turning gear speed

G

Thread Starter

getdy

the turning gear speed of gas turbine is faster than steam turbine. the reason is whether temperature in gas turbine is higher than in steam turbine?

And what does the setting of the turning gear speed consider?
can the lowest turning gear speed of 9E gas turbine reach?
 
A

Andrew turner

You should not assume that the turning gear speed of all gas turbines is higher than that in all steam turbines just because that is what you observe in your installation.

The main function of turning gear in both types of turbine is to prevent rotor sagging when hot, that is distorting under its own weight when static and still hot from operation. The speed required to do this depends on the construction of the rotor and turbine, the distance between the bearings, the weight, rigidity and construction of the rotor. Sometimes ratcheting is sufficient, that is turning just 45degress every two minutes sometimes a speed of 8 rpm is required. The 9 e can be ratcheted but sometimes is slow rolled at 8 rpm. It depends on the installation, combined cycle, simple cycle.

In any event you should not consider trying to change the slow roll speed. This cannot be done on a slow roll turning gear motor without changing gear ratios, you should not alter time between ratchet cycles, and if the slow roll is provided by the LCI again you should not alter this speed, you may damage the LCI.
 
Thank you to answer my question. What is LCI?

My company has 3 combined cycle units in which gas turbines are 9E. one 9E gas turbines turning gear speed is 0.21% TNH. the others are 4.2~4.7%. I don't know what is the reason for differences of turning gear speed.

the higher of turning gear speed, the more heat of boiler loses. I want to slow down turning gear speed.
 
getdy,

There is so little air flow caused by the rotation of the axial compressor at these low speeds (approximately 0.21%, or approximately 6 RPM, and remember: the IGVs are also closed (approximately 32 DGA) while on cooldown.

It's natural convection that draws air through the inlet air filters, the IGVs, the axial compressor, the combustion section, the turbine section, the gas turbine exhaust, and the exhaust transition to the HRSG (boiler) and up the exhaust stack. The height of the exhaust stack also has a lot to do with the amount of draft created by the HRSG and exhaust stack.

It's like a fireplace with a chimney. If you build a fire quickly without waiting for the chimney to heat up, the house will fill with smoke. But, once the chimney gets warm a natural draft is created that draws air into the fireplace, across the burning material, and up through the chimney.

Even if the GT is not turning at all there is a very large natural draft when there is an HRSG (boiler) and exhaust stack.

Many sites have installed roll-up doors like are used on many warehouses and mechanical shops between the inlet filter compartment and the inlet duct work. When the unit reaches cooldown speed they then close the doors to restrict the air flow through the turbine and HRSG to keep heat "bottled-up" in the HRSG.

But, the speed of rotation is not your problem. Even if the shaft were only rotated 45 degrees every three minutes, there would still be a natural draft created by the heat in the HRSG and exhaust stack. That's just natural physics, and the speed of rotation of the axial compressor rotation while on cooldown isn't going to appreciably increase the draft.

The speed of rotation of any turbine while on cooldown is generally determined by available components and the amount of energy to be consumed to rotate the turbine (and generator) shaft, and the cost of the available components (as opposed to having to design and manufacture a mechanism from scratch). Sure, there may be some manufacturers who will say it has to turn at this or that speed, but in reality, as long as the shaft isn't allowed to be stationary while it's hot/warm to protect against bowing/warping--that's the only real consideration.

An LCI (Load-Commutated Inverter) is a device that outputs a variable frequency AC voltage and current that is used to power the synchronous generator, using it as a synchronous motor, to rotate the turbine, usually during starting and acceleration. I've only heard of a couple of GE-design heavy duty gas turbines that use the LCI (sometimes called SFC--Static Frequency Converter) as the cooldown means.

If you want to design your own cooldown mechanism ("turning gear"), if you own the machines you are free to do so. You just have to accept the responsibility and consequences of doing so. Or, you can find some company who will do the engineering for you.

But, I maintain that slowing down the speed of rotation while on "turning gear" isn't going to solve the problem you didn't tell us about in the original posting.
 
LCI is Load Commutated Inverter, and is used when the generator is also used as a starting motor. This arrangement is mainly used on 9F and 7F gas turbines.

Do you have a stack closure to help bottle up the boiler? Closing this may also slow down the turning gear speed by reducing the natural draft through the turbine and HRSG ductwork.
 
Thank you.

I don't change turn gear, but I can change control logic sequences to lower rotor rotational speed during cooldown.

Our combined cycle units don't have stack closures.

Each 9E gas turbine rotor rotational speed is 0.21%TNH, 4.2%TNHand 4.7%TNH when cooldown.0.21%TNH is contrast to 4.2%TNH,I think 4.2%TNH and 4.7%TNH are high leading to more natural draft of following boilers. So I just want to reduce the rotor rotational speed, for example form 4.2%TNH to 2.5%TNH.

I don't why rotor rotational speed are different during cooldown, they are 9E gas turbines.
 
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