ISO 21487:2022

Diesel Non-Metallic Tank Compliance Checklist

Non-Integral Tank — Installed Outside Engine Room

With Enhanced Type Test Documentation

Small Craft - Permanently Installed Petrol and Diesel Fuel Tanks

Third Edition 2022-11

Project Information

Enter the value permanently marked on the tank by the manufacturer per Clause 8. See the guidance note below for a full explanation of what this value means and how it must be determined.
Peak ambient temperature expected at the tank's installed position, accounting for solar gain, proximity to engine room bulkheads, exhaust runs, or other local heat sources. Enter this value to automatically check it against the declared maximum exposure temperature.
Guidance for Tank Manufacturers — Maximum Exposure Temperature (Clause 8): ISO 21487:2022 requires the maximum exposure temperature to be permanently marked on every non-metallic tank. This value must be determined by the manufacturer based on the thermal properties of the specific material and resin system used. It represents the highest continuous ambient temperature at which the tank will retain its structural integrity, dimensional stability, and fuel containment performance throughout its service life. It is not the temperature of the fuel itself — it is the temperature of the surrounding air or structure in direct contact with the tank exterior.

For thermoplastic tanks, the value is governed by the heat distortion temperature (HDT) of the polyethylene grade used, typically between 40°C and 60°C for HDPE and somewhat lower for LDPE. The manufacturer must test or obtain certified material data to confirm the HDT of the specific resin formulation, apply an appropriate safety margin, and declare the resulting maximum exposure temperature on the tank marking. A value that is simply estimated or assumed from generic material data without reference to the actual resin batch is not acceptable for CE marking purposes.

For FRP tanks, the maximum exposure temperature is governed by the glass transition temperature (Tg) and heat distortion temperature of the cured resin system. Polyester resins typically have a lower Tg than vinyl ester or epoxy systems. The manufacturer must obtain the Tg from the resin supplier's cured laminate data or through testing, apply a safety margin appropriate to the application, and declare the result on the marking.

The declared value directly determines where the tank may be installed. A low declared value restricts the tank to cool, well-ventilated spaces and excludes it from use near engine room bulkheads, under dark decks exposed to tropical sun, or in any location where heat soak or proximity to exhaust systems could raise the ambient temperature above the marked limit. Manufacturers who declare a realistic and well-evidenced maximum exposure temperature give the boat builder and inspector the information needed to confirm safe installation. Manufacturers who leave this field blank or mark an unrealistically high value without supporting data expose themselves to liability if a tank failure is attributed to thermal degradation.
About Type Tests for Non-Metallic Diesel Tanks Outside the Engine Room: Type tests are laboratory tests performed on representative tank samples to verify that the tank design meets all safety and performance requirements of ISO 21487:2022. For non-metallic non-integral diesel tanks installed outside the engine room, TWO type tests are required: Leakage Test and Pressure/Strength Test. The fire resistance test applies only to non-metallic tanks installed in an engine compartment — tanks located outside the engine room are exempt from fire testing. The Pressure-Impulse Test (Clause 7.3) is not required for diesel tanks of any material or installation position. Once a tank design has successfully passed type testing, individual production tanks only require a leakage test.

6.2 Type Tests for Non-Metallic Diesel Tanks — Outside Engine Room (Table 2)

Type Test Overview: Per ISO 21487:2022 Table 2, non-metallic non-integral diesel fuel tanks installed outside the engine room must undergo TWO mandatory type tests: Fire Testing (Clauses 7.4 and 7.5) is NOT required — the fire test obligation is linked to engine compartment installation, not to non-metallic material alone. The Pressure-Impulse Test (Clause 7.3) is NOT required for diesel tanks of any material.

Type Test 1: Leakage Test (Clause 7.2.1)

Test Purpose and Objective

The leakage test verifies the integrity of all tank seams, joints, moulded connections, fittings, and closures under normal operating pressure conditions. The standard requires a positive detection method — either an ammonia-free soap solution or total water immersion. Pressure-drop monitoring alone is insufficient.

Test Procedure

Step 1: Test Setup

  • Tank must be tested as a complete assembly including the fuel pick-up tube, fuel fill pipe, and fuel gauge/sender unit as they would be installed in service (Clause 7.1)
  • All openings except the pressure inlet must be capped or sealed
  • Tank must be supported to prevent distortion during pressurisation
  • If inspection hatches are fitted, ensure gaskets are properly installed and torqued to specification
  • If bottom drain fittings or side fittings with shut-off valves are installed, test in operational configuration

Step 2: Pressure Calculation

Calculate the required test pressure as the GREATER of:

  • 20 kPa (minimum absolute value)
  • 1.5 times the highest hydrostatic pressure to which the tank will be subjected in normal service
  • For sealed systems: 1.5 times the maximum hydrostatic head at designed fill level plus the system relief valve pressure

Step 3: Pressurisation

  • Use clean dry air or inert gas — nitrogen is recommended
  • Gradually increase pressure to the calculated test level; avoid sudden pressure application
  • Monitor the pressure gauge continuously during pressurisation

Step 4: Static Pressure Hold

  • Maintain the test pressure for a minimum of 5 minutes without pressure drop or rise
  • Record initial and final pressure readings with timestamps
  • Any pressure change may indicate leakage or a temperature effect — investigate before accepting

Step 5: Leak Detection

  • Apply an ammonia-free soap solution to all joints, seams, moulded fittings, and inspection hatches and look for bubbling
  • Total immersion in water is an acceptable alternative detection method
  • Do NOT rely solely on pressure-drop — this is explicitly insufficient under the standard
  • Do NOT use solutions containing ammonia, which attacks brass fittings; the resulting damage can be invisible initially and cause failure months after testing
Pass Criteria
  • No visible leakage at any point during the 5-minute test period
  • No pressure drop or rise during the static pressure hold
  • No bubbles at any seam, joint, moulded connection, fitting, or inspection hatch gasket
  • Tank structure shows no signs of distortion or damage
  • All gaskets and seals remain intact and functional
Failure Indicators
  • Visible leakage of test gas at any location
  • Continuous pressure drop indicating a leak
  • Bubbles forming at seams, joints, moulded connections, fittings, or gaskets
  • Structural deformation or damage to tank walls
Special Considerations for Non-Metallic Tanks
  • Non-metallic tanks can exhibit creep under sustained pressure — monitor carefully for slow pressure loss that may not register immediately on a gauge
  • Pay particular attention to moulded-in fittings and integrally formed pipe connections, which are common on thermoplastic tanks and are frequent leak initiation points
  • Inspection hatches of minimum 120 mm diameter are required for diesel tanks (Clause 6.1.6) and must be leak-tested with gaskets properly installed
  • Surface-treated or fluorinated tanks — verify that treatment has not compromised any joint or seam integrity prior to pressurising
Test 1: Leakage Test - Data Recording Form

Type Test 2: Pressure/Strength Test (Clause 7.2.2.4 / 7.2.2.2)

Test Purpose and Objective

The pressure/strength test verifies that the non-metallic diesel tank structure can withstand pressures significantly higher than normal operating conditions without cracking or catastrophic failure. For thermoplastic tanks a mandatory fuel soak preconditioning phase must be completed before the pressure test. This simulates the long-term effect of fuel contact on the material. Permanent deformation is acceptable, but the tank must not crack or leak.

Test Method Selection

The applicable test method depends on tank material:

  • Thermoplastic (HDPE or LDPE) — Clause 7.2.2.4: Mandatory fuel soak preconditioning required. Hold duration: 60 minutes for high-density material, 5 hours for low-density material.
  • FRP Non-Integral — Clause 7.2.2.2: No fuel soak preconditioning required. Standard 5-minute hold applies.
Fuel Soak Preconditioning (Thermoplastic Tanks — Clause 7.2.2.4)

Acceptable soak fuels — use one of the following:

  • The actual fuel for which the tank is fabricated (diesel)
  • Liquid C blended to 10.0% ± 1.0% ethanol by volume (Liquid C per ISO 1817)
  • CE10 fuel — 90% Liquid C and 10% fuel ethanol by volume

Soak conditions:

  • Fill the tank to maximum capacity and store for at least 28 days at ambient temperature of not less than 21°C
  • Alternative: minimum 10 weeks at 43°C ± 5°C
  • Maintain a continuous temperature and duration record throughout the soak period
  • The pressure test must be performed immediately after emptying — no delay is permitted
Pressure/Strength Test Pressure Calculation

Test pressure must be the GREATER of:

  • 20 kPa (minimum absolute value)
  • 1.5 times the highest hydrostatic pressure to which the tank can be subjected in service
  • For sealed systems: 1.5 times the maximum hydrostatic head at designed fill level plus the system relief valve pressure
Test Procedure
  • Conduct AFTER successful completion of the leakage test (7.2.1)
  • For thermoplastic tanks: conduct immediately after emptying the soak fuel — no delay whatsoever
  • Gradually increase pressure to the calculated test pressure
  • Maintain test pressure for the full required hold duration per Table 2
  • Monitor continuously for cracks, ruptures, and leakage throughout the hold period
  • Permanent deformation is acceptable — the tank does not need to return to its original shape
Pass Criteria
  • No cracks develop in the tank wall, seams, joints, or at any fitting
  • No leakage detected during or after the pressure hold period
  • Pressure is maintained for the full required hold duration
  • Permanent deformation is permissible
  • Inspection hatch gaskets remain sealed (if applicable)
  • Bottom/side fittings and shut-off valves show no leakage (if applicable)
Failure Indicators
  • Any crack formation in tank wall, seams, or at fittings
  • Rupture or catastrophic structural failure
  • Leakage of test medium at any location
  • Inability to maintain test pressure for the required hold duration
  • Gasket failure at inspection hatch
Special Considerations for Non-Metallic Tanks
  • Thermoplastic tanks may show visible wall bulging during the hold — this is permanent deformation and is acceptable provided no cracking or leakage occurs
  • The transition from soak to test must be immediate — any delay allows the material to partially recover and invalidates the preconditioning
  • FRP tanks should be visually inspected for delamination before testing; pre-existing delamination is a disqualifying condition
  • Moulded-in fittings and bosses are common stress concentration points — monitor these areas throughout the hold period
Important Note on Thermoplastic Preconditioning:

The fuel soak in Clause 7.2.2.4 is mandatory for HDPE and LDPE tanks and cannot be omitted or shortened. The pressure test must begin immediately after the soak fuel is emptied. Plan a minimum of 28 days lead time before the scheduled pressure test date. If you are unsure whether your tank material is high-density or low-density, confirm with the manufacturer before selecting the hold duration, as the difference between 60 minutes and 5 hours is significant.

Test 2: Pressure/Strength Test - Data Recording Form
Fuel Soak Preconditioning Record (Clause 7.2.2.4 — Thermoplastic Tanks Only)

Complete this section for HDPE and LDPE tanks. For FRP tanks select N/A in each field and proceed directly to the pressure test data below.

Important Note - Non-Metallic Diesel Tank Type Testing Outside Engine Room:

Non-metallic non-integral diesel tanks installed outside the engine room require TWO type tests — the same number as metallic diesel tanks. The fire test requirement in Table 2 is linked to engine compartment installation, not to non-metallic material alone. The key difference from the metallic version is the fuel soak preconditioning step in the pressure/strength test, which is mandatory for all thermoplastic tanks and requires a minimum of 28 days planning lead time before the pressure test can begin.

The following tests are NOT required for non-metallic diesel tanks outside the engine room:

Once both type tests have been passed, individual production tanks of the same design require a leakage test only.

Type Test Summary and Compliance Status

Type Test Results Overview: Review the results from both mandatory type tests. The non-metallic diesel tank design (outside engine room) can only be considered compliant with ISO 21487:2022 if BOTH required tests have been passed successfully.
Test Number Test Name ISO Clause Required for Non-Metallic Diesel Outside Engine Room? Status Test Date
1 Leakage Test 7.2.1 YES - REQUIRED Not Tested -
2 Pressure/Strength Test (with fuel soak for thermoplastic) 7.2.2.4 / 7.2.2.2 YES - REQUIRED Not Tested -
3 Pressure-Impulse Test 7.3 NO - Not Required for Diesel Exempt N/A
4 Fire Resistance Test 7.4 / 7.5 NO - Outside Engine Room Exempt N/A
Overall Type Test Status: Not Complete

Test Report Certification

ISO 21487:2022 - Small Craft - Permanently Installed Petrol and Diesel Fuel Tanks

Third Edition 2022-11

This checklist references ISO 21487:2022 for informational purposes only.

ISO standards are copyright of the International Organization for Standardization.

Users must obtain the official standard from ISO or authorized distributors.

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