Hi guys,

M new here,
currently I am working with GE supplied Gas Turbine Frame 6 machine. Recently we had shutdown for one of the gas turbine, and after 3 hours of rachting <i>[moderators note: I think he means ratcheting]</i> "Rachet trouble" alarm appeared. And on machine racthing was not happening.

We checked starting clutch solenoid valve 20CS-1, and found one of its drain port choked with foreign materials.

Lastly we found 20CS-1 SOV -ve supply fuse blowned off.

I want to know the cause of "Rachet trouble" alarm. Did fuse blowing caused the ratching to stop???
Does 20CS-1 need to be remain energised for ratching??

 

If the ratchet mechanism is one of the "self-sequencers" then the likely cause of the alarm is a failure of the 33HR-1 limit switch to indicate that an end of forward stroke had been reached in some period of time (some machines were set for 6.0 minutes, some shorter, some longer).

If 20CS-1 can't be energized (such as when the fuse is blown), then the self-sequencer won't operate and port oil to the proper actuator pistons to make the shaft rotate through it's normal periodic cycle. When the Mark VI detects that the shaft hasn't rotated (as indicated by reaching an end-limit of the forward stroke) for some period of time, then it lets an alert operator know there is some problem with the ratchet mechanism and that the shaft likely isn't being rotated as it should be.

All of this should be contained in the application code in Toolbox.

Typically, there is a filter in the oil supply line to the self-sequencer, but since there typically isn't a differential pressure gauge or switch, most sites don't bother to replace the filter until it ruptures and lets a lot of crud go downstream.

Have a look at the Starting Means Piping Schematic (P&ID) for the locations of piping components in the system, then go to the Access. Compartment and locate them. If there is a filter and it's never been changed, there's no time like the present.

If you're trying to understand why the fuse blew, it was because of excessive current being drawn through that fuse. There could be several causes for excessive current draw, and one of them could be debris/blockage you describe.

Others could be wiring practices, grounds, a faulty or intermittent 20CS-1 solenoid, loose terminations or wire terminal crimps, etc. In other, the usual culprits.

I would also ask if there is a wire at the SO terminal of the 20CS-1 solenoid output, and if so, are there any jumpers to any other terminals in the Mark VI panel (SO terminals of other solenoid outputs)? In other words, is there a field wire at 20CS-1? And, is there a jumper there (whether or not there is a field wire)? If there is a jumper, where does the other end of the jumper terminate?

 

the overly short answer (for the Germans and terse people who like bullet point answers )

The RATCHET trouble will come when the ratchet cycle ie one forward and one reverse stroke is not completed within 30 sec (the usual time delay) since the start of the ratchet motor and the energization of the 20CS-1 clutch engage solenoid. The 20cs-1 must always be energized during the forward and the reverse stroke so that oil can be ported to the self sequencing valve assembly and the Hydraulic ratchet. without this the cycle will not be completed (and the incomplete sequence is sensed using L33HRF hydraulic ratchet limit switch).

The overly long answer (for Indians, Americans and to people who like to read and read a little more)

How does the ratcher operate?
well to know about the ratchet is operated you need to know about how the ratcheting mechanical assembly and the logic sequencing in the speedtronic control system. I have included two diagrams for explanation . here they are

a. A typical Hydraulic ratchet mechanism
http://www.2shared.com/photo/jds7LUoP/ratchet_mechanism.html

b. A typical Hydraulic logic sequencing mechanism
http://www.2shared.com/photo/5qfwzBf2/RATCHETING.html


as you can see in the first pic the The hydraulic ratchet consists of DC ratchet supply pump, hydraulic self sequencing valve assembly, torque converter mounted ratchet, starting clutch. The most important part of the ratchet mechanism is the hydraulic self sequencing assembly which controls the sequencing of the hydraulic ratchet. The self sequencing assembly consists of

1. Three way poppet solenoid valve (PV)
2. Four way fully pilot operated valve (POV-1)
3. Four way fully pilot operated valve with limit switch (POV-2)

so given that the lO is functioning well. the ratchet motor is in top condition, the clutch is engaged (ok just assume that conditions are perfect for the ratcheting cycle to start )

step 1 : the ratchet cycle starts with the ratchet timer L2HR going high. this in turn will energize the L20CSX (command to clutch solenoid) and L4HR (the master signal to the hydraulic ratchet motor). this starts the forward stroke in the ratchet cycle. The 20CS solenoid is energized and ports oil to engage the clutch. only is the clutch is engaged will the three way poppet valve (PV) will allow oil to port to POV-1. the oil also ports to POV-2 causing the hydraulic ratchet to operate on forward stroke.this happens for about 12 sec.

Step 2 : Once the forward stroke is completed, the limit switch L33HRF goes high, signaling the end of the forward cycle. upon completion a port is opened inside the ratchet assembly which allows a pilot signal to shift POV-1. This causes a shift in POV-2 which will then port oil to the reverse stroke port.

Step 3 : the reverse stroke when completed a port opens inside the ratchet assembly allowing a pilot signal to shift POV-1 back to its original position. this in turn shifts the POV-2 back to its original position. This takes about 8-10 sec and the Limit switch L33HRF switches back to zero. signifying the end of the reverse stroke and completion of the POV's returning to the initial position. This is when the Ratchet timer (3 min usually) is initiated.

Step 4 : two sec after the L33HRF comes back to the initial 0 position, the clutch solenoid and the hydraulic ratchet motor are stopped. this cycle continues after 3 mins once again.

compare what i have written here, the two diagrams you will be able to comprehend about what actually happens during the hydraulic ratcheting.

coming to the actual question

Question 1 : I want to know the cause of "Rachet trouble" alarm? - in the above explanation, you will see that the forward and the reverse stroke takes about 20 sec to finish. if from the time the motor is running, ie the start of the ratchet forward stroke, if the motor is energised for more than 30 sec the ratchet trouble alarm comes. now there are two possibilities. the system may come back to its previous position. it the ratchet is operating slowly. this is caused due to the fact that the oil pressure in the ratchet and the self sequencing assembly is low. this may be due to leak in the circuit, or more commonly choke of the suction and discharge (40 mic) filter in the circuit. sometimes the ratchet will not complete its cycle at all, this may be due to the severe choke in the filter, or sometimes also due to the limit switch problem. you have to be in the field to diagnose this problem. but these are the
more common trouble areas.

Question 2 : Did fuse blowing caused the ratcheting to stop??? Does 20CS-1 need to be remain energised for ratcheting?? - Yes , without the solenoid energised the ratchet cycle will not get completed and the fuse blowing is the reason for the ratcheting to stop midway. The actual reason for the fuse blowoff is many, but did it occur again, intermittent faults are difficult to diagnose.

What you can do
1. Replace the 40 mic filters in the ratchet mechanism

2. If possible try to flush the ratchet line with lube oil. your mechanical people will do it for you. since you had so much choke this would be a good thing to do. time consuming yes but well better be sure right. choke in the small pilot lines will be a big headache so do the flushing as a precautionary measure.

3. check the wiring and replacement of the solenoid if the fuse blows again and again

NOte * - the ratchet is a shortduty motor and should not be allowed to run continuously for long periods.

 

Dear PV,

Very well explained. Thank you.

Link 2 is not opening, though we referred our GE logic diagram.

If L33HRF is problem, how to service or replace the limit switch as it's internal part.

G.Rajesh

 

Although PV's drawing showed the 33HR limit switch as internal, in fact it is mounted external to the Ratchet Module and is replaceable

 

Dear Rajesh,

if L33HRF limit switch is the problem, it is replaceable. I work for a company who represents Young&Franklin, who is the OEM as far as hydraulic self sequencing ratchets are concerned. Should you have any need you can contact me.

Omar
[email protected]