ISO 28520:2009 – Ships and Marine Technology – Lubricating Oil Systems Cleanliness and Flushing

1. Introduction to ISO 28520 and Marine Lubricating Oil System Cleanliness

ISO 28520:2009 provides comprehensive guidance for flushing of lubricating oil systems in ships and marine installations, and for grading the resultant cleanliness. The standard addresses a critical engineering challenge: during the construction and installation of marine piping systems, contaminants including welding slag, oxide scale, sand, and machining debris inevitably accumulate. If not removed before commissioning, these contaminants cause catastrophic bearing failure, scored cylinder liners, and turbocharger damage. The standard distinguishes between cleaning processes based on the Reynolds number — washing through (Re < 3,000) versus flushing (Re ≥ 3,000), with turbulent flow being essential for effective contaminant removal.

The Reynolds number threshold of 3,000 is a key engineering parameter in this standard. Below this value, flow remains laminar with insufficient shear force to dislodge adhered particles. Achieving turbulent flow (Re ≥ 3,000) requires careful balancing of pipe diameter, flow velocity, and oil viscosity — a classic fluid dynamics optimization problem.

2. Pipe Cleaning Degrees and Surface Treatment

The standard specifies recommended cleaning levels for prefabricated pipes before installation:

Cleaning Stage	Method	Requirement	Verification
After welding	Chemical acid-cleaning or mechanical blast-cleaning	Sa 2½ per ISO 8501-1	Visual inspection against standards
Blast medium	Copper (Cu) slag — NOT steel sand	Prevents magnetic adhesion and rusting	Material certification
Sealing surfaces	Mechanical protection during blasting	No damage to joint faces	Visual inspection
Post-cleaning treatment	Appropriate oil product application	Maintains cleanliness until installation	Surface condition check

The prohibition of steel sand for shot blasting is a critical detail often overlooked in practice. Steel particles can become magnetically adhered to pipe walls, later detaching during operation to cause rapid bearing wear. Copper slag is the recommended alternative, being non-magnetic and providing equivalent cleaning performance.

3. Component Cleaning and Flushing Procedures

The standard provides detailed procedures for cleaning each system component before the main flushing operation:

3.1 Engine Crankcase Cleaning

For large marine engines, the crankcase must be cleaned before flushing. The procedure involves washing down with oil at regular intervals while turning the crankshaft, using thin non-drying oil (SAE 10-30 cSt) or heated system oil (SAE 30-90 cSt at 55-60°C). Approximately 200-400 litres per cylinder are used. Crucially, the flash point of the cleaning oil must be monitored for fire safety. After cleaning, the crankcase is wiped with lint-free cloths and inspected before flushing commences.

3.2 Gearbox Cleanliness

Gearboxes are categorized as sealed or non-sealed, with different procedures for each. The standard includes a quantitative cleanliness classification system (Table 1) linking gear type to required ISO 4406 cleanliness codes:

Gear Type	Application Example	Cleanliness After Flushing (ISO 4406)	Max Service Contamination (ISO 4406)
Type 1	High-speed ferries, gas turbines, high-load gears with PTO	16/14/11	18/16/13
Type 2	Trawlers, coupled main engines, integrated CPP systems	17/15/12	19/17/14
Type 3	Separated gear and CPP control systems	18/16/13	20/18/15
Type 4	Low rpm/load, low service time	21/19/16	23/21/19

The cleanliness classification system enables engineers to specify appropriate filtration and flushing targets based on the criticality of the application. A Type 1 gearbox (high-speed ferry) requires much tighter contamination control than Type 4 (low-speed auxiliary), with commensurate differences in filter specification and flushing duration.

4. Engineering Design Insights for Effective Flushing

Several engineering principles underpin successful flushing operations:

Flow Velocity and Reynolds Number: The standard provides worked examples showing that for a 200 mm pipe with 80 cSt oil, a velocity of 2.5 m/s yields Re = 6,250 (turbulent). However, with 85 cSt oil in a 100 mm pipe at 1.8 m/s, Re = 2,118 (laminar). This demonstrates that pipe diameter, oil viscosity, and flow velocity must be carefully matched. Using two parallel pumps can nearly double flow velocity.

Temperature Management: Heating flush oil to 55-60°C reduces viscosity and enhances contaminant removal. The standard explicitly warns against exceeding 90°C to avoid oil oxidation. For cold-weather operations, supplementary heating may be necessary.

Two-Stage Flushing: The standard prescribes a first flushing session (external piping, with blanked-off bearings) followed by a second session (with retainers and flushing bags installed). Filter bag mesh size of 47-50 μm is specified for the concluding verification phase.

The relationship between contaminant particle size, hardness, and bearing damage is quantitatively addressed in the standard. Particles harder than approximately 800 HV (Vickers hardness) and larger than the oil film thickness (~0.4 μm) are directly harmful to bearings. This means that even sub-micron particles of hard materials (silica, wear debris) must be filtered out for bearing protection.

5. Frequently Asked Questions

Q1: When do OEM flushing requirements take precedence over ISO 28520?
A: The standard explicitly states that original equipment manufacturer requirements for flushing take precedence. ISO 28520 serves as the default guidance when OEM specifications are not available.

Q2: How is flushing approval obtained?
A: Approval should be done in concert with representatives of the shipyard/plant, the owner, and the equipment manufacturer. Oil samples are taken and analyzed to verify that the required ISO 4406 cleanliness code has been achieved.

Q3: What is the recommended approach for flushing turbines and turbo generators?
A: Flushing practice for turbines and turbo generators is not addressed in ISO 28520. The standard explicitly refers to manufacturer specifications for these systems.

Q4: How does ISO 28520 relate to ISO 28521?
A: ISO 28520 covers lubricating oil systems; ISO 28521 covers hydraulic oil systems. They share common principles (Reynolds number, ISO 4406 cleanliness coding, flushing methodology) but differ in the specific requirements for each system type.