Q – What is starting air line explosion and its preventive measures?
Ans – Main line contain 30 bar air but this air is not pure it also contain some traces of the oil so, we have the fuel which is oil and we also have the sufficient air so for the fire to to place we only need a heat or spark which usually come from the leaky air starting valve which is the cause of starting airline explosion.
Preventive measure
1. Air compressor to be maintained in a good condition
2. Air bottle is to be drain regularly in every watch
3. Starting air line is to be drain once the engine has been stopped
4. Make sure air starting valve on each cylinder head is properly overhauled and inspected at regular interval of time.
5. Relief Valve: It is installed on the common air manifold which supplies air to the cylinder head. Normally installed at the end of the manifold & it lifts the valve in the event of excess pressure inside the manifold. The advantage of relief valve is it will sit back after removing the excess pressure & thus continuous air is available to engine in case of manoeuvring or traffic.
6. Bursting Disc: It is installed on the starting air pipe & consist of a perforated disc protected by a sheet of material which will burst in case of excessive pressure created due to the air line explosion. It also provided with a protective cap such constructed that if the engine is required to run even after the disc has been ruptured, the cap will cover the holes when it is turned. This will assure that in manoeuvring or traffic air is available for engine at all time.
7. Non Return Valve: Positioned in between the Air Manifold & Air Receiver, it will not allow the explosion & its mixture to reach the air bottle because of the unidirectional property of N.R. valve.
8. Flame Arrestor: It is a small unit consisting of several tubes which will arrest any flame coming out of the cylinder through leaking start air valve. It is installed on every cylinder before the start air valve.
Note – In sulzer we have the relief valve and in man b&w we have the bursting disc
Q – lf air starting valve leaks during manoeuvring, how will you get to know out and what action you will take.
Ans – If air starting valve leaks you get to know out from the following locations:-
1. Air starting line becomes hot (just by hand touching you get to know out if excessive leaks it becomes red hot)
2. Relief valve lifts again and again with a high sound
3. Bursting disc burst out and relieve all the pressure
4. From the drain line smoke will come out
Then tell to bridge that you have the problem with engine and ask to stop the engine and change the air starting valve
But if it was not possible to stop the engine then cut off fuel oil supply to that unit so that no combustion should take place in that unit
In the mean time you should also ready the other air starting valve so that as soon as the engine stops you should replace it with in a short span of time
Note – Sometime surveyor ask you that if your bursting disc burst out then it was not possible to give a air kick because the air should go out from the bursting disc. If you ever overhaul the bursting disc then you can easily answer to this question.
The safety cap provided with a bursting disk enclosed by a perforated cylinder & a perforated cover in order to save any bystanders, in the event of a burst. The cover is installed with a tell tale, which shows if the bursting disc has been damaged. If the bursting disc of the safety cap is damaged due to excessive pressure in the starting air line, overhaul or replace the starting valve which caused the burst, & mount a new disk.
If a new disk is not available, or cannot be fix immediately, then the cover can be turned in relation to the perforated cylinder, in order to decrease the leakage of starting air.
Note – Some time surveyor might ask you that your engine is stopped then how will you get to know out that your air starting valve is leaking or not?
Most of the time surveyor check your intellectual knowledge so before giving answer to these type of questions always think for a second and then answer because your answer tell your thinking ability to the surveyor
So to check this you need to open the indicator cock if your air starting valve is leaking then air come out from the indicator cock which give you an indication that your air starting valve is leaking because air is always available at 30 bar but to allow the air to go to the unit 7 bar pilot air come from the distributor but if starting valve is leaking then 30 bar air keep going inside the unit.
Q – Draw starting air line diagram and explain the same.
Ans –
The reason of providing the negative air distributor cam is –
1. Positive closing of the valve if in case it stuck
2. If some one manually open auto start valve, still it wont allow the air to go in because control air is not present and all rollers are already lift up.
When engine start running all roller on distributor will lift up because of the spring action so air distributor negative cam will keep on rotating but no wear and tear because all rollers are already lift up
Tuning direction interlock:
Interlocks are the blocking devices which assure that the engine is started or reversed only when some conditions are met. Running direction interlock is an important interlock that restrict the injection of fuel to the engine when the telegraph does not synchronize with the running direction of the engine. It is an important application in the cash manoeuvring when the starting air is used to apply brake on the engine by reversing the operation.
Turning gear interlock
Turning gear interlock is another important device that restrict the admission of starting air to the engine cylinders when the turning gear is engaged. If the starting air is allowed with the turning gear engaged, then the turning gear along with the motor will fly off puncturing the bulkhead. Thus the interlock is important to prevent such accidents.
Other interlocks are –
1. lube oil low pressure interlock
2. Control air interlock
3. Camshaft lube oil low pressure interlock
4.Spring air low pressure interlock (for exhaust valve)
5. Aux. blower interlock (at the time of starting aux blower is to be put on auto mode
because of no scavenge air else it is difficult to start the engine without air that was also the reason why we get black smoke at the time of starting the engine because of less quantity of air)
6. Safety air interlock
7. Reversing completed (it was the direction safety valve if engine some how starts in
opposite direction, so it will cut off the fuel supply)
8. Camshaft Interlock – It is a device, which restricts the engine from starting if the camshaft has not shifted or changed its end position in compliance with engine order telegraph.
9. Starting Air Distributor In End Position Interlock – This device prevents starting from taking place if the shifting of the distributor has not been completed.
10. Fuel Pumps Interlock – It prevents reversing of engine if the fuel rack is not in its zero injection position.
Note – In sulzer if engine start in opposite direction fuel supply will be cut off but in man b&w engine won’t start because of the direction interlock. So that was the reason why this interlock is given after the telegraph.
For slow turning a small line is provided as shown in the diagram which give you a very small kick of 2-3 turns (slow turning automatically takes place and at the time of slow turning your indicator cock is closed), but if you didn’t get the kick may be because of water accumulated over the piston crown so in that case you will get the alarm.
Note – Sometime surveyor might ask you why to turn the engine on turning gear –
1. the reason for this is if in case excess water is accumulated over the piston crown then at the time of blow through that much quantity of water unable to come out from the small indicator cock
2.Second reason is at the time of turning the engine on turning gear if your motor ampere is increasing it simply give you an indication that excess water is accumulated over the piston crown. So never ever try to blow through in that case because you may break the engine parts.
Some time surveyor might ask you that you give the kick but engine fails to come on alr-
1. Start air distributor has not activated its end stop valve
2. No air pressure at engine manifold. The valves from the air receiver would be checked, and opened if found shut.
3. Low air pressure at engine manifold. This could indicate that the air compressors are either not working or excess air is being used. All air compressors would be started, & air usage restricted to engine manoeuvring only.
4. Turning gear engaged. The turning gear position & the interlock switch would be visually checked. The gear would be removed if found engaged.
5. Control air valves faulty or less control air pressure
6. Bursting disc burst out on starting airline
7. Auxiliary blower is not running or not to be put on auto mode
8. Spring air pressure is low so exhaust valve is unable to close and the air kick goes out from the exhaust valve
9. Fuel lever on manoeuvring stand not on remote position.
Q – Draw the Manoeuvring diagram and explain the same (GA)
Ans – I try to explain the basic working of the manoeuvring diagram so while reading this keep checking the manoeuvring diagram which has been given below because at your level Surveyor won’t expect much from your side but still I try to cover all important points where surveyor can pin point you.
1. Normally at the time of run we have to give the fuel to the engine but we don’t give in the starting unless he won’t attain the minimum start rpm, once engine attain the start rpm so for 2-3 seconds both the micro switches has been pressed up (start and run ) so that the engine should attain minimum momentum and keep running on fuel, but before this control air goes to the telegraph and according to the command receive from the micro switch it reverse the position of fuel pump roller through its control cylinder.
2.Two micro switch will be there in the air distributor if air distributor is properly reversed
with respect to the telegraph so from non-return value air goes to reversing interlock and then from reversing interlock control air goes to cylinder of auto start valve and operate it. This control air first check all the other interlock and wait for the reversing interlock command to operate so that it goes forward to the auto start valve
3.So due to this air @ 30 bar pressure from air bottle goes to starting air manifold through the auto start valve and NRV.
4. Now distributor and fuel pump roller is set for the operation so pilot air @ 30 bar come from the distributor and operate the air starting valve so that 30 bar air should go inside the unit and give the sufficient air to the engine so that it will over come the inertia.
Note – Some time surveyor might ask you that below the balance piston 30 bar air is
present and above operating piston 30 bar air is there so how it will operate the valve
downward so this thing will happen because of the area difference.
5.Dotted lines in the diagram represents the venting line because if air goes to the cylinder to rotate the piston so the air to the other side must be vented out unless it is difficult to operate the piston inside the cylinder
6. Now sometime surveyor might ask you how to increase the speed of the engine so in this case again micro switches play a very important role when you keep on pressing the micro switch they keep on giving the signal to the governor and governor keep on increasing the fuel quantity
7.Surveyor mostly ask this question that at the time of starting we don’t have any scavenge pressure so governor never release the fuel because of the interlock so tell to surveyor that at the time of starting false signal is to be fed to the governor so that it will release the fuel in excess quantity ( just for 3-4 seconds ) so now engine started properly and from now onward it keep getting the signal from turbocharger only.
8. In case of emergency if you want to stop the engine then stop signal for emergency operate the puncture valve of the fuel pump and stops the engine.
Q – What do you understand by their crash manoeuvring & how to perform it?
Ans – Crash manoeuvring is turning the engine in opposite direction to reduce the heading speed of the ship. After some time, the ship stops & starts streaming in astern direction. This is performed by supplying starting air at about 30 bars from the air receiver to the engine. The stopping air is called as brake air.
The brake air when sudden inserted inside the engine cylinder, will try to resist the motion of the piston and the rotation of the crankshaft and propeller.
Note – When you give the brake air in the cylinder it try to resist the piston to come upward but when piston about to reach to TDC so this air will compressed and again you get the kick because of this compressed air so due to this you will unable to stop the engine so for that reason relief valve is provided on the cylinder head or in some new engine there was the switch for crash manoeuvring which operate the exhaust piston to open when the piston is about to reach to TDC
Procedure
Following Procedure is to be performed when a navigational Officer calls engine room & says that we have to stop immediately to prevent collision
When there is an emergency like collision, grounding etc. the controls are transferred immediately in to the Engine room controls.
The bridge will give astern direction in the telegraph, follow the same. When the telegraph is acknowledged only the starting air cam will reverse its direction but the fuel cam will remain in its running position due to running direction interlock since the engine is still running in the ahead direction
The fuel lever in the engine control room is brought to the ‘0’ As soon as the RPM of the engine drops below 40 % of the Maximum Continuous Rating or MCR rpm of the engine, give break air few times in short time frame
The break air will inject with astern time setting inside the ahead moving piston which will resist the piston motion
Since fuel will not inject until running direction interlock opens, as soon as the rpm drops near to 0, give fuel & air kick by bringing fuel lever to minimum start setting
When carrying out Crash Manoeuvring, some safeties need to be bypassed to avoid tripping of engine in mid of emergency When the ship stops and situation is under control, a detailed Main engine inspection is to be carried out when there is a chance.
