Contents
About this tool
The Rim Seal Gap Survey records gap measurements between a floating roof’s seals and the tank shell, position by position around the tank. It totals the gaps, checks them against the limits of the regulation you choose, records the visual condition checks, adds the API 653 (Addendum 4, 2025) seal inspection and repair items when you want them, and works out from the rim gap whether the roof is centred and the shell round.
Every survey you save joins that tank’s history. The history shows as a gap map, a plan view and a trend chart, so a spot that keeps coming back stands out from one-off seal damage.
This is an indicative field-record tool, not a regulatory submission tool. Limits are filled in from the regulation text where we could verify it, and stay editable. Confirm the applicable limits and procedure with your permit or regulator before relying on a result for compliance.
The tool covers external floating roofs with a primary seal (mechanical shoe or liquid-mounted) and, usually, a secondary seal. It works in US units (inches, feet) or metric (mm, m); rim gap is always in millimetres.
Quick start
A first survey takes about nine steps. Anything not mentioned can stay at its default.
- 1Survey Setup: enter the Tank ID, diameter and survey date. The point count fills in.
- 2Set the 0° reference point (for example the rolling ladder) and whether angles run clockwise or anticlockwise seen from above. Use the same on every survey of this tank.
- 3Pick Units: US (in, ft) or Metric (mm, m).
- 4Regulatory Profile: choose the regulation. Limits and the probe size fill in.
- 5On the roof, open Field entry and work round the tank point by point: rim gap, then primary and secondary width and length. Tap No gap at either where there is none.
- 6Mark the condition checks (shoe extension, tears, coverage, and fittings under Kc). For an API 653 inspection, choose it under API 653 Seal Inspection & Repair and mark those items too.
- 7Read the Compliance Summary and the overall inspection result. Fix anything listed under Check these readings.
- 8Save Draft (Ctrl/⌘+S). When the record is final, Submit Survey to lock the readings.
- 9Print / PDF Report, then Load Survey History to see this survey alongside earlier ones.
What is being measured
The floating roof sits a little inside the shell. The space between the roof rim and the shell is the rim space. The primary seal closes it near the liquid: usually a steel shoe pressed against the shell and dipping into the liquid. The secondary seal, mounted above, wipes the shell and covers the primary.
| Reading | What it is | Unit |
|---|---|---|
| Rim gap | Distance from the roof rim plate to the shell at that position. Not a seal gap: it shows where the roof sits and the shape of the shell. | mm (always) |
| Primary / secondary width (W) | The widest opening between that seal and the shell at this gap, measured with probes of increasing size. | in or mm |
| Primary / secondary length (L) | How far that gap runs around the tank (along the circumference). | in or mm |
| Gap area | L × W for each gap; the tool adds them up for each seal. | in² or cm² |
Setting up a survey
Tank ID, diameter and survey date are required to save. Company and Tank ID together identify the tank’s history, so spell them the same way every time.
- Point spacing is your inspection procedure’s distance between measuring points, not a regulatory value. Min. points sets a floor for small tanks.
- 0° reference point and Angles run make positions comparable between surveys. If two surveys in the history use different references, the gap map warns you.
- Gap probe width: a gap only counts if the probe passes freely. The profile fills 1/8 in (3.175 mm) for US EPA; enter your own if your procedure differs.
- Units: switching converts every value on screen in place. Readings are always saved in inches, so the setting only changes what you type and see. Each device remembers its own choice.
- Record Type: Just for research surveys are visible to you and admins only; Official surveys are visible to your company.
Regulatory profiles
The profile fills the limits in the regulation’s own units, then converts them to yours, so nothing is lost to rounding. An info panel under the selector shows the source, the measurement method, how often to measure and what happens on a failure.
| Profile | Primary seal | Secondary seal | Probe | Frequency |
|---|---|---|---|---|
| US EPA — NSPS Kb 60.113b · built 23 Jul 1984 – 4 Oct 2023 | 3.81 cm (1.5 in) widest; 212 cm²/m (10.016 in²/ft) | 1.27 cm (0.5 in) widest; 21.2 cm²/m (1.0016 in²/ft) | 0.32 cm (1/8 in) | Primary every 5 years; secondary yearly |
| US EPA — NSPS Kc 60.113c · after 4 Oct 2023 | 1.5 in widest; 10 in²/ft | 0.5 in widest; 1 in²/ft | 0.125 in | Primary every 60 months; secondary every 12 |
| India — MoEF Gazette 2012 | 4 cm widest; 200 cm²/m | 1.3 cm widest; 20 cm²/m | not stated | Every six months |
| India — EIL 6-13-0053 Rev. 6 Annexure-I (project specification) | 4 cm widest; 200 cm²/m | 1.3 cm widest; 20 cm²/m | not stated | not stated; at handover and for the 5-year seal guarantee |
| EU — EN 14015 / Dir. 94/63/EC | No numeric gap limit in either document. The fields are left blank: enter your permit or company limits. | |||
| Custom | Whatever you type. Changing to Custom keeps the values already in the fields. | |||
The India figures are our own translation of a Hindi-only Gazette notification, not an official English text. Kb and Kc were checked against the eCFR text in October 2026. All limit fields stay editable.
Leave a limit blank to skip that check. Limits stay editable after a survey is submitted (Update Limits), because the readings are the record and the limit is the yardstick you hold them against.
Read from EIL Standard Specification 6-13-0053 Rev. 6 (July 2010), Annexure-I 4.4 and 5.1, the copy on file; later EIL revisions have not been compared. EIL is a purchase specification for new tanks, not a regulation: the same 4 cm / 200 cm²/m and 1.3 cm / 20 cm²/m limits as the MoEF profile, a 200 mm rim space, and a roof gap within ±13 mm of nominal after erection (10.2). Choosing it sets the expected rim gap range to 187 – 213 mm.
Expected rim gap range
Optional. API 650 asks seals to cope with ±100 mm (±4 in) of local roof-to-shell deviation, and 200 mm is a common design rim space, which gives a 100–300 mm band. The India profile fills it on tanks over 30 m; a link fills it for any tank. The design rim space on your roof drawings is the better reference if you have it.
Recording readings
Each row is one position. Work round the tank and enter the rim gap and, for each seal, the width and length of any gap at that position.
Blank, zero and below the probe
Blank = not measured
The seal only passes once every point has a width. Until then it shows Incomplete.
0 = measured, no gap
A positive record that the seal was checked there. Use the Blank widths → 0 buttons once you’ve walked the whole tank.
Below the probe
Shown struck through and left out of the totals: the probe couldn’t pass, so it is not a gap.
Gap length and overlaps
A gap is taken as centred on its row’s position and running its length around the shell. If one gap is long enough to reach the next point, record it once. Two gaps that overlap on the same seal are flagged, since they are probably the same gap entered twice.
Readings between points
Add reading between points adds a row (numbered +1, +2…) placed in the widest unmeasured stretch; type the actual angle. Use it for a gap that falls between survey points. Remove it with ×.
Readings stay at the angle where they were measured
If you change the diameter or spacing after entering data, readings are never slid to new angles. A reading whose angle is still a survey point stays put; any other is kept as a + row at its own angle, and a blue note says how many. Change the spacing back and they fold in again. The new survey points start blank.
Field entry and shortcuts
Field entry, one point at a time opens a large panel built for a phone on the roof, gloves included. On phones a bar at the bottom of the screen shows both seal verdicts and opens it in one tap.
- It opens at the first point without both widths.
- Enter moves to the next field; on the last number it moves to the next point.
- No primary gap, No secondary gap and No gap at either fill zeros in one tap; the last also moves on.
- Next blank jumps to the next unfinished point.
- A status line shows each width against its limit as you type.
- Esc closes the panel.
In the table
- Enter / Shift+Enter move down / up the same column.
- Ctrl/⌘+S saves a draft.
- Phones show the number keypad for every number field.
Paste, import and export
Paste from Excel or a gauge log. Click the cell where the block should start and paste. Columns fill in the table’s order: position, rim gap, primary L, primary W, secondary L, secondary W, note. Rows that would run past the end are reported, not lost silently.
Import CSV reads column headings flexibly and converts mm or inches to the units you are using:
| Heading contains | Goes to |
|---|---|
| angle pos deg | Position |
| rim | Rim gap (mm) |
| primary + width / length | Primary W / L; a unit in the heading (mm, in) is converted |
| sec + width / length | Secondary W / L |
| note remark | Note |
A reading whose angle matches a survey point fills that point; any other becomes a + row. A file without headings is read in the tool’s own export order. Export CSV writes the table in that order, with the unit in each heading.
Condition checks
Visual checks made at the same time as the gap measurement. Mark each OK, Failed or N/A. Under Kb and Kc a failed check is an inspection failure, just like a gap over the limit.
- Shoe in the liquid and at least 61 cm (24 in) above it (N/A for a liquid-mounted seal)
- No holes or tears in shoe, fabric or envelope
- Secondary covers the whole rim space
- No holes or tears in the secondary
- The four seal checks above
- Roof openings project below the liquid
- Vacuum breakers and rim vents gasketed and closed
- Hatch, sample well and gauge float covers gasketed and closed
- No tears in sleeves, wipers or fabric covers
India, EU and Custom show four good-practice checks with no regulatory reference.
API 653 seal inspection and repair
Below the condition checks, API 653 Seal Inspection & Repair adds what API 653 5th edition (2014), Addendum 4 (July 2025) asks of the floating roof seal. Choose the inspection; the items follow the seal type. These are inspection and maintenance findings: they do not change the regulatory gap verdict.
| Item | What the tool does | Clause |
|---|---|---|
| In service (external) | Eight seal items: condition and gaps recorded, shoe fabric not pulling shoes off the shell, fabric, metal parts, vapour openings, protruding bolts or rivets, seals pulled back all round, secondary not buckled; wedge wiper seals for a wiper type. | C.1.4.7 a, c – j |
| Rim space readings | From the Rim Gap column: widest spacing at most 9 m (30 ft), all four quadrants. Readings without one 180° across are noted: an even, equally spaced point count gives every point its opposite. | C.1.4.7 b |
| Out of service (internal) | Shoe assembly (foam logs at 90°, hangers and clips, shoes, fabric, clearance at nozzles and mixers) or toroidal assembly (fabric, hold-downs, foam), plus the rim-mounted secondary (bolting bar, gaps charted, light through the seam, fabric, tip, obstructions). | C.2.9.1 – C.2.9.3 |
| Roof rim thickness | Under 2.5 mm (0.10 in.) the rim is replaced with plate at least 5 mm (3/16 in.) thick before a seal is installed. | 9.14.6.2 |
| Seal fabric length | Shoe-top-to-rim fabric against the largest rim space measured: shorter fabric pulls the shoes off the shell. Check it at other roof levels too. | C.2.9.1 g |
| Repairs | When gaps exceed the limit: adjust shoe hangers or add foam filler; lengthen a rim-mounted secondary; replace the primary (possibly with a rim extension) only after checking the annular space from low pump-out to high level — the tool counts the roof levels recorded for the tank. After damage: correct the cause, replace buckled parts and torn fabric. In service: at most a quarter of the seal out at a time, with spacers to keep the roof centred. | 9.14.1 – 9.14.6 |
The items are saved with the survey, reopen with it, and print in the report. API 650 14th edition (2025) C.3.13.3 says seals are installed to meet the jurisdiction’s gap requirements for new construction and the Purchaser’s; H.4.4.3 (±100 mm / ±4 in. local deviation) is unchanged in that edition.
Reading the results
For each seal the tool adds up the area of every gap that counts and divides by the tank diameter, then compares that and the widest single gap with the limits:
Primary gaps: 0.8 in × 60 in = 48 in², and 1.2 in × 30 in = 36 in². Total 84 in² ÷ 150 ft = 0.56 in²/ft against 10.016, and widest 1.2 in against 1.5: Within limit. Secondary: one gap of 0.6 × 100 = 60 in² → 0.40 in²/ft passes on area, but 0.6 in is wider than 0.5 in: Exceeds limit.
Seal verdicts
Inspection result
Below the summary, one box combines both seal verdicts and the condition checks: inspection failure, not complete yet, or no failures found. For Kb and Kc a failure gives the 45-day repair-or-empty date, notes the 30-day extension, and lists the next primary and secondary measurement dates and the 30-day advance notice.
Roof centring and shell fit
The rim gap readings show where the roof sits in the shell. The tool fits a smooth curve to them and splits the pattern three ways.
- Roof offset: how far and toward which angle the roof has moved. The gap is tightest on that side. It also says whether re-centring the roof alone would bring every point inside the expected range.
- Out-of-round: roughly how much two diameters differ, and on which axis the gap is widest. It can’t tell whether the shell or the roof is the oval one.
- Local deviations: points well off the curve, suggesting a dent, bulge or out-of-plumb shell there. One bad point is set aside so it doesn’t distort the fit.
- Any point more than 100 mm from the mean gap is flagged, beyond what API 650 expects a seal to accommodate.
At least 6 rim gap readings with no unmeasured stretch over 120°; the out-of-round part needs 8 with none over 90°. Flags use judgement thresholds: offset ≥ 10 mm, out-of-round ≥ 20 mm, local ≥ 10 mm, each also standing out from the reading scatter. A survey at one roof level can’t separate a tilted roof from a leaning shell; repeat at another level to check.
Plan view
The tank seen from above, using your 0° reference and direction.
- Seal gaps are arcs on the shell at their real position and length: inner ring primary, outer ring secondary. Thicker means wider; colour shows the share of the limit.
- Roof edge (gold) is traced from the rim gap: a wider gap sits further from the shell. It is stretched radially so millimetres show; the key states by how much.
- The shaded ring is the expected range; the blue arrow is the roof centre’s offset.
- Tick boxes switch layers on and off. Hover any arc or point for its values.
Gap map and history
Load Survey History fetches saved surveys for this Company and Tank ID. Each row is a survey; each block is one reading at its true position, spanning halfway to its neighbours, so nothing is merged or hidden. The top row (▶ Live) is the survey on screen, updating as you type.
- Tabs: primary seal, secondary seal, rim gap. Each block is coloured against that survey’s own limit, or shown hatched (not measured) or outlined in green (measured, no gap).
- Rim gap, tighter / wider: colours each reading against that survey’s average, so an off-centre roof shows as an orange band on one side and blue opposite even when every point is inside the range.
- Repeat hot spots usually point to the shell (dent, bulge, out-of-plumb, weld distortion) rather than seal damage.
- Click a date to open that survey. Your drafts reopen for editing; submitted surveys open locked; another user’s open read-only.
- Tick boxes hide the live row or research surveys.
Surveys saved before October 2026 stored blank widths as 0, so a 0 in them may mean “not measured”. The map draws those cells dashed grey.
Trend across surveys
Once two or more surveys are shown, the trend card plots accumulated gap area and widest gap as a percentage of each survey’s own limit, so surveys under different profiles share one chart, plus the roof offset where it was saved. The live survey is drawn hollow.
With three or more rising surveys it draws a straight line and gives the date it would reach the limit. Seal gaps seldom grow in a straight line: treat it as a reason to look sooner, not a forecast.
Saving, locking and reports
- Kept on this device. After a closed tab, phone restart or dropped connection, a blue banner offers Restore or Discard. It is cleared once a save succeeds, isn’t shared between devices, and starting a second survey before saving replaces it.
- No connection. A failed save says so and keeps the readings on the device; press Save again when back online. The browser also warns before you leave with unsaved changes.
- Print / PDF Report gives a summary strip, compliance table, inspection result, condition checks, the API 653 seal items and findings (when used), plan view, roof fit, trend (when history is loaded), all readings, reading checks and signature lines. Use Save as PDF in the print dialog.
Messages and what to do
Problems with the readings are listed under Check these readings below the table and printed on the report.
| Message | Meaning and what to do |
|---|---|
| overlap Primary gaps at point 4 and point 5 overlap | Two gaps reach into each other. If it is one gap, record it once at its centre. |
| no length width … has no gap length | The gap’s area counts as 0 until you enter its length. |
| units width … is very wide — was it entered in mm? | A width over 4 in is likely mm typed in inch mode. Switch Units or correct the value. |
| units width … is under 1 mm — was it entered in inches? | Inches typed in metric mode. |
| rim gap rim gap … mm looks unlikely | Over 600 mm or under 5 mm. The rim gap column is always mm. |
| probe No probe width entered | US EPA counts only gaps the 1/8 in probe passes; enter the probe used. |
| position position is blank / should be 0–359.9° | Enter an angle within one turn. |
| notice Point count changed … readings were kept | Informational; see Recording readings. |
| save No connection — not saved to the server yet | Readings are on the device. Save again when online. |
Limits and good practice
- Measure with the roof floating off its legs, at one or more roof levels, and note the level in Roof / Liquid Level.
- Pass the probe without forcing. Width is the widest probe that passes; length is how far round the shell that probe passes.
- Keep the same 0° reference and direction for every survey of a tank.
- Record zeros as you go, or use Blank widths → 0 only after walking the whole circumference.
- Under Kb and Kc, give the Administrator 30 days’ notice before gap measurements. The tool reminds you but does not send anything.
- The tool records and evaluates; it does not certify. Reportable results remain your responsibility.
- Photos per reading aren’t stored yet; reference photo numbers in the Note field.
Glossary
| Accumulated gap area | Sum of length × width of every counted gap on one seal. |
| Mechanical shoe | Steel band pressed against the shell, dipping into the liquid; the usual primary seal. |
| Liquid-mounted seal | Foam- or liquid-filled envelope touching the liquid; an alternative primary seal. |
| Probe | Uniform rod (1/8 in for US EPA) used to find and size gaps. |
| Rim gap / rim space | Distance between the roof’s outer rim and the shell. |
| Secondary seal | Seal mounted above the primary, usually a wiper, covering the rim space. |
| NSPS Kb / Kc | US federal standards for volatile organic liquid storage vessels; Kc applies to vessels built or modified after 4 Oct 2023. |
| Repeat hot spot | A position flagged in two or more surveys of the same tank. |