Hello

What direction is the total thrust in a gas turbine?
What is the reason for that direction.

If the direction of the Thrust is towards the aft end than it should remain towards the forward end and should continuously be on the active side of the thrust bearing. Generally in gas turbines initially the thrust is towards the forward side but when the turbine becomes on Full load or FSNL the position of the shaft changes(mostly it remains in the centre). Can some one please explain the reason for this.

Thanks

 

Replying to Izhar's 13 July, 2014 - 2:59 pm post

This gonna be a lengthy reply!

What direction is the total thrust in a gas turbine?

Well, that depends, either the turbine is under start-up/shutdown or normal running condition. Read on to find how.

QUICK ANSWER:

Start-up/Shutdown: Towards Forward direction.
Normal running: Towards aft direction.

Q#1. What is the direction of thrust in individual rotary machines(in compressor and turbine mounted individually, on different shafts)? and what if they're mounted on a single shaft?

Q#2. What does "active" and "non-active" side of a thrust bearing mean?

A#1: The direction of thrust in a rotary machinery is always from HP(high pressure) side to LP(low pressure) side.

For Compressor the pressure of inlet gases is lower than that of outlet gases, thus the inlet side becomes LP and discharge side becomes HP. Therefore, thrust will act towards inlet(LP). Thus it will require a thrust bearing on inlet side.

For Turbine, the pressure of inlet gases is higher than that of outlet gases, thus the inlet side becomes HP and exhaust side becomes LP. Thus, thrust will act towards the exhaust side. Thus it will require a thrust bearing on outlet side.

From this, we can deduce that, the thrust in a compressor and turbine acts in opposite direction to each other.

Now, if we mount this compressor and turbine on single shaft (as is the case in GE-design heavy duty gas turbines), the resultant thrust of the arrangement will be ALGEBRAIC sum of the individual thrusts of compressor and turbine. As this two thrust are acting in opposite direction they'll tend to cancel each other.

Now, We'll look at three different operating conditions of the turbine.

1. Start-up: During start-up the starting means (diesel engine or steam turbine or electric starting motor) will run the turbine up to a minimum firing speed. Up to this speed, air is continuously flowing in the compressor and getting compressed, thus creating LP and HP side on inlet and outlet of the compressor respectively. No fuel is admitted to the combustion chambers during this, thus no flue gases flow to the turbine. This means that the air compressed by the compressor is passing to the exhaust through the turbine. As there is no flue gases passing through the turbine, HP and LP sides are not created, and hence no thrust in the turbine. Therefore the resultant thrust will be towards the FORWARD (inlet) end during start-up.
(Note: Need a thrust bearing in the forward side)

2. Normal running: During normal running, the compressor is compressing air, fuel is admitted in the combustion chamber and flue gases are passing through the turbine. As flue gases are passing through the turbine, the HP and LP sides are created, and hence thrust the in turbine. As the resultant thrust is algebraic sum of the two thrusts, they'll tend to balance each other. As the turbine thrust is more than compressor thrust the resultant thrust will be towards the AFT (outlet)side in normal running condition.
(Note:Need a thrust bearing in aft side).

3. Shutdown: During shutdown, the fuel flow is ramped down (decreased) and finally cut-off at a particular speed. Thus the thrust will be towards FORWARD end (as there is no turbine thrust).

As the compressor and turbine thrust balance each other, the resultant thrust can be "taken" by relatively small thrust bearings. Instead of providing two thrust bearings on forward and aft end, thrust bearing assembly is provided on the forward end. This assembly consist of a thrust collar and two thrust bearings mounted on either side of it.

A#2. Active side: the side of thrust bearing will "take" the thrust during normal condition.

Non-active side: this side will "take" the load in case of "thrust reversal" (may be there is no such term) i.e. when the thrust reverses its direction as in start-up/shutdown.

Coming to the end, the author wants to know how did you come up with this question? (I also had the same question when I started working on a GE-design heavy duty gas turbine, in my case it was the "small" size of the thrust bearing.)

Hope this helps you. Write back if you have any questions or to give feedback. Your feedback is important.
==========================================
"Feedback is a respondents feed"

 

SB

Thanks for the so detailed reply. Its very clear now. This question just came into my mind while I was wondering that compressor must have thrust towards the forward end but the turbine must have a thrust towards the aft end (I didn't know that but i thought it should be like this, used my common sense).

Your reply is very helpful. I hope to get more help and increase my knowledge on gas turbines.

Thanks a lot!

 

Replying to izhar's 14 July, 2014 - 2:46 pm post

Thanks for the feedback!

You're welocome for any help, essentially we're here to help each other.

=========================================
"one who shares his knowledge expands it."

 

SB,
Thank you for your detailed and clear reply. It is very helpful for me as I had electrical background.

Regards,
Hameed

 

SB,

I have one more question (or you can call it confusion).

1) Regarding the inactive and active side of bearings shouldn't the active side of the bearing be towards the forward. Because more thrust acts during start up and shutdown as there is no turbine thrust.

2) During the normal load operation the turbine thrust is more than compressor thrust, I think 3 out 4 of the thrust is taken by the compressor and the rest by the turbine (almost balancing each other but still there will be thrust in the aft direction). But as the turbine stages are impulse, it will further reduce the thrust (won't it?).

3) Is it a confusion? or am I not very clear about the definition of active and inactive side of thrust bearing?

4) Last but not the least, our frame 9E gas turbine has its active side of the bearing towards the forward end (I think).

Hope to hear from you.

Thanks

 

Replying to Izhar's 15 July, 2014 - 4:53 pm post

Active/loaded: take thrust during normal operation
Inactive/unloaded: take thrust during "thrust reversal"

well,

let me introduce a new term related to axial movement of the shaft, called CROSSOVER.

Under various operating conditions, the RESULTANT thrust of the machine changes its direction(as described in my previous post). Due to change in the direction of thrust, the shaft will move in the direction of the thrust, and that is called crossover. At the point of crossover, the shaft will move axially.Crossover is undesirable as large axial movement of the shaft is not permitted. As, for efficient operation of gas turbine, the clearance between compressor stator blades and rotor blades is minimum. But due to axial movement of the shaft, the rotor blades tend to engage with the stator blades, and thus cause wear and eventually inefficient operation of the turbine.

How to overcome this problem?

The rotor assembly (compressor+turbine) is designed in such a way that the resultant thrust remains unidirectional under wide range of operation of the machine. During stat-up and shutdown the resultant thrust acts in the forward direction and the direction is changed near rated load (acts towards aft direction), and also the magnitude of the resultant thrust acting in the forward direction is higher than that in the aft direction. It is that, there will be more thrust in the forward direction during start-up and shutdown, and the thrust will remain in the same direction for a wide operating range. Thus you can call the forward side as active side. As per the author's experience with frame 5 machine, there should be two different types of thrust bearings. Tilting pad equalizing type thrust bearings are commonly employed as "loaded" thrust bearings and taper-land type bearings are employed as "unloaded" thrust bearing.

CONCLUSION: Based on the design philosophy, active thrust bearing should be towards forward side. This author is not familiar with the particular arrangement on GE-design heavy duty frame 9E gas turbine.

And for the turbine stages, there is no comment from the author at the time, but it should be remembered that nowadays there are no pure impulse or pure reaction stages in a gas turbine, instead a blade in a stage is a combination of the two (Isn't this interesting?)

Hope this helps!

=========================================================================================
"Today, machines are more reliable than humans"

 

Replying to hameed's 14 July, 2014 - 10:46 pm post

Thanks for the feedback!

 

The author hopes that other members of the forum will also share their in-sights on the topic.

 

SB

Thanks again for the detailed response.

Our GT has the loaded thrust bearing at the forward end.That means that during the normal operating conditions the thrust should act in the forward direction isn't it? But you said earlier that direction of thrust during normal operation is towards the aft end.

Regarding the start up I understand that thrust has direction in the forward end and this thrust is more than in normal operating conditions.

My question is that shouldn't it be the other way round i-e active side should be at the aft end if and only if the thrust direction in normal operating condition is towards the aft end.

Current configuration of the our thrust bearing shows that during normal condition the direction of thrust should be towards the forward end(which is the not the case as per your previous explaination).

Inactive side being at the aft end shows that during cross over i-e during start up and shutdown the thrust should be towards the aft end which actually is not the case and as explained by you the thrust has the direction towards forward end during start up and shut down.

I think if we change the definition of active and inactive then only can we make things clear.

Lets suppose:

Active side: That side which has the maximum load.
Inactive side: That side which has the minimum load.

Now according to definition forward end has the maximum thrust during start up and shut down therefore we have active side at the forward end.

And during the normal operation the thrust is in the aft direction but has the lowest value therefore we have inactive bearing at the aft end.

I think the problem lies in the definition only. The concept of the direction explained by you is quite clear.

Looking forward to hear from you.

Thanks

 

Izhar,

During start up & at low load operation of a heavy duty industrial gas turbine, the "active" side of the thrust bearing is the forward end collar. As the load increases, the loading pattern of the thrust bearing changes & the aft end collar becomes "active". This can be verified by monitoring the forward & aft end temperature of the thrust bearing at different loads.

Next time when you start the turbine, observe the temperatures & note down the load at which the pattern starts changing. You can even retrieve the data from the trends on your HMI. Share your observations with us.

 

what is explained by the author in the very first post is true in theory, but industrial gas turbine rotor assembly (compressor + turbine) is designed in such a way that the resultant thrust remains unidirectional (in the forward direction) for a wide range of operation (as the author tried to explain in the second post). Thrust acts in the forward direction during start-up, shutdown and also for most part of normal running. At one point (may be near base load) the direction of thrust reverses towards aft direction. And the magnitude of thrust in the aft direction is less compared to that in the forward direction.

Hence, the active/ loaded thrust bearing is in forward direction and inactive/ unloaded thrust bearing is in aft direction.

Hope this addresses all your question.

===========================================
"looong night shift can make you feel dizzy the whole day"

 

Thank you all for such a great replies. During next start up I will observe the temperature readings and will share my observation.

Thanks again for sharing your knowledge.

 

hi all,

is there any specific reason to place the thrust bearing at bearing no.1? is expansion of rotor or stator has got any thing to do with the positioning of the thrust bearing?

 

I have a basic question regarding the direction of thrust force. Thrust is nothing but a reaction force. For eg, in an axial compressor, If the force due to pressure difference is from HP to LP (terminology from previous posts) or aft to fwd, then direction of reaction force must be in the opposite side, i.e, fwd to aft?

please correct me if I am wrong.

 

There have been several statements made in this thread, that, while not entirely correct, are confusing and do not apply to all gas turbines--particularly all GE-design heavy duty gas turbines.

For every action there is an equal and opposite reaction--that much we can all agree on, right? When a GE-design single-shaft (axial compressor and turbine sections coupled together to form a single shaft) is running at or near rated speed the compressor is pushing the air axially from the inlet towards the exhaust (from "forward" to "aft"???). This is the action: air being pushed (compressed) as it flows through the axial compressor from the axial compressor inlet towards the turbine exhaust (parallel to the axis of the shaft).

The reaction is that the shaft (axial compressor and turbine) is pushed axially in the opposite direction--from the gas turbine exhaust exhaust towards the axial compressor inlet (from "aft" to "forward"???). The thrust bearing is usually located in the #1 bearing area where it's much cooler than if it were located in the exhaust area (the #2 or #3 bearing area, depending on the design of the machine).

Yes; some of the shaft thrust created by the flow of hot combustion gases through the turbine section does offset some of the shaft thrust created by the axial compressor, but not much. It's not even very equal (the axial thrust forces developed by the flow of hot gases through the turbine section), as the turbine "buckets" (blades) are a combination of impulse and reaction blades performing the expansion and primarily turning the forces created by the flow of hot gases into rotational forces (torque). This is one of the differences between aircraft gas turbines and power generation gas turbines (or those used to produce torque to drive mechanical loads, including generators). Aircraft gas turbines want most of their energy to develop axial thrust to propel the aircraft forward, while gas turbines used to drive mechanical (including electrical) loads want to produce rotating torque--not axial thrust.

F-class GE-design heavy duty gas turbines do operate a little differently during starting and acceleration from their older, smaller brethren in that they have a "reverse" thrust during starting and acceleration from what is normally experienced during full speed operation, and the prevailing belief is that it's due to the starting means providing most of energy for accelerating and compressing the air during starting and acceleration. (Myself, it seems the axial compressor is doing the same thing--compressing air and pushing it towards the gas turbine exhaust which should still cause the shaft to move towards the axial compressor inlet, so this doesn't seem to fully explain why F-class turbines behave the way they do during starting and acceleration.) But, once an F-class unit reaches rated speed, the normal direction of thrust is just like every other GE-design heavy duty gas turbine--pushing the shaft towards the #1 bearing (axial compressor inlet) as the air is compressed and pushed out of the axial compressor discharge area towards the gas turbine exhaust.

Lastly, I have an issue with using HP and LP to describe the direction of shaft thrust forces; the air entering the axial compressor of a gas turbine is "low pressure"--as is the exhaust leaving the gas turbine (usually just a few mm of H2O). It would seem to be more descriptive to describe which direction the shaft is being moved--towards the axial compressor inlet or towards the gas turbine exhaust (axially) by the thrust forces developed (and slightly offset) by the axial compressor and turbine which are coupled together.

That's my two pence on this subject, for what it's worth.

 

Number of interesting explanations have been already given.

An assumption has puzzled me regarding the net thrust, The GE 9E gas turbine at our power plant is 11mm below the generator. For the GT and generator to couple together the the gas turbine has to tilt, The net result will be that the inlet of the compressor is lower that the turbine exhaust. This will result in a net axial thrust towards the Compressor inlet side.

Kindly share you views whether this 11mm height difference between GT and the generator will have an effect on axial thrust or not.

Thanks in advance.

 

rohan_ram,

About the only time this might have an effect on "thrust" would be when the unit was at zero speed, or on cooldown (hydraulic ratchet; slowroll; turning gear).

I would venture to say that any intentional difference in height between the turbine and generator would be to minimize the effects of heat during running. All materials (okay; 99.9999% of materials) grow/expand when subjected to heat--and the turbine section of the unit gets darned hot. (Most GE-design Frame 9E heavy duty gas turbines have water cooling jackets on the aft support legs just to help reduce the effect of growth of the support legs due to the heat radiating from the turbine casing.) And, when aligning equipment misalignments like this are intentional so that when the unit is up to rated speed and temperature the units are more closely in line (aligned) and so vibration due to misalignment is minimized and wear on the coupling and coupling bolting is minimized.

Hope this helps!

 

This answer is very clear and very helpful for newcomers in the area of Gas turbines.

Mechanical Engineer- Sri Lanka

 

Dear CSA,

I understood the compressor section (action & reaction). But about what happens in the turbine section still not clear, can you explain in another way.

Thank you