LCI Vs Hydraulic pump starting - Gas turbine 2

[NickParker]

There are several methods to start the GTG, primarily:
- LCI (Load Commutated Inverter)
- Hydraulic Pump Starter
How is the decision made between using LCI or a hydraulic pump starter? If LCI is used, is a turning gear still necessary?

 

[wcaseyharman]

Most of our older, smaller gas turbines use an electric starting motor through a torque converter. Only the areo-derivative gas turbines use a hydraulic motor.

The newer, larger 7FA gas turbines have an LCI - I had just assumed it was more modern and cheaper since a separate gearbox, torque converter and starting motor are not needed.

 

[crshears]

In my experience, a separate turning gear will almost always be required on large machines.

Turning gear uses very low horsepower, rotates the machine at a very low speed, and is used to [a] eliminate sag and eccentricity from machines that have been at rest during extended overhaul, or to prevent permanent and irreversible shaft damage and warping due to uneven cooling following machine shutdown; the entire turbine shafting assembly is kept in slow rotation until the temperatures throughout the machine's mass have equalized.

Depending upon design, turning gear can also be used to facilitate machine repositioning for inspection or during repairs.

The horsepower and speed requirements of gas turbine starting systems are completely different, and to me are not in any way interchangeable with turning gear.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wcaseyharman]

My utility has a mix of turning gear and “ratchet” systems to prevent shaft sag. Turning gear typically runs somewhere between 3 and 6 RPM. Ratchet systems in my company rotate a quarter turn every 3 minutes.

Even LCI equipped turbines have turning gear.
One problem with turning gear is it can wear the root of the turbine buckets and the wheel due to the rocking action of the blades while on gear if the turbine is on turning gear for extended periods.

 

[edison123]

Interesting. Has it been known to happen since barring gear is normally continuously running and how it rocks the blades at such low speeds?

Muthu

http://www.edison.co.in

 

[wcaseyharman]

We’ve had to re-coat turbine wheels at several sites for this rocking action due to the weight of the blades/buckets. Granted, these turbines are 40 years old, but the 6 rpm turning gear is not fast enough to lock the blades in place, I’ve heard turning gear speed has to be upwards of 50 rpm for that.

 

[crshears]

Is this sloppy fit a gas turbine thing? I was always taught that steam turbine blades are tightly mounted, with the last one installed being solidly pinned.

This blade rocking thing on turning gear is complete news to me; I've never heard it mentioned before.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wcaseyharman]

Yes, gas turbine issue. Of the 5 steam turbines at my utility I have never heard them having issues with bucket rock, and we haven’t spent any time thinking about reducing turning gear speed either.

We have been toying with the idea of adding VFD and slowing downing the turning gear speed further on some of the turbines to reduce wear.

Excessive turning gear operation also can impact generator rotors, but in our experience it’s not as big of an issue.

Casey

 

[edison123]

I have also never seen either turbine or generator developing any issues in barring gear in my 40 years of servicing them.

Any specific generator incident, caseyharman?

Muthu

http://www.edison.co.in

 

[wcaseyharman]

GE 7E 74MW gas turbines with 6 RPM turning gear speed, installed 1981/1982. They’ve developed excessive wear on the turbine wheels from turning gear. Of the four installed I think two of them have had to have the turbine wheels flame sprayed.

At least 10 years ago we also had issues with a 1983 vintage Westinghouse 501D5 with migrating locking pins on the turbine which also was attributed to turning gear - those unit have enormous amounts of turning gear time.


We’ve had no obvious issues in the generators that we can attribute to turning gear operation.