In MMD Exam there are so many questions asked about starting air line Explosion , How it occur ? What are preventive measures taken ? What are the safety devices fitted ? Some Cross Question related to this Topic. So, Let us start Step by step about this topic.
What is starting air line explosion ?
Explosion in the air starting line due to fire occur in the starting air line of main engine.
or, a fire occur in the air starting line of M/E.
How starting air line explosion takes place ?
As we know that to take fire, a fire triangle need to be complete. What is fire Triangle ? Combustible Substance , Heat and Air called as Fire triangle. If All these three are available then fire occurs.
Oxygen , Heat and Fuel are Required for occuring a fire.
Fire Triangle in the case of air starting line explosion.
- Oxygen :- In the air starting line or manifold , Air for starting (oxygen ) is present in bulk.
- Heat source :- Due to leaky air starting valve. During operations of engine, heat leak from air starting V/V is act as a heat source.
- Combustible Substance :- In the Air starting line , air is available at 30 bar. But this air is not pure. It contains some amount of oil.
How oil come in the air of air starting line ?
Main air compressor used to produce 30 Bar. Lube oil used for lubrication of liner and piston. Some amount of lube oil mix with air. This air finally come to air starting manifold become combustible liquid for explosion.
Combustible fuel is available as lube oil carried out from main air compressor. Oxygen is available in bulk. Due to leaky air starting valve heat from combustion chamber igniters the lube oil, Thus Starting air line explosion Occur.
What are the causes of starting air line Explosion ?
- Leaky Starting Air Valve or Air starting Valve get stuck in open positions.
- Oil comes with air from main air compressor.
- During maneouvring Time, high compressed air come in contact with fire and explosion occur.
Preventive measures of starting air line explosion
- Always maintain the main air compressor in good conditions Feed rate of lube oil for lubrication of cylinder id adequate.
- Regularly drain the air bottle once in the every watch by duty officer.
- Also drain the the air starting line or manifold, when the engine is stop.
- Regular overhaul of Air starting valve of each cylinders.
- Leaky test of Air starting valve before departure.
- Oil separator at discharge side of main air compressor working effectively.
what are the safety devices fitted on starting air line to prevent Explosion?
1.Flame arrestor :- It is fitted before air stating valve for each cylinder in the air starting pipe. A flame arrestor is a device which allow to pass a gas through it and did the work of stopping a propagation of flame. or, It is a small unit or device which consist of many tubes which arrest any flame or spark coming from the cylinder due to leaky air starting valve.
2.Bursting Disc :- Bursting disc is installed in the starting air pipe and consists of a perforated disc protected by a sheet of materials that will burst in the event of an air line explosion. It also includes a protective cap that is designed in such a way that if the engine is required to run even after the disc has been ruptured, the cap will cover the holes when turned. This ensures that manoeuvring or traffic air is always available to the engine.
Bursting discs are pressure relief devices with a defined breaking point that respond to a specific pressure and are used in a wide range of applications. They are used to protect against overpressure or vacuum within a process, thereby protecting man, the environment, and the machine. It is also known as rupture discs , Burst disk and diaphagram disk.
3.Non return valve :- Because of the unidirectional property of the non-return valve, it will not allow the explosion and its mixture to reach the air bottle if it is located between the Air Manifold and the Air Receiver.
4.Relief Valve :- It is installed on the common air manifold, which provides air to the cylinder head. It is normally installed at the end of the manifold and lifts the valve if there is too much pressure inside the manifold. The benefit of a relief valve is that it will sit back after removing the excess pressure, allowing continuous air to be available to the engine in the event of manoeuvring or traffic.
5.Air starting manifold drain :- Drain the manifold of starting air when engine is stopped.
How will you know your air starting valve is leaking
Here, two conditions arises, how will you know if the engine is stopped or when engine is running.
1.When engine is running
In the running conditions , we get to know leakage by following methods :-
- Air starting line or manifold become too hot due to leaky air starting valve. By touching hands. If too much leaking then air starting line become red hot.
- Relief Valve which is fitted at the end of air starting manifold lifts again and again with a high sound.
- Bursting disc fitted in the air starting line get burst. It cause all the pressure release
- Smoke started coming from the Drain fitted in the air starting manifold.
2. When is stopped
Surveyors in MMD exam generally ask this questions that when your is stopped then howl will you know your starting valve is leaking.
In the air starting line air is always present. Pilot air open the air starting valve then air into cylinder combustion chamber. Pilot air responsible for opening air starting valve. If air starting valve leaks, air available in the manifold enters into cylinder. We know leaking by opening of indicator valve. When indicator valve open air start coming.
Sometimes surveyor asked why there is negative cam is provided in air starting distributor ?
Why negative cam is provided in air starting Distributor ?
The reason for negative cam is :-
- If air starting valve get stuck then it get close positively. Positive closing of valve in the case it get stuck.
- If any one open the air starting line of air bottle then it does not allow to open the valve because pilot air is not available and all rollers are already lift up.
- When main engine is in the running conditions then all rollers of air distributors will lift up due to spring action. An thus, negative cam of distributor will rotate continuosly but no wear down of rollers takes place because they are already lift up.
Action in case of starting air line explosion
- Inform the bridge and ask them for stopping the engine for changing the air starting valve.
- If not possible for stopping the engine, cut off the fuel supply so that combustion will not take place in that cylinder.
- In the mean time , we have to ready air starting valve so that change as soon as possible if u get the time.
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