Contents
About this tool
The Leak Detection & Grillage calculator checks the undertank leak detection and subgrade protection of API 650 Annex I: the performance requirements of I.2, the leak barrier (thickness, permeability, cover), the perimeter drain pipes, double steel bottoms, sumps, detection wells and testing; and tanks supported by grillage with the I.7.3 equations for member spacing, bottom thickness and deflection.
Scope
Annex I is a requirement only when the Purchaser specifies one of its options or the whole annex; the owner decides whether the undertank area is built for leak detection and which method (I.1.3). It follows API 650, 14th edition (August 2025). Grillage members (AISC Chapter K), anchorage and shell buckling are for the designer.
Quick start
Pick a sample: a 40 m tank on a concrete ringwall with a liner (nothing fails), a 30 m double steel bottom with grating and an unsealed perimeter (I.4.2 fails until corrosion protection is ticked), or an 8 m tank on grillage (the bottom plate passes I.7.3).
- 1Section 1: bottom support (leak detection or grillage), the arrangement of Figures I.1 – I.7, and D.
- 2Section 2: the leak barrier, its thickness and permeability, and the sand cover.
- 3Double steel bottom (when chosen): how the primary bottom is supported, what separates the bottoms and whether the perimeter is sealed.
- 4Section 3: drain pipes, sumps, detection wells and sensors.
- 5Grillage: shell, temperature, liquid and test levels, Fy, tg, CA and the member spacing.
Leak barrier, cover and performance (I.2, I.5.1 – I.5.4)
| Item | SI | US customary |
|---|---|---|
| Fiber-reinforced membrane (I.5.1) | at least 800 µm | at least 30 mils |
| Unreinforced membrane (I.5.1) | at least 1000 µm | at least 40 mils |
| Permeability (I.2 d) | at most 1 × 10⁻⁷ cm/s | at most 4 × 10⁻⁵ mils/s |
| Sand cover over the barrier (I.5.4) | at least 100 mm | at least 4 in. |
- The two systems are not exact conversions (30 mils = 762 µm, 4 in. = 101.6 mm): the tool checks the set you choose, as 1.1.3 asks. 762 µm passes US and fails SI.
- A clay liner has no thickness rule: it must be thick enough to meet the permeability (I.5.1). An applied coating (Figure I.6) and the ACI 350 slab (Figure I.7) have no membrane check.
- Cathodic protection between the bottom and the barrier may need more cover (I.3, I.5.4): a Note.
- The rest of I.2 — leaks seen at the perimeter and collected, chemical resistance, subgrade suitability, one-piece barrier or tested joints, sumps keeping the detection — is in the record.
Drain pipes, sumps and wells (I.5.5 – I.5.8)
| Rule | Requirement | Example |
|---|---|---|
| Number (I.5.5) | spacing at most 15 m (50 ft) around the perimeter, at least 4; 2 allowed for D up to 6 m (20 ft) | 40 m: π × 40 = 125.7 m ÷ 15 → 9; 20 m → 5; 6 m → 2; 60 m → 13 |
| Size (I.5.5) | at least NPS 1, Schedule 40 | NPS 3/4 or Schedule 10 fails; a double bottom with the NPS 1/2 coupling of Figure I.5 is a Note to agree with the owner |
| Spacing | equal spacing assumed; enter the largest spacing if they are not equal | 10 drains on 40 m: 12.6 m |
| Sumps (I.5.8, Figure I.9) | a drain line from each sump to the perimeter; a center sump at least 300 mm (12 in.) | |
| Wells (Figure I.10) | at least 100 mm (4 in.) diameter, top above the maximum high water level in the dike | |
| Sensors (I.2 b) | fail-safe or with provision for calibration; leaks still visible at the perimeter |
Double steel bottoms (I.4)
- I.4.1: the primary bottom and shell are evaluated for all operating conditions (design metal and maximum design temperature, fill, hydrotest, seismic, settlement), unless the primary bottom is uniformly supported on both sides of the shell and not attached to the secondary bottom or its support.
- I.4.2: with steel members (grating, shapes, wire mesh) between the bottoms and an unsealed perimeter, water gets in and corrodes locally: corrosion protection of the bottoms is required.
- With sand, pea gravel or grooved concrete between them (Figure I.4), an unsealed perimeter is allowed but may accelerate bottom-side corrosion (figure note).
Tanks supported by grillage (I.7)
Scope (I.7.1): shell at most 13 mm (1/2 in.) including CA — thicker by agreement — and maximum design temperature at most 93 °C (200 °F). The bottom is a single butt-welded steel bottom on steel or concrete members.
| Equation | I.7.3 | Sample |
|---|---|---|
| p | the greater of the product weight plus internal pressure, or the test water, plus the bottom plate weight | 8 m water: 0.07845 + plate 0.00046 = 78.9 kPa |
| Largest spacing | b = [1.5Fy(tg − CA)² / p]0.5 (I.7.3.1-1) | [1.5 × 250 × 5² / 0.07892]0.5 = 345 mm ≥ 300 ✓ |
| Required thickness | tg = [b²p / (1.5Fy)]0.5 + CA (I.7.3.2-1) | [300² × 0.07892 / 375]0.5 + 1 = 5.35 mm ≤ 6 ✓ |
| Deflection | d = 0.0284pb⁴ / (Es(tg − CA)³) ≤ (tg − CA)/2 (I.7.3.3-1) | 0.0284 × 0.07892 × 300⁴ / (199,000 × 5³) = 0.73 mm ≤ 2.5 ✓ |
- The equations are consistent in either unit system; the US set uses its printed limits and E = 28,800,000 lbf/in² (the API 650 value at 93 °C / 200 °F, Table M.2), SI 199,000 MPa. Enter E to override.
- Flush-type fittings (I.7.11): a circumferential member (Figure 5.12 Method A) and radial supports at most 300 mm (12 in.) apart.
- Not calculated: the members (AISC Chapter K, I.7.5), bracing and anchorage (I.7.7, I.7.8; Tank Anchorage tool), shell buckling at the members (I.7.9), loads under roof columns (I.7.10). They are record items.
The checks
| Check | Clause | Fails when |
|---|---|---|
| Membrane thickness | I.5.1 | under 800 / 1000 µm (30 / 40 mils) |
| Barrier permeability | I.2 d | over the limit |
| Sand cover | I.5.4 | under 100 mm (4 in.); a Note with CP in the space |
| Primary bottom support / corrosion between bottoms | I.4.1, I.4.2 | not evaluated / unsealed with steel separators and no protection |
| Perimeter drains, size | I.5.5 | too few or too far apart; under NPS 1 or Schedule 40 |
| Sumps, wells, sensors | I.5.8, Figure I.10, I.2 b | no drain line or small sump; small well or top below the dike high water; sensors neither fail-safe nor calibratable |
| Scope of I.7, spacing, thickness, deflection, flush-type supports | I.7.1, I.7.3, I.7.11 | outside I.7.1; over the I.7.3 limits; radial supports over 300 mm |
Construction and testing record
| Clause | Item |
|---|---|
| I.1.3, I.1.6 | Owner’s decision and method; regulatory implications of contaminated pad material. |
| I.2 a, c, e, f, g | Leaks seen and collected at the perimeter; materials; subgrade; barrier joints; sumps and pipes. |
| I.5.2, I.5.3, I.5.5 – I.5.7 | Barrier protection; drains protected from clogging, visible, sleeved where needed, outlets protected. |
| I.6.1 – I.6.3 | Visual inspection and vacuum-box test of seams; seam samples each shift per operator and machine; penetrations leak tight. |
| I.7.2, I.7.4 – I.7.10, I.7.12 | Grillage: Figure 4.1, butt welds over the members, member design and CA, bracing and anchorage, column loads, corrosion protection. |
How the numbers were checked
- Annex I was read from the 14th edition (August 2025); every limit is the printed SI or US value.
- The drain counts and the grillage sample are recomputed in this manual’s build and in the test suite (37 checks), including the SI/US differences at 762 µm, 101 mm and 6.05 m.
- Each rule was made to fail in turn.
Saving, reopening and reports
- Inputs are kept on this device as you type. Calculate & Save stores the check in SI in the
annexILeakCalcscollection; Show saved checks lists them for the tank and Open reloads one. - Print / PDF report: summary, arrangement, barrier, drains with the plan, or the grillage results with the chart, and the record list.
- A release prevention barrier built to Annex I earns the 10-year credit of API 653 Table 6.1 (iv) in the Inspection Interval tool.
Limits and interpretations
- p includes the bottom plate weight with the liquid, as the definition of p says; the plate weight uses 7850 kg/m³.
- Drains are assumed equally spaced unless the largest spacing is entered.
- The figures are typical arrangements (I.8); their dimensions are not checked except the 300 mm sump and 100 mm well that the figures mark as minimums.
- Secondary containment of diked areas is outside Annex I (I.1.7): see the Dike / Bund tool.
Glossary
| RPB | Release prevention barrier: a steel bottom, synthetic liner, clay liner or other barrier that stops and channels a leak (I.1.1 Note). |
| Grillage | Steel or concrete members supporting the bottom plate in place of a continuous foundation. |
| tg, b, p | Bottom plate nominal thickness, member spacing, uniform pressure on the bottom. |