Hi ,

May i know the machine delatch from the turning gear at what condition basis .I mean the follow up of the parameters .

Actually a day back in my facility one of the GTG was kept stand still for 6 hrs with the wheel space temperature of less than 80 deg C .After that machine was tried to engage on turning gear but shaft fail to break away alarm was appeared and conformed physically shift doesn't rotate .It was checked torque converter filter , torque converter SOV and other parameters but it seems all are normal and again start command initiated to engage turning gear but again fail .

It was decided to ratchet the shaft 180 deg .After that start command initiated it was quit normal start with the starting motor as well in the first attempt and engaged turning gear .

with this above scenario there are so many comments like shaft bow due to this reason this was not break away .

Please let me clear if any of them having same experience in their own facility .As well advice me at which wheel space temperature or any other criteris has to look up before stopping the turning gear .

Thank you
Reg
Saravanan

 

Saravanan

Turning has gear has two uses,
First is to keep "rotating" (as you know it is not exactly a rotation) the turbine when it is warm to avoid bending shaft due to high temperature and second is to help the starting device to break away the turbine shaft a first start.

For the first one also called "Cool down sequence", it is normally an automatic sequence which last few hours (depending of the size of the turbine but usually between 10 and 24 hours after fired turbine reach again zero speed). it is also possible to occasionally stop (I cannot say this is a good practice even if it done in the field and everyone knows that it is done mostly for maintenance purpose) this cool down sequence before this timing expire. we use to say whenever ALL wheelspace thermocouples read less then 100DegC. Normally at the end of this cool down sequence (full or aborted) the clutch is still engaged and should stay like this unless you manually move the shaft.

In your case, it seems that your clutch is no more engaged for some reason. you will need to check why! I don’t know PG 9171E so I cannot really tell where to find but normally there are two pistonsto pull the clutch to engage it, unless it is a self engaging clutch. but in both case you should had a limit switch (magnetic type) to detect that the position of the clutch (engaged/not engaged). if this switch does not detect the correct position of this clutch this can lead to "fail to engage" alarm. when you did ratchet as you said 180deg you engage, for sure, the clutch which solve the problem. Check again the correct positioning of this switch and be sure it can detect the correct position of the clutch they are sometimes very sensitive and few millimeters apart for the correct position they do not detect anything.

Also when you said it can be due to "Shaft ending" (shaft bow is you correct term) NO! This cannot be.

Even if you stop the cool down sequence before the wheelspace temperature are well above 100DegC the only problem you will eventually face will be some high vibration on the shaft as the shaft will be EVENTUALLY bend between the bearing. and the ratcheting system including the clutch are BEFORE the first bearing. in all case this should not avoid the ratcheting system to break the shaft away especially if your ratcheting/cool down system is correctly working, meaning the shaft and wheels are not stuck in the casing.

In case this happen (it is called a banana shaft) and that you have some high vibrations on the shaft due to failure (and aborted) cool down sequence. I hope you know what to do to solve the situation and safely start the turbine again. if not tell me I will explain you MY technique (this is maybe not the best but at least it is working) but this is another subject for another thread eventually

 

Perhaps otised can help with decoding the designation, but a 9171E basically denotes a single-shaft Frame 9, "model" E heavy duty gas turbine. I have heard of early Frame 9s with actual turning gears (electric motor-driven gear mechanisms) used for cooldown operation, and I've even heard of a couple of Frame 9s with jaw-type clutches and ratchets for cooldown operation and starting assist.

But I believe the majority of Frame 9Es (particularly those with hydrogen-cooled generators) were supplied with torque converters and "oversized" Aux. L.O. Pumps/motors for cooldown operation. In this configuration the torque converter gets no assist from a ratchet or other means to break the shaft away from zero speed. A solenoid-operated valve, 20TU-2, is used to port Aux. L.O. Pump discharge pressure to the torque converter mechanism to maintain shaft speed during cooldown operation.

Unfortunately, the originator did not tell us what kind of starting means arrangement is in use on the Frame 9E that's experiencing trouble at his site so we are left to make guesses.

Torque converters do require maintenance over time. They need to be refurbished to restore them to their original condition to be able to transmit torque with the most efficiency. The earlier thread had some of the same issues, with the plant crew having to provide some assist to get the shaft to break away from zero speed. bel

We don't know how old this Frame 9E is, nor how it's operated (cyclic starting and stopping; base load operation; etc.) or has been maintained.

If the starting means is the typical arrangement described above (torque converter without ratchet), then the torque converter has a motor-operated torque adjustor drive mechanism to control the opening angle of the torque converter nozzles. There are limit switches to control the opening angle and sometimes these require adjustment. Sometimes the mechanism fails or is damaged.

To supply the maximum amount of torque during starting, the torque converters must be open to "maximum" which is typically defined as between 150-160% of the nameplate rated current of the starting means electric motor. So, the originator can monitor the current drawn by the starting motor during breakaway and it should be between 150-160% of nameplate rated current. If it's less than that, then the torque adjustor drive mechanism likely requires some adjustment. If it's 160% or more and the shaft won't break away then it's likely the torque converter is "tired" and requires some maintenance.

So, there's really too much we don't know about the unit in question and how it's configured. All Frame 9s are not created equally, and they can have a variety of different auxiliaries and yet all of them still produce power similarly.

 

9171E decoded:

first character (9) is frame size

next 2 characters, followed by sufficient number of zeros, is approximate rated horsepower

last character (1) is number of shafts.

So, this would be frame 9, 170,000 HP (126 MW), one shaft (HP rotor only).

Most of the frame 9's (E's and F's) I have worked on (mainly combined cycle applications) did use a turning gear, both for cooldown and for breaking away on start up. The turning gear is actually located on the generator.

Starting means on frame 9E's is almost always an electric motor; on the 9F's it is usually the generator used as a motor, with an LCI. I believe the LCI arrangement was used on some 9E's.