ISO 28522:2009 — Ships and Marine Hydraulic Oil Systems: Guidance for Assembly and Flushing

Introduction to ISO 28522

ISO 28522:2009, developed by ISO/TC 8 (Ships and marine technology), provides essential guidance for the assembly and flushing of hydraulic oil systems aboard ships. The standard addresses two critical objectives: removing debris introduced during erection and installation, and verifying that pipes and the complete system achieve sufficient cleanliness before commissioning. Hydraulic systems on modern vessels power critical functions including steering gear, winches, hatch covers, and stabilizers, making contamination control a matter of operational safety.

Assembly Guidelines and Preparation

Before assembly begins, all blank-offs on pipes and components must be inspected for damage. If any blank-off is compromised, the cleaning procedure must be repeated. After the pipe system is assembled, it must be blown through with dry high-pressure air or nitrogen to remove loose debris. Lint-free cloths may be pulled through pipes before connecting them to components. These seemingly simple steps are critical — even small amounts of welding slag, machining chips, or dust left in the piping can destroy精密 hydraulic components within minutes of system start-up.

Shock Testing

The pipe system must undergo a minimum of 25 shock test cycles at the specified test pressure while components are bypassed. Pump stations and pre-assembled units that have not been pre-tested for cleanliness must be flushed separately unless delivered in a certified clean condition. Shock testing validates the mechanical integrity of welds, flanges, and pipe supports under transient pressure conditions that simulate real operating stresses.

Flushing Procedures and Contamination Control

Flushing is the most critical phase of hydraulic system commissioning. The flush-rig must be equipped with a filter having the correct beta ratio, defined as the ratio of particles upstream to particles downstream for a given particle size x (in micrometres). A beta ratio of 75 or higher at the required micron rating is typically specified for critical marine systems. Circuits are always connected in series to avoid large pressure losses that would compromise flow velocity and cleaning effectiveness.

Key flushing parameters include: flow velocity per ISO 28521 Table 2, oil temperature at least matching expected service temperature (not exceeding 60 °C, or 80 °C if the oil is disposed of after flushing), and maintaining the flushing procedure until the cleanliness targets in ISO 28521 Table 1 are achieved. The use of a vibrating unit during flushing can help dislodge stubborn particles adhering to pipe walls. Flush oil should be checked periodically for particle count trending.

Engineering Design Insights

From a design perspective, the standard reveals several practical engineering truths. First, the beta ratio specification highlights that filter selection must be matched to system sensitivity — finer filtration increases pressure drop, so there is always a trade-off between cleanliness and flow performance. Second, the requirement to bypass critical components during flushing protects delicate servo-valves and pumps from flushing debris, but adds complexity to the flushing circuit design. Third, the commissioning sequence — replacing non-system oil, verifying alignment, adjusting pressure regulators — mirrors best practices across all fluid power systems, not just marine applications.

Commissioning Sequence

When commissioning: (a) replace non-system flush oil with designated system oil using a filter, (b) verify alignment and oil levels, (c) start pump units, (d) adjust pressure regulating and relief valves, (e) invite the system supplier representative to acknowledge cleanliness, and (f) begin the system performance test per ISO 28521. Documentation of achieved cleanliness levels should be maintained as part of the vessel’s quality records.

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