high exhaust temperature in DLN1

A

Thread Starter

abdi

Hi,

In my company there is two type of gas turbine:

1. DLN1 TYPE: Frame 9 Mark V
Water injection Type: Frame 9 Mark V

When DLN1 Type working the exhaust gas temperature is higher than the Water injection Type. after that this two type of gas turbine used for HRSG boilers and in each boilers there is a supplemental burner that after this burner there is three thermocouples to sense the gas temperature after this burner so in boilers work with Water injection Type gas turbine. there isn't any problem around the difference between these three thermocouples but the boilers work with DLN1 TYPE. these thermocouples have a difference about over than (50 dc)that cause trip the boilers .

I think when the exhaust gas temperature in DLN1 type is higher than the Water injection Type these thermocouples has a problem. Am I true?

The Water injection Type's gas turbine don't work with water injection. All of time it works without water injection. because I think when this works with water injection, the combustion chamber will destroy. and I want you guideline me about that to find a way to reduce the nox without water injection.

Thank's a lot.
 
While many similar GE-design heavy duty gas turbines are called Frame 9E machines, that basically just refers to the design of the machine (number of combustors; number of stages of both the axial compressor and the turbine; number of turbine bearings; etc.).

The actual hot gas path components (primarily the turbine nozzles and -buckets; combustion liners; and transition pieces) and in some cases also the design and materials used in the axial compressor actually define the optimum internal firing temperature, the compressor discharge pressure and characteristics, and also the exhaust temperature of any specific gas turbine.

Unless the two machines were built at the same time with the same components and if upgraded have each been upgraded with the same parts there is very little likelihood the exhaust temperature at Base Load will be the same--regardless of the combustion system (conventional (diffusion flame only) or DLN-I).

In fact, it's typical of machines built consecutively in the factory one right after the other, installed and commissioned on the same site, with the same number of fired hours and operating history to have different compressor discharge pressures and exhaust temperatures when operating at Base Load.

The internal clearances, the cleanliness (or lack thereof) of the turbine inlet air filters and axial compressors, and the back pressure on the exhaust can all have an effect on the performance of the machines. There is very little likelihood that two "identical" machines will ever have exactly the same compressor discharge pressure, exhaust temperature or power output at Base Load--even when they're both new and clean during commissioning. It just isn't common, nor is it to be expected. They are very large machines with a lot of mechanical tolerances and parameters (including instrumentation tolerances) that all have to be the same.

Performance guarantees for machines (much to the surprise and frustration of many owners and inexperienced plant managers!) reflect this and also have a range of guaranteed performance limits. Sometimes two "identical" Frame 9E machines being installed and commissioned at the same time will have outputs that differ by two percent (!) or so--and yet both will meet performance guarantees. (The implication is also that the compressor discharge pressure and exhaust temperature will not be the same either.)

So, there should be no expectation that two such dissimilar machines as you describe will have the same exhaust temperature when operating at even the same load at Base Load.

As for tuning water injection, that would take a whole chapter in a book--too much for this forum. Injecting water is not the best thing to do to a gas turbine, but when done correctly and tuned to meet the emissions guarantee the effects are calculated into the parts life and maintenance intervals of the machine--so that shouldn't be a problem, when tuned correctly.

If you would describe the problems you have when injecting water we might be able to make some recommendations or suggestions or offer some guidance. But we would need to understand the problem(s) first.

A good place to start understanding water injection is to study the P&ID for the system, and to read Section 9 or 10 of the Control Specification document provided with the machine.
 
Abdi,

I have some questions and comments:

You say there are 2 types of gas turbines. Both are Frame 9 with Mark V controls. One is water injection NOx control and the other is DLN1 NOx control. Also, both types have an HRSG with supplemental firing. Is there more than 1 unit of each type?

2. Was the DLN1 unit originally water injection and later converted to DLN1 or has it always had the DLN1 combustion system?

3. For the water injection unit, which is being run without the water injection on, why do you think you will destroy the combustion chambers if you use water injection? As far as NOx control, there really isn't any other way to reduce the NOx emissions unless you convert to DLN1 or add a catalytic emissions reduction system to the HRSG, which is a rather expensive option.

4. For the DLN1 unit and the supplemental firing problem: I assume the three thermocouples you are talking about are used by the burner management system. Do they show this high differential temperature when the burner is off? If so, are you getting any high spread alarms from the Mark V? If not, the problem is likely with the burner system. Are these thermocouples measuring duct gas temperature or superheater metal temperature? Are the burners the same on both types of units? Are there multiple burner elements (runners)? Have you compared burner fuel gas pressure readings between the two units? Have you had the HRSG or burner supplier look at this?

5. Do you only use the burner when the gas turbine is at (or near) base load?
 
Otised,

I have answer your questions :
> Is there more than 1 unit of each type?

yes , there are 5 units of water injection type , and there are two DLN type.

>2. Was the DLN1 unit originally water injection and later converted to DLN1 or
> has it always had the DLN1 combustion system?
there two DLN1 unit have had always the DLN1 combustion system

> 3. For the water injection unit, which is being run without the water injection
> on, why do you think you will destroy the combustion chambers if you use water
> injection? As far as NOx control, there really isn't any other way to reduce the
> NOx emissions unless you convert to DLN1 or add a catalytic emissions reduction
> system to the HRSG, which is a rather expensive option.

Because we thinking can't do really correct and then destroyed the combustion chamber.

> 4. For the DLN1 unit and the supplemental firing problem: I assume
> the three thermocouples you are talking about are used by the burner management
> system. Do they show this high differential temperature when the burner
> is off?
About these questions, the three thermocouples used by the burner management system. they don't show this high differential temperature when the burner is off.

> If so, are you getting any high spread alarms from the Mark V?

We aren't getting any high spread alarms from the Mark V

> Are these thermocouples measuring duct gas temperature or superheater metal temperature?

These are thermocouples measuring duct gas temperature.

> Are the burners the same on both types of units?

The type of burners the same on both types of units, but the place of the these thermocouples aren't the same on both types of units.

> Are there multiple burner elements (runners)?

There are 9 ramp burner at the vertical position.

> Have you compared burner fuel gas pressure readings between the two units?
fuel gas pressure readings between the two units are the same.

> Have you had the HRSG or burner supplier look at this?

> 5. Do you only use the burner when the gas turbine is at (or near) base load?

No, we use the burner at any load of gas turbine.
 
Abdi,

HRSG duct burner differential temperature problem on the DLN1 units:
When I asked about the fuel gas pressure on the burners, I did not just mean the supply pressure. You say there are 9 runners. Are the pressures at the runners the same for all units (assuming you have gauges on them all)? Basically, I think you need to involve the HRSG and burner suppliers. It would seem that either the thermocouples are not located correctly or the burner runners are not balanced. That is, I do not think the gas turbine combustion type has anything to do with the differential temperatures downstream of the burners.

For the water injection units:
If using the water injection is destroying combustion chambers, then something is wrong! I am a controls engineer and not a combustion engineer, but water injection is a proven technology, and as long as the fuel and water meet GE's specifications and the control system is properly tuned it should not destroy the combustion chambers. Has it ever worked properly?
 
Hi,Martin

> Are the machines all 9E?
Yes

> Do they use water injection or steam injection?
None of them

I want to say you,I Measurement the distance between three thermocouple with themselves.

This is for DLN type:
1. upper thermocouple with middle thermocouple is:
2.973 meter

2.middle thermocouple with down thermocouple is:
3.545 meter

And for water injection type is:

1. upper thermocouple with middle thermocouple is:
3.32 meter

2.middle thermocouple with down thermocouple is:
2.96 meter

Please say to me about Is it true to the change the place of thermocouples at the all of unit to be same as the water injection type.

Thanks a lot.
 
Abdi,

> Please say to me about Is it true to the change the place of thermocouples at
> the all of unit to be same as the water injection type.

I do not think you should change the position of the thermocouples without first getting agreement of the change from the HRSG supplier (assuming the burner was provided by the HRSG supplier).
 
Have you checked TTXM on each gas turbine and compared it to the relevant duct temperature?

That is where I would look first. I believe that the post from otised is correct - this is a problem with your duct burners and duct thermocouples, not with your GTs.
 
Otised,

> I do not think you should change the position of the thermocouples without first getting agreement of the change
> from the HRSG supplier (assuming the burner was provided by the HRSG supplier).

We can't to get agreement from the HRSG supplier.
because he hadn't the temperature profile from GTG exhaust then he can't to determine the corrected places of three thermocouple.

I have a question:
Can we determine the place of these three thermocouple without the temperature profile from GTG exhaust, or there is another way?

Please help me to solve this problem, because my unit must be at low load.
 
Hi,Martin

> Have you checked TTXM on each gas turbine and compared it to the relevant duct temperature?

Yes,TTXM in GTG's DLN type about 100 centigrade higher than GTG's water injection type.

I have a question:
Is there at the HRSG boiler logic with name post combustion trip (limit the increase the load of burner's boiler at any load of GTG).

I attention you, I FORCE this logic to get high steam load at any load of GTG.

Thanks.

 
Abdi,

> Can we determine the place of these three thermocouple without the
> temperature profile from GTG exhaust, or there is another way?

I do not know if this can be done without a temperature profile from GTG exhaust. I am a controls engineer, not an aeronautical engineer. I do know that the exhaust gases leaving the GTG are rotating (swirling) and this has sometimes caused problems with HRGS's and duct burners. However, it should not be any different between the water injection units and the DLN1 units. Also, you said you are not getting temperature spread alarms from the GTG and so even with the rotation of the gases I would think the temperatures are fairly uniform from the GTG. You also said that the three thermocouples downstream of the duct burners do not have a large differential when the burners are off, so I think the burners themselves cause the differentials you are experiencing.

At some sites where we experienced problems with the duct burners, they finally fixed the problem by installing straightening vanes of some sort upstream of the burners.

I do not see an easy way out of this problem without involving either the equipment suppliers or an independent consultant.

Also note that while you may be able to find alternate locations for the 3 thermocouples that avoid the temperature differential trip, that does not mean you have solved the problem - the differentials would still exist and could cause and equipment failure! You really need to get some appropriate expertise on site. This is NOT a controls problem.
 
Otised,

You said,this discrepancy of three thermocouples this is a problem with my duct burners and duct thermocouples, not with my GTs.

I have a question.
Is there any distribution plate after GTG exhaust before HRSG inlet?

Thank you
 
Abdi,

> Is there any distribution plate after GTG exhaust before HRSG inlet?

I do not know. It is not standard equipment furnished by GE. If it is there, the HRSG supplier would have provided it, unless it was upstream of any exhaust silencer upstream of the HRSG. It should show up on exhaust system drawings.
 
Otised,

Do you know about post combustion trip logic in HRSG?

In my document this logic as it:
At any load of gas turbine we can't have any load of burner's boiler,so we forced this logic to can increase the load of burner at any load of GTG.

In my company there isn't any equilibrium between consumption of the power and the steam,the consumption of the steam higher than the power,so we need to increase the load of burner's boiler.

I want to say another thing about the positions of these thermocouples:
At the HRSG worked with water injection GTG, the positions of these thermocouples is wrong . It isn't the same of document,but in the HRSG worked with DLN GTG,is the same of document.
 
Hi,

Please define the post combustion & temperature profile.
What relationship there is with them?

Thank you,
 
Abdi,

Duct burners are rarely intended to be used at any load other than Base Load. That's why there are interlocks to prevent using duct burners at loads less than Base Load.

One of the defining conditions of Base Load is that the IGVs are at their maximum operating angle ("full open"). That's when the excess air from the gas turbine is at it's maximum--meaning the most oxygen is available for burning fuel with the duct burners.

Base Load is also when the mass flow of air, fuel and exhaust gases is at it's highest, which is the best condition for operating duct burners.

Operating duct burners at loads less than Base Load can be very destructive to HRSGs, and turbines, as well. The combustion of fuel in the exhaust can raise the back pressure on the gas turbine which can lead to performance decreases and hot gas path part degradation. The high heat which can be generated at lower air flows can cause problems with the HRSG components, in particular the superheater.

If steam production is the most important aspect of operating the gas turbines then it's--usually--best accomplished at Base Load with the duct burners running, not at Part Load. Especially when the IGVs are at not fully open.

Bypassing the interlocks/permissives to run the duct burners at loads less than Base Load is ultimately going to lead to problems--likely with both the gas turbine(s) <b>AND</b> the HRSGs.

Please work with the packager of the GE-design Frame 9E heavy-duty gas turbines <b>AND</b> the HRSG/duct burner supplier to understand how the turbines and duct burners are designed to be operated. Involve the entity that consumes/purchases the electric power that is produced by the turbine-generators so that they understand the optimum operating conditions for the turbines and duct burners. This is the best way to solve the problems that leads to the best outcome over the long term for everyone.

There's little we can do via this forum to help you because we can't see the drawings or the equipment. Having someone knowledgeable come to site to help with the problem(s) <b>AND</b> getting all parties to understand the limitations and the capabilities of the equipment will lead to the best outcome for everyone.
 
Top