We are haveing 3no's GE Frame -III machines which are equipped with NT 855C diesel engine & Twin Disk Model: 4-SG-1607-2 torque convertor. The jaw clutch is getting disengaged at TNH of 36% speed. Diesel engine complete servicing was done. Torque convertor pressure checked & found to be 4.0 kg/cm2. Alignment between torque convertor output shaft & GT input shaft checked & found to be with in 0.12 mm radially & 0.15 mm axially which are with in tolerable limits. Still the clutch is getting disengaged at this speed. If we force the clutch disengagement limit switch signal machine going to FSNL, but with a bit slow acceleration. How to sort out the problem?

 

Do you know if the 4 Kg/cm2 is according to spec? I have no experience of this particular torque converter on a Frame 3 but, normally on Frame 5 & 6, it should be at least 6 Kg. Have you checked the foot valve and trash screen on the torque converter?

 

PRASAD DEVU,

Is this same problem occurring on all three of your GE-design Frame <b>3</b> heavy duty gas turbines--or just one of them?

When did this problem start? After a maintenance outage? After a trip from load?

I have been to several sites where the hydraulic actuators for the diesel speed control were not adjusted properly after the service--because generally diesel engine service technicians are NOT familiar with the type and number of actuators used on some GE-design heavy duty gas turbine diesel starting means. Sometimes, the solenoid operators fail or get dirty or stick partially open.

Lastly, I have seen plugged fuel filter pass just enough fuel to make rated speed during unloaded testing, but not enough to maintain rated speed under load (accelerating the turbine).

The jaw clutch disengages when the turbine side of the coupling over-runs the torque converter/starting means side of the clutch. Or, said another way, when the torque being supplied by the starting means is insufficient the spin it's side of the jaw clutch faster than the turbine side of the jaw clutch the jaw clutch opens. It's not anything else that opens the jaw clutch except low torque from the starting means.

The fact that you say when you fool the turbine control system (I note you did not select 'GE Mark V/VI' as the control system vendor) into believing the jaw clutch is still engaged that the turbine accelerates slowly is even more proof that insufficient torque is being supplied by the starting means (diesel and/or torque converter). Not knowing for sure what turbine control system is in use, it can only be presumed it's programmed as a Speedtronic would be.

This is most likely not a controls-related issue, specifically. When you force the limit switch telling the turbine control system to keep the starting means running the turbine does accelerate--but slowly because there's really not a sufficient torque assist from the starting means. Again, when the starting means torque is insufficient to keep the jaw clutch teeth engaged (i.e., when the turbine side of the jaw clutch spins faster than the starting means side of the jaw clutch) the clutch opens. Unless this is some unusual version of clutch and engagement, and some packagers of GE-design turbines have been known to try different components.

You say the diesel has been checked. I've seen many diesels that will make rated speed during testing--when there is no load on the diesel. But, under load the fuel rack is/has been misadjusted such that it will not open sufficiently to increase the fuel to supply sufficient torque under load. So, when you say the diesel was tested, how was it tested: Loaded or unloaded?

What is the diesel speed when it's trying to accelerate the turbine? Is it "rated", or slightly less than rated?

The diesel may have new valves and pistons and rings and bearings, but if it's not getting enough fuel to provide sufficient torque under load, then it's not going to keep the jaw clutch engaged until the unit reaches self-sustaining speed (usually somewhere between 50-60% of rated HP turbine speed).

You provided a pressure from the torque converter, but we have no way of knowing if that's the same pressure as on a working unit at your site (many torque converters operate at slightly different pressures). Some torque converters have orifices in various drain lines; have you consulted the 'Starting Means' P&ID (GE calls them Piping Schematics) to determine if all the components of the system are in place--including solenoid-operated valves and orifice(s)?

Something is amiss. It's not the fuel control, because fuel does accelerate the turbine when the logic signal is forced--just very slowly. A torque assist is necessary during acceleration to prevent putting too much fuel into the machine and causing an exhaust over temperature by trying to accelerate the turbine using more fuel than torque.

Please write back and let us know what you determine.

 

Well explained!!

 

> Do you know if the 4 Kg/cm2 is according to spec? I have no experience of this particular torque converter on a
> Frame 3 but, normally on Frame 5 & 6, it should be at least 6 Kg. Have you checked the foot valve and trash screen
> on the torque converter?

checked. 6g is too much