Turbocharger RTflex Question

[lazygenius]

I sailed on a Bclass containership this summer, which ran on a RTflex engine. We had one turbocharger blanked off, and the reasoning for this was fuel consumption was too high. Looking back now I can't quite understand the effect blanking off a turbocharger would have on the fuel consumption, compared with controlling the speed in the normal way.

Is this a common practice?

[Chiefy]

It's coming in as more ships / companies derate their engines and take up slow steaming.

As for the theory......it's complicated and even I am a little shaky but it's roughly along the lines of......
At lower speed the t/c spins slower, therefore there is less scavenge pressure, so the peak pressure is lower and fuel consumption higher, also you may not even reach the point where you can stop the auxy blowers (ie scav pressure doesn't go above .4bar) so engine isnt breathing right.

So you cut out a t/c and now you have more exhaust gas for the other 2 thus they spin faster and scav pressure goes up. Auxy blowers cut out and engine can breathe, peak pressures go up as the scavenge pressure rises (basic engine theory) higher peak pressure is lower fuel consumption, HOWEVER you may need to go flat out occasionally (above 80rpm in the flex's case) which would over speed the 2 remaining t/c's so you need to be able to easily cut in and out the 3rd t/c......however Wartsila claim you can leave it at all times cut out and then the auxy blowers will stop while still on stand-by however chiefs arent impressed with that idea

Also there is the saving from not having the Auxy Blower running all the time, 250amps 24/7 is quite a saving and add the wear and tear on the auxy blower too (where never meant to run for extended periods ie >24-48 hours though occasionally it is ok)

Right will that do for you?!

Any further questions ask away

[lazygenius]

I thought it was a bit of a tricky question, I asked the chief and second a couple of times, which was met with lots of paper shuffling and "im dead busy" body language!

Do I have the basic principle of the auxy bowers wrong? I was under the impression that if we had to flat out, the auxy blowers supplimented the two turbos.

When you say there is a big saving by not having the auxy blowers running all the time, when would that be the case?

I thought it was a false economy having one turbo blanked off ( which in our case was completely blanked off, and wouldn't be ready to go back into service without a bit of work), and then running across the Atlantic at top speed with the blowers on!

[lazygenius]

However I can understand now, that the engine will be more efficient at slower speeds!

[Chiefy]

Riiiight.....you are well muddled up....

Auxy blower are there to supply air when the engine is at low power and the turbos dont pull in enough air to maintain proper combustion. Once you get higher than .4bar scavenge they shut down and off (flap valves). Once you have enough power on the engine it breathes well and so no need for blowers.

Where you really running across the Atlantic at full power (what RPM where you doing)?

Where the remaining t/c's surging? What was the scavenge pressure, what was wrong with the blanked off one? Was it broken? Often on a Wartsila you can run the blowers to help settle the blowers and stop them surging HOWEVER you have to watch the Amps they are pulling due to the way they are in the system at higher pressures they pull more amps.

So Auxy Blowers are for Start up, maneuvering and LOW Power running...just hang on to that fact

[lazygenius]

Normally the scavage pressure was 0.8ish under normal conditions i.e eco speed. Yeah we were on a MSC charter so the schedule was changed often which left us up against it to get to Port Everglades in time, hence why we were going flat out. By flat out I mean we were definatly doing over 80RPM but I cant remember the exact figure!

I know it had been blanked off for ages and the reason it was taken out of service was to improve fuel economy.

Cheers for the help

[Chiefy]

Yeah I know about that charter.....without wanting to know it maybe the Blowers *should* have been off at that point, unless it was an anti-surging matter, but it ought not to be as the electrical load is quite high.......however not my problem

And the fuel economy is UPTO 8% on like for like power

the K's are half way though getting adjustable valves for this...ie we can cut and un cut without wielding big spanners, just have to press a button and the computer does *stuff*

[SpannerMonkey]

B_Class container ships - Maersk by any chance - i sailed on the most of their 'quickest container ships in the world' B Class fleet - and then quickly layed 5 of them up in scotland for a year