Our gas turbine is a GE MS5002D model, designed to run three centrifugal compressors BCL in series (gas compression process).

My question is: if the discharge pressure of the load increase what will happen to the engine (the speed of LP shaft). I mean the turbine request more energy or less.

for example my machine was running at 99%TNL, and the discharge pressure of the header was 220 barg.

if the pressure in the header will be 250, what will happen to the speed TNL?

thank you in advance

 

To answer your question the first thing we need to know is how are you controlling your compressor load (suction pressure, discharge pressure, or what?).

Normally, if you are controlling on Discharge pressure, and the scenario that you explained happened, then the system would see the discharge pressure as high and back off the LP to correct it.

Tell us a bit more, what control system are you using for turbine and compressor control? What caused the discharge pressure to go high?

 

Before going into the exact question here, if we need to increase discharge pressure or flow from any compressor, there are two ways.

1. Increase the speed
2. Close the ASV if it is recycling any gas

Similarly to reduce discharge pressure/ flow
1. Decrease the speed
2. Open the ASV to recycle some gas

These two parameters are managed by the load control system to optimize the machine operation.

Any disturbance or variation in the process parameters will be primarily managed by the Compressor load control system. Depending on the control philosophy, it may ask for a reduction or increment of LP shaft speed(TNL) according to any change in process parameter.

Since you have a configuration of 3 compressors working in series, any disturbance on the parameter of one compressor will be primarily managed with its own Anti-surge valve. The speed of driving equipment (here a GT) will be regulated at the most conservative limit (minimum required) of the three Loads/anti-surge curves. This is in order to keep the machine operating in most efficient state (less fuel burned and less recycling).

If your load control philosophy is to maintain, say 220 BarG at third compressor discharge, and due to some process disturbance this pressure changes to 250 BarG, immediate action will be taken by Load controllers to open the third ASV and then to reduce TNL gradually while closing the ASV to achieve a steady operating state. Again there will be other corrective actions depending on the transient effects/disturbances on the first 2 compressors.

Please share the type of Anti-surge and Load controllers you are using and the Load control parameter you are using in the site in order to understand the system better.

 

First thank you for your comments. before you answer my question I want to clarify some Inputs.

our gas compression plant is managed by CCC series 3 plus, including three ASV for each compressor stage, one load sharing controller for the hole train, and one master controller for the suction header, to keep all machines in most efficient state (load divided equally, suction pressure from operators) because they are four identical trains.

the main thing is all the ASV Are closed, all trains are running in control temperature, and the speed for all the train are close to each other between 99%-92%.

so we are controlling suction pressure. what I have remarked, our most powerful train which is MS5002D model when the discharge pressure is about 220, the machine enter TEMP control over 102% with same conditions (flow constants, ambient temp the same, ...) but when we closed an injection well for maintenance, the discharge header pressure rises and reach 250 barg, which is normal, but all the machine enter in Temp control earlier (TNL les 2-3%)

hope this will help you to investigate.

 

Looking at the last comment from you, I believe the question you are asking should be, "Why GT is reaching temperature control at lower speeds when the Discharge pressure of compressors are increased?"

Answer to this is: Temperature control mode of GT is a protective control to regulate fuel flow depending on the exhaust temperature. In case of higher discharge header pressure conditions, load on the GT is higher. You will be able to see that exhaust temperature of GT is also higher in this case. This will cause GT to reach temperature control at lower speeds.

You can also read some detailed explanation here below:

The scenario explained here seems quiet normal considering the process situations. Since the application is re-injection of gas into the well, the critical parameter to be controlled is suction pressure and Load control does not bother about regulating the discharge pressure value (Still there will be a protection for very High Disch. pressure).

For a Gas turbine, load is any torque which is acting against the rotation of its shaft. In case compressor drive machines, load on the GT is basically the work to be done or Torque needed by the compressor. One parameter which is closest equivalent of the load in this application is Compressor pressure ratio.To make it simple, In your site purpose of the compressor is to push enough gas out at the discharge header(into the well) in order to maintain the suction pressure at given set point.

This means when Suction pressure increases above set point, more gas has to be pushed to the discharge i.e,

-GT speed to be increased
And when Suction pressure decreases below set point,

-GT speed to be reduced
(These actions will be according to the compressor operating curves)

You have not mentioned any range of the suction pressure to be maintained, lets say it is 20 BarG. When the Discharge header pressure is 220 BarG, i.e, Compressors are creating a pressure ratio of 11 (220/20). In this case machine is reaching Temp. control at ~102% TNL as per your observation.

When the compressor discharge header pressure is increased to 250 BarG, the compressor pressure ratio to be created is 12.5 (250/20).
At any given speed, the work done by the compressor(GT LOAD) will be directly proportional to the compressor pressure ratio(CPR). Taking your example in original post, when machine is running at 99% speed, GT had to burn "X" amount of fuel when CPR was 11. When CPR is increased 12.5 due to change in process, GT will have to burn "X+y" amount of fuel to maintain 99% speed.

More the fuel you burn, more will be the exhaust temperature.
So you will reach temperature control in comparatively lower speeds(<102%) when CPR is higher.

 

well explained, what you've said is correct
to answer your question about the suction pressure is 30 barg.
as a rule the GT need more energy to face the discharge pressure increasing in the header, so more fuel, and as a result the gas turbine reach temperature control curve in lower speed.

my first question is an introduction to an other complicated one:
I started working in this plant long time ago. what we have observed our GT's speed start decreasing. I remember in the first time when we started the plant, all the machines reach 100% TNL, even 105% with type D machine. what have changed.

when we started the plant we re-inject the process gas, and to reach the full capacity of re-injection we add a quantity of light gas.

with the time we start decreasing the volume of light gas injected, because our field produce more gas.

so with the same machine, same condition, the GTs reach temp control, in low TNL (over 95%), even il less than ISO ambient temp (15°c). and some times when the ambient temp increase, the master controller can not maintain the suction pressure setpoint, and the process start flaring. and to avoid this, we close some producer oil wells with high GOR.

to understand what happen the our GT's we did a test. decrease the process gas (heavy gas), and we let more light gas to the suction header. and all GTs ACCELERATE and reach 102% TNL with speed control.

My question is light gas need more or less energy from gas turbine to reach the desired discharge pressure?
if the answer is more energy, is that mean Higher TNL or Lower TNL?

My last question is, 3 of our GTs are MS5002C model and one D model. by converting C to D model our machines, will this solution avoid flaring the gas and respecting the suction pressure setpoint?

Thank you.

 

>My question is light gas need more or less energy from gas
>turbine to reach the desired discharge pressure?
>if the answer is more energy, is that mean Higher TNL or
>Lower TNL?

I am not an expert on gas handling plants operation or centrifugal compressor dynamics to give an accurate answer to this point. But From the details of the test you conducted at site, it looks obvious that with light gas, GT is going into temperature control at lower speeds. That means, Light gas needs less energy and you can run on high TNL levels.

>My last question is, 3 of our GTs are MS5002C model and one D model.
> by converting C to D model our machines, will this solution avoid
> flaring the gas and respecting the suction pressure setpoint?

There is no straight forward answer to this query with the limited data we are discussing here. In order to understand if upgrading the 3 GTs will solve your problem, we need to properly calculate how much additional load is to be handled and how much additional power is needed. This depends on the variations in suction header pressure, Discharge header pressure, site ambient conditions and other factors.

Upgrading 3 GTs from MS5002C to MS5002D, you can get additional power of min 12MW (4MW x3) as per the ISO ratings. There are improved versions of MS5002D which can provide even more power. They call it Power crystal upgraded MS5002D which has a higher temp control margin as well.

But as I said before, whether it will be enough for the site, is not a calculation we can do in this forum. Sorry about that.

 

> My question is light gas need more or less energy from gas

From your data, the light gas would seem to require less energy to compress than the heavy gas. (You can achieve a higher speed when light gas is used to dilute the heavy gas. And, while it has been quite a while since I worked on mechanical drive gas turbines, my recollection is that the energy vs. speed relation for centrifugal compressors is a cubic function.)

> My last question is, 3 of our GTs are MS5002C model and one D model.

I am not sure you can upgrade a MS5002C to a MS5002D, except by replacing the whole gas turbine, and maybe the centrifugal compressor as well. You did not say if the "C" load compressors are identical to the "D" load compressor.

The MS5002C gas turbine (an upgrade to the MS5002A) I believe uses the MS5001R compressor and 1st stage turbine for the HP rotor, with a separate turbine wheel & shaft for the LP rotor. This axial compressor has fixed inlet guide vanes and (not exactly sure here) 14 compressor stages.

The MS5002D gas turbine (an upgrade to the MS5002B) uses the MS5001P compressor and 1st stage turbine for the HP rotor, with a separate turbine wheel & shaft for the LP rotor. This axial compressor has variable inlet guide vanes (open-close variety) and (again not exactly sure here) 17 compressor stages.

Now, I suppose there might be some way to fit new rotors, compressor stators, turbine nozzles, etc. into the existing casings, but I doubt it. Also, that additional 4 MW for each gas turbine translated to about 5350 HP, and it is likely that the combustion system, load couplings and probably the load compressors would have to also be replaced.

I also believe you indicated that you have HRSG's on these units, which could well be impacted by the higher air flow of the "D" machine.

Have you contacted the supplier (GE, I assume) about this?