22 replies to this topic

[thesavager12345]

Hello everyone,

 

I wanna know if it is possible to run the centrifugal compressor and gas generator using one gas turbine. and if so what is the cons and pros?

 

Thank you

[Bobby Strain]

gas generator?

[thesavager12345]

Sorry, I mean an electrical generator.

[Bobby Strain]

It's not a good idea.

 

Bobby

[thesavager12345]

could you explain the reasons.

[thorium90]

The control requirements for the gas compressor and electrical generator is different.  There would come a time when the gas turbine would be given conflicting instructions.

 

Say the electrical generator needed the gas turbine to speed up to maintain line frequency or risk de-sync from the grid, but the compressor wants the gas turbine to slow down as the discharge pressure is too high.  What should the gas turbine do then?  Speed up and slow down at the same time?  Obviously it can't do both together.

[thesavager12345]

the gas turbine is gonna be associated with a gear box that comes with two output shafts, one for electrical generator and one for centrifugal compressor. so each shaft is gonna have its own speed.

[Bobby Strain]

Sounds like you have already committed to the arrangement. We'll probably hear from you again when you try to operate it.

 

Bobby

[thesavager12345]

No. It's just a proposition. No arrangement is done. That's why I want to know more info about it.

[thorium90]

Gear boxes with continuously variable transmission for both the compressor and generator?  That's interesting...

 

I still think its going be hard to balance the torque output to both at the same time.

 

Is there a significant advantage to combining them together?  My thinking is that all three equipment are expensive and putting them all in potential deadlock with each other has a potential to cause serious damage to each other one fine day in the lifetime of the plant.

 

I would think safety would be more important and having them segregated would ensure the utmost control and equipment safety.

[Art Montemayor]

anouarthebest:

 

I fully agree with both Bobby and Thorium.  I've never seen or heard of this kind of integrated drives on a gas turbine driver.  That doesn't mean that I'm correct in suspecting it doesn't make for a reliable and proven system.  I am willing to learn - even in my old age.

 

However, bear out the important features that would be in conflict with such an integrated system:

• the demands or needs imposed on both deliveries - electrical power output and process gas pressure and capacity - can easily be in conflict in a normally operating process plant;
• don't forget about the inherent operating characteristics of both driven services: the surge feature of a centrifugal compressor would be difficult to control when subjected to the demands of a power generation;
• What about the novelty of having both services tied up in one machine installation?  Does this mean you are content in stopping power generation when the compressor is down for maintenance or is undergoing an operational upset?  What about the reverse effect?
• Any spare replacement(s) for the associated installation would be a "special" design - with the associated higher costs and scarce availability in the future.
• Which is the most important and critical product for your unit's profit picture - gas compression or power generation?

[Saml]

I've been involved with such an arrangement. It is workable:

 

The reason we had for this arrangement is as follows:

 

Gas turbines come in fixed power values.  ie: GE has the LM-2500 at 24 MW and the next one is TM-2500 at 34 MW. Now suppose you need a base load of 21 MW with peak power of 25 MW  with a derating of 2 MW in turbine power during  summer conditions. The LM is small, the TM is oversized and will run at lower efficiency, specially during winter. So you add a generator to use that extra power.

 

In the case I know, it made further sense (from an economic standpoint) to add the generator because there was an HRSG with supplemental fire at turbine exhaust. If you reduce the turbine power, exhaust gas is coming in cooler and you need more supplemental firing to maintain the steam generation. So the marginal efficiency of electricity generation (at constant steam production) is very high. .

 

Gas turbines and centrifugal compressors are reliable machines and the arrangement works. The main problems come from the interactions.

 

If the generator is running connected to the grid, the speed of your system is governed by the grid frequency. The grid we were connected to is orders of magnitude bigger than our generator, so no way we could influence the grid frequency. Our only options were to follow (keep running at the grid frequency) or disconnect (to be able to recover the speed)

 

Disconnecting has a couple of problems:

 

-  probably you are contractually obligated (as we were) to shed load in case of low grid frequency to prevent a national/regional blackout. Say the grid is running 60hz. probably you are contractually obligated to reduce you consumption instantaneously by a certain ammount if requency goes, say, below 58.5hz. This happens when a major line or generator goes down somewhere in the grid. Now, if you disconnect your generator, you have to reduce the load by the contractual ammount PLUS the power you were generating or be subject to fines, because during those seconds of crisis in the grid, instead of reducing your load you are reducing the generation!.

 

-  You produce a "load rejection" where turbine speed varies for a few seconds. Turbine vendors desing for this. Just be sure that you don't trip your process during the transients.

 

Following the grid has it's own problems too: The height developed by the compressor is quadratic with speed. The seriousness of this problem depends on how critical the discharge pressure is to your process. The variables you have to control the discharge pressure are IGV and recirculation. Those are not fast, at least compared to the velocity of grid frequency variations. Imagine that the grid goes from 60 to 59 hz in 10 seconds. That is a reduction in speed of 1.7% or a reduction in head of 3.3%. Say your compressor is normally rising pressure from 10 to 50 bar. In 10 seconds the pressure falls to 48.7 bar. If that is critical to your process, then you are in trouble, If your process can handle it, you are probably OK.

 

If you are looking at an actual proposal, please contact me privately with your details. Because a former employer of mine is the owner of the information, I cannot post here the nitty gritty details of the actual implementation. Just the broad issues that are already known in our particular industry. However,  I may put you in touch (depending on your case) with the people operating that unit.

[thesavager12345]

in our case we don't have a grid. we produce our own electricity. it is a CPF plant. we want to install 3 process trains (3 working and 1 standby) + 3 turbo generators (2 working and 1 stadby) for electricity. the options are as follow:

 

1. each compressor and each generator comes with it's own gas turbine.

2. combine the compressor and EG in one gas turbine

 

obviously option two will minimize the number of Gas turbines. Hence the project CAPEX and OPEX.

 

Answering some the your questions about option 2:

 

1. According to the proposer, this system has been made in the past and it works.

2. it is gonna be a clutch installed at each shaft to the driven machine, so when one of them goes down for maintenance (say the centrifugal compressor) the EG will be operating.

3. both the products are important (Electricity and gas compression)

4. there is no special design (except the gear box)

[Technical Bard]

Althought slighty more expensive, it is far more operable to have the gas turbine drive a generator, and drive the compressor with an electric motor.

[Bobby Strain]

Nowhere is there a mention of the power required for the proposed service. This makes a big difference. And, some of the LNG plants use a combined turbine (single shaft) and motor drive for compressors. If you are generating your own power, best separate the compressor and generator services. Otherwise, you are in for problems for a long time.

 

Bobby

[thesavager12345]

quick update:

 

the centrifugal compressor and the electrical generator will be on the same shaft. A VFD at the outled of the electrical generator will be implemented to stabilize all the inflectuation that can be caused by the centrifugal compressor.

 

what are your thoughts?

Edited by anouarthebest, 17 September 2018 - 09:28 AM.

[thorium90]

The gas turbine? On another shaft. Clutch to connect the them?

Never seen a VFD being used to maintain grid frequency... Thought it would be by the excitation voltage. How would the reactive power be controlled? Would there be an issue to the compressor side if the generator goes into severe leading power?

Also, what about the VFD induced harmonics? For such a large load, the harmonic filter would be large and expensive.

Edited by thorium90, 17 September 2018 - 10:21 AM.

[thesavager12345]

You will find in the attached file a drawing of the whole system. there is no gear box. the shaft of gas turbine is connected to the shaft ogf the centrifugal compressor. the shaft of the centrifugal compressor is connected to the shaft of the electrical generator. the electrical generator is connected to the VFD to regulate the freequency and keep it constant at 60 HZ.

In our project we don't have a grid. we produce our own electricity in order to use in the process facility.

and there is no clutch, no gear box. just one shaft connecting all of the major equipment.

Attached Files

•  Sans titre.png   5.22KB   5 downloads

Edited by anouarthebest, 19 September 2018 - 04:53 AM.

[thorium90]

In that case, how do you startup the gas turbine?

In order to start the GT, you have to use the generator to turn the GT to the lightup speed so you can turn on the flame.

Your description of running on island mode is not quite correct.

Firstly, if there is no link to the outside grid, how do you start the turbine to reach lightup speed? Presumably you will have a GCB for this function. Note that the rest of your plant will then have to be in complete shutdown before this step.

Secondly, if they are all on the same shaft including the compressor, the starting load will also include the compressor. Presumably this can be considered in the startup load. The compressor will have to be forced to run in full recycle mode during the startup.

 

I can see there is a lack of electrical understanding of how a generator is controlled. Including stuff such as control of terminal voltage and reactive power control. As already mentioned, I foresee using a VFD to control the output frequency of the generator has major drawbacks. The harmonics problem will be large, which you have not understood how to address. It can be addressed but there is significant investment involved. Which makes this a less attractive option than having separate shaft.

 

Perhaps you can also consider the compressor to sit on the opposite side of the shaft connected by clutch. So it will be GT, generator, clutch and compressor. This will allow independent control and startup of GT then compressor and disconnection of the compressor in case of problems.

Edited by thorium90, 19 September 2018 - 10:13 AM.

[thesavager12345]

Black start is expected by using a Diesel Engine Generator.

For the harmonics problem, The high technology VFD proposed may outcome this problem.

[thorium90]

High technology VFD? Do elaborate.

[thesavager12345]

VFD with IGBT's

[thorium90]

All VFD's have IGBT's. IGBT is the basic component in a VFD. That is not advanced at all... That is how a VFD works...

You should be looking for AHF or passive ones. Also look for AC line reactors and DC link chokes.

Nothing to do with a VFD having IGBT's.