Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API MPMS Chapter 6.7 1991 (2012): Measurement of Liquid Hydrocarbons by Turbine Meters
Authoritative Guidelines for Turbine Meter Applications in Petroleum Measurement
Scope
API MPMS Chapter 6.7, originally published in 1991 and reaffirmed in 2012, provides standard practices for the dynamic measurement of liquid hydrocarbons using turbine meters. This document is part of the broader API Manual of Petroleum Measurement Standards (MPMS) and is widely referenced for custody transfer and allocation measurement applications.
The standard encompasses criteria for the selection, installation, operation, calibration, and maintenance of turbine meters. It applies to meters used in pipelines, terminals, and refineries where precision and reliability are paramount. The guidelines accommodate a range of liquid hydrocarbons, including crude oils, refined products, and natural gas liquids, over varying viscosity and flow conditions.
Technical Requirements
Meter Design and Construction
API MPMS 6.7 specifies that turbine meter materials must be compatible with the process fluid and withstand operating temperatures. Typical materials include stainless steel for the rotor and body, and tungsten carbide or ceramic for bearings. The meter’s internal components must minimize intrusion while ensuring smooth flow. The standard provides guidance on rotor blade design, magnetic pick-off types, and signal output specifications.
Performance Criteria
The standard defines performance parameters including accuracy, repeatability, linearity, and rangeability. For custody transfer applications, turbine meters must meet strict tolerances. The following table summarizes typical performance requirements per the standard:
| Parameter | Requirement | Remarks |
|---|---|---|
| Accuracy | ±0.25% of reading | Under reference conditions |
| Repeatability | ±0.05% of reading | Over successive runs |
| Linearity | ±0.5% or better | Across calibrated flow range |
| Rangeability | 10:1 to 20:1 | Typical for liquid service |
| Pressure Drop | 0.2–1.0 bar (3–15 psi) | At maximum flow |
Calibration and Proving
API MPMS 6.7 requires that turbine meters be initially calibrated and periodically proved using traceable methods. The standard refers to other MPMS chapters for specific prover designs (e.g., Chapter 4 for proving systems). Calibration frequency is influenced by service conditions, but proactive testing is recommended. Prover runs are to be conducted at normal operating flow rates, and the meter factor must be determined from multiple runs within specified repeatability limits.
Viscosity and Flow Effects
Viscosity has a direct impact on meter performance as it affects blade drag and bearing friction. The standard provides correction methods or recommends dedicated calibration at different viscosities. For low-viscosity fluids like LPG or condensates, special bearing materials may be required to reduce wear and ensure accuracy.
Caution: Operation outside the calibrated viscosity range can result in significant measurement errors. Always adjust calibration or select a meter factor for the prevailing fluid properties.
Implementation Highlights
Installation Requirements
Proper installation is critical for achieving rated accuracy. The standard mandates a straight pipe run of at least 10 nominal pipe diameters downstream and 5 diameters upstream of the meter, or more if flow conditioning devices are not used. Swirl eliminators and flow straighteners are recommended when installation space is limited. The meter should be installed with an upstream block valve and a downstream flow control valve, avoiding control valve placement immediately upstream to prevent flow disturbances.
Tip: Use a flow straightener with a minimum of 10D upstream length to ensure a fully developed velocity profile. For high-accuracy applications, increase upstream length to 20D or integrate a proven flow conditioner.
Danger: Cavitation can damage the rotor and render measurements inaccurate. Ensure sufficient back pressure to prevent flashing of the liquid. A general rule is to maintain back pressure at least twice the meter’s pressure drop at maximum flow.
Maintenance Practices
Regular maintenance of bearings, rotor, and electronics extends turbine meter service life. The standard recommends periodic inspection for wear, corrosion, and debris accumulation. Bearing replacement should follow manufacturer guidelines and must be documented with a comparison of meter factors before and after repair. Electronic signal pick-offs should be checked for proper gap and signal strength.
Success Factor: Implementing a predictive maintenance schedule based on measurement deviations rather than fixed intervals can maximize uptime and minimize uncertainty. Many operators track meter factor trends to schedule calibrations proactively.
Compliance Notes
Although API MPMS 6.7 is a voluntary standard, it is frequently incorporated into regulatory requirements, contractual agreements, and company specifications for fiscal measurement. Compliance involves not only using the standard’s technical specifications but also documenting all aspects of installation, calibration, and maintenance.
Key compliance elements include:
- Ensuring that turbine meters are designed, manufactured, and tested according to the standard.
- Calibrating with traceable references and maintaining calibration records for audit.
- Adhering to installation guidelines to minimize flow profile errors.
- Performing periodic re-verification at intervals appropriate for service conditions (e.g., every six months for high-usage custody transfer meters).
- Establishing documented procedures for meter proving and factor updates.
Non-compliance can result in measurement discrepancies that affect revenue and regulatory standing. Many organizations require certification of measurement personnel in accordance with API MPMS practices.
Q: What types of liquids are covered by API MPMS 6.7?
A: The standard applies to all liquid hydrocarbons, including crude oils, refined products (gasoline, diesel, jet fuel), NGLs, and biofuels, provided they are in the liquid phase and within the meter’s operating envelope.
A: The standard applies to all liquid hydrocarbons, including crude oils, refined products (gasoline, diesel, jet fuel), NGLs, and biofuels, provided they are in the liquid phase and within the meter’s operating envelope.
Q: How does viscosity affect turbine meter accuracy?
A: Higher viscosity increases drag on the rotor, causing the meter to slow down and under‑register. The standard recommends calibration at actual operating viscosity or using viscosity correction models derived from testing.
A: Higher viscosity increases drag on the rotor, causing the meter to slow down and under‑register. The standard recommends calibration at actual operating viscosity or using viscosity correction models derived from testing.
Q: What is the typical calibration interval recommended by the standard?
A: The standard does not prescribe a fixed interval; rather it suggests basing the interval on meter performance history and the criticality of the measurement. Common practice for custody transfer is every 1 to 3 months initially, extending as data demonstrates stability.
A: The standard does not prescribe a fixed interval; rather it suggests basing the interval on meter performance history and the criticality of the measurement. Common practice for custody transfer is every 1 to 3 months initially, extending as data demonstrates stability.
Q: Is a flow conditioner always required upstream of a turbine meter?
A: Not always, but API MPMS 6.7 emphasizes that adequate straight pipe lengths are necessary. When space constraints or existing piping configurations cannot provide the recommended 10D straight pipe, a flow conditioner is strongly recommended to achieve performance.
A: Not always, but API MPMS 6.7 emphasizes that adequate straight pipe lengths are necessary. When space constraints or existing piping configurations cannot provide the recommended 10D straight pipe, a flow conditioner is strongly recommended to achieve performance.
This article is based on API MPMS Chapter 6.7 (1991, reaffirmed 2012). For the latest updates and specifics, refer to the current API publication.
© 2026 — All rights reserved.