Main Engine Safety Devices

Main Engine Safety Devices

Main Engine Safety devices

The main engine safety devices are provided in the system to safe guard the main engine
in case of any damage in the system Mainly are fitted in engine when all the alarm and trips fails to work then the last means for safe guarding the engine and components is these safety devices fitted in the main engine.

These are mainly of the type of pressure relief valve which release the excess pressure built in case of malfunctioning of the engine and no action is well in
advance to rectify or to avoid the situations.

Main Engine Safety DevicesMain Engine Alarms and TripsMain Engine Shut Down Trips Main Engine Interlocks
1. Crankcase Relief Door1. Over Speed Trip1. Low Lube Oil Pressure1. Turning Gear Engage Interlock
2. Scavenge Space Relief Door2. Lube Oil Low Pressure Trips2. JCW High Temp.2. Running Direction Interlock
3. Cylinder Head Relief Valve3. Camshaft Lube Oil Low Pressure Trip3. JCW Low Pressure3. Auxiliary Blower Off Interlock
4. Starting Air Relief Valve4. JCW low pressure trip4. Thrust Bearing High Temp.4. Starting Air Distributor In end Position
5. Starting Airline Flame Trap5. Piston Cooling Low Pressure Trip5. Over speed5. Air Spring Pressure Interlock
6. Oil Mist Detector6. Thrust Bearing High Temperature trip  
7. Rotation Direction Interlock7. Main Bearing High Temperature Trip  
8. Turning Gear Interlock8. Oil Mist Detector Alarm  
 9. Spring Air Low Pressure Trip  
 10. Manual Emergency Trip  

Here We have provided Description of Main Engine Safety Device and for others Main Engine Alarms and Trips, Shut Down Trips, and Interlocks Please visit the Given Link:


  • Crank case Relief Door
  • Cylinder head Relief valve
  • The Starting Air relief valve
  • Flame trap.
  • Oil mist detector.
  • Turning gear Interlock.
  • Running Direction Interlock

Explanation of main engine safety devices

1 .Explosion relief valve :-

The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber.

In common with
all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the
travel of the piston between its extreme points.

Each stroke is accomplished in half a revolution of the crankshaft.
As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase.

They must also be self closing to stop the return of atmospheric air to the crankcase.

Various designs and arrangements of these valves exist where, on large slow-speed
diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used.

One design of explosion relief valve is shown in Figure. A light spring holds the valve closed against its seat and a seal ring completes the joint.

A deflector is fitted on the outside of the engine to safeguard personnel from the
out flowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase. After operation the valve will close automatically under the action of the spring.

2. The cylinder relief valve:-

The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal.

A spring holds the valve closed and its lifting pressure is set by an appropriate thickness of packing piece .

Only a small amount of lift is permitted and the escaping gases are directed to a safe outlet. The valve and spindle are separate to enable the valve to correctly seat itself after opening.

The operation of this device indicates a fault in the engine which should be discovered and corrected. The valve itself should then be examined at the earliest

3.Relief valve :-

Its purpose is to relieve excess pressure in the air start

It consists of a spring loaded valve disk which locates on a mating seat which is bolted to the end of the air start manifold.

When the force exerted on the disk due to excessive pressure is
greater than the spring force holding the valve closed, the valve will open and release the excessive pressure.

4. Flame Arrestors :-

The flame trap is manufactured from brass or aluminium alloy which both have a high specific heat capacity.

A number of holes are bored through the thick circular form to allow the air to pass through.

They are fitted in the main air line immediately before the air start valve to restrict the risk of a
flame in the cylinder propagating back to the main air start manifold, by dissipating the heat energy in the flame.


The presence of an oil mist in the crankcase is the result of oil vaporization caused by a hot spot.

Explosive conditions can result if a buildup of oil mist is allowed.

The oil mist detector uses photoelectric cells to measure small increases in oil mist

A motor driven fan continuously draws samples of crankcase oil mist through a measuring tube.

An increased meter reading and alarm will result if any
crankcase sample contains excessive mist when compared to either clean air or the
other crankcase compartments.

The rotary valve which draws the sample then stops to indicate the suspect crankcase. The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air.

The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for cross head type engines and the level model for trunk piston engines.

6. The Turning gear interlock:

The turning gear interlock is a valve which will not allow
starting air to operate the system when the turning gear is

7 . Running direction interlock :-

Running direction interlock prevents engine to be started in reverse direction until all cam rollers of fuel pump have shifted.

Telegraph position interlock allows engine to be started only in the direction intended by bridge.

Distributor position interlock.

checks whether distributor is
whether in start or stop direction.

It has to be in either in one direction to commence starting.