Recently our Frame 5 GT ratchet system failed during shutdown after the purge credit. This happened at night and there was some speculation as to a wait time as we weren't able to fix it within a few hours. The decision was made to wait 30 hours before restarting or attempting to ratchet. We didn't run these often at before this so a failure of the ratchet during shutdown wasn't a big issue as months would pass before it needed to run again. This did become a big issue this summer as we were running it every afternoon. Some of the concerns were the differential expansion on the blades and the housing may be an issue. Any input into this would be appreciated.
Frame 5 GE turbine wait time after ratchet failure?
- Thread starter jacarey67
- Start date
Where or which points did the staff/team investigate on troubleshooting this issue..
Are you suspecting a Mechanical or Controls systems failure...
Are you suspecting a Mechanical or Controls systems failure...
Any alarm/trip has been annunciated /dipslayed before or during the shutdown ..
I fixed it the following morning but was procedurally prevented from moving the rotor not a written but a verbal. Logically nothing was holding me out. . The concern isn't the fix it's the missed day of power generation because no one knew if it there was a time frame that it couldn't be rolled after sitting for a few hours.
The limit switch on the end of stroke contacts had fallen out. it was intermittently coming in and out.
jacarey67, Some of my reply may be information you already know, and some comments are just comments based on past experience, I can't back them up with specific documentation.
The purpose of turning gear or ratchet systems is to slowly rotate the rotor to prevent rotor bowing, most common for long shafts on steam turbines. Aeroderivative turbines typically have a cooldown crank time to attempt to cool the turbine rotor/blades/buckets and turbine frame at the same rate so you don't have an issue with uneven casing shrinkage causing blade tip rubs. Frame type gas turbines may use a turning gear or a ratchet. It depends on the starting means, generator type, (Air Cooled vs Hydrogen Cooled), and overall unit size.
The Frame 5 heavy frame units do not have that long of a rotor and in my experience do not have issues with rotor bow events. Casing expansion/shrinkage is also typically not a big problem as long as turbine compartment doors are not kept open after a shutdown that could cause differential cooling of the casing. Frame 5 tip clearances are not as tight as the never Frame 7 and Frame 9 units that have more issues with tip rub.
So to not directly answer your question, I would say that there is not a specific time that you should wait before rotating or ratcheting the machine after a failure to ratchet. Based on past experience assuming the turbine compartment temperature remains stable you should not need to wait. If this is a combined cycle unit with a HRSG that has the ability to induce a draft through the machine, that can increase the possibility of a rub due to uneven case cooling. Simple cycle units with a short stack, not so much, no draft.
It is virtually impossible to know how long if any you need to wait. Unless you have tip clearance probes measuring the case to blade tip you don't know if your going to rub tips if you ratchet or crank. But past experience says this is nearly impossible on a standard Frame 5 unit, these were not a tight clearance super efficient machine. They are built like a tank, and take the abuse of one.
The purpose of turning gear or ratchet systems is to slowly rotate the rotor to prevent rotor bowing, most common for long shafts on steam turbines. Aeroderivative turbines typically have a cooldown crank time to attempt to cool the turbine rotor/blades/buckets and turbine frame at the same rate so you don't have an issue with uneven casing shrinkage causing blade tip rubs. Frame type gas turbines may use a turning gear or a ratchet. It depends on the starting means, generator type, (Air Cooled vs Hydrogen Cooled), and overall unit size.
The Frame 5 heavy frame units do not have that long of a rotor and in my experience do not have issues with rotor bow events. Casing expansion/shrinkage is also typically not a big problem as long as turbine compartment doors are not kept open after a shutdown that could cause differential cooling of the casing. Frame 5 tip clearances are not as tight as the never Frame 7 and Frame 9 units that have more issues with tip rub.
So to not directly answer your question, I would say that there is not a specific time that you should wait before rotating or ratcheting the machine after a failure to ratchet. Based on past experience assuming the turbine compartment temperature remains stable you should not need to wait. If this is a combined cycle unit with a HRSG that has the ability to induce a draft through the machine, that can increase the possibility of a rub due to uneven case cooling. Simple cycle units with a short stack, not so much, no draft.
It is virtually impossible to know how long if any you need to wait. Unless you have tip clearance probes measuring the case to blade tip you don't know if your going to rub tips if you ratchet or crank. But past experience says this is nearly impossible on a standard Frame 5 unit, these were not a tight clearance super efficient machine. They are built like a tank, and take the abuse of one.
This is exactly what I needed to hear. there was some concerns about differential expansion from the enigneering department despite not having LVDT's on the areas where they had concerns. The manual basically says give it hell but some old rumors were preventing us from giving it the go ahead.
