Dear All,

We have in our plant 2 turbine 5002C (under Mark VI). Since that's our production is in lowest needs, we have decided to operate with only one machine and let the second one in standby. Each month we permit between them, at the end One working and the other one in standby.

My questions are:
#1_What will be the risks if we let the machine
(the one in standby) in cooldown PERMANENTLY?

#2_Are their risks for the pumps (Ratched and QA)?

Thank you in advance for your technical support!

 

etoilesh,

The first question I have is: Why do you feel it is necessary to have the standby unit continuously on Cooldown? If it's because someone believes that the unit MUST be on Cooldown prior to EVERY start, that's false. If the unit has been off Cooldown for 24 or even 48 hours and it's started from zero speed, that's probably not going to cause any damage to the unit. If it's been off Cooldown for a week or a week-and-a-half or more, that will probably result in high vibration during acceleration, maybe even a trip.

The longest distance between bearings includes the axial compressor--which is VERY heavy. At zero speed, the axial compressor will cause the rotor to sag over time. How much time? That depends on several factors, the most important of which is: Was the unit fully cooled before taking it off Cooldown? Or, was the unit taken off Cooldown at the earliest possible time, which the axial compressor rotor was still warm? Of course, that's very difficult to say, because there is no temperature sensor(s) in the axial compressor rotor. It all depends on what the ambient temperature is (air that's being drawn into the axial compressor by the hot turbine exhaust stack), the temperature in the turbine compartment, and how hot the unit was when it was shutdown--as well as the length of time the unit was left on Cooldown.

Is it best to have a GE-design heavy duty gas turbine on Cooldown for some time before starting it? Yes. Is it critical to have a GE-design heavy duty gas turbine on Cooldown for some time before starting it? No--unless the unit has been at rest for a long time (more than several days) AND the L.O. temperature is below approximately 80 deg F. Again, left at zero speed for a long period of time the axial compressor weight will eventually cause the rotor to begin to bow or sag, which will cause higher than normal vibration during starting and acceleration. If the L.O. temperature is also less than approximately 80 deg F the cold oil can and likely will cause oil whip in the bearings--which can make any shaft vibration worse.

If you put the standby unit on Cooldown for one or two hours every day, which would held to keep the L.O. temperature higher also, this would be more than enough to help keep the shaft straight and prevent high vibrations and tripping due to high vibration during starting and acceleration.

To answer your question #1, it's probably not really ideal to have the unit on Cooldown continuously for a month at a time. The turbine buckets can shift in the dovetails which can cause increased wear on the dovetails. Probably not a real problem for units with hydraulic ratchet, but it certainly is a problem for units with turning gear or slowroll Cooldown mechanisms.

To answer your question #2, keeping the unit on Cooldown continuously for a month at a time is definitely going to increase the wear on the pumps and motors, which is going to require more frequency inspections. If the hydraulic ratchet pump motor is a DC motor, it will have brushes which will need to be inspected and replaced more often. The commutator will also need to be inspected more frequently. But, is it going to cause irreparable damage to run the Aux. L.O. Pump (and motor) and Hydraulic Ratchet Pump (and motor) continuously for a month at a time? No. Does it increase the likelihood of pump or motor failure because they are running a month at a time? Yes. But, to estimate by how much is virtually impossible. If you or someone at your site is concerned about this, it would be best to get a vibration/machinery expert to site and add some instrumentation to the equipment to monitor the condition to try to be able to anticipate impending failure before it occurs. Probably wouldn't be very difficult or expensive, and would probably give many people some piece of mind. Is it 100% accurate and foolproof? Nothing is.

So, talk about why it is felt the standby unit should be on Cooldown continuously for a month at a time--and consider the above. Again, it is certainly a good idea to have a GE-design heavy duty gas turbine on cooldown for some period of time before it is started and accelerated. But, unless the unit has been at zero speed for several days AND the L.O. temperature is less than approximately 80 deg F AND the unit was "prematurely" taken off Cooldown when it was shut down there is little likelihood of damage to the unit if it is started from zero speed with no time on Cooldown. Yes, the vibrations during starting and acceleration will be higher than if the unit had been on Cooldown for an hour or two before starting, but it probably won't trip on high vibration (unless the L.O. is cold) and there probably won't be any lasting damage to the unit if it does trip on high vibrations. The fact that the unit is spinning during acceleration, and even deceleration if it tripped on high vibration during acceleration, means the shaft is straightening and the heat of compression and combustion is helping to straighten the shaft also.

If you want a definitive answer about what kind of lasting damage can occur to the turbine and axial compressor by having the unit continuously for a month at a time you should be asking GE Oil & Gas (the former Nuovo Pignone) or the packager of the unit.

 

CSA,

One thousand thanks for your rich technical explanation.
Thanks a lot!