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API MPMS 5.6 (2002, Reaffirmed 2013): Measurement of Liquid Hydrocarbons by Coriolis Meters
Technical requirements, implementation guidelines, and compliance considerations for Coriolis metering in petroleum measurement systems
API MPMS 5.6, originally published in 2002 and reaffirmed in 2013, is the sector’s definitive standard for the application of Coriolis mass flow meters in the measurement of liquid hydrocarbons. This article provides a thorough examination of its scope, core technical requirements, implementation best practices, and compliance implications for custody transfer and operational measurement systems.
Scope of API MPMS 5.6
The standard defines the minimum design, installation, calibration, and performance requirements for Coriolis flow meters used to measure liquid hydrocarbons in both upstream and downstream operations. It covers meters used for custody transfer, allocation metering, and process control where the mass flow rate of liquid hydrocarbons is the primary variable. The scope specifically includes:
- Meters with nominal diameters typically ranging from DN 15 to DN 400 (½ inch to 16 inches).
- Liquid hydrocarbons such as crude oil, refined products, NGLs, and LPGs within defined viscosity and density ranges.
- Single-phase liquid flows at temperatures and pressures consistent with typical pipeline and terminal operations.
- Requirements for materials of construction, pressure ratings, and temperature tolerances to ensure compatibility with hydrocarbon fluids.
The standard does not apply to gas or multiphase measurements, nor to Coriolis meters used solely for non-financial applications where meter performance does not affect revenue or regulatory reporting.
Technical Requirements and Specifications
Meter Selection and Design
API MPMS 5.6 specifies that Coriolis meters must be designed and manufactured in accordance with recognized industry standards such as OIML R 117 or ISO 10790. The flow meter must be capable of meeting the accuracy requirements over its entire rated flow range for the intended fluid. Key design criteria include:
- Materials: Wetted parts must be compatible with the measured hydrocarbon and not cause contamination. Stainless steel (316L) is common, with Hastelloy options for corrosive streams.
- Pressure and temperature: The meter must have an MAWP (Maximum Allowable Working Pressure) equal to or greater than the maximum operating pressure of the system. Temperature compensation for fluid density changes is mandatory for custody transfer volumes.
- Safety: Meters must meet hazardous area classification requirements (e.g., ATEX, IECEx, or NEC/CEC) for the installation environment.
Tip: When selecting a Coriolis meter for custody transfer, consider the fluid’s viscosity and operating temperature range. Most Coriolis meters perform best with low-to-moderate viscosity liquids where the flow profile is fully developed. Very high viscosity may affect accuracy unless the meter is specifically designed for such service.
Installation and Flow Conditioning
The standard mandates that Coriolis meters be installed in a mechanically stable piping system that minimizes external vibration and stress. Unlike many inferential meters, most Coriolis designs require no upstream or downstream straight pipe runs for flow conditioning, provided the meter is installed such that flow enters the sensor without excessive swirl or asymmetrical velocity profiles. However, the standard advises that piping immediately upstream and downstream of the meter should be supported independently to avoid imposing bending or torsional loads on the meter body. Where block valves or flow control valves are present, they should be located downstream to avoid flow disturbances.
Calibration and Verification
API MPMS 5.6 requires that Coriolis meters used for custody transfer be initially calibrated using a traceable high-accuracy master meter or a gravimetric calibration system. The calibration must cover the expected operating flow range, fluid conditions (density, viscosity, temperature), and pressure environment. The standard references API MPMS Chapter 4 for general meter calibration procedures and API MPMS Chapter 12 for calculation of uncertainty. Periodic re-verification (proving) is required, typically not to exceed 12 months, although more frequent verification may be needed for critical applications. The table below summarizes the principal performance metrics specified by the standard:
| Parameter | Requirement |
|---|---|
| Maximum Permissible Error (MPE) at calibration | ±0.15 % of mass flow rate for custody transfer |
| Repeatability | ±0.05 % of reading |
| Calibration interval (proving) | Not to exceed 12 months (per API MPMS Chapter 4) |
| Temperature compensation | Mandatory for volume conversion (MPMS Chapter 11.1) |
| Density measurement uncertainty | ±0.3 kg/m³ or better for integrated density output |
| Zero stability | Must be within manufacturer’s specification for the installed fluid |
Caution: Coriolis meters can be sensitive to two-phase flow. The standard requires that the installed system be capable of maintaining single-phase liquid flow at all times; any gas breakout or flashing can cause errors that fall outside the MPE. A properly sized backpressure valve may be necessary.
Implementation Highlights and Best Practices
Experience with the standard reveals several common practices that enhance measurement security and operational reliability:
- Secondary instruments: Temperature transmitters and pressure gauges located immediately upstream of the meter provide essential data for fluid property corrections. API MPMS 5.6 advises that these instruments be calibrated at least twice during the proving interval.
- Zeroing procedure: The meter electronics must allow a factory-recommended zeroing procedure to be performed with the meter at operating temperature and isolated from flow. This zero is critical to maintaining the stated repeatability.
- Uncertainty analysis: Operators should develop an uncertainty budget per the Guide to the Expression of Uncertainty in Measurement (GUM) that accounts for meter calibration uncertainty, drift, fluid property uncertainty, and rounding errors.
- Data resolution: Custody transfer meters must be equipped with an electronic output capable of resolving flow increments to 0.001 kg or 0.001 L for volumetric calculations.
Critical: Never attempt to re-zero a Coriolis meter while there is any flow or vibration present. Even slight flow during zeroing can introduce a systematic bias that degrades accuracy for the entire proving interval.
Compliance and Reaffirmation Notes
API MPMS 5.6 was reaffirmed in 2013 without substantive revisions, indicating that the 2002 edition’s technical provisions were deemed still current and adequate. The reaffirmation process ensures that the standard retains its status as an industry-recognized reference in the Manual of Petroleum Measurement Standards. Regulatory bodies worldwide, including the U.S. Bureau of Land Management (BLM) and state oil and gas commissions, often cite this standard as a mandatory requirement for hydrocarbon measurement in custody transfer. Auditors and inspectors will typically seek evidence that the installed Coriolis meter system has a valid calibration certificate issued by an accredited laboratory within the previous 12 months, and that proving records are maintained for at least six years under typical regulatory rules.
Operators should also ensure that their measurement system complies with the chain of traceability requirements of API MPMS Chapter 7.1 (Calibration and Verification of Provers) and Chapter 13 (Statistical Aspects of Measuring and Sampling) where additional uncertainty constraints apply. Failure to maintain compliance can lead to measurement discrepancies that have significant financial and contractual consequences.
Q: What is the current revision status of API MPMS 5.6?
A: The standard is in its 2002 edition, reaffirmed in 2013. While it has not been revised, it remains the active edition and is widely accepted for custody transfer measurement of liquid hydrocarbons.
A: The standard is in its 2002 edition, reaffirmed in 2013. While it has not been revised, it remains the active edition and is widely accepted for custody transfer measurement of liquid hydrocarbons.
Q: Can Coriolis meters approved under API MPMS 5.6 be used for fluene?
A: The standard primarily addresses liquid hydrocarbons. For fluene or other non-hydrocarbon liquids, the user should verify that the meter materials, viscosity range, and calibration procedures are suitable; however, API MPMS 5.6 is not specifically written for such fluids.
A: The standard primarily addresses liquid hydrocarbons. For fluene or other non-hydrocarbon liquids, the user should verify that the meter materials, viscosity range, and calibration procedures are suitable; however, API MPMS 5.6 is not specifically written for such fluids.
Q: How often must a Coriolis meter be proved under the standard?
A: Typically every 12 months, but the standard allows more frequent proving based on the required uncertainty, historical drift, or contractual agreements. The proving interval should be established using statistical performance data.
A: Typically every 12 months, but the standard allows more frequent proving based on the required uncertainty, historical drift, or contractual agreements. The proving interval should be established using statistical performance data.
This technical article reflects the state of API MPMS 5.6 (2002, Reaffirmed 2013) as recognized in 2026. Users are advised to check with the American Petroleum Institute for any future editions or addenda that may supersede this version.