Safety
Starting Air Compressors, Receivers & Air Bottle Safety
Two compressors, the 15-to-25 bar in 30 minutes rule, the 12/6-start capacity and the mountings that keep an air bottle safe.
Key Principles at a Glance 6 points
- Two starting air compressors must be fitted, of sufficient total capacity to meet the engine requirements.
- Each compressor must be able to press up the air receiver from 15 bar to 25 bar in 30 minutes.
- Two air receivers must be provided, with total capacity sufficient for twelve (12) starts of reversible engines and six (6) starts of non-reversible engines.
- Air bottle mountings include the fusible plug, relief valve, spring-loaded safety valve, compensation ring, manual or automatic drain valve, pressure gauge, manhole, main starting air valve, auxiliary starting air valve, filling valve and general service line.
- Safety on the air bottle is provided by the fusible plug, pressure relief valve, low air pressure alarm and drain valve.
- A fusible plug melts at a set temperature to release pressure if the bottle is exposed to fire, while the relief/safety valve prevents over-pressure during filling — both are essential statutory safety devices.
1. Starting Air Compressors & Capacity
- Two starting compressors must be fitted, of sufficient total capacity to meet the engine requirements.
- Each compressor must be able to press up the air receiver from 15 bars to 25 bars in 30 minutes.
The Number to Memorise
15 bar → 25 bar in 30 minutes by each compressor. The rule guarantees that, even with one compressor out of action, the starting air can be replenished quickly enough to keep the main engine startable.
2. Air Receivers & Starting Capacity
- Two air receivers must be provided.
- The total air receiver capacity is to be sufficient for twelve (12) starts of reversible engines and six (6) starts of non-reversible engines.
| Engine Type | Minimum Starts from Full Receivers |
|---|---|
| Reversible engines | 12 starts |
| Non-reversible engines | 6 starts |
Why Two Receivers?
- Two receivers give redundancy — if one is being drained, repaired or isolated, the other still supplies the starting air.
- They allow the total capacity (12/6 starts) to be split between two smaller vessels, which is more practical to install and inspect.
- Isolating valves let each receiver be used independently or together.
Reversible vs Non-Reversible
A reversible engine needs 12 starts (it must be capable of starting in either direction, e.g. for manoeuvring), while a non-reversible engine needs 6 starts. Remember this pair: 12 and 6.
3. Mounting on the Air Bottle
An air bottle/receiver is fitted with the following mountings and fittings:
- Fusible plug — melts at a set temperature to release pressure if the bottle is exposed to fire.
- Relief valve.
- Spring-loaded safety valve — prevents over-pressure.
- Compensation ring — around penetrations to compensate for the hole in the shell.
- Manual or automatic drain valve — removes accumulated oil and water.
- Pressure gauge — indicates receiver pressure.
- Manhole — for internal inspection and cleaning.
- Main starting air valve, auxiliary starting air valve, filling valve and general service line.
Mounting vs Safety
The mountings are all the fittings on the bottle (gauge, manhole, valves, compensation ring, drain). The safety devices are specifically the fusible plug, pressure relief valve, low air pressure alarm and drain valve. Keep the two lists distinct in an oral answer.
4. Safety on the Air Bottle
Safety on the air bottle is provided by:
- Fusible plug — melts at a set temperature to relieve pressure if the bottle is involved in a fire.
- Pressure relief valve — relieves over-pressure and protects the receiver from being over-pressurised.
- Low air pressure alarm — warns the bridge/engine room when starting air pressure falls below the safe value.
- Drain valve — allows accumulated oil and water (condensate) to be drained, preventing internal corrosion and carry-over to the engine.
Why These Safety Devices Matter:
- A high-pressure air receiver stores a large amount of energy; over-pressure or fire can cause catastrophic failure.
- The fusible plug protects against fire (it melts before the shell weakens), while the relief valve protects against over-pressure.
- The low air pressure alarm ensures the engineers know when starting capability is compromised.
- Regular draining of oil and water protects the bottle and the starting air system from corrosion and contamination.
Surveyor Asks
Surveyors commonly ask: "What is the regulation for starting air compressors / air bottles?" Answer in three parts:
- Two compressors, each pressing up 15 → 25 bar in 30 minutes.
- Two receivers with total capacity for 12 (reversible) / 6 (non-reversible) starts.
- Safety devices — fusible plug, relief valve, low air pressure alarm and drain valve.