Glad I could help.
With the starting air it has been standard to use 30 bar/400 psi air for starting and 7 bar/100 psi air for control systems for a very long time.
It is very wasteful to compress air to 30 bar and then expand it through a reducing valve to 7 bar, obviously it takes more power to run the compressor delivering 30 bar than it does to run it at 7 bar and the volumetric efficiency is less because the the air trapped in the clearance volume of the compressor at the end of the delivery stroke is at 30 bar rather than 7 bar so during the suction stroke it will expand to occupy more than 4 times more voume of the cylinder reducing the amount of air being drawn in. It will also require a larger motor to drive the compressor at 30 bar than would be required to drive the same compressor at 7 bar and it would require more cooling, while as I said previously it would deliver a smaller mass flow of air.
That is why most ships have a seperate starting air system and one for control air although they often have a reducing valve from the 30 bar system to supply the 7 bar system in the event of an emergency.
With direct reversing Engines it is a basic safety requirement that the air Receivers should hold enough air to enable safe manoevring of the engine, you would not want to use all the air in the receivers at each start and then wait for half an hour to charge up the receivers to get the next start! SOLAS lays down the precise number of starts that the system has to be able to make in rapid succession. (I cant remember it now, but I am sure someone will tell us!)The engine Maker will say what the maximum and minimum air pressures required to start their engine is and the volume of air required for each start. The Naval Architect can then calculate the volume of air receiver required, to give the required number of starts before the pressure falls too low to start the engine.
If the engine is hot, the air starting system is in good condition and the pistons and valves are sealing tightly you should be able to start a slow speed engine with as little as 12 bar air pressure. When we used to manoevre these engine manually ( Before Bridge Control) we all developed a technique whereby the lower the start air pressure the more fuel we put on to get the engine to start more quickly and thus use less air. You could sense the engine starting to fire by a dull thud felt through your hands on the controls and your feet and immediatly reduced the fuel lever to stop the relief valves blowing (it used to cost you a case of beer if they did).
There can be numerous reasons why an engine is reluctant to start:-
Engine is cold (Jacket water and L.O.)
Fuel is cold (if running on HFO)
Poor compression - piston rings leaking due to wear or carbon build up or lack of lubrication (cylinder Lubricators not operated after a spell shut down)
Poor Compression due to valves leaking
Poor compression pressure due to auxiliary blower not being effective- or due to poor condition of the scavenge valves.
Start air pressure too low
Control air pressure too low.
Main Air Start valve not opening properly- requires overhaul or control air pressure to it too low- air leakage or start solenoid valve not opening fully
Start air distributor incorrectly timed, so that there are dead bands ( I have seen this with a 6 cylinder Bergen Medium speed engine where the rotor that was fitted was for a different number of cylinders, no wonder it was tempermental when it came to starting!!) with some designs it is also possible for the rotor to slip on theshaft as they are held on a taper not a keyed or splined shaft. Some distributors use piston type valves operated by cams and these can be seized or leaky and require overhaul.
Cylinder head air start valves require overhaul.
Air start pipework leaking, this is only pressurised for the few seconds that starting air is on the engine, if everyone is in the control room during starting no one will realise that all the starting air is blowing across the engine room instead of going into the engine. It is a good idea to make up sticks with small pieces of rag on the end and run them along the pipes and round the couplings while the engine is being started, you often find a number of small leaks that way although large leaks should be more obvious to anyone walking around the engine while starting.
I am sure other people will have some more ideas, but there is enough there to keep you busy checking for a while when you get back on that ship!
It is always better to ask a stupid question than to do a stupid thing.