API MPMS 3.1B (2001): Automatic Tank Gauging Standard for Liquid Hydrocarbon Measurement

1. Scope and Application

API MPMS Chapter 3.1B—officially titled Standard Practice for Level Measurement of Liquid Hydrocarbons in Tanks by Automatic Tank Gauging (2001 Edition)—establishes the minimum performance, design, installation, calibration, and verification requirements for Automatic Tank Gauging (ATG) systems employed in the petroleum and petrochemical industries. Unlike manual tank gauging (covered in Chapter 3.1A), ATG systems provide continuous, remote level measurement, significantly reducing manual tank entry and providing high-resolution data for inventory reconciliation and custody transfer.

Applicable Tank Types

This standard applies primarily to stationary, atmospheric storage tanks found at refineries, terminals, and pipeline facilities, including:

• Fixed-roof tanks
• External floating-roof tanks
• Internal floating-roof tanks

The standard specifically addresses the measurement of total liquid product level (image), interface levels (e.g., water bottom), and average liquid temperature.

Exclusions and Scope Boundaries

API MPMS 3.1B does not cover dynamic measurement systems, pressurized vessels (spheres, bullets), or hybrid measurement configurations that fall outside defined validation protocols. These systems are addressed elsewhere in the MPMS, such as Chapter 3.6 for pressurized tanks.

2. Core Technical Requirements and Performance Criteria

The central feature of API MPMS 3.1B is its strict classification of performance requirements based on application. The standard differentiates between custody transfer (highest accuracy) and inventory control (operational accuracy). The overall system—including the gauge, temperature sensors, and transmitter electronics—must be proven to meet these specifications in situ.

Level Measurement Accuracy

For custody transfer applications, the ATG system must demonstrate an overall level measurement uncertainty of within ±3 mm (±0.125 in) at a 95% confidence interval. For inventory control applications, a wider uncertainty of ±6 mm (±0.25 in) is typically permissible.

Temperature Measurement Requirements

Volumetric correction for thermal expansion demands precise temperature measurement. The standard requires averaging temperature probes (ATPs) to provide a representative average liquid temperature with an uncertainty of ±0.3 °C (±0.5 °F) for custody transfer and ±0.5 °C (±1.0 °F) for inventory control.

Water Interface Detection

Many ATG systems are required to detect and measure the free-water level at the bottom of the tank. The standard mandates a water interface detection uncertainty of ±3 mm for custody transfer tanks to ensure accurate net oil volume (NOV) calculations.

Table 1: Summary of Accuracy Specifications

Functional Design Requirements

The standard mandates that a compliant ATG system provide the following capabilities:

1. Continuous Level Output: A method for transmitting the level reading remotely—typically via 4–20 mA loop, digital fieldbus, or proprietary serial protocol—to a DCS or custody ticketing system.
2. Self-Diagnostics: The ability to detect and annunciate a device fault or failure, ensuring that a failed instrument does not produce unchanging or erroneous output.
3. High-Level Alarm Integration: While overfill protection may be a separate system, the ATG must support independent high-level alarm contact interfaces for safety system connectivity.

3. Implementation, Verification, and Calibration Protocols

Successful implementation of API MPMS 3.1B demands a rigorous verification protocol that begins before the tank enters service and continues throughout its operational life.

Initial Calibration and Baseline

The ATG level reading must be compared against a manual reference gauge (per API MPMS 3.1A or using a Portable Automatic Level Gauge, PALG) at multiple tank strapping points over the entire tank height. A statistical linear regression of these data points is performed to verify system linearity within the required specification. Any systematic bias must be corrected before the system is placed in service.

Routine Field Verification

Verification confirms the system remains within tolerance; it is not a full recalibration. A common procedure involves comparing the ATG reading against a confirmed reference dip at a single operational point. Tolerance is typically set to the standard uncertainty (e.g., ±3 mm). If drift exceeds this threshold, a full recalibration is triggered.

Vapor Correction Factors

When installed on tanks containing volatile liquids, the ATG must compensate for vapor-phase influences. The 2001 edition specifies requirements for correcting level readings based on vapor pressure, temperature, and product density to ensure the observed level accurately represents the true liquid volume, particularly in tanks operating above their Reid vapor pressure.

4. Compliance Notes and Industry Impact

Adherence to API MPMS 3.1B is often written directly into custody transfer contracts. Auditors and regulatory bodies examine compliance with this standard rigorously, particularly during terminal and refinery verifications.

Key Audit Points

• Traceability: All calibrations and verifications must be traceable to national standards (e.g., NIST in the United States).
• Environmental Safeguards: The standard requires that ambient environment—temperature extremes, vibration, moisture ingress—does not degrade the instrument accuracy below the specified tolerance.
• Documentation Package: A complete set of records must include the Factory Acceptance Test (FAT), Site Acceptance Test (SAT), initial baseline data, and all subsequent verification records. Gaps in this documentation are the most common audit findings.

Impact on Operations

The industry-wide adoption of ATG systems under MPMS 3.1B has improved operational efficiency by eliminating dependence on manual tank gauging, enhanced safety by reducing personnel exposure to tank-top hazards, and increased data reliability for batch tracking, loss control, and regulatory reporting.