During my first week in the Engine Room onboard the BW xxxx, the following anomalies in the DGs were apparent –
- D/G # 2 T/C RPM is lower than DG # 1 and DG # 3 for the same load.
- DG # 2 prime mover load is lesser than DG # 1 and DG # 3, and shop trial records, for similar switchboard loads.
- DG # 2 appears to take more time to stop as compared to DG # 1 and 3, and has a distinct knock during start.
- DG # 3 prime mover load is higher than normal.
- Frequent load shifting b/w two DGs whenever DG3 is in parallel with 1 or 2.
Since all these problems can be attributed to various causes, and since this has been the running condition for quite sometime, I decided to keep the DGs under observation and log down all the parameters twice a day to zero-in on the faults before adjusting any settings.
Later, while measuring the monthly peak pressures for DG # 3, it was observed that the DG is unable to load beyond 680 KW.The rated load is 900 KW.
This DG had a faulty PMS and was being run on MSBD mode (vsl was awaiting spares back then)
The engineers onboard assumed that this has something to do with the faulty PMS.
The workdone history revealed that the Governors for both these DGs had been recently renewed with overhauled ones received from shore workshop.
Closer inspection of the Governor revealed that both the Governors had been incorrectly fitted.
So much so, that DG # 3 was fitted back with the same Governor that was removed, and yet so..
What can be the problems due to the above scenario –
CASE 1 (DG# 3)-
- If a Generator is not able to load upto its rated KW, and it is coupled to another DG in parallel, the power output of other DG is also limited. (explained in detail below)
- While running standalone,it can cause preferential trip or a blackout, if heavy load consumer is started. This can be very serious during Cargo Ops or while the vessel is manoeuvering in congested waters
- Damage to the Governor itself.
- Start Failures of the Aux Engine.
- Unequal load sharing when synchronised with another genset.
CASE 2 (DG # 2) –
- The Governor on this engine was adjusted in such a way that even after giving stop command, fuel was being admitted to the cylinders (fuel pump racks not going to 0).
This small amount of fuel could not produce sufficient torque to keep the engine going, so, ultimately, the engine would stop.
But during this time, the fuel pumps keep pumping and the fuel gets injected inside the cylinder.
However, this is not burnt completely, resulting in fouling of exhaust gas passageways.
This can be very deterimental to the health of Turbochargers.
Aux Engine turbocharger failures have occurred in the past (on our fleet) because of unburnt fuel accumulation
- Excessive carbon deposits, deteriorating the general health of engine over a period of time.
- Excessive fuel during START sequence –knocking- puts additional load on the bearings.
- Rate of Rise of pressure – known reason for piston ring breakage.
- A 4S engine used for electricity generation duties is a constant speed engine.
It is thus balanced at the rated speed.
If the engine takes more time to stop, it means the crankshaft spends more time in the Barred Speed range.
Barred Speed range is provided on engines so that it does not operate in the range where Torsional Vibrations will be in resonant conditions.
The very case for which the governor is fitted can become the cause of its damage!
(according to most governor makers,Poor Lube Oil Quality and Torsional Vibrations are the most prominent causes for Governor failure, whereas Air in the system is mostly responsible for malfunction)
- Unequal Load Sharing
Primary Cause –
a.)Incorrect fitting of output shaft to fuel linkage.
b.)Lambda Controller incorrect settings.
Explanation of Above –
The below explanation is for STX MAN 6L28/32 (900 rpm, 900 kw engine, 1150 KVA alternator) fitted with a woodward UG-8 Governor with droop settings of about 5%
(however, valid for almost all medium speed generating set installations onboard)
Below is an excerpt from the Woodward Instructions manual –
The maximum travel of output (terminal) shaft is 42°. The recommended travel of output shaft is 28° from NO-LOAD to FULL-LOAD, which allows sufficient overtravel at each end so that the governor can shut down the prime mover and also give maximum fuel when required.
Governer and Fuel Linkage relationship –
What happens if the above is not followed ?
Apart from the effects mentioned earlier, if the governer output shaft does not use the full 30° of available travel from “NO-LOAD” to “FULL LOAD”, the DROOP also reduces proportionately.
Why is it important that droops should be similar for machines running in parallel ?
For Generators with different droop characteristics and running in parallel, the load shared following a load change will not be equal.
Why is it important that generators of same rating should supply equal shares of the system load ?
When one machine supplies more load than the other, the TOTAL CAPACITY of the supply is effectively reduced.
Coming back to the case of DG # 3, it can be mathematically proved that if the original Droop was 5%, the new Droop is 3.6% .
Now consider paralleling this generator with DG # 1 during cargo discharging where we are running the IG system, 2 cargo pumps, BWTS system and 1 ballast pump.Lets say the total switchboard load is 1150 KW.
So, a 1.4% reduction in droop has reduced the total system capacity by 36%.
If the governor droops are unequal, the machine with the smallest droop will take up a larger share of increased load.
In above case, if an additional load is started, a pref trip or a blackout will occur.
Load Hunting on switchboard
The PMS system will try to balance the load between the two DGs. To do this, it increases the reference speed for the machine at lower load and decreases the reference speed for the machine at higher load, thus shifting the droop curves.
Even if at some point the load is balanced, following a disturbance, it will unbalance again.
THINGS TO NOTE WHEN FITTING A NEW GOVERNER– (apart from those mentioned in the manual)
- Compare both governers and be sure the ratings and specifications are same.
There has been cases where the model is same, but gov speed is different.
- Change the bearing if the R/Hrs is due
- Do not insert a gasket on the mounting foot of the governer as it is a metal to metal contact and by inseerting a gasket, the height is increased. A chart paper can be used if there is a doubt of LO leakages.
- There should be no free movement or lost motion in the linkages.
- Run the engine for 1 min. Drain and Refill the oil . This ensures air is not trapped.
- Push the fuel linkage by hand to minimum fuel (zero index on fuel pump racks), while keeping the linkage pressed,move the “LOAD-LIMIT INDEX” on Governer to Zero.
- Release the fuel linkage and check the fuel injection pump racks are at zero.
- If not, re-adjust.
- Move the “LOAD-LIMIT INDEX” to Maximum (10).
- Pull out the fuel linkage in the direction of maximum fuel.
- Match and confirm the Fuel Injection Pump Racks and Governer Index are at maximum.
- If not, re-adjust.
- Make sure the fuel racks go to ZERO when stop command is given.
- Take the generator on-load and at various loads, confirm Governer Index,
- For example, at 450 KW, index should be 5 (50%), 7 at 700 kw, 2 at 200 kw …etc
CHECKS DURING ROUTINE ROUNDS –
- Oil level
- Hunting/Frequent Load shifting
- Fuel Index
- Turnbuckle – no free movement or lost motion
Checks during c/c inspection –
- Inspect the gears.
3/E, BW Maritime
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