Diesel Engines 3

[jacktheripper]

Does anyone has any background information on Turbochargers:

I'm a plant operator for Two 20 MW slow speed[100rpm]diesel engines mitsubishi made, but lately we are having turbocharger problems such as heavy and continuously surging hence causing/forcing us to derate the units.

What could be the possible causes?
What effect can this continuos surging have on the units?
Can the units run at full load with a surging turbocharher?
What problems could lead to turbocharger surging?

We have done cleaning of the air coolers,scavenge air spaces and cleaning of the turbocharger[exhaust and compresser side]but still the problem exist.

Any ideas are welcome.

 

[ingore]

we are encountering the same problem of turbocharger surging in our operation. will the imbalance speed of the tow turbochargers contributory to this problem?

 

[tommy75]

Jack,

When the turbo surges do you see an increase in engine R.P.M.?

 

[jacktheripper]

tommy75 to be honest a increase in engine speed is not observed, i dont know if that has anything to do with the fact that the engines are slow speed[100 rpm].

 

[tommy75]

Jack,

I did googled for 100 rpm mitsubishi engines and I could not find one below 1200 rpm. I must say that is a first for me and I find very hard to fathom a 20 MW diesel generator runs at 100 rpm flywheel speed, most generators run at 1800 rpm (4-pole) or 3600 rpm (2 pole) so if in fact the engine does run at 100 rpm there must be one hella gearbox on that unit and I would have to say surging would be due to an unordinary loading on the motor via gearbox or lead and lag on the generator sync. I would say at first chance pull down the turbo and inspect cold air compressor and hot end drive for any signs of hot end errosion or cold side spool damage.

Keep me posted with what you find...
Tommy

 

[tommy75]

Wow...

I must retract above response, I did find in fact diesel engines that run in the 60 to 100 rpm range. I just learned something today!! So with that in mind and the fact that you are seeing surging in both units I would suspect that your problem could be caused by loading on the generators and you would not see much of an increse in speed due to fact of the crazy low rpm speed. Now are these unit tied to the grip or are supply power for yourself??

If tied to the grid, I would check your sync vectors to see if your are pushing or pulling on the units in an abnormal way.

The turbo sound is the first detection for loading on the units. Also are you EGT'S abnormally high when the turbos surge??

Again keep me posted cuz I'm real interested in this now!!! Tommy

 

[jacktheripper]

hi tommy

yes we are tied to the grid.

I will have to get more information with regards to 'sync vectors'. Probably you could shed some more light on it's effect specifically.

Nomally when the turbo surges oneside has a speed increase while the other decrease in speed.
The liner temperatures nomally increased.
exhaust temperatures increase a bit,but i would not say dramatically.
This happens when the surging is continous hence forcing us to reduce load on the unit. If the surging is intermitent only a increase in T/C speed was observed.

This is the specs of the M/E and Generator
M/E: Mitsubishi heavy industries
Misubishi-Sulzer 8RLFB90
Cylinders-8
MCR speed -100rpm
Horsepower-28160
Piston stroke-1900mm
Cylinder bore-900mm
Generator:
Meidensha electric Mfg C/o Ltd
Type-synchronous
#pole-60
Output-20,000Kw
Rated Voltage-13.8
Synchronous speed-100rpm

Presently we did cleaning of the T/C and replacement of parts on the #2 unit.
The Number 1 unit is only running with 1 T/c as we have lost the B bank T/C. The T/C Housing is cracked presently we are awaiting spares.Hence it is operating at 10Mw.

 

[MikeHalloran]

Are the two banks' turbos supposed to be interconnected all the time, or is the cross- connect intended only for operation with one turbo out?



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

The reason you aren't seeing any change in shaft speed is that the grid is too large for you to significantly alter its frequency. What you probably will see is a change in load angle if the machine surges. This is a shift in electrical phase angle of the generator relative to the grid which depends on load. You might pick it up on a disturbance recorder as a transient acceleration or deceleration of the shaft if you configure it correctly.

A simple way to think of how the generator and grid interact is to picture the grid as being represented by a disc rotating at 3000rpm and the generator being another disc rotating at the same speed and connected to the first disc by a tension spring. The spring represents the generator field. If the generator is supplying no torque the spring is slack and the two discs rotate at the same speed. If the generator accelerates it advances ahead of the grid slightly and the spring tension increases as the generator tries to accelerate the grid, which it can not do. Similarly if the generator decelerates the generator drops behind the grid slightly and the grid pulls the generator around. The generator has become a synchronous motor.

Under prime mover surge conditions the generator can oscillate between generating and motoring causing enormous stress on the shaft and gearbox as it does so.

Tommy: 'Sync vectors'? Do tell more...


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[jacktheripper]

hi mike

The turbochargers are fed with exhaust gas from a common exhaust gas manifold which in turn is fed with exhaust gas from all eight cylinders: that is both T/C are on the exhaust gas manifold, so yes they are interconnected all the time mike.

With the one Side of T/C out the other side is being fed with exhaust gas from all eight cylinders.

Scotty

I agree with the point you have made.

 

[catserveng]

At about what percent of engne load does the surge start?

Were there any operational or mechanical changes made before this problem started to occur?

What is the trend in the engine exhaust temperatures before this issue occurs?

Some things that can cause turbo surge,

Excessive exhaust backpressure
Excessive turbo inlet restriction
Excessive turbo outlet restriction
Turbo outlet pressure leak
Erratic engine or engine control operation

When the turbo goes into to surge, the turbo outlet pressure "dumps" and then builds back up, the fuel control system responds to maintain the desired output, the charge air temperature will change, and can cause higher than normal combustion temperatures and over-fueling.

Depending on the severity of the turbo outlet pressure fluctuations, the turbo shaft coould be banging back and forth against it's thrust bearing, causing excesive wear, leading the a catastrophic failure.

As a turbo operates over it designed load range, you can plot the compressor outlet pressure versus the compressor outlet air temperature rise, the point at which there is an increase in turbo outlet temperature on the compressor, with no increase in pressure is the point we used to call choke, stall is close and that is what is likely causing the surge, the turbo hits a stall, unloads, and then builds pressure back, this typically occurs in engines with multiple turbo's fed as you describe.

Above was a short list of possible contributors, I have also seen this caused by the following,

A muffler with a large amount of water in it.

A collapsed air cleaner filter element.

Several cylinders misfiring.

A badly worn fuel control linkage.

An improperly adjusted governor.

A worn governor actuator.

A voltage regulator with an intermittant output fault.

Damaged turbine and/or compressor blades on the turbo.

We have also (rarely) seen problems with turbo's returned from overhaul with updated or incorrect parts that caused the problem after overhaul.

Lots of potential causes, but continuing to run in surge can cause severe engine and turbo problems.

The turbo manufacturer can usually provide better assistance based on his experience with a particular design. Usually on engines that size, the turbo is not made by the engine manufacturer.

Hope that helps

 

[ActiveSolar05]

Are you running a waste gate to control turbo intake pressure via dumping exhaust?

Is W/G sticking or slow to react?

You say there are two turbos on each engine both running off a single exhaust manifold; but on the turbo compressor side are they one-after-the other for more pressure or parallel for more flow volume ?

jrr

 

[jacktheripper]

We are not running a waste gate : what is a waste gate exactly?

#1 to #4 cylinders feed exhaust to the A T/C
and the #5 to #8 cylinders feed exhaust to the B T/C

The T/C are at each end of the engine on top of the exhaust gas manifold[which is cylindrical in shape which runs the full lenght of the engine] does this answer your question on there position.

Presently the parts for the T/Cs have arrive and we are in the process of rebuilding the B side T/C,i will keep you guys updated.

 

[catserveng]

A wastegate is basically a high temperature valve that allows exhaust gas to bypass the turbocharger and dump into the exhaust, reducing the amount of air compressed by the cold side of the turbo. Wastegates can be fairly simple and controlled by a spring and diaphragm with a pressure sensing line, or electro-mechanically or electro-hydraulically controlled.

 

[chief]

I have experience of slow speed diesels and the main reason I have found for surging in exhaust turbines is a fault in the exhaust valves , either timing or leaking by. One other long shot to check, is there a restriction in the exhaust trunking or stack. Is the exhaust stack diameter correctly sized, without restrictive bends or an exhaust boiler or silencer choked.
Another problem may be electrical, whereby one engine is 'motoring' the other via the power management system. This obviously overloads one engine and underloads the other and will certainly cause turbo surging.