Main engine safety devices are essential for ensuring the safe operation of marine engines, protecting both the engine and the ship from potential damage or failure. These devices help prevent catastrophic accidents by monitoring critical parameters and automatically taking corrective actions. Below are some of the main engine safety devices used in marine diesel engines, along with detailed explanations:
1. Crankcase Explosion Relief Valve
- Purpose: To prevent explosions in the crankcase due to the ignition of oil mist, which can form from oil vaporizing and mixing with air.
- Working: It is fitted on the crankcase doors and opens to release pressure in case of an explosion, preventing damage to the crankcase and engine components.
- Mechanism: Contains a spring-loaded valve that opens when pressure inside the crankcase exceeds a certain limit. It also has a flame arrestor to prevent the external spread of fire.
2. Oil Mist Detector (OMD)
- Purpose: To detect the presence of oil mist inside the crankcase, which could lead to an explosion if ignited.
- Working: The device continuously samples the crankcase air for oil mist. If the concentration exceeds safe levels, it triggers an alarm and may automatically stop the engine to prevent an explosion.
3. Main Bearing Temperature Sensors
- Purpose: To monitor the temperature of the main bearings.
- Working: Thermocouples or Resistance Temperature Detectors (RTDs) are placed near the bearings to detect any abnormal rise in temperature, which may indicate a lubrication failure or overloading. If the temperature crosses a preset threshold, alarms are triggered, and the engine may shut down to prevent damage.
4. Low Lube Oil Pressure Alarm and Trip
- Purpose: To protect the engine from damage due to insufficient lubrication.
- Working: Pressure sensors monitor the lube oil pressure. If the pressure drops below a safe level, an alarm is activated. If it continues to drop, the system will automatically shut down the engine to avoid damage due to metal-to-metal contact between moving parts.
5. Cooling Water Temperature Alarm
- Purpose: To monitor the temperature of the engine cooling water.
- Working: Temperature sensors are placed in the cooling water circuit. If the water temperature rises above the safe operating range, alarms will sound. Continuous high temperature can lead to overheating and damage, so in critical cases, the engine might be tripped.
6. Overspeed Trip
- Purpose: To prevent the engine from running at an unsafe speed, which can lead to mechanical failure.
- Working: The overspeed trip device is linked to the engine governor. If the engine exceeds its rated speed, the device automatically shuts down the fuel supply to stop the engine.
7. Air Starting Safety Valve
- Purpose: To prevent backflow of combustion gases into the air starting system, which can cause an explosion.
- Working: The valve allows compressed air to enter the cylinders during starting but prevents any gases from flowing back into the starting air line by closing once the engine starts.
8. High Exhaust Gas Temperature Alarm
- Purpose: To monitor the exhaust gas temperature of each cylinder and the overall engine.
- Working: Thermocouples placed in the exhaust system measure the temperature of gases exiting each cylinder. A high temperature could indicate improper combustion or an overloaded engine, triggering an alarm.
9. Engine Shutdown System
- Purpose: To shut down the engine automatically in case of critical failures.
- Working: This system is designed to stop the engine in case any critical parameter (like overspeed, low lube oil pressure, or high crankcase pressure) reaches dangerous levels. This prevents engine damage or accidents due to the continuation of unsafe operation.
10. Scavenge Fire Detector
- Purpose: To detect fires in the scavenge spaces of a two-stroke engine.
- Working: Temperature sensors are fitted in the scavenge space. If a fire is detected due to fuel or oil accumulation igniting, the system raises an alarm, and corrective action can be taken, such as shutting down the engine or activating the fire-fighting system.
These safety devices work together to ensure the reliable and safe operation of the main engine, preventing accidents, reducing downtime, and minimizing damage to the engine components. Regular maintenance and testing of these devices are critical to ensure their proper functioning during emergencies.