Tank Gauging Fundamentals

1. Overview & Purpose

Tank gauging is the process of measuring the liquid level in a storage tank and converting that level measurement to a volume. In the petroleum industry, tank gauging serves three primary purposes: custody transfer (determining the quantity of product bought or sold), inventory management (tracking product quantities for accounting and operations), and loss control (detecting leaks, evaporation, or theft through inventory reconciliation).

Why Accuracy Matters

For a typical 100,000-barrel crude oil tank at $70/bbl, every 0.01% measurement error represents $700 of product. Over a year with monthly transfers, systematic measurement bias of just 0.05% can result in losses exceeding $42,000 per tank. This is why API MPMS establishes rigorous standards for every step of the measurement chain: gauging, calibration, sampling, temperature measurement, and volume computation.

2. Gauging Methods

There are two fundamental approaches to measuring liquid level in a tank: innage (measuring the depth of liquid from the tank bottom) and outage or ullage (measuring the empty space from a reference point at the top to the liquid surface). API MPMS Chapter 2.1A covers manual tank gauging procedures.

Innage Gauging

Innage is the measurement of liquid depth from the tank datum plate (a small metal plate welded to the tank bottom at the gauge point) to the surface of the liquid. A graduated steel tape with a bob (plumb weight) is lowered through the gauge hatch until the bob touches the datum plate, and the tape is read at the liquid surface where it shows a wet/dry interface (using gauge paste that changes color on contact with petroleum).

Outage (Ullage) Gauging

Outage measures the empty space from a reference point (gauge reference point at the top of the tank) down to the liquid surface. This method is preferred for volatile liquids where immersing a tape in the product may cause vapor release or safety concerns. The innage is calculated by subtracting the outage from the known reference height.

Automatic Tank Gauging (ATG)

Modern terminal operations use automatic tank gauging systems that provide continuous level measurement. API MPMS Chapter 2.1B covers automatic methods.

3. Strapping Tables

A strapping table (also called a tank capacity table or tank table) is the calibrated relationship between liquid level and volume for a specific tank. Every tank has a unique strapping table because it depends on the as-built dimensions, which differ from nominal due to construction tolerances, foundation settlement, and shell distortion.

How Strapping Tables Are Developed

API 2550 (API MPMS Chapter 2.2A) defines three methods for calibrating upright cylindrical tanks:

• Manual strapping: Measuring the external circumference of the tank shell at multiple heights using a calibrated steel tape. The most traditional method, still widely used. Circumference is converted to internal diameter by subtracting twice the shell thickness and dividing by pi.
• Optical reference line (ORL): Using a vertical reference line and measuring the distance from the line to the shell at multiple heights and angles. More accurate for large or distorted tanks.
• Electro-optical distance ranging (EODR): Using laser or optical devices to measure internal diameters directly from inside the tank. The most accurate modern method.

Table Format

A strapping table typically provides volume in barrels for each increment of liquid level, usually in 1/4-inch or 1/8-inch increments for the main body, with finer increments for the critical zone (bottom 12 inches where deadwood, bottom geometry, and datum plate affect accuracy). The table also includes the reference height, datum plate location, and any deadwood corrections.

4. Tank Geometry Formulas

When a calibrated strapping table is not available, volume can be calculated from tank dimensions using geometric formulas. This provides a reasonable estimate for operational purposes, though it lacks the accuracy of a physical calibration.

Vertical Cylindrical Tank

Horizontal Cylindrical Tank

Deadwood Corrections

Deadwood is any structure inside the tank that displaces liquid volume, such as internal piping, heating coils, mixers, support columns, or floating roof legs. The strapping table typically accounts for deadwood by subtracting the displaced volume at each level. For geometric calculations, deadwood must be estimated and deducted separately.

5. Shell Temperature Correction (CTSh)

The strapping table is developed at a specific shell temperature (typically the ambient temperature during calibration). When the actual shell temperature differs from the calibration temperature, the steel shell expands or contracts, changing the tank diameter and therefore the volume per unit height.

6. CTL — Temperature Correction for Liquid

CTL (Correction for Temperature of Liquid) adjusts the observed volume at the actual liquid temperature to the volume at standard temperature (60 °F / 15 °C). This is typically the largest correction factor and is essential because petroleum liquids expand significantly with temperature.

Magnitude of CTL Correction

7. CPL — Pressure Correction for Liquid

CPL (Correction for Pressure of Liquid) accounts for the compressibility of the liquid under elevated pressure. For atmospheric storage tanks, CPL is essentially 1.0 and can be omitted. For pressurized tanks, bullets, or spheres operating above atmospheric pressure, CPL becomes significant.

8. Volume Calculation Procedure

The complete volume calculation follows a standardized sequence defined in API MPMS Chapter 12. Each step builds on the previous one, applying corrections in a specific order to ensure consistent results.

Floating Roof Considerations

For floating roof tanks, the volume displaced by the roof structure must be deducted from the gross volume. The roof displacement depends on the roof weight and the liquid density, and is typically provided in the tank table as a fixed barrel value. Additionally, the liquid on top of the floating roof (rainwater) and liquid trapped in the roof seals must be accounted for.

Tank Bottom and Heel

The bottom zone of a tank (typically the lowest 6 to 12 inches) requires special attention because the tank bottom is not perfectly flat. Sump pits, drain piping, bottom plates, and datum plates all affect the volume at low levels. The strapping table should include these corrections, but geometric calculations will be inaccurate in this zone.

9. Custody Transfer Practice

Custody transfer is the formal measurement of petroleum quantities for commercial transactions. Both parties (buyer and seller) typically witness the gauging and agree on the measurements. API MPMS Chapter 18 defines the standard terminology and procedures.

Opening and Closing Gauges

A custody transfer by tank measurement requires an opening gauge (before the transfer begins) and a closing gauge (after the transfer is complete). The difference in net standard volume between the two gauges is the transferred quantity.

Sources of Measurement Uncertainty

• Gauge reading: Manual tape reading accuracy is typically ±1/8 inch, which for a 100-ft diameter tank is about ±7 barrels per 1/8 inch.
• Temperature: A 1 °F error in temperature measurement causes approximately 0.04–0.07% volume error depending on product gravity.
• Strapping table: Calibration uncertainty is typically ±0.02–0.05% for a properly calibrated tank.
• Sampling (BSW): Representative sampling is critical. Bottom sampling, running samples, and composite samples all have different accuracies.
• Tank condition: Shell settlement, bottom deflection, and internal deposits change the tank calibration over time.

10. Industry Standards