We have two 9E dual fuel gas turbines.Both the machines were in standstill condition for 12 to 14 hours and maximum wheelspace temperature were 33 degC for each machine before going to standstill. We made the Turning System of both GT normal and put them into turning after 14hours. But GE T/A here at our site advised both machines must be four hours on Turning before start. Can anyone please help me regarding this issue?

I have the experience with Siemens V94.2 & MHI701F. In case of Siemens V94.2 we can start the machine after one or two hour turning and in case of MHI701F we can start the machine two/three hours turning.

 

Rokan Uddin,

GE TAs typically have extremely little experience operating GE-design heavy duty gas turbines, and they usually cite what the "Company line" is with respect to issues such as this.

The idea behind putting the unit on turning gear for some time before starting it if the unit has been off turning gear for some time is to try to help relieve any sag in the axial compressor rotor which may have occurred just due to the weight of the axial compressor section.

The axial compressor section is heavy--much heavier than the turbine section of the rotor--and over time gravity will naturally cause some sag even if the rotor was cool when taken off turning gear. If the rotor was not "cool" when taken off turning gear then any sag due to gravity might be slightly accentuated.

Wheelspace temp's are used as the criteria for taking a GE-design heavy duty gas turbine off cooldown (turning gear; ratchet; slow roll; etc.) because they are closest to the bearings and bearing babbit melts at about 300 deg F and heat from the turbine section does radiate out and to the bearing pedestals.

Bearing metal temp's are not a good indicator because of the lube oil flowing when on cooldown. And, again, in addition to trying to protect the bearings from heat damage during--and <b>after</b>--cooldown, the axial compressor rotor needs to be cool to prevent sag when cooldown is stopped. But, there aren't any temperature sensors (T/Cs) in the axial compressor to sense the axial compressor rotor temperature (the axial compressor discharge temperature T/Cs are actually mounted in the turbine section frame so they're not useful for this purpose).

If a machine was taken off cooldown less than approximately 24 hours before it is re-started, unless the ambient temperature was very cold when it was taken off cooldown, it's probably not necessary to put in on turning gear for very long (if at all) before starting the turbine. The vibration may be a little higher than normal, but it should still be well within allowable during starting.

Even if the turbine has been sitting for days or weeks there is no start-check permissive that would prevent starting the turbine without putting it cooldown for any period of time. In an emergency it's important to be able to start the turbine.

The four hour recommendation is just to try to minimize any axial compressor rotor sag and thereby minimize any vibration excursion due to same during starting. If you know you're going to be starting the unit and it's been a couple of days or more since it was on cooldown it's just recommended to put the unit on turning gear for a couple (four?) hours to, again, try to minimize any natural sag in the rotor just due to gravity and the weight of the axial compressor rotor.

I've started many GE-design Frame 9E heavy duty gas turbines without any cooldown period at all, some which had been sitting for a week or two, and other than some higher-than-normal vibrations during the acceleration nothing untoward happened.

This is not a hard and fast, concrete recommendation--and it has to be tempered with actual conditions experienced when the unit was last shut down and taken off cooldown, how long since it was taken off cooldown, and consideration of ambient temperature versus machine temperature when it was taken off cooldown.

Another often over-looked reason for recommending some period of cooldown before starting is to raise L.O. temperature. It should be a minimum of 80 deg F, but many L.O. tank heater temperature switches are set to stop the heaters when the tank temperature gets above 70-72 deg F, which is marginal.

Low L.O. temperature can increase oil whip in the bearings which can increase vibrations during starting and acceleration. So, if the turbine has been sitting for a long time and the ambient is at or below approximately 70 deg F, it's a good idea to put the unit on cooldown for some time prior to starting the turbine just to start raising the L.O. temperature so that cold L.O. doesn't cause high vibration during starting and acceleration.

Hope this helps!

By the way, the above is true for most heavy duty gas turbines--regardless of manufacturer/packager. Axial compressor rotor sections are heavy, and cold L.O. causes high vibrations during starting and acceleration--regardless of machine manufacturer/packager. Some manufacturers have slightly different recommendations--but they are for the same reasons (reduce axial compressor rotor sag and increase L.O. temperature before starting and acceleration).

 

Well explained by CSA

Take care
G.Rajesh

 

Thank you very much CSA for your explanation.