API RP 2556-1993 (2013): Recommended Practice for Calibrating Liquid Hydrocarbon Storage Tanks

1. Scope and Application

The document applies to tanks with nominal capacities typically used for storing crude oil, refined products, liquefied petroleum gases (LPG), and other liquid hydrocarbons. It covers three primary calibration methodologies:

• Strapping (Manual Taping) Method – Direct measurement of tank circumference and shell plate dimensions using calibrated steel tapes.
• Optical Method – Indirect measurement using theodolites or electronic total stations to determine tank geometry from external reference points.
• Liquid-Level (Volumetric) Method – Calibration by adding or removing known volumes of liquid and correlating level changes to incremental capacity.

The recommended practice is applicable to both new tanks requiring initial calibration and existing tanks where calibration verification or correction is needed due to structural modifications, foundation settling, or suspected inaccuracies. It is closely aligned with API MPMS (Manual of Petroleum Measurement Standards) Chapter 2 (Tank Calibration) and Chapter 3 (Tank Gauging).

2. Technical Requirements and Calibration Methods

API RP 2556-1993 (2013) details specific technical requirements for each calibration method. The choice of method depends on tank type, access, desired accuracy, and operational constraints. Below is a summary of key technical elements.

2.1 Strapping Method

The strapping method requires direct physical measurement of the tank’s circumference at multiple horizontal courses. Key requirements include:

• Use of calibrated steel tape with known tension and temperature corrections.
• Measurement at intervals not exceeding 2.5 m (8 ft) vertically or at every plate course.
• Correction for out-of-roundness by taking multiple circumferential readings at cross‑sections.
• Accounting for shell expansion due to hydrostatic head and temperature.

2.2 Optical Method

This method uses a theodolite or electronic total station to measure distances and angles from a baseline. Technical requirements include:

• Establishment of permanent reference points and control networks.
• Measurement of tank radius at increments typically every 2.0 m (6.5 ft) vertically.
• Correction for atmospheric refraction and instrument calibration.
• Applicable to floating-roof tanks where roof position may affect calibration.

2.3 Liquid-Level (Volumetric) Method

This field‑based method involves introducing or removing a metered volume of liquid and measuring the corresponding level change. Requirements include:

• Use of calibrated flow meters or gravimetric measurement.
• Sufficient number of volume steps to define the tank capacity curve (typically a minimum of 10–15 steps).
• Temperature and pressure corrections for liquid density.
• Applicable to small tanks or those where other methods are impractical.

3. Implementation Highlights

Successful implementation of API RP 2556-1993 (2013) requires attention to the following elements throughout the calibration project lifecycle:

3.1 Preparation and Pre-Calibration Checks

• Obtain tank construction drawings, previous calibration reports (if any), and data on tank geometry (diameter, height, shell courses, roof type).
• Inspect tank for obstructions, deformations, corrosion, or deposits that may affect measurements.
• Select the appropriate method considering safety, accuracy needs, and facility constraints.
• Calibrate all measurement equipment (tapes, theodolites, flow meters) against recognized standards.

3.2 Measurement Execution

• Perform measurements under stable ambient conditions (light wind, temperature within ±5°C of reference).
• For strapping: maintain tape tension within ±0.5% of nominal; record temperature at each course.
• For optical: sight each point twice; use the average for radius calculation; check closure by re‑observing reference points.
• For liquid‑level: ensure no thermal stratification; record liquid temperature and density at each increment.

3.3 Data Processing and Tank Table Generation

• Apply corrections for thermal expansion of the tank shell and measuring tape/material.
• Account for hydrostatic deformation for fixed‑roof tanks (liquid head expansion).
• Calculate incremental volumes per millimeter (or per 1/8 inch) and produce a table of capacity vs. liquid level.
• Perform numeric integration using trapezoidal or Simpson’s rule; verify consistency with sum of theoretical plate volumes.

4. Compliance Notes and Industry Application

API RP 2556-1993 (2013) is a recommended practice, not a mandatory code. However, its principles are widely adopted as an industry norm for ensuring custody transfer measurement accuracy. Key compliance considerations include:

• Contractual adoption: Many purchase agreements and terminal tariffs require adherence to API RP 2556 for tank calibration.
• Regulatory context: While not government regulation, API RP 2556 aligns with the requirements of many national metrology agencies (e.g., NIST Handbook 44 in the US).
• Reaffirmation status: The 2013 reaffirmation confirms the document’s technical validity; it must be used in conjunction with current API MPMS chapters and equipment specifications.
• Re‑calibration intervals: The recommended practice suggests that tanks be re‑calibrated at intervals not exceeding 10 years, or sooner when structural changes, tank repair, or uncertain accuracy are identified.
• Quality assurance: The calibration results should be reviewed by a qualified third party; uncertainty analysis per the ISO Guide to the Expression of Uncertainty in Measurement (GUM) is encouraged.

Frequently Asked Questions (FAQs)

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