Contents
About this tool
The Hydrostatic Test calculator plans and checks the shell hydrostatic test of a new tank the way API 650 7.3.6 – 7.3.8 set it out: the test water level, the maximum filling rates when settlement is measured, the holds and the six sets of settlement readings, how many shell and bottom points to survey, and the settlement limits that stop filling. It also checks the test water: the shell temperature against the design metal temperature for carbon steel, and the Annex S and X water quality and exposure limits for stainless.
This is an indicative planning and check tool, not a certified test procedure or a regulatory submission. It follows API 650, 14th edition (August 2025), and for existing tanks API 653, 5th edition with Addendum 4 (July 2025), 12.3 – 12.5. A written, site-specific procedure still governs. The optional OISD-STD-129 stages follow its Revised Edition (November 2006), Annexure III.
Inputs work in SI (m, mm, mm/h, m³/h) or US customary units (ft, in, in/h, bbl/h). The units also choose the printed values API 650 gives for each system (1.1.3); calculations and saved records are always in SI.
Quick start
Press Load illustrative sample: a 36 m × 14.4 m fixed-roof tank with a 13.8 m design liquid level, a 14 mm bottom course, a 2.4 m top course, filled at 300 m³/h and emptied at 400 m³/h, settlement measured, carbon steel at 12 °C against a 5 °C design metal temperature. Every check passes; enter settlement readings to see section 4 at work.
- 1Identification: Tank ID, location, record type and units.
- 2Section 1: diameter, shell height, design liquid level, roof and the test level basis.
- 3Section 2: bottom course thickness, top course width, pump and emptying rates, any venting limit or Purchaser minimum rate, the hold at test height; optionally the OISD stages.
- 4Section 3: read off the survey points and reading sets.
- 5Section 4: during the test, enter each set’s settlement at every shell point.
- 6Section 5: shell material and test water; section 6: tick the record items, then Calculate & Save and Print / PDF report.
Identification and units
- Tank ID is required to save. Company and Tank ID together identify the tank’s saved calculations.
- Record Type: Just for research is visible to you and admins only; Official is visible to everyone in your company.
- SI or US customary: the US rule set uses 12 / 18 / 9 in./h, 7/8 in., 32 ft, 10 ft, 2 ft, 1/2 in. per 32 ft and 2 in. — API 650’s printed US values, not conversions of the SI ones.
Test water level (section 1)
7.3.6.1.1 gives four ways to fill the tank. Levels c) and d) leave part of the shell above the water, and 7.3.6.2.1 then requires those joints to be examined another way: penetrating oil on the inside, a vacuum box, or internal air pressure above the water (7.3.8).
| Basis | Level | Sample |
|---|---|---|
| a) | Maximum design liquid level H (blank H = shell height) | 13.8 m |
| b) | Gas-tight roof only: 50 mm (2 in.) above the roof-to-shell weld (blank weld = shell height) | 14.45 m |
| c) | A lower level agreed by the Purchaser and Manufacturer — overflows, an internal floating roof or other freeboard | as entered |
| d) | Seawater giving the same first-course hoop stress as fresh water at a): 0.3 m (1 ft) + (H − 0.3) ÷ Gsea | 0.3 + 13.5 ÷ 1.025 = 13.47 m |
Hydrostatic test exemption (7.3.6.2.2): only when water is not sufficient and the Purchaser specifies it. The tool lists the four conditions — foundation bearing on the greater of 1.1 × product G and 1.0, every shell T-joint radiographed, the first product filling run as a hydrotest under 7.3.7, and all NDE and welding finished first.
Existing tanks: API 653 Section 12 (section 1A)
Choose Existing tank after repair, alteration or reconstruction under Tank in section 1. Section 1A then works out whether API 653 (5th edition 2014 with Addendum 4, July 2025) requires a hydrostatic test, and the rest of the tool plans the test with the API 653 rules added.
| Work on the tank | API 653 | Result |
|---|---|---|
| Reconstructed tank | 12.3.1 a) | test required |
| Major repair or major alteration | 12.3.1 b) | test required, unless exempt by 12.3.3 |
| More severe service: higher G, lower temperature (Figure 5.2), damaged tank | 12.3.1 c) | test required |
| Repair or alteration that is not major | 12.3.1 | no test, unless an engineering evaluation says otherwise |
| Floating roof only | 12.3.4.2 | no test |
| Bottom outside the critical zone | 12.3.4.3 | no test if the subgrade is accepted by the inspector and the annular ring (or critical zone) and its support stay intact |
Exemption for a major repair (12.3.3) always needs both written approvals of 12.3.3.2: an engineer experienced in API 650 tank design who concurs, and the owner/operator who authorizes it. Then either the repair meets 12.3.3.3 – 12.3.3.6, or a fitness-for-service evaluation (12.3.3.7) is made; API 653 gives no procedure for the latter.
| Repair | What the tool checks |
|---|---|
| Shell, critical-zone bottom, minor jacking | Existing material: a) API 650 7th edition or later, b) safe-for-use area of Figure 5.2 (Brittle Fracture tool), or c) S = 4.9 H D G / t ≤ 48 MPa (SI) or 2.6 H D G / t ≤ 7000 lbf/in.² (USC), H the fill height above the bottom of the repair |
| Shell-to-bottom weld (12.3.3.5.1) | a) any length, up to 50 % of the weld cross-section; b) one side fully replaced, up to 300 mm (12 in.) long, repairs over 50 % at least 300 mm apart including the other side; c) both sides only for D ≤ 9 m (30 ft) in the Figure 5.2 safe-for-use area |
Example: the sample tank (D 36 m, H 13.8 m, G 0.9) with a shell repair whose bottom is 9.6 m up in 8 mm plate gives S = 4.9 × 4.2 × 36 × 0.9 / 8 = 83.3 MPa, over 48: test the tank. The same repair 12.0 m up gives 35.7 MPa and can be exempted with the approvals.
| Test rule | API 653 | In the tool |
|---|---|---|
| Test level | 12.3.2 1) | the API 650 7.3.6.1.1 a) level, or the lower maximum test height from 4.3.3.2 (from the Shell Evaluation tool, typed or read with the button below); joints above are examined |
| Hold | 12.3.2 2) | at least 24 h, checked even with no settlement survey |
| Survey needed | 12.5.1 | yes, unless the tank has a documented history of acceptable settlement and none is expected |
| Survey points | 12.5.2 | N = D/10 with D in ft, at least 8, rounded up to even, at most 10 m (32 ft) apart: 36 m → 12, 35 m → 12 (API 650 alone gives 11), 55 m → 20, 61 m → 22 |
| Readings | 12.5.2, 12.5.3 | tank empty before the test (the baseline; without it the tank is taken as initially level), during filling and at 100 % of the test level |
| Reinforcing plates | 12.4 | new or altered plates air leak tested to API 650 7.3.5 (record item) |
Reading the linked records. With a Tank ID entered, Read the latest Shell Evaluation and Brittle Fracture records for this Tank ID looks up the records you can see for that tank (yours, and official ones of your company):
| Record | What is filled in | Notes |
|---|---|---|
| Shell Evaluation, the latest with a tank test height | Maximum test height (4.3.3.2); D and H when they are blank | Shows its allowable liquid level and any failing checks; a diameter that differs from the one entered by more than 0.5 % is flagged. Older one-course records have no tank test height and are skipped. |
| Brittle Fracture, the latest of the current tool | The Figure 5.2 safe-for-use box: ticked when the metal temperature is at or above the Figure 5.2 boundary, or the plate is within the thickness with no limit; cleared when below | A record that used API 650 Figure 4.1 (known material) or a stainless shell leaves the box as it was, with the reason. The Figure 5.1 outcome is quoted; when safe, material route b) is suggested. |
- The values can be edited afterwards; the saved record keeps which records were read, the values, and whether the test height was changed.
- Nothing is read automatically: press the button after entering or changing the Tank ID.
- API 653 sets no filling rates: the API 650 7.3.7.6 rates and 7.3.7.7 limits are shown as the usual practice; the settlement readings go to the Tank Settlement tool (Annex B) for evaluation.
- Record items add the inspection during filling, the exemption approvals, the NDE of each repair type (12.3.3.3 – 12.3.3.6), the 12.4 leak test, the initial survey and, for a reconstructed tank, the 13.1 nameplate.
- This is how a contractor shows the owner, before mobilizing water, that the test is required, or exactly which conditions the exemption rests on.
Filling rates and schedule (section 2)
When settlement measurements are specified, 7.3.7.6 caps the filling rate by how thick the bottom course is:
The tool cuts the filling into bands at every place a limit changes, and in each band uses the lowest of the 7.3.7.6 rate, the venting limit you enter (5.8.5 — the vents must pass the air the filling pushes out), the OISD stage rate and the pump (pump rate ÷ tank area).
| Quantity | How | Sample |
|---|---|---|
| Level rise at the pump rate | Q ÷ (π D² ÷ 4) | 300 ÷ 1,017.9 = 295 mm/h |
| Filling time | Σ band height ÷ band rate | 13.8 m at 295 mm/h = 46.8 h |
| Hold at the test height | At least 24 h before reading set 5, longer if the Purchaser specifies | 24 h |
| Emptying | test volume ÷ emptying rate | 14,047 ÷ 400 = 35.1 h |
| Total, water in the tank | fill + holds + emptying | 105.9 h ≈ 4.4 days |
| Same tank at 600 m³/h | pump 589 mm/h, throttled to 460 then 300 | 32.1 h filling |
- The Purchaser may lengthen the 24 h hold for i–iv: the first tank in the area, larger or more heavily loaded than any other, or settlement in question. Ticking one gives a Note until the hold is longer.
- A Purchaser minimum filling rate (Data Sheet Line 23) above a band’s maximum fails: the two have to be reconciled.
- Filling stopped for each reading set: 0 keeps filling, which 7.3.7.6 allows as long as the water rises no more than 300 mm (12 in.) during the set.
- With measurements waived (Data Sheet Line 14) the 7.3.7.6 rates and reading sets drop out; the pump, venting and OISD limits still apply.
OISD-STD-129 stages (optional)
Choosing API 650 7.3.7 with OISD-STD-129 stages adds the staged procedure of OISD-STD-129, Revised Edition (November 2006), Annexure III on top of the API 650 limits. The values were checked against that text:
| Item | OISD-STD-129 Annexure III | Sample |
|---|---|---|
| Stage levels | 0.5, 0.67, 0.83 and 1.0 H | 6.9, 9.25, 11.45, 13.8 m |
| Stage filling rate | 70 % of the design product-filling rate | 0.7 × 1.5 m/h = 1,050 mm/h (the API limits and pump are lower) |
| Maximum filling rate | Up to 1.5 m/h, even on a stable foundation (rock, piled slab) or a repeat test after a major repair | cap 1,500 mm/h |
| Holds | 24 h minimum after stages 1–3; at full height 24 h with no settlement, 48 h or longer with predicted settlement, until the rate of settlement diminishes; editable | 72 h + 24 h |
| Floating roof | Not more than 0.25 m/h (250 mm/h) until the roof floats — external or internal floating roof | — |
| Test temperature | 4 °C and above (checked from the lowest shell metal temperature) | 12 °C ✓ |
| Shell levels | Empty, 50 %, 67 %, 100 % full and empty again; with predicted settlement every 24 h while filling to stage 1, then every 12 h, with the bottom profile at 0.25 m of water before and after | listed in section 3 |
Sample with OISD stages: 46.8 h filling + 96 h holds + 35.1 h emptying = 177.9 h. OISD also asks for fresh clean water where possible (pH 6 – 8.3 otherwise, after a chemistry test), clean water with an aluminium internal floating cover, no pipework but the fill line during the test, a written procedure, diagonal filling across a tank farm, and care with vacuum relief and roof landing when emptying; these appear as record items in section 6.
The 1999 amended edition used 2.5 m stages held for 7 days; the 2006 revision replaced that with the staged procedure above. Confirm the edition your site works to.
Settlement survey (section 3)
| Item | Rule | Sample |
|---|---|---|
| Shell points (7.3.7.6 a) | Equally spaced, at most 10 m (32 ft) apart, at least 8 | 12 points, 9.42 m apart |
| Reading sets (7.3.7.6 b) | 1 before filling; 2–4 at ¼, ½ and ¾ of the test height (± 600 mm / 2 ft); 5 after the hold at the test height; 6 after emptying | sets at 0, 3.45, 6.9, 10.35, 13.8 m and empty |
| Sets 2–4 | May be omitted by the Purchaser (7.3.7.6 Note) | — |
| Bottom elevations (7.3.7.9) | Before and after the test, at most 3 m (10 ft) apart on diametrical lines; lines at most 10 m (32 ft) apart at the shell, at least 4 | 6 lines × 13 points = 73 (the centre shared) |
All readings refer to a permanent benchmark outside the influence of the tank and go into the Manufacturer’s post-construction package (7.3.7.10, W.1.5).
Settlement readings check (section 4)
Enter the settlement at each shell point since reading set 1, downward positive. 7.3.7.7: if the settlement is more than 50 mm (2 in.) uniform, or the difference between neighbouring points is more than 13 mm per 10 m (1/2 in. per 32 ft) of circumference, filling stops and the Purchaser is told.
| Quantity | How | Sample |
|---|---|---|
| Uniform settlement | Mean of the points | 24.9 mm ≤ 50 ✓ |
| Differential | |si+1 − si| ÷ spacing × 10 m, around the shell | 15 ÷ 9.42 × 10 = 15.9 mm > 13 ✗ |
- The column To next, per 10 m fills in as you type, so a bad pair shows at once.
- A tilted tank settles in a smooth single wave; a large tilt can trip the differential limit on its own. The limit is applied as written; Evaluate these readings in Tank Settlement opens the Tank Settlement tool (API 653 Annex B) with these points loaded, as elevation changes in ft with settling points negative, to separate tilt from out-of-plane settlement.
Test water and temperature (section 5)
| Material | Check | Rule |
|---|---|---|
| Carbon or low-alloy | Shell metal temperature | Not colder than the design metal temperature (Figure 4.1), water kept from freezing; the Manufacturer heats the water unless the Data Sheet says otherwise (7.3.7.5). |
| Carbon or low-alloy | Water contact over 14 days | Note: the Purchaser states extra limits on Data Sheet Line 14 — oxygen scavenger, biocide, caustic (7.3.7.4). |
| Annex S or X | Water quality | Potable treated (≥ 0.2 ppm free chlorine), pH 6 – 8.3, below 50 °C, chloride below 50 ppm, unless the Purchaser specifies otherwise (S.4.10.2, X.4.10.2). |
| Annex S or X | Exposure time | Potable water at most 21 days, other fresh water 7 days (S/X.4.10.3, .4), from the schedule in section 2. |
| Annex S or X | Seawater test level d) | Fails: seawater does not meet the 50 ppm chloride limit. |
The checks
| Check | Clause | Rule |
|---|---|---|
| Test water level | 7.3.6.1.1 | The level from the basis chosen; fails if H or the test level is above the shell (b: weld + 50 mm). |
| Shell above the test water | 7.3.6.1.3, 7.3.6.2.1 | Note with the band to examine. |
| Hydrostatic test exemption | 7.3.6.2.2 | Note with the four conditions. |
| Maximum filling rates | 7.3.7.6 | Pump against each band; says where to throttle. |
| Purchaser minimum filling rate | 7.3.7.6, Line 23 | Fails above a band maximum. |
| Hold before reading set 5 | 7.3.7.6 b 5 | At least 24 h; Note for i–iv. |
| Shell settlement points | 7.3.7.6 a | Count and spacing. |
| Settlement during the test | 7.3.7.7 | 50 mm uniform, 13 mm per 10 m differential. |
| Shell metal temperature | 7.3.7.5 | Carbon steel: not below the design metal temperature. |
| Water quality / exposure | S.4.10, X.4.10 | Stainless limits. |
| OISD minimum test temperature | OISD-129 Annex. III | With OISD stages: 4 °C and above. |
| Test record and procedure | 7.3.6 – 7.3.8 | A Note until every visible item is ticked. |
Test procedure and record (section 6)
| Clause | Item | Shown for |
|---|---|---|
| 7.3.6.1.2 | Tank inspected frequently while filling. | All |
| 7.3.7.3 1) | Tank and roof clean before the test. | All |
| 7.3.7.1 | Test before permanent piping; flexible fill line; what may be welded during and after the test. | All |
| 7.3.7.2 | Coatings over shell joints after the test. | All |
| 7.3.7.4 | Water source, disposal and quality set by the Purchaser. | All |
| 7.3.6.2.1 | Joints above the test water examined. | Levels c) and d) |
| 7.3.6.2.2 | Exemption conditions met. | Exemption |
| 7.3.7.6, 7.3.7.10 | Benchmark, instrument set-ups, readings in the W.1.5 package. | Settlement measured |
| 7.3.7.9 | Bottom elevations before and after. | All |
| 7.3.7.8 | Floating roof annular space before flotation and at the top. | Floating roofs |
| 7.3.7.3 7) | Wind girders checked for drainage. | All |
| 7.3.8.1 / 7.3.8.2 | Roof test: gas-tight methods, or visual for vented roofs. | By roof |
| OISD-129 Annex. III | Fill line only; water quality and chemistry; written procedure and diagonal filling; emptying precautions and bottom profile. | With OISD stages |
| S/X.4.10.1 | Penetrant leak test of the shell-to-bottom weld. | Stainless |
How the numbers were checked
- 7.3.6, 7.3.7 and 7.3.8, S.4.10 and X.4.10 were read from the 14th edition (August 2025) text; every value in the tool is the printed SI or US value.
- The sample’s volume, pump rise, band times, holds, emptying, seawater level, survey counts and readings are reproduced by hand in the test suite, in both unit systems.
- Each limit was made to govern in turn: 460 / 300 for thin and 460 / 300 / 230 for thick bottom courses, a venting limit, the OISD stage rate, the OISD 1.5 m/h cap and the floating-roof 0.25 m/h; the OISD 4 °C test temperature was tested on both sides.
- The readings check was tested passing and failing on both limits; the Tank Settlement handoff was tested end to end.
Reading the results and graphics
- The filling timeline plots water level against time: the 7.3.7.6 bands shaded on the level axis, holds as flat parts of the line, the numbered reading sets, OISD stage levels and any shell left above the water hatched.
- The survey plan shows the shell points numbered clockwise from your datum and the bottom lines and points, with any stretch of shell over the differential limit in red.
- The settlement profile (once every point has a reading) plots settlement downward around the shell with the mean and the 50 mm line; red segments are over 13 mm per 10 m, with their value.
What changed from the older tool
| Item | Older tool | This tool (2025 edition) |
|---|---|---|
| Basis | OISD-STD-129 staged filling only | API 650 7.3.6 – 7.3.8 (14th edition), with OISD stages as an option |
| Test level | Tank height | The four 7.3.6.1.1 bases, the band above the water, the exemption |
| Filling rate | 70 % of a design rate | The 7.3.7.6 bands by bottom course thickness, pump, venting, Purchaser minimum |
| Settlement | Text only | Survey points, reading sets, bottom lines, the 7.3.7.7 check and a handoff to Tank Settlement |
| Test water | — | Carbon steel temperature and contact days; Annex S and X quality and exposure |
| Saving | Save only | Autosave, reopen (including older records), print / PDF report and graphics |
Saving, reopening and reports
- Kept on this device. After a closed tab or lost connection, a banner offers Restore or Discard; inputs older than 30 days are dropped. A successful save clears it.
- Show saved calculations for this tank lists this tool’s records with the earlier page’s, newest first, with the procedure, test height, duration and failing checks. Open reloads every input and reading; saving again makes a new record noting what it was based on.
- Older records (OISD staging only) bring the tank height as shell height and test level, the design rate and the settlement setting into the OISD option; a fixed roof opens as gas-tight, a floating roof as external. Enter the diameter, bottom course thickness and pump rate to apply 7.3.7. The message quotes the old total hours.
- Print / PDF report: summary, test level, filling bands and timeline, survey and reading sets with the plan, the readings table and profile, test water, the record list and an appendix of inputs.
Messages and what to do
| Message | Meaning and what to do |
|---|---|
| 7.3.6.1.1 level above the shell | Correct H or the weld elevation; b) allows only 50 mm above the roof weld. |
| 7.3.6.2.1 shell above the water | Plan the oil, vacuum box or air test for the band shown. |
| 7.3.7.6 minimum rate conflicts | Agree a lower minimum with the Purchaser, or drop the settlement measurements. |
| 7.3.7.6 b 5 hold under 24 h | Hold at least 24 h before reading set 5. |
| 7.3.7.7 stop filling | Stop, report to the Purchaser, and evaluate (Tank Settlement) before continuing. |
| 7.3.7.5 metal colder than the DMT | Heat the test water or wait for warmer conditions. |
| S/X.4.10 water or exposure | Change the water, treat it, or shorten the holds and drain sooner. |
| reopen older format | Enter D, the bottom course thickness and the pump rate. |
Limits and interpretations
- Test levels, rates, holds, reading sets, survey counts and the 7.3.7.7 limits follow API 650 7.3.6 – 7.3.7 (2025) as printed.
- The venting limit (5.8.5) is entered, not calculated — size the vents in the API 2000 venting tool.
- The differential check compares neighbouring points as written; it does not separate tilt. Use Tank Settlement for evaluation.
- The timeline assumes the pump rate is constant within each band and that holds happen where shown.
- OISD-STD-129 values follow its Revised Edition (November 2006), Annexure III; the 1999 edition’s 2.5 m / 7-day stages are not used.
- Annex F tanks are tested to Annex F; roof tests (7.3.8) appear only as record items.
- API 653: the material route and a fitness-for-service evaluation are taken as entered; the Figure 5.2 box can be filled from the latest Brittle Fracture record, but only a record that used Figure 5.2 (unknown carbon steel) sets it.
Glossary
| Test height | Water level the shell is tested to (7.3.6.1.1). |
| Reading set | One round of shell elevation readings at every point (7.3.7.6 b). |
| Uniform settlement | Average settlement of the shell points. |
| Differential settlement | Difference between neighbouring points, per 10 m (32 ft) of circumference. |
| Benchmark | Fixed reference level outside the influence of the tank. |
| Design metal temperature | Lowest metal temperature the shell is designed for (3.8, Figure 4.1). |
| Hydrotest exemption | Testing with product on first filling when water is not available (7.3.6.2.2). |
| Major repair / alteration | API 653 3.25: a shell penetration over DN 300 (NPS 12) below the design liquid level; a bottom penetration within 300 mm (12 in.) of the shell; a shell or annular plate replaced or added over 300 mm (12 in.) long; more than 300 mm of a vertical shell or radial annular weld replaced; a new bottom; shell-to-bottom weld replaced beyond 12.3.3.5.1 a); jacking the shell. |
| Critical zone | Bottom within 75 mm (3 in.) of the inside of the shell (API 653 Section 3). |