GE Frame-5 Gas turbine Ratcheting system Hydraulic pressure low Problem

We are facing a problem with the GE Frame-5 Gas turbine on ratcheting system. When the ratchet starts, it runs every three minutes without an alarm, but on the third cycle, the ending time, the ratchet clutch is disengaging and reengaging frequently. The following have been checked.

  • We checked the hydraulic pressure and found it to be low, with a maximum of 32 bar. Then suspected a pump issue, so we replaced it with a new one. We started it again, but the hydraulic pressure was still low.
  • We then checked the DC motor and replaced it with new one, We started it again but the hydraulic pressure still low with a maximum of 32 bar.
  • 20CS: Clutch solenoid valve checked (coil, tested, supply is ok 120vdc) and found normal.
  • The clutch piston was working normally (if the 4-way valve was operated, it was found to engage and disengage normally).
  • The hydraulic line filter was checked and replaced, and found to be normal.
  • We inspected all fittings and found no physical leakage.
  • During the Ratcheting Sequence, there were no alarms, and 33HR, 33CS were checked and working normally.
  • During Forward stroke end, the 33HR is working normal and it checked contacts
  • Sequence valve is working normal because there is no alarm. Physically shaft is rotating.
  • I noticed that while the Ratcheting Shaft should always be engaged with the Turbine shaft, during the sequence the ratcheting was pushing forward to the Turbine shaft, but the Turbine shaft was moving quickly and the ratcheting shaft was coming slowly, So it makes a small space, after sequence stopping time, then shaft is disengaged.
  • But only one problem during ratcheting time the shaft is disengage and engage.
  • We suspected ratcheting system some internal passing oil and it not building pressure, I don't know where is passing oil. Kindly give some ideas to rectify the issue.

Thanks & Regards,
Selvalagan. A
The normal sequence is that every three minutes the ratchet mechanism rotates through approximately 1/8th of a turn (approximately 45 degrees). That usually occurs in a "jerky" motion through the forward stroke, then when the end of the forward stroke is reached the ratchet goes through a return (retraction) stroke and begins another forward stroke. That partial forward stroke ends very quickly; it's purpose is to try to ensure that the clutch halves remain engaged during the next three minute period.

Now, GE-design Frame 5 heavy duty gas turbine shafts are relatively light and the bearings are filled with oil which makes them relatively easy to turn. The turbine output shaft is coupled to the input shaft of the reduction (load) gear box, and the output shaft of the load gear box is coupled to the drive end of the generator rotor. There is some "backlash" in the load gear box, AND the turbine shaft and the generator shaft have different inertias. So, it's pretty common for the clutch halves to separate a little bit during the jerky movements of the forward stroke--but, again, the short forward stroke is designed (intended) to make sure the clutch is engaged when the ratchet operation shuts down for the three minute period.

The teeth of the two jaw clutch halves have smooth and slightly rounded edges and a slight bevel--all designed to help keep the clutch halves engaged. HOWEVER, over time, and with some abuse, the jaw clutch teeth begin to get damaged to the extent that they will not stay engaged as they should under normal circumstances. (The abuse usually takes the form of attempting to start the turbine before the turbine shaft has come to rest. Repeatedly in some cases, since people don't think the clutch halves are susceptible to damage. Ever.) Finally, the teeth can become damaged just during normal operation over a period of decades to the point they won't stay engaged properly.

So, it would seem the ratchet system and jaw clutch halves have probably worked satisfactorily for many years, maybe even decades. And, now it's not working exactly correctly. Something could also be amiss with the ratchet pistons--worn cylinders, worn piston seals, worn mechanical connections. Worn jaw clutch halve teeth, ratchet piston/connection issues, and two dissimilar inertias aggravated by the backlash in load gear teeth.

You don't say you've replaced the hydraulic ratchet pump. [EDIT: Sorry I missed that bit in the original post.] Usually the hydraulic oil for the ratchet comes directly from the L.O. Reservoir--and is only filtered by the filter you replaced and say was found to be okay. But what about filters before the one you replaced? Were they all good and intact? The hydraulic ratchet pump is pretty small and is usually a positive displacement pump--meaning that everything that enters the pump has to exit the pump. And, they take a pretty good beating over the years, too. And usually get very little respect. (Kind of like the jaw clutch... and the ratchet filter.) If the pressure is still low (How low is it--compared to other machines you have data for?), and you've replaced the pump, then there must be a reason elsewhere that hasn't been discovered yet.

So, if the ratchet system worked for some time (years; decades) and has just begun to have issues--you now have the knowledge you need to troubleshoot and resolve the problem. It might require replacing the jaw clutch, or maybe having it machined to clean up the teeth and clutch faces. It might also require replacing the hydraulic ratchet pump. It might require opening up the torque converter to see/inspect the ratchet pistons and mechanical connections. But, you can return it to normal operation.
I guess no need to replace pump with immediately if pressure is low
  • If pressure is low first try to replace FH-1(HR supply filter)
  • Test the integrity of VR-5 verify the limit switches operation ( pump pressure relief valve)
  • If still pressure low then replace FH-5( HR hydraulic supply filter)
  • Make sure pressure and instrument devices are ok
  • Measure the clearance between jaw clutch’s
  • Test the integrity of jaw clutch hydraulic cylinders
  • Once try to manual ratchet by using jog switch (43HR) and verify the ratchet operation
Dear WTF? ,

Thank you for your valuable information,

I got some usefull information from your post to rectify this issue.

Our ratcheting system working but hydraulic pressure low, after some time disenagage, so i analysed with fluid flow diagram, Ratcheting system working but system is not building back pressure, that means oil is leaking somewhere in internally. i tested all individual parts like VR-5, NRV, SOV, 4 Way valve for Jaw Clutch piston, Jaw Clutch piston, Sequencer valve, Cylinder HM-1.
Finally we got one thing, The sequencer valve output to Cylinder (forward storke line), we put put plug and started ratcheting system, found getting 80 bar hydraulic pressure. that means the Cylinder piston seal is in weak condition. our maintenance crew checking the cylinder.

Yes, you are Correct, this Gas turbine morethan running 20 years,
Thank you for your quickly.

Dear Vscontrol,

Thank you for your reply,

  • If pressure is low first try to replace FH-1(HR supply filter) - After found pressure low, first we replaced filter only.
  • Test the integrity of VR-5 verify the limit switches operation ( pump pressure relief valve)- Yes, we tested VR-5 also, we put pulg at pump discharge line and only VR-5 is path for discharge the oil, we tested found hydraulic pressure reach 80 bar pressure. That means VR-5 working normally.
  • If still pressure low then replace FH-5( HR hydraulic supply filter)- We replaced filter also
  • Make sure pressure and instrument devices are ok- We tested all instruments SOV, limit switches (33CS, 33HR) and working normal
  • Measure the clearance between jaw clutch’s- Not yet checked, we will check
  • Test the integrity of jaw clutch hydraulic cylinders- We checked Jaw Clutch hydraulic cylinders found normally working
  • Once try to manual ratchet by using jog switch (43HR) and verify the ratchet operation- Ratchet operation running normally.

We got some issue with cylinder, our maintenance crew checking the cylinder, i will update you soon,

Thank you for your valuble information

Selvalagan. A