diesel engine starting a gas turbine 2

[mectarek]

hi, I'm a mechanical engineer..I'm new to turbines, I don't know alot about them....we have an old gas turbine that is started by a diesel

engine coupled through a torque converter... it's a very old engine and we don't have any documents about it..I'm trying to understand the

starting sequence of the turbine.. i found in the torque converter documents this chart but i couldn't understand it. So basically I need

an explanation of this chart and of the turbine starting sequence by a diesel engine

 

[LittleInch]

This is simply a chart showing the input and output of the torque converter. As the engine speed rises, the output torque drops off, but between about 800 and 3000 rpm the output powwer is relatively flat.

The only interesting thing about this the hand written note in french at 1250 rpm which appears to say speed before firing. it seems a reasonable speed for the turbine to be at a minimum before you introduce the fuel and start the ignitor.

I don't know what this is driving, but most GT's start on no load or recycle if driving a gas compressor so that the turbine doesn't take a huge engine to wind it up to speed.

Basically you turn on the diesel engine, connect it to the turbine which then slowly winds up to speed flushing any fuel out of the system (seemingly minimum speed here is 1250, but normally you go a bit more), introduce the fuel, ignite it and then hopefully you get fire in the turbine and then you can de-couple your diesel engine. There is normally an automated start sequence in the turbine controller

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mectarek]

what i can't undestand is the starting torque...if you look at the chart the starting input torque is about 400 lb.ft and the input power

is about 305 hp...if we calculate the input speed at the starting according to this torque and power we find it N= 3990 rpm which does not

make sense...

 

[LittleInch]

Err, maybe I'm missing something but I can't understand your issue. This graph is all about the torque convertor. At the start, your engine is going full throttle, but low output speed from the convertor, hence huge output torque.

How are you "calculating the input speed"?. Are you using another page of this manual? There's nothing in this graph that I can see about the input speed.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mectarek]

P = T ω (1)

where

P = power (W)

T = torque or moment (Nm)

ω = angular velocity (rad/s)

N(rpm) = π/30ω

 

[LittleInch]

Ok. That's the input speed into the TC. A little high for a diesel engine, but not out of this world. Is there a gearbox between the engine and the TC?

Does that make more sense?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mectarek]

There is no gearbox between the engine and the TC. I'm starting to thing that maybe the values at the chart are not accurate?

In the starting sequence, does the engine starts to turn alone then it's coupled to the TC when it reaches a some rpm or they are

coupled frome the beginning?

 

[LittleInch]

The values are accurate, but may not match the actual input into the TC. I don't know your particular system, but I would imagine you have some sort of clutch mechanism to allow the diesel engine to start and warm up before you connect the TC and then go full throttle on the engine. If you get a lower torque output the turbine will simply take a bit longer to wind up to it's minimum firing speed, providing it doesn't drop so low that the turbine won't speed up.

You will then need some sort of decoupling once the turbine fires to slow the Fidel engine to stop and not overspeed. How it does that is within your system somewhere.

Hope this helps.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[tbuelna]

I believe the chart represents the various operating limits of the torque converter device based on different parameters. There is no relationship between the curves shown, which is why there seems to be disagreement between the input torque and input power curves.

Torque converters and fluid couplings are hydrodynamic devices, and their ability to transmit torque over a wide range of input/output speed differential comes from converting varying amounts of the input power to hydraulic flow losses (or slip). Right at startup, when the output speed is zero and the input speed is nominal, the TC has 100% slip and 100% of the input power is converted to heat in the hydraulic fluid. Torque converters are usually limited by their capability to reject the heat transferred to the hydraulic fluid due to slip. That's probably why the input power curve shown in the OP is mostly flat over the operating range.