While starting the gas turbine, after applying the start signal the diesel engine starts. Initially for 2 minutes at 600 RPM then on the energization of 20DA-2 SOV, the speed of diesel engine goes to 2100 RPM. At the speed of about 2900 RPM gas turbine takes over the control and diesel engine disengaged from the Turbine.

At speed 2100 to 2900 RPM, does the diesel engine rotate either at its own maximum speed 2100 or with turbine speed 2900 RPM?

If it rotates at 2900RPM then we can say that gas turbine takes over after 2100 RPM which is the maximum speed of the diesel engine but disengage at 2900 from the turbine. What is the function of torque converter in all this activity?

Gas turbine frame 5, capacity 25 MW, diesel engine makes/ model, Cummims HP 465.

 

It is believed that the output shaft of the hydraulic torque converter is "directly" connected to the turbine (and generator) shaft (through the jaw clutch) via the load gear. In other words, there is no speed increase or reduction of the torque convert output through the Accessory Gear.

However, the speed of the output shaft of the hydraulic torque converter is NOT directly proportional to the speed of the diesel engine starting means.

The turbine shaft speed remains at zero until (with the assist of the Hydraulic Ratchet mechanism to overcome the inertia of the turbine shaft when starting from zero turbine shaft speed) the TORQUE output of the diesel (through the hydraulic torque converter) is enough to accelerate the turbine shaft.

The purpose of the hydraulic converter for the starting means (either a diesel engine- or an induction electric motor) is to provide a means to transmit torque from the starting means to the turbine shaft at variable speed. The shaft of a diesel engine or induction motor starting means cannot be directly coupled to the turbine shaft (the diesel couldn't be started, ... and the the in-rush for an electric motor would be extreme), so a torque converter is used.

The initial operation of the diesel is at what's called "warm-up" speed to allow the diesel a period of time to warm up prior to loading it to accelerate the turbine. After the warm-up period, the diesel is accelerated to rated speed, and through the hydraulic torque converter (with the assist of the Hydraulic Ratchet mechanism when starting from zero turbine shaft speed) the turbine shaft accelerates. The torque converter allows a speed differential (between the starting means and the turbine shaft) as TORQUE is being transmitted from the starting means to the turbine shaft.

After ignition and warm-up, the turbine shaft speed accelerates as fuel is increased. When the turbine shaft speed exceeds the speed output of the hydraulic torque converter, the jaw clutch "opens" (due to the speed differential) and the diesel speed is reduced for a "cool down" period. As the turbine shaft speed increases, the amount of torque being transmitted through the hydraulic torque converter decreases.

After warm-up, some diesel engine starting means are accelerated to to an intermediate speed to accelerate the turbine shaft to purge speed, and then after ignition and warm-up the diesel is accelerated to rated speed. Still others purge the turbine at a higher speed, decelerate to a lower speed for ignition and warm-up, and then accelerate to a higher speed to assist with accelerating the turbine.

F-class (and H-class) GE-design heavy-duty gas turbines use the generator as an electric starting means--by varying the frequency of the applied current to the generator during starting.

markvguy

 

Thank You Very much. Please explain in term of speed for this question for better understanding.

Does the diesel engine rotate either at its own maximum speed 2100 or with turbine speed afetr its maximum speed befoer diesel engine disengage from gas turbine at speed of 2900 RPM.

 

You're welcome, very much.

As was said earlier, there is no correlation between the speed of the diesel engine and the turbine (and generator) shaft(s)--they are linked through a hydraulic torque converter.

The key word is: torque. It's not about diesel speed with respect to turbine speed--it's about the amount of torque the diesel produces. It doesn't produce enough torque during warm-up to break the turbine away from zero speed; it has to be accelerated to develop the torque necessary to accelerate the turbine shaft.

If you have a Mk IV- or Mk V- or Mk VI Speedtronic turbine control system, there should a diesel speed signal--usually PN_RPM or PN or something similar. This is usually from a magentic speed pick-up on the diesel engine's ring gear perimeter--so it's actual diesel speed. If the diesel is at 2100 RPM and the turbine is at 2900 RPM when the jaw clutch opens, that's about normal.

The diesel governor keeps the diesel from overspeeding--in fact, it maintains the speed at the preset setpoints. That's why the diesel speed stays at 2100 RPM while the turbine continues to accelerate with the assist of the torque from the diesel being transmitted through the hydraulic torque converter, until the torque decreases to the point that the speed output of the torque converter is less than the speed of the turbine shaft and the jaw clutch opens.

While the turbine shaft is accelerating, the diesel is running at rated speed (2100 RPM?).

While the speed of the diesel is important, it's the torque output of the diesel that's more important--torque is what accelerates the turbine (and generator) shaft(s), NOT the speed of the diesel. The diesel probably produces maximum torque at its rated speed.

markvguy

 

I just want to ask... at what speed of the diesel engine will the torque be converted to rotate the turbine shaft from still condition? Is it at diesel engine rated speed?

 

It depends--on L.O. temperature, age of the equipment, condition of the equipment, whether or not the turbine is being re-started with a warm/hot rotor or is starting with the rotor at ambient temperature. There is no set diesel engine speed at which the torque required to accelerate the shaft speed is sufficient to "roll" the turbine (and generator) shaft.

Also, an assist is required from the Hydraulic Ratchet to overcome the inertia of the shaft when it's at zero speed--so the "position" of the ratchet mechanism can also have an affect of the timing of the shaft breakaway with respect to diesel speed.

In this author's experience, with all conditions "normal" the turbine shaft usually breaks away before the diesel reaches rated speed--but some older units with diesels which needed rebuilding or torque converters which needed refurbishing didn't break away until after the diesel had reached rated speed....

markvguy

 

Several people have written to say that it's not just the torque output of the diesel that's important, but it's the torque output of the torque converter that's important. (Remember--anyone can contribute to control.com--and anonymously, too!) The diesel produces torque on the input shaft of the torque converter. The torque converter then transmits that torque through to its output shaft, which is connected to the turbine (and generator) shaft.

The torque converter is kind of like a big hydraulic pump and -motor in one unit--the diesel is coupled to the input shaft of the torque converter which drives the "pump," which supplies high-pressure hydraulic oil to the "motor" which drives the output shaft of the torque converter.

So the "efficiency" of the torque converter--its ability to convert the torque coming in at one speed to the torque going out at another speed--is very important. The torque converter is really like a variable speed "automatic" transmission in a car. It's always been explained to this author that it's torque that accelerates a race car or a superbike; it's horsepower that makes them able to go at high speeds.

In any case, it's torque which accelerates the turbine rotor--regardless of whether it comes from a diesel engine, an electric motor, or an expander turbine.

markvguy

 

In our GE Frame V, PG 5361, the starting means is Detroit Allison diesel engine 12V71. When the DE is run through logic, the first 4 minutes the DE is run on idle speed for warm up period before accelerating to transmit torque.

Your appended answer is true with regard to the sequencing of diesel engine & torque converter, but my query is should the output turbine shaft also begin to rotate with the DE during warm up duration of 4 minutes or should it start to rotate after the DE accelerates.

<i>"It is believed that the output shaft of the hydraulic torque converter is "directly" connected to the turbine (and generator) shaft (through the jaw clutch) via the load gear. In other words, there is no speed increase or reduction of the torque convert output through the Accessory Gear.

However, the speed of the output shaft of the hydraulic torque converter is NOT directly proportional to the speed of the diesel engine starting means."</i>

---- snip ----

<i>"After warm-up, some diesel engine starting means are accelerated to to an intermediate speed to accelerate the turbine shaft to purge speed, and then after ignition and warm-up the diesel is accelerated to rated speed. Still others purge the turbine at a higher speed, decelerate to a lower speed for ignition and warm-up, and then accelerate to a higher speed to assist with accelerating the turbine.

F-class (and H-class) GE-design heavy-duty gas turbines use the generator as an electric starting means--by varying the frequency of the applied current to the generator during starting."</i>

HSF

 

markvguy hasn't been heard from in a while here on control.com. While I won't presume to speak for him, I will put my two cents' worth in on this question.

The diesel is not producing sufficient torque to break the turbine-generator shaft away from zero speed during warm-up. Also, the sequencing does not usually start the ratchet (used to assist with initially breaking the shaft away from zero speed) until the diesel warm-up sequence is complete and the diesel is being accelerated up to a higher speed.

Remember: The torque converter is a variable torque transmission device with "slip". The turbine shaft can't just be accelerated to rated speed in just a couple of seconds. And most combustion turbine starting means (a diesel engine or an electric motor or an expander turbine--wow, that's a grammatically incorrect series if there ever was one!) either run at one speed (electric induction motor) or don't develop rated torque until running at a higher speed (diesel engine, some expander turbines) and so there needs to be a means of allowing the starting means to run at different speeds than the turbine-generator shaft.

And, just because there is power being input to the torque converter does not mean there is power being output by the torque converter.

The torque converter is essentially an "automatic clutch" mechanism, just like an automatic transmission in an automobile (which has a torque converter, or at least they used to! it's been a long time since I've crawled under an automobile to perform any work!) or a motor scooter. It allows for a variable speed difference between the driving device and driven device. It's an "indirect" coupling device.

I guess one could used a "manual clutch" (pressure plate, clutch plates) for a variable speed/torque transmission device between a starting means and a turbine-generator shaft, but that would require much more maintenance and be much more complicated a torque converter, though both do exactly the same thing.

Hope this helps!

 

> Does the diesel engine rotate either at its own maximum speed 2100 or with
> turbine speed after its maximum speed before diesel engine disengage from gas
> turbine at speed of 2900 RPM.

The turbine in concern disengage at the of 2450 rpm