(a)
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As been depicted in the diagram when the flow comes below the valve lid, the fluid will lift the lid and pass but when the flow is in reverse direction the lid will sit on the seat and stop the flow.

On the other hand when the spindle is screwed down the lid will not open in any case. This is what is called SDNR and screw down lift valve.

b) Quick closing valves are fitted in Lube Oil and Fuel oil lines, generally at the tanks on the lines which take the suction from those tanks. It is used to cut-off the oil flow to those line in case of fire.

It could be mechanically or pneumatically activated and when activated it shuts the valve. The shutting of the valve is achieved by collapsing the bridge of the valve which has internal threads on which the treaded spindle of the valve move up and down.

Thus if this internal thread (fitted in the bridge of the valve) itself is brought down it will shut the valve even if it is in open condition.
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For reversing the Main engine first the engine has to be started in reverse firing order, this is achieved by the changing the cam direction in the mechanical pilot air distributor of the main engine, which is done by the engine's pneumatic system on receiving the appropriate Signal from Engine order telegraph operated from the ship's wheelhouse.

a) Three methods of reversing the main engine Aftyer the engine is started on air in reverse order as mentioned it can be run on fuel by following either of the three methods as following:-
1} Reversing the firing order of the fuel pump:- It means the sequence in which the fuel is injected to each unit of the Main engine is reversed by its cam drive (it could have a separate cam for each cylinder for reverse firing order or the cam has special heart shape which when rotates in reverse direction due to running of engine in reverse direction due to air will work for reverse firing order) or electrical signal(as in the modern electronic engines)
2} Reversing the Actuation of the exhaust valves:- In this method the actuation of the valve opening and closing is reversed, only. This method is old and can run the engine safely in reverse order for small RPMs only.
3} Reversing the Fuel pump and Exhaust valve actuation: This is the most commonly used system where above mentioned both methods are applied.

b) The lost motion is the turning of the cam by an angle with respect to the cam shaft (In this there is a small servomotor put inside the cam shaft which shifts the cam in its axis)so that when the cam shaft is rotating in the reverse direction the cams would be pushing the roller of the fuel pumps in different firing order or alternatively both the shapes are forward and reverse directions are combined together to create a heart shaped cam (as shown in the video below)
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