What are the Safety Devices on the Refrigeration System of a Ship?

A reefer system is the back bone of ships carrying refrigerated cargo. A malfunction of any of the components of the system can lead to degradation and wastage of perishable and cold storage cargo including provision for a ship. It is therefore important to maintain and run the refrigeration plant properly to avoid any breakdowns.

In this article we will learn about the safety feature of the refrigeration plant. Before that, you must read the construction and working of the ship’s refrigeration plant.

To have an efficient and hassle free operation of a ship’s refrigeration plant and its components, a good maintenance schedule and safety systems are adopted.

A safety system includes alarm, cut offs, and trips which safeguards the machinery and its parts from getting damage.

The main safeties adopted for refrigeration plants are

  • Low Pressure or LP cut off: This is a compressor safety which cut off the compressor in the event of pressure drop in the suction line. The pressure of the suction line is continuously sensed by the control unit and when it goes below the set value, which means the room is properly cooled, the LP cut out will auto trip the compressor. When the pressure rises, indicating there is flow of refrigerant in the line due to increase in room temperature, the LP switch will start the compressor.

Refrigeration System of a Ship

  • High pressure or HP cut out: As the name suggests, the high pressure cut out activates and trips the compressor when the discharge side pressure increases above the limit value. The HP cut out is not auto reset and has to be done manually. The reason behind it is to manually attend the fault which is leading to rise in pressure, else this situation can lead to overloading of compressor parts and may damage the same


  • Oil differential cut out: This safety is again for compressor as it is the only machinery in the circuit having rotational parts which requires continuous lubrication. In the event of low supply or no supply of lube oil to the bearing, the differential pressure will increase and activates a trip signal to safeguard the bearing and crankshaft.
  • Relief valves: Relief valves are fitted in discharge side of compressor and will lift and safeguard the compressor in the event of over pressure. One relief valve is also fitted in the condenser refrigerant line to avoid damage to the condenser if there is high pressure in the discharge line.
  • Solenoid valves: Master solenoid valve is fitted in the common or main line after the condenser discharge. It closes when compressor stops or trips to avoid over flow of refrigerant in to evaporator. All holds or rooms are fitted with individual solenoid valve which control the flow of refrigerant to that room.
  • Oil heater: Oil heater is provided for the compressor crank case oil and prevents compressor from getting excessively cold which may effect the lubrication of the parts.

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References: Marine Aux Machinery by H.D.McGeorge

Image Credits: acr-news, maritime

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  1. what are the steps taken when a ship is entering a region known for its cold climatic conditions??

  2. How the pressure of the refrigerant suddenly decreases to 2-3 bar from 12-13 bar after entering the expansion valve?

  3. A valve pin is provided in the thermostatic valve which restricts the flow of the refrigerant leading to drop in pressure.

  4. @ Shrikant:

    – There is no O2 available inside the crankcase of reefer plant which will lead to explosion
    – Due to low temp of incoming refrigerant, emerging of hot spot chances are less
    – Compressor is taking suction from the crank case. If relief valve is fitted and malfunctioning, it will draw air in the system

  5. Is solenoid valve controlling the flow to each room or Expansion valve? Master solenoid valve will shut the flow during trip. What Individual solenoid valve does?

  6. hy people.refrigerant compressor dont stop and run continiously.set point is 16 and not stoping.plz help.what is a reazon to rizing tempriture?

  7. Y haven’t u told about oil separator which is fitted just after compressor and wt about defrosting system…….I think those are safely features too…..

  8. Compland ac plant has fitted one lp pressure switch and one HP pressure switch (which have dual pressure switch and right side switch have manual reset function
    CanAny one help , what is the function of dual pressure switch function in this plant.

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