Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO 28521:2009 – Ships and Marine Technology – Hydraulic Oil Systems Cleanliness and Flushing
Comprehensive guidance for pipe cleaning, flushing, and cleanliness verification in marine hydraulic systems
1. Introduction to ISO 28521 and Hydraulic System Cleanliness
ISO 28521:2009 specifies pipe cleaning and cleanliness levels for hydraulic oil systems in ships and marine technology. Hydraulic systems are particularly sensitive to contamination because their tight clearances (typically 5-50 μm in servo valves and proportional valves) are easily blocked by particles. The standard provides a comprehensive methodology covering everything from prefabrication pipe cleaning to final system start-up, with the goal of ensuring trouble-free operation of marine hydraulic systems including steering gear, stabilizers, cranes, winches, and propeller pitch control systems.
Modern marine hydraulic systems operate at pressures exceeding 160 bar with servo valve clearances measured in microns. A single grain of sand (typically 100-500 μm) circulating through such a system can cause catastrophic valve failure. This is why ISO 28521 specifies cleanliness levels down to ISO 4406 code 15/13/10 for the most critical applications.
2. Pipe Cleaning Levels and Surface Treatment
The standard distinguishes between two categories of pipes based on their manufacturing process:
| Pipe Category | Description | Cleaning Method | Cleanliness Target |
|---|---|---|---|
| Black-steel pipes | Oxide scale from welding/heating | Chemical pickling or mechanical blow cleaning | Sa 2½ per ISO 8501-1 |
| Precision-steel pipes | Delivered free of oxide scale | Chemical cleaning, pressurized air, or lint-free cloth pull-through | No residual contaminants |
For black-steel pipes, steel shot must not be used due to the risk of magnetic adhesion and subsequent rust seizure. Copper (Cu) slag is the recommended blast medium. Sealing faces must be mechanically protected during blow cleaning operations.
3. Filter Selection and Sizing Methodology
The standard provides an engineering methodology for selecting flushing filters based on system parameters:
3.1 Filter Capacity Calculation
The filter flow capacity Q1 is calculated as Q1 = Q2 × K1, where Q2 is the system flow requirement and K1 is a factor typically between 2.5 and 3.5 (higher values for high-pressure systems). Q2 itself is derived from the required flow velocity W and pipe cross-sectional area A: Q2 = W × A.
The required flow velocity W is determined from the Reynolds number: W = (Re / 1000) × (v / d), where v is kinematic viscosity and d is pipe diameter. By fixing Re at 3,000 (the threshold for turbulent flow), the necessary velocity and therefore pump capacity can be calculated for any pipe size and oil viscosity.
3.2 ISO 4406 Cleanliness Targets by Application
| System Application | Pressure | Flushing Unit Target (ISO 4406) | Delivery After Test Run | Max In-Service | Filter Requirement |
|---|---|---|---|---|---|
| Stabilizers with servo valves | >160 bar | 15/13/10 | 16/14/11 | 18/16/13 | 3-5 μm |
| Steering gear, variable pumps | >160 bar | 15/13/10 | 16/14/11 | 18/16/13 | 3-5 μm |
| Gantry cranes, proportional valves | >160 bar | 16/14/11 | 17/15/12 | 20/17/14 | 5-10 μm |
| Bow/stern thrusters | <160 bar | 18/16/13 | 19/17/14 | 21/18/15 | 10-20 μm |
| Low-pressure winches | <160 bar | 18/16/13 | 21/17/14 | 22/19/16 | 10-20 μm |
The standard provides a practical rule: find the system component with the finest cleanliness requirement. For example, if a proportional valve requires 18/16/13, the service target becomes 17/15/12, and the flushing unit target becomes 16/14/11. This three-step margin ensures the final system cleanliness meets the most sensitive component’s requirements.
4. Flushing Procedures and Cleanliness Verification
The standard details a complete flushing methodology with practical engineering guidance:
4.1 Special Flushing Techniques
For small-diameter pipes where achieving turbulent flow is difficult due to high pressure drop, the standard describes a specialized gas/oil pulsation technique. A hydraulic accumulator and pulsation valve send pressure/flow pulsations through the pipes, creating intermittent turbulent flow even when continuous turbulent flow is impractical. This represents an elegant solution to a challenging fluid dynamics problem.
4.2 Cleanliness Verification Methods
Four methods are described for verifying cleanliness levels, ranging from simple pressure drop monitoring across filters (Method 1) to laboratory particle counting analysis (Method 4). The standard recommends Method 4 as the most reliable for final verification, supplemented by Methods 1 and 3 for in-process monitoring.
4.3 Flushing Oil Selection
Either system oil or special flushing oil can be used. Special flushing oils have cleaning effects on sticky particles without attacking sealing materials. The standard recommends a viscosity of approximately 15 cSt at 40°C for flushing oils. If system oil is used, it should be heated to reduce viscosity and enhance cleaning. However, the oil temperature must not exceed 60°C (or 80°C maximum) to prevent oxidation.
If flushing oil becomes dark or blurred during flushing, chemical analysis must be performed. The oil is considered unfit if viscosity changes by more than ±15%, water content exceeds 0.05% mass fraction, or the neutralization number exceeds 1.2 mg KOH/g. Continuing to flush with degraded oil can introduce contaminants rather than removing them.
5. Frequently Asked Questions
Q1: Can the permanent system filters be used for flushing?
A: No. Permanent filters should not be used for cleaning flushing oil. Special flushing filters with increased capacity should be used together with the portable pump station. Using permanent filters risks damage and reduces flushing efficiency.
A: No. Permanent filters should not be used for cleaning flushing oil. Special flushing filters with increased capacity should be used together with the portable pump station. Using permanent filters risks damage and reduces flushing efficiency.
Q2: How is the required flushing time determined?
A: Flushing continues until the filter element no longer shows an increasing pressure drop. The time depends on filter efficiency, flow velocity, temperature, and initial contamination level. Higher efficiency filters (higher x value) reduce flushing time but cost more — the standard notes that the extra cost is outweighed by time savings.
A: Flushing continues until the filter element no longer shows an increasing pressure drop. The time depends on filter efficiency, flow velocity, temperature, and initial contamination level. Higher efficiency filters (higher x value) reduce flushing time but cost more — the standard notes that the extra cost is outweighed by time savings.
Q3: What happens to the flushing oil after flushing?
A: Flushing oil can be reused after cleaning if analysis shows it still complies with additive specifications, viscosity requirements, and water content limits. Otherwise, it must be disposed of per environmental regulations.
A: Flushing oil can be reused after cleaning if analysis shows it still complies with additive specifications, viscosity requirements, and water content limits. Otherwise, it must be disposed of per environmental regulations.
Q4: How do I handle dead areas in hydraulic systems during flushing?
A: Dead areas must be avoided during the design phase by considering flushing grouping and circuit design. Circuits should always be connected in series, and relevant coupling options should be evaluated to ensure uniform pipe diameters and avoid excessive pressure losses.
A: Dead areas must be avoided during the design phase by considering flushing grouping and circuit design. Circuits should always be connected in series, and relevant coupling options should be evaluated to ensure uniform pipe diameters and avoid excessive pressure losses.
