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Posted: Sat Feb 16, 2013 11:13 pm
i have some doubts regarding class 4 exams . expecting answers guys thanks in advance .
1. difference b/w main engine and auxiliary engine tie bolts
2 how tie rods are pretensioned . i had a friend explained me that by using hydraulic jack the threads of tie rods elongate and so tie rods are holding the cylinder head A frame bed plate tightly . can some one explain about how hydraulic pre tensioning is happening
3 connecting rod checks . we check for ovality , straightness , scorings in bearings faces . what else checks we do .
once again thanks for your assistance in advance guys
Re: tie rod
Posted: Sun Feb 17, 2013 1:21 am
When hydraulically tightening any nut, the bolt or tie rod is put into position and the nut is screwed up hand tight.
A sleeve or collar is then placed around the nut, the hydraulic jack is screwed onto the thread above the nut and collar until it sits just above the collar with minimum extension of the jack. (If you screw the jack down tight onto the collar it will be impossible to unscrew the jack after tightening)
When hydraulic pressure is applied to the jack it pulls the top of the stud up and pushes the collar down onto the engine. The entire length of the Bolt/ Tie Rod is stretched.
When you have achieved the correct hydraulic pressure (Pressure x area of hydraulic ram = force.) small Tommy bars are placed through cut outs in the collar into holes drilled into the side of the nut and these are used to tighten the nut hand tight. NEVER hammer the Tommy bars or put extension handles on them, this will damage the holes in the Nuts.
The hydraulic pressure is then released, and the elasticity of the steel tie Rod/bolt pulls the Nut tight against against the Engine.
The hydraulic pressure can then be released and the jack pushed back into the minimum extension position, the hydraulic hoses can then be removed ( I have seen people hammering jacks trying to get them to retract AFTER THEY HAVE REMOVED THE HYDRAULIC HOSE!!!) the jack, hose and nut can then be removed.
There is always a correct sequence in which the individual Tie Rods/Bolts should be tightened in order to avoid distorting the engine. In most cases you have to tighten ALL the Tie Rods/Bolts to an initial pressure to ensure everything is properly pulled down into the correct position and then go round a 2nd and even a third time to increase the tension in stages.
With connecting Rods, you have to crack detect them in way of the serrations and bolt holes also check for fretting around the bolt holes and bearing shells also the absolute dimension of the Bottom End bearing housing, there is an absolute limit on this as well as the limit for ovality.
Re: tie rod
Posted: Wed Feb 20, 2013 8:35 pm
Thanks a lot for your assistance BP. I have another doubt in starting air line. The ship I worked have Man B&W MC type engine. Through books I find starting line control air is 7 bar, pilot and main starting air pressure acting on cylinder head starting valve is 30 bar. I remember seeing two pressure reducing valves and two pressure gauges showing 7 bar. Am not sure with what I observed. But main doubt is in the ship I worked we have had starting failures. We even had given air kicks with air bottles showing pressures of 15 bar. Can u explain what is happening there.
Again thanks a lot for your assistance
Re: tie rod
Posted: Thu Feb 21, 2013 2:18 am
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!
Re: tie rod
Posted: Thu Feb 21, 2013 1:27 pm
Check your control air filter on the engine. When it got dirty on the last ship I was on, the engine would take a long time rolling over to kick.
Re: tie rod
Posted: Mon Feb 25, 2013 8:33 pm
B&W MC engine? Check the control air pilot valve located under the control room console. Water and that white goo tends to collect there and the valve shuttle sticks. Same thing on the Sulzer starting system on the pilot air valve located above the engine control stand at the middles.
The former stuck on me while transiting past Hong Kong to Chekou in "line astern" in a short convoy on the way upriver. We got a bit too close to the guy in front (we were SH) so the bridge called down S and SS. (this after coming along the grooved highway from Sri Lanka to Hong Hong) Engine stopped, then nada.....no kick... The old man had to go hard over to avoid smacking the other ship's arse! He was not amused when we lost our place in the convoy and delayed for 12 hours. Guess who was blamed?
The Sulzer pilot valve stuck while coming alongside in Montreal.
Re: tie rod
Posted: Tue Mar 12, 2013 9:26 pm
thanks for your valuable comments jk jolly , its been really helpful .....
Re: tie rod
Posted: Tue Feb 23, 2016 6:29 am
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