated in accordance with API MPMS Chapter 4 (Proving Systems). Key requirements include:
- Prover loop size must allow at least four sphere passes (for ball provers) to ensure repeatable data.
- Temperature and pressure conditions during proving should closely match operating conditions to minimize correction errors.
- Master meter provers must have a higher accuracy class than the meters they are proving (typically four times better).
- Prover volumes must be traceable to national standards via water draw calibration or displacement calibration.
Ancillary Equipment and Installation
API MPMS 6.5 provides detailed guidance on the design and placement of supporting equipment:
- Air Eliminators & Strainers: Required upstream of meters to protect against air entrainment and debris that could cause measurement errors or meter damage.
- Temperature and Pressure Compensation: Real-time correction using RTDs (100 Ω platinum) and pressure transmitters; density determined by Coriolis or online densitometer for mass-based systems.
- Sampling Systems: Automatic, proportional-to-flow sampling for quality determination (API gravity, BS&W, sediment). The sample probe location and line mixing must ensure a representative sample.
- Flow Conditioning: Straight-pipe lengths of at least 10 pipe diameters downstream and 5 upstream of the meter (or use of flow conditioners) to ensure a developed flow profile for turbine and ultrasonic meters.
- Back-Pressure Control: A back-pressure valve must maintain minimum pressure above vapor saturation to prevent flashing, especially at the meter outlet.
Tip: Always install a downstream control valve rather than an upstream valve to regulate flow without affecting flow profile at the meter. This practice minimizes measurement uncertainty.
Implementation Highlights for Marine Environments
System Architecture and Safety
Marine loading and unloading terminals present unique challenges: tidal changes affecting static head, ship movement, wide flow ranges, and hazardous zone classifications. API MPMS 6.5 addresses these by recommending:
- Meter runs arranged in parallel to handle variable flow demands (e.g., for different vessel sizes).
- Electrical components rated for the appropriate hazardous area (Zone 0,1,2 as per IEC 60079-10-1 or API RP 500).
- Emergency shutdown (ESD) integration so that metering systems can safely isolate upon detection of abnormal conditions.
- Data acquisition systems (DCS/PLC) that record flow totals, instantaneous rate, temperature, pressure, density, and density of water for automatic back-flushing of sample loops.
Shore vs. Shipboard Metering
The standard acknowledges both configurations. For shore-side systems, the meter station is fixed and can be more elaborate. For shipboard systems, space and weight constraints are more restrictive, and meters must be capable of marine classification (e.g., Lloyd’s, ABS, DNV). Proving shipboard meters often requires a portable prover or a master meter. API MPMS 6.5 provides additional provisions for such cases, including reduced proving frequency with added statistical monitoring.
| Feature | Shore-Side | Shipboard |
|---|---|---|
| Typical Meter Size | 4" – 24" (100-600 mm) | 3" – 16" (80-400 mm) |
| Prover Type | Ball prover or small volume prover | Master meter or portable small volume prover |
| Power Supply | Grid-based; unlimited | Ship generator; limited capacity |
| Accessibility for Maintenance | Easy | Constrained by ship layout |
Caution: Temperature gradients between ship and shore piping can cause significant measurement differences if not compensated. Use insulated piping and multiple temperature sensors to reduce uncertainty.
Compliance Notes and Best Practices
Adherence to API MPMS 6.5 ensures that marine custody transfer measurements meet contractual accuracy and legal metrology requirements. Key compliance considerations include:
- Reaffirmation Status: Although originally published in 1991 and reaffirmed in 2012, this edition remains in common use. Engineers should verify with API whether a newer edition (e.g., API MPMS 6.5:2016, 2021) has superseded the 1991 version for new installations. Local regulations may mandate the latest edition.
- Integration with Other API MPMS Chapters: The standard works in concert with API MPMS Chapter 4 (Proving Systems), Chapter 5 (Metering), Chapter 7 (Temperature Determination), Chapter 8 (Sampling), Chapter 9 (Density Determination), and Chapter 12 (Calculation of Tank Volume). Full compliance requires referencing these documents.
- Third-Party Verification: Many contracts require independent inspection of the metering system by a recognized body (e.g., SGS, Bureau Veritas) to certify conformance with API MPMS 6.5 before first use.
- Documentation: Detailed records of prover calibrations, meter factor trends, and maintenance must be retained for audit. The standard recommends keeping at least 2 years of data for fiscal systems.
Compliance Tip: Implement a structured meter verification program that includes monthly meter factor checks and annual prover calibrations. This proactive approach demonstrates due diligence and supports dispute resolution.
Important: Failure to comply with the minimum proving frequency or neglecting to correct for temperature and pressure can lead to measurement discrepancies exceeding 0.5%, potentially causing significant financial losses and contractual disputes.
Frequently Asked Questions
Q: Is API MPMS 6.5 applicable to LPG and LNG marine transfer?
A: No. API MPMS 6.5 specifically addresses liquid hydrocarbons (crude oil, refined products, etc.). LPG and LNG are governed by other standards such as API MPMS Chapter 14 (Natural Gas Fluids) or ISO 13398. For cryogenic cargoes, specialized metering standards apply.
A: No. API MPMS 6.5 specifically addresses liquid hydrocarbons (crude oil, refined products, etc.). LPG and LNG are governed by other standards such as API MPMS Chapter 14 (Natural Gas Fluids) or ISO 13398. For cryogenic cargoes, specialized metering standards apply.
Q: How often must meters be proved under this standard?
A: The standard does not prescribe a universal fixed interval; instead it recommends a proving frequency based on the meter’s performance history and the application (fiscal vs. monitoring). Common practice is to prove at the start of each cargo transfer for fiscal meters, or at least monthly for continuous operations. The prover must also be recalibrated annually at a minimum.
A: The standard does not prescribe a universal fixed interval; instead it recommends a proving frequency based on the meter’s performance history and the application (fiscal vs. monitoring). Common practice is to prove at the start of each cargo transfer for fiscal meters, or at least monthly for continuous operations. The prover must also be recalibrated annually at a minimum.
Q: Can a shipboard metering system use a master meter for proving instead of a pipe prover?
A: Yes, API MPMS 6.5 permits the use of a master meter (provided it is traceable to a primary standard and has a fourfold accuracy improvement over the meter being proved). Portable master meters are often the preferred solution for proving meters on vessels due to space constraints.
A: Yes, API MPMS 6.5 permits the use of a master meter (provided it is traceable to a primary standard and has a fourfold accuracy improvement over the meter being proved). Portable master meters are often the preferred solution for proving meters on vessels due to space constraints.
Q: Does API MPMS 6.5 require automatic temperature compensation (ATC) at the meter?
A: Yes. The standard requires real-time temperature and pressure compensation for volume-based meters to correct to reference conditions (typically 15°C or 60°F) or, for mass flow meters, to directly report mass. ATC is essential for achieving the required system accuracy of ±0.25%.
A: Yes. The standard requires real-time temperature and pressure compensation for volume-based meters to correct to reference conditions (typically 15°C or 60°F) or, for mass flow meters, to directly report mass. ATC is essential for achieving the required system accuracy of ±0.25%.
“