hi,

What's the meaning of free rotor use in cool down fail procedure & if in ge-frame9 we haven't ratchet device we can use turning gear start about two hours for between 15 minutes & 48 hours after cool down fail to restart again?

 

Frame9E's have a barring motor, not a ratchet. Our cool down speed is about 120rpm.

If your rotor fails to go onto barring after shutdown you have approx 15 minutes to fix the problem and get it turning again. During this time the rotor if free to turn. If it takes any longer the rotor will potentially rub or seize and do more damage if it is turned. If you can't get the rotor turning then it's best to leave it stopped for 48 hours to cool before turning it again. You should ensure the lube oil remains running to keep the bearings cool. In practise we monitor our wheel space temperatures and crank again when within 50C of ambient. It's very hard to actual times from GE.

Frame 9E's are well known to rub after tripping if not going straight on to barring gear 88TG. And sometimes still will even if they do. Other important points are to keep the doors closed, vent fans on, and the flexible vent hoses positioned where they should be.

By default the Speedtronics will continuously try to place the unit on cool down by energizing the cranking motor 88CR. The sequence if broken by the protection relay allowing 3 starts and then issuing a starter motor trip signal L3SMT. By far our main failure to go onto cooldown is lift oil pressure failures.

 

Hi,

In procedure for gas turbine cool down fail said if start gas turbine at max. 15 minutes check vibration velocity and if vibration velocity is higher than 25.4 mm/s , shutdown the unit and remain idle for at least 30 hours or until the rotor is free I can't understand what's the meaning the if the rotor is free , please explain for me.

Thank you

 

abdi,

I haven't read the procedure you are referring to. Was it provided by GE, or by some third-party organization or independent contractor?

If I understand your post correctly, you are asking what actions should be taken (and NOT taken!) if the turbine cannot be put on cooldown (turning gear; hydraulic ratchet; etc.) when the shaft is hot. If that's not correct, you will have to clarify.

Let's presume the unit was running for several hours at some load and the axial compressor and turbine sections of the shaft were both hot. The turbine is either shut down normally by the operator, or is automatically shut down by some unsafe condition, or it is tripped (immediate shut-off of fuel).

When the shaft gets to zero speed, the cooldown mechanism (ratchet; turning gear) is not working or has failed shortly after reaching zero speed, and can't be repaired. In this case, if the rotor can't be rotated in approximately 15-30 minutes then what will happen is that the compressor section of the shaft will begin to sag (bend; deform) under it's own weight. The axial compressor (rotor) is MUCH longer (usually 15-17 stages, versus 2-3 stages for the turbine section of the rotor), and is MUCH heavier than the turbine rotor section. When the compressor rotor metal is hot and is in its elastic range, it will bend if not rotated periodically (ratchet) or continuously (turning gear; cranking).

If the shaft is very hot, then the compressor rotor will begin to sag quicker if it can't be turned periodically or continuously (cooldown; cranking). It will actually sag to the point that the compressor rotor blades will come into contact with the stationary compressor casing. When the rotor blades are in contact with the casing it prevents the shaft from being rotated. If the shaft is rotated--or attempted to be rotated--when compressor rotor blades are in contact with the compressor casing then <b>damage WILL result</b>.

The good news is (and there IS good news!), the hot compressor rotor metal is only in it's elastic range, and even though it sags (bends; deforms), as it cools it will also "unbend" itself. All of the deformity will not disappear, but the majority of it will. This means that the compressor blades which had come into contact with the compressor casing will "rise up" and separate from the compressor casing as the compressor rotor cools and returns to near its normal shape. The shaft can then be safely rotated, using the ratchet/turning gear, or even by using the starting means to crank the turbine, which will then help to remove most of the remaining the sag/deformity.

So, the <b>BEST</b> thing to do when the shaft can't be rotated with the ratchet/turning gear after 15-30 minutes of being at zero speed when the shaft is hot is to <b>WAIT</b>. Wait at least 24 hours if the shaft was very hot/warm; if the cooldown mechanism failed after several hours on cooldown, then you might have a little more time to get the shaft back on cooldown--<b>but in no circumstances should you "force" the shaft to turn!</b>

Many sites try to get bottle jacks and hydraulic rams and long levers to pry against coupling bolts to <b>MAKE</b> the shaft turn after a half-hour or more at zero speed when the shaft is hot. And those sites that do that always spend a LOT of money to repair the axial compressor. And lose a lot of production (electricity; steam).

Again, patience is the key.

The whole thing about "free rotor" seems to be a reference to a rotor that is not "bound up", which is one reference to when the axial compressor rotor blades are in contact with the compressor casing and not allowing the shaft to turn by the ratchet or turning gear, or even the starting means.

So, it's CRITICAL to just sit and wait. Keep the Auxiliary L.O. Pump running to keep oil flowing to the turbine (and generator, and load gear if present) bearing to cool them. Temperature from the shaft can melt/deform the bearings if oil flow is stopped.

I also recommend "forcing" the cooldown system (ratchet or turning gear) OFF from the control system. It's NOT good to keep trying to turn the shaft if compressor blades are in contact or close proximity to the compressor casing. NOT GOOD at all. If the shaft does "free up" enough to start turning the rotor blades will rub and scrape the compressor casing and increase clearances which can decrease efficiency.

Also, opening the turbine compartment doors and/or turning on the turbine compartment vent fan while waiting for the compressor rotor to cool is among the worst things one can do to try to accelerate the rotor cooling. The compressor casing will shrink faster than the compressor rotor and that makes it even MORE likely that blades will come into contact with the casing. Opening the turbine compartment doors and/or turning on the turbine compartment vent fans does NOTHING to help cool the compressor rotor inside the compressor casing!

Most turbines require an assist from the cooldown mechanism (either the turning gear or the ratchet) to assist with breaking the shaft away from zero speed. So, if the cooldown mechanism isn't working then the starting means can't be used to crank the machine for several hours to prevent rotor sag/bow.

If the starting means can break the shaft away from zero speed to crank it at higher speed than the cooldown mechanism, then that's an option--but it's <b>STILL</b> only an option if the shaft hasn't been at zero speed longer than about 15-30 minutes while hot.

Once the appropriate time (usually a minimum of 24 hours) has expired waiting for the compressor to cool while maintaining L.O. flow to the bearings, then the shaft can be rotated, or attempted to be rotated, using the cooldown mechanism. It's best to use the cooldown mechanism to try to help remove the most of the remaining sag/bow; sometimes cranking immediately will result in high vibrations, and possibly even damage.

Rotate the shaft with the cooldown mechanism for about four hours or so, and most of the deformity will be removed. The turbine can then be re-started, and accelerated to FSNL. There may be a higher vibration than normal during acceleration but as the compressor heats up and the speed increases the last of the sag/bow/deformity will be removed.

Again: Patience is key. Everything will be alright after a minimum of 24 hours--which will allow time to troubleshoot/repair the cooldown mechanism. But, haste will almost always result in damage--very expensive damage.

If, when you try to turn the shaft with the cooldown mechanism, it doesn't rotate (i.e., NOT a free rotor), just wait. Unless L.O. flow to the bearings hasn't been maintained and the shaft has deformed/flattened the bearings, it just needs a little longer to cool and unbend naturally.

I hope this helps. It's a delicate subject, and people always want to ask, "But, what if ... ?" And the answer is: "Just be patient, and everything will be alright. Don't be patient, and sorrow will ensue."

 

Hi CSA,

if emergency shutdown is happen and don't rotate the gas turbine for 48 hours according my document must be made as following:

first stage :
1.rotation at minimum ( few rpm)by pulsing of starter motor
check / listen the group during rotation and slowing-down

second stage:
1. turning the unit down for half a day
2.cranking at 600 rpm (without firing)

-if vibration is too high ( up to 20 mm/s)trip the group , then turning down for half a day
- else cranking for 10 minutes then turning down

third stage:
1. normal startup
2. trip the group if vibration is too high (up to 20 mm/s)
3.turning operation for one hour

I don't understand exactly what' the meaning of turning the unit down. I think mean's start the turning gear but other people in my company say mean's turn off the unit in your procedure say after minimum time (24 hours)the shaft can be rotate for four hours or so.

please help me to write the correct procedure according my document.

Thank you

 

abdi,

My guess is this was translated from another language, and "turning down" really means 'turning gear' or similar.

I can tell you--it's overkill (translation: too complicated and involved).

It would be VERY unlikely that the rotor would still be unable to be rotated in 48 hours. You mentioned GE Frame 9, but there are GE-design Frame 9Es, and GE-design Frame 9Fs, and GE-design Frame 9FAs, and GE-design Frame 9FA+s, and GE-design Frame 9FA+es, and so on. The GE-design Frame 9Es are three-bearing machines, with a bearing between the axial compressor and turbine sections. The GE-design Frame 9F-class machines are two-bearing machines with NO bearing between the axial compressor and turbine sections. They would be most likely to need more time (more than 24 hours) if the rotor were very hot and it was not possible to use cooldown (ratchet; turning gear) when the unit reached zero speed, especially if the unit was tripped from load and not shut down normally (fired shutdown).

You need to provide more information about the turbine you are referring to. I can understand you want to understand the "procedures" you have been given or have access to, but a Frame9 is not a Frame 9.

And translated documents are never perfect, and most technical documentation is also never perfect even in it's native language.

Best of luck!