Answer
For an MEO Class 1 Orals examination, your answer must sharply distinguish between structural/habitability vibration (hull, accommodation, workspaces) and machinery-specific vibration (auxiliaries, pumps, reciprocating engines). Assessors look for exact ISO standard references, the correct units of measurement (mm/s RMS velocity), and how classification societies (like IRS, DNV, LR) enforce these limits.
The acceptable limits are governed under two distinct standard frameworks.
1. Hull & Structural Vibration Standards (Habitability)
The primary standard used by classification societies for structural vibration, human comfort, and habitability on merchant ships is ISO 20283-5 (which replaced the older ISO 6954 standard).
Frequency Range: Measured over a range of 1 to 80 Hz.
Measurement Metric: Expressed as overall frequency-weighted Root Mean Square (RMS) velocity in mm/s.
Application: Applied during sea trials under "free route" conditions (steady-state speed and course).
Classification societies classify maximum acceptable levels based on space usage:
Maximum Structural & Habitable Vibration Limits (ISO 20283-5)
| Space Category | Specific Areas | Maximum Acceptable Velocity (mm/s RMS) |
|---|---|---|
| Crew Passenger Accommodation | Sleeping cabins, medical spaces, hospitals, offices | 3.5 mm/s |
| Recreation & Mess Rooms | Crew mess, lounges, recreation spaces | 4.5 mm/s |
| Work Spaces | Workshops, galleys, laundries | 5.5 mm/s |
| Navigation Spaces | Wheelhouse, bridge wings, chart room, radio room | 5.0 mm/s |
| Machinery Spaces | Engine Room, steering gear flat, purifier room | 5.5 to 6.5 mm/s |
Note for Orals: Exceeding these values in the accommodation or bridge indicates localized structural resonance or serious propeller/hull interaction. If paint is flaking or welds are cracking on localized bulkheads, the structure is experiencing high dynamic stress.
2. Marine Machinery Vibration Standards
For rotating and reciprocating auxiliary machinery, classification societies look to the ISO 10816 series (specifically Part 3 for industrial/auxiliary machines and Part 6 for reciprocating machines) and the updated ISO 20816 series.
Vibration is assessed on non-rotating parts (such as bearing housings) and categorized into Severity Zones:
Zone A: New, commissioned machinery condition (Excellent).
Zone B: Satisfactory for long-term continuous operation.
Zone C: Marginal/Unsatisfactory. Represents a warning or Alarm state; corrective action must be planned.
Zone D: Critical. Represents a Trip or immediate shutdown state; high risk of catastrophic failure.
ISO 10816 Machine Classification & Limits
Auxiliary machinery on board is split into classes based on power output and type of foundation:
| Machine Class | Description (Onboard Examples) | Satisfactory Limit (Zone B - mm/s RMS) | Alarm Limit (Zone C - mm/s RMS) | Critical Limit (Zone D - mm/s RMS) |
|---|---|---|---|---|
| Class I | Small motors and pumps up to 15 kW (e.g., small domestic water pumps, accommodation fans) | Up to 1.8 | 1.8 to 4.5 | Above 4.5 |
| Class II | Medium machines, 15 kW to 75 kW, or up to 300 kW on rigid foundations (e.g., fuel oil transfer pumps, sea water cooling pumps) | Up to 2.8 | 2.8 to 7.1 | Above 7.1 |
| Class III | Large prime movers with rotating masses on rigid foundations (e.g., Main Air Compressors, large Turbochargers, Main Engine blower motors) | Up to 4.5 | 4.5 to 11.2 | Above 11.2 |
| Class IV | Large prime movers with rotating masses on flexible/soft foundations (e.g., Turbo-alternators, resiliently mounted Aux Engines) | Up to 7.1 | 7.1 to 18.0 | Above 18.0 |
3. Reciprocating Machinery (Auxiliary Engines & Compressors)
For diesel generators and reciprocating compressors, ISO 10816-6 applies. Because these machines have inherent structural unbalance due to pistons and connecting rods, their acceptable vibration thresholds are significantly higher.
Measurements are taken at the engine base, block, and bearing structures across three axes:
Displacement: Up to 180 μm (microns) Peak-to-Peak for low-speed engines.
Velocity: Typically, an auxiliary diesel generator engine block is considered Satisfactory up to 11 to 18 mm/s RMS depending on its size and mounting (rigid vs. resilient). Anything above 28 mm/s RMS is considered highly critical (Zone D).
Chief Engineer’s Checklist for the Surveyor
If a Classification Surveyor is called on board to verify a vibration complaint, you must have the following data ready:
Vibration Spectrum Report: Showing peak frequencies (1×,2× RPM) to prove whether it is an unbalance, misalignment, or structural resonance issue.
Calibration Certificate: Ensure the handheld vibration analyzer’s calibration is valid and approved by Class.
Trend Data: Evidence from the Planned Maintenance System (PMS) demonstrating when the baseline shifted from Zone A/B to Zone C.