1 April 2026·Leide team
DNV-RU-SHIP Pt.2 Ch.4: Ship Piping Systems
DNV-RU-SHIP Pt.2 Ch.4 piping classes I/II/III by fluid hazard, Barlow wall thickness formula, corrosion allowance, valve requirements
DNV-RU-SHIP Part 2 Chapter 4 governs piping systems serving ship machinery — fuel, seawater cooling, lubricating oil, hydraulic, steam, compressed air, and related services. Its piping class framework sets wall thickness, material, valve, and testing requirements according to the hazard level of the fluid carried. Correct class assignment at design stage is the most common audit finding; misclassification flows through to inadequate wall thickness, wrong valve types, and missed hydrostatic tests.
This chapter works closely with DNV-RU-SHIP Pt.2 Ch.2 (machinery system requirements) and DNV-OS-D101 (marine and machinery systems for offshore). The lifting appliances piping interface is covered in DNV-RU-SHIP Pt.4 Ch.6.
Contents
1. Piping Class Designations (Class I / II / III)
DNV-RU-SHIP Pt.2 Ch.4 classifies all machinery piping into three classes based on two parameters: fluid hazard category and design pressure/temperature. The class drives every downstream requirement — wall thickness method, allowable joining method, valve specification, and test pressure.
| Class | Fluid Category | Design Pressure (gauge) | Design Temp. | Scope |
|---|---|---|---|---|
| Class I | Flammable, toxic, or high-pressure steam | > 16 bar OR any pressure if toxic/flammable at elevated temp | Any | Fuel oil high-pressure lines, hydraulic at >16 bar, steam >300°C, refrigerant piping |
| Class II | Flammable at ambient temp, or steam/hydraulic below Class I threshold | 7–16 bar | ≤ 300°C | Lubricating oil mains, fuel oil low-pressure transfer, hydraulic 7–16 bar, steam ≤ 16 bar |
| Class III | Non-flammable, non-toxic at ambient, low pressure | ≤ 7 bar | ≤ 60°C | Seawater cooling, fresh water, compressed air ≤ 7 bar, bilge and ballast |
Dual-criteria rule: a piping system must satisfy both the fluid hazard criterion and the pressure criterion. A cooling water line that passes through a space at >16 bar falls to Class I even though water itself is non-hazardous. Always check both.
Fluid Hazard Categories
The standard defines fluid hazard by flash point and toxicity:
- Flammable liquids: flash point ≤ 60°C — includes most marine diesel, HFO, lube oil (some grades), hydraulic fluids
- Highly flammable: flash point < 23°C — gasoline, methanol, certain refrigerants
- Toxic: classified per MARPOL / IMDG; CO₂ flooding medium is treated separately under safety system rules
- Steam: always at least Class II; Class I when T > 300°C or p > 16 bar
2. Material Selection by Fluid Service
Material requirements vary by class and service. The standard specifies minimum requirements; higher grades are always permitted.
| Service | Preferred Material | Restrictions |
|---|---|---|
| Seawater cooling (Class III) | Copper-nickel (90/10 CuNi), galvanised steel, GRP/GRE composites | Plain carbon steel only if internally coated; aluminium alloys permitted below waterline with corrosion allowance |
| Fresh water cooling (Class III) | Carbon steel, CuNi, austenitic SS | Galvanising optional; trace copper leach into engine water system should be confirmed with engine maker |
| Fuel oil (Class I/II) | Seamless carbon steel (EN 10216-1 or equivalent), SS 316L | Copper alloys not permitted for HFO > 130°C; rubber-lined pipe not permitted in Class I |
| Lubricating oil (Class II) | Seamless carbon steel, SS 316 | Zinc, cadmium, and magnesium alloys prohibited (react with sulphur-bearing oil) |
| Hydraulic (Class I/II) | Seamless carbon steel, SS 316, approved high-pressure hose assembly | Threaded connections (NPT) not permitted in Class I above DN50; only flanged or socket-weld permitted |
| Steam (Class I) | Alloy steel (P11/P22 for >300°C), carbon steel for lower temps | Cast iron bodies for valves not permitted in Class I steam; ductile iron not permitted > 230°C |
| Compressed air (Class III) | Carbon steel, CuNi, composite (GRE) | GRE permitted to 7 bar max if DNV-approved product; above 7 bar → Class II, metallic only |
Pt.2 Ch.4 §3: All materials shall have mill certificates to EN 10204 type 3.1 minimum for Class I piping. Class II and III may use 2.2 test reports where traceability can be established by marking.
3. Wall Thickness Calculation
The required minimum wall thickness is calculated using a form of the Barlow (thin-wall hoop stress) formula, with a safety factor applied through the allowable stress:
DNV-RU-SHIP Pt.2 Ch.4 — Required wall thickness (pressure containment)
treq = (p × Do) / (2 × fall + p)
where:
p = design gauge pressure [bar]
Do = outside diameter [mm]
fall = allowable stress = Rm / Sf [N/mm²]
Sf = safety factor (3.5 for Class I; 4.0 for Class II; 4.0 for Class III at elevated temp)
The nominal wall thickness ordered must add a corrosion allowance and account for the negative manufacturing tolerance on wall thickness:
Nominal wall thickness — ordering tolerance
tnom = (treq + c) / (1 − δ)
c = corrosion allowance [mm] (see table below)
δ = negative manufacturing tolerance (typically 0.125 for seamless steel pipe = 12.5%)
| Service / Environment | Corrosion Allowance c [mm] |
|---|---|
| Seawater, externally exposed | 3.0 (internally coated) or 2.0 + coating |
| Seawater, internally — uncoated CS | 3.0–5.0 depending on flow velocity |
| Fuel oil / lube oil, internally | 1.0–2.0 |
| Fresh water, internally | 1.0 |
| Steam, internally | 1.0 (low-alloy steel), 0 (SS/alloy steel) |
| Hydraulic fluid | 0.5–1.0 |
| CuNi, SS, GRE/GRP | 0 (corrosion-resistant material) |
Minimum Wall Thickness
Regardless of the Barlow calculation, DNV-RU-SHIP Pt.2 Ch.4 specifies absolute minimums by pipe size and class:
| Nominal Bore | Class I min. t [mm] | Class II min. t [mm] | Class III min. t [mm] |
|---|---|---|---|
| DN ≤ 15 | 2.3 | 2.0 | 1.6 |
| DN 20–50 | 2.6 | 2.3 | 2.0 |
| DN 65–150 | 3.2 | 2.9 | 2.3 |
| DN 200–350 | 4.0 | 3.6 | 2.9 |
| DN ≥ 400 | 5.0 | 4.5 | 3.6 |
4. Pressure Ratings and Design Pressure
Design pressure is the maximum continuous working pressure under normal operating conditions. The standard distinguishes:
- Design pressure (pd): the sustained maximum pressure used for wall thickness and valve rating selection
- Maximum allowable working pressure (MAWP): the highest pressure the system may reach under any operating mode, including transients
- Relief valve set pressure: shall not exceed 1.1 × MAWP for Class I; 1.15 × MAWP for Class II/III
Pressure surge / water hammer: For pumped systems with fast-closing valves (e.g. solenoid-actuated fuel valves), the surge pressure must be estimated and either included in the design pressure or mitigated by surge suppression. DNV-RU-SHIP Pt.2 Ch.4 requires the surge analysis to be documented for Class I fuel systems on vessels with high-speed injection pumps.
Pressure-Temperature Rating for Flanges and Valves
Standard flanges to ASME B16.5 or EN 1092-1 are rated by pressure class (PN or ANSI #). The operating envelope (p, T) must fall within the flange material group pressure-temperature table:
| ASME Class | PN equivalent | Max. gauge pressure at 100°C (CS) | Typical Class I application |
|---|---|---|---|
| #150 | PN 20 | 19.6 bar | Low-pressure Class I margins only |
| #300 | PN 50 | 51.1 bar | Fuel injection return, lube oil mains |
| #600 | PN 100 | 102.1 bar | Main fuel injection, high-pressure hydraulics |
| #900 | PN 150 | 153.2 bar | High-pressure hydraulic cylinders |
| #1500 | PN 250 | 255.4 bar | Extreme HP hydraulic applications |
5. Valve and Fitting Requirements
Valve Type Requirements by Class
| Application | Class I | Class II | Class III |
|---|---|---|---|
| Isolation valves | Flanged or butt-weld; ball, globe, or gate; minimum PN 40 rating | Flanged or screwed (≤ DN50); ball or gate | Any type including screwed; butterfly permitted ≥ DN80 |
| Check valves | Swing check or piston check; cast iron body not permitted | Swing check or piston check; DI body permitted to 230°C | Swing check; wafer type permitted |
| Relief valves | Full-bore spring-loaded; ductile iron or steel body; DNV-type-approved | Spring-loaded; type-approved for marine use | Spring-loaded; position indicator recommended for accessible locations |
| Remote-operated (ROVSV) | Required on fuel oil tanks at hull penetration and at machinery space boundaries | Required at fuel transfer pump inlets if > 500 L tank | Not mandatory |
| Body material | Carbon steel, LCS, SS — no grey cast iron | DI, LCS, SS, CS — grey cast iron only ≤ 25 bar, ≤ 150°C | GCI permitted ≤ 10 bar, ≤ 120°C |
Fittings and Connections
- Threaded connections (NPT/BSP): permitted only in Class III, and in Class II up to DN50 and 16 bar; prohibited in Class I above DN15 (seal-weld required for instrument taps in Class I)
- Compression fittings: permitted for instrument impulse lines ≤ DN12 in Class II/III; Class I requires tube fittings rated for the service pressure with DNV acceptance
- Socket-weld fittings: permitted in Class I up to DN50; preferred over threaded in fuel oil and hydraulic Class I applications
- Butt-weld (BW) fittings: required for Class I above DN50; all welds subject to non-destructive examination (NDE) per weld category
6. Flexible Hose Criteria
Flexible hoses and expansion joints are permitted in specific circumstances only. DNV-RU-SHIP Pt.2 Ch.4 imposes strict criteria:
Where Flexible Hoses Are Permitted
- Between fixed pipework and machinery subject to vibration (engines, pumps, compressors) — to absorb relative movement
- At temporary connections for loading/unloading or maintenance access
- Where pipe misalignment at final connection is unavoidable
Maximum Length and Pressure Limits
| Piping Class | Max. Hose Length per Run | Max. Design Pressure | Approval Requirement |
|---|---|---|---|
| Class I | 0.5 m (connection piece only) | Per hose type approval, not to exceed system MAWP | DNV type approval mandatory; end fittings integrated, not field-assembled |
| Class II | 1.0 m | Up to 40 bar for hydraulic/fuel hose assemblies | Type approval required; field assembly of end fittings permitted if tested as assembly |
| Class III | Practical limitation; support at max. 1.5 m intervals | As rated; 7 bar max. for seawater cooling expansion joints | Marine-grade product; manufacturer datasheet sufficient for Class III |
Hose Installation Requirements
- Bending radius ≥ manufacturer's minimum bend radius (MBR), typically 6–10× outside diameter for wire-armoured fuel hose
- No kinking, no torsional loading — hoses must be installed in their natural plane
- Fuel oil hoses in machinery spaces must be shielded or have drip trays if above ignition sources
- Visual inspection interval: every 5 years or at each class renewal docking, whichever is sooner
- Replacement interval: 10 years maximum regardless of condition; 5 years for Class I fuel oil hoses
Common non-conformance: Using a multi-metre flexible hose run to route around obstacles instead of installing proper rigid pipework with bends. Only short connection pieces to absorb vibration are permitted. Long flexible runs are a survey finding in almost every machinery space inspection.
7. Hydrostatic and Leak Testing
All piping systems shall be pressure-tested before commissioning. The standard distinguishes between workshop/shop testing and onboard testing after installation.
Test Pressure Requirements
| Piping Class | Shop Test Pressure | Onboard Test Pressure | Hold Time |
|---|---|---|---|
| Class I | 1.5 × design pressure (min. 4 bar above design) | 1.5 × design pressure OR max. working pressure after pressure relief verified | 30 min minimum; no pressure drop permitted |
| Class II | 1.5 × design pressure | 1.5 × design pressure (can be combined with functional test) | 10 min minimum |
| Class III | 1.5 × design pressure (min. 4 bar) | Hydrostatic OR pneumatic leak test at design pressure; pneumatic only with DNV surveyor permission | 10 min minimum |
Testing Conditions and Exceptions
- Test medium: water (clean, treated where corrosion is a concern); pneumatic test only by specific approval
- Components not suitable for water testing (expansion joints, certain instruments) may be blanked off and tested separately or after installation
- Valves tested closed position: gate/globe valves tested with pressure applied in both directions; check valves tested in reverse direction for seat tightness
- After onboard installation: where full hydrostatic test is not practical (e.g. long seawater systems), visual leak test at design pressure during initial sea trial is accepted for Class III
- All test pressures and results shall be recorded by a surveyor or by a DNV-approved QA procedure with documentation available for class review
8. Joining Methods and Welding
Acceptable joining methods depend on piping class and size:
| Method | Class I | Class II | Class III |
|---|---|---|---|
| Butt weld (BW) | Required for DN > 50; NDE mandatory (radiography or TOFD per weld category) | Preferred for DN > 50; NDE per QA plan | Permitted; NDE not mandatory unless system flagged |
| Socket weld (SW) | Permitted DN ≤ 50; gap 1.5 mm before welding | Permitted DN ≤ 80 | Permitted all sizes if adequate weld access |
| Threaded (NPT/BSP) | Prohibited; seal-weld all instrument taps | Permitted DN ≤ 50, ≤ 16 bar | Permitted all sizes within Class III limits |
| Flanged | Permitted all sizes; raised-face preferred; RF/RTJ for Class I steam | Permitted all sizes | Permitted all sizes |
| Compression / tube fitting | Only for instrument impulse lines ≤ DN12 with DNV-type-approved fittings | DN ≤ 12, instrument lines | Permitted for instrument lines |
| Brazing | Not permitted | CuNi piping only, in non-flammable services | CuNi piping only |
Weld Procedure Qualification
- All welding on Class I and Class II piping shall be performed by qualified welders with valid WPQ (welder performance qualification) for the specific process and position
- Welding procedures shall be qualified by WPS/WPQR in accordance with ISO 15614-1 (or ASME IX for projects on ASME code basis)
- Post-weld heat treatment (PWHT): required for carbon and low-alloy steels above the thickness and composition thresholds in the applicable standard; mandatory for P11/P22 alloy steels in steam Class I service
9. Common Pitfalls
- Classifying a fuel oil low-pressure transfer line as Class III because the pressure is <7 bar — fuel oil is flammable so the minimum is Class II regardless of pressure
- Forgetting to add manufacturing tolerance (12.5%) when ordering seamless pipe — ordering treq results in under-thickness pipe after the mill tolerance is applied
- Using grey cast iron valve bodies in Class I steam piping — prohibited regardless of pressure/temperature; replace with ductile iron or steel
- Installing flexible hose runs longer than 0.5 m in Class I fuel systems to route around obstacles — only short vibration-absorbing pieces are acceptable
- Threaded NPT joints in Class I hydraulic piping above DN15 — only socket-weld or butt-weld permitted; NPT in Class I is a direct non-conformance
- Using ASME #150 flanges on a 19-bar Class I system because "it's rated to 19.6 bar" — the #150 P-T table must be checked at the actual operating temperature, not just ambient; at 250°C the allowable drops significantly
- Blanking off expansion joints during hydrostatic test and forgetting to test them separately — the test record must show all components were verified
- Documenting all material certifications to EN 10204 3.1 for Class I pipework at procurement stage — makes class review surveys straightforward and avoids shutdown-forcing material traceability disputes
- Running weld NDE at fabrication stage rather than after installation — access for radiography is far easier in the workshop and non-conformances can be repaired before the system is insulated
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