Leak Detection & Grillage (API 650 Annex I)

Undertank leak detection and subgrade protection to API 650 Annex I: the I.2 performance requirements, the leak barrier thickness and permeability (I.2 d, I.5.1), sand cover (I.5.4), perimeter drain pipes (I.5.5), double steel bottoms (I.4), sumps and detection wells, testing (I.6), and tanks supported by grillage with the I.7.3 spacing, bottom thickness and deflection

New here? Read the User Manual — the arrangements, each requirement, the grillage equations with a worked example.

◉ API 650 14TH ED. (AUGUST 2025) ANNEX I · FIGURES I.1 – I.11 · NOT A REGULATORY SUBMISSION TOOL

⚠ Indicative tool only. Follows API 650, 14th edition (August 2025), Annex I, checked against that text. Annex I is a requirement only when the Purchaser specifies one of its options or the whole annex; the owner decides whether leak detection is provided and which method (I.1.3). The units choose the printed values (1.1.3): 15 m / 800 µm / 100 mm in SI, 50 ft / 30 mils / 4 in. in US customary. Grillage members (AISC Chapter K), anchorage and shell buckling at the members are for the designer; this tool checks the bottom plate equations of I.7.3.

Identification

Company

Tank ID

Location

Record Type

Units and rule set

Annex I prints each limit in both systems (1.1.3): the tool checks against the set you choose. Records are kept in SI.

Summary of checks

Choose the arrangement and enter the tank.

1. Tank and Arrangement (I.1)

Bottom support

Arrangement (I.1.2)

Tank diameter, D m

2. Leak Barrier and Cover (I.2, I.5.1 – I.5.4)

Leak barrier

Membrane thickness µm

Permeability of the barrier cm/s

Sand cover over the barrier mm

Cathodic protection in the space between the bottom and the barrier (I.3, I.5.4: more cover may be needed)

Membrane laid over a steel bottom (I.5.3)

Double Steel Bottom (I.4)

Primary bottom and shell support (I.4.1)

Separation between the bottoms

Perimeter of the bottoms

Corrosion protection of the bottoms provided (I.4.2)

3. Drain Pipes, Sumps and Detection Wells (I.5.5 – I.5.8, Figures I.8 – I.10)

Drain pipes at the perimeter

Drain pipe size NPS

Wall

Largest spacing between drains m · blank = equal spacing

Sumps in the liner below the bottom (I.5.8, Figures I.8, I.9)

Center sump diameter (Figure I.9) mm · blank if none

Drain line from each sump to the perimeter

Drains piped to leak detection wells (Figure I.10)

Well diameter mm

Well top above the maximum high water level in the dike mm · negative if below

Electronic vapour or liquid sensors used (I.2 b)

Sensors

2. Tank Supported by Grillage (I.7)

Maximum nominal shell thickness mm · incl. CA

Maximum design temperature °C

Purchaser and Manufacturer agree to apply I.7 to a thicker shell (I.7.1)

Design liquid level m

Specific gravity, G

Internal pressure kPa · blank = 0

Hydrostatic test water level m

Bottom plate yield strength, F_y MPa

Bottom plate nominal thickness, t_g mm · incl. CA

Bottom corrosion allowance, CA mm

Spacing between grillage members, b mm · centre to centre

Modulus of elasticity, E_s MPa · blank = 199,000

Flush-type cleanouts or flush-type shell connections (I.7.11)

Spacing of the radial supports mm

Construction and Testing Record (I.2, I.5, I.6, I.7)

Tick each item once it is confirmed. Items that don’t apply are hidden.

Save & Report

Inputs are kept on this device as you type and offered back if the page is closed before saving. An API 650 Annex I release prevention barrier earns the 10-year credit of API 653 Table 6.1 (iv) in the Inspection Interval tool.