Overview
Marine air compressors are equipped with several safety devices to ensure optimal performance and protect the machinery from damage.
Key devices include intercoolers, aftercoolers, bursting discs, fusible plugs, relief valves, and unloader systems. Learn about their functions and how to maintain these critical components.
- Intercooler Air Temperatures: Inlet air is 130°C, and outlet air is 35°C.
- Aftercooler Air Temperatures: Inlet air is 130°C, and outlet air is 35°C.
- Intercooler: Single-pass type; Aftercooler: Double-pass U-tube type.
1. Low Lubricating Oil Pressure Cut-Out
This safety feature activates when lubricating oil pressure drops below a set threshold, preventing damage to components like the liner, piston, and bearings.
If oil pressure falls, the system shuts down to avoid wear on these critical parts.
2. Bursting Disc on Intercooler (Water-Side)
Each intercooler and aftercooler shell is equipped with a bursting disc to prevent over-pressurization.
When a high-pressure air tube bursts, the disc releases the pressure, preventing damage. Made from copper, this disc needs regular annealing to maintain correct relief pressure.
What to Do If a Bursting Disc Bursts During Manoeuvring:
- Set the compressor to manual mode.
- Ensure a watchkeeper or oiler is present.
- Close water valves before starting the compressor, then reopen them after starting.
- Monitor the compressor expansion tank and refill as needed.
3. Bursting Disc and Fusible Plug (121°C) on Aftercooler
Fusible plugs, typically made of tin, antimony, and bismuth, melt at 121°C to release excess pressure and prevent overheating.
When temperatures exceed this threshold, the metal melts, relieving pressure and protecting the system from damage.
4. Non-Return Valve in Delivery Line
A non-return valve prevents air from flowing back into the compressor if low pressure develops. This ensures consistent air flow and protects the compressor from backflow.
5. Discharge Unloader (Automatic Moisture Drain Valve)
The discharge unloader facilitates unloaded starting and stopping of the compressor, while also draining accumulated water and oil. It’s crucial for maintaining system efficiency and preventing moisture buildup.
6. Relief Valves on LP/HP Stages and Crankcase LO Pump
These relief valves are set to lift at 10% above normal stage pressure. They release excess air if high pressure is generated, usually due to valve malfunction or a choked intercooler. Regular maintenance of these valves is essential for preventing compressor failure.
Maintenance Tips:
- Fully slacken the spindle.
- Inspect and clean the disc.
- Replace O-rings and clean the seating area.
- Reset the valve to lift at 10% above maximum working pressure.
7. Delivery Air High Temperature Cut-Out and Alarm (93°C)
At 93°C, an alarm signals high air temperature at the aftercooler outlet. If the temperature exceeds this, the system will cut out, preventing damage from overheating.
8. Cooling Water Supply Failure Alarm and Trip
If the cooling water supply fails, the compressor may overheat. This system trips the compressor to prevent seizing due to insufficient cooling.
9. Water High-Temperature Trip
A high water temperature trip is activated if intercoolers become choked or water flow is reduced. This prevents the compressor from overheating by cutting off the system when necessary.
10. Motor Overload Trip
If the unloader isn’t open when the compressor starts, or if mechanical issues arise, the current may spike, causing motor damage. The motor overload trip protects the motor by cutting power in case of excessive current draw.
Conclusion
Regular maintenance of safety devices like bursting discs, relief valves, and fusible plugs is essential to prevent marine air compressor failure. Understanding how each component works and taking the necessary precautions will ensure smooth operation and prolong the lifespan of your equipment.
