we have two GE units frame 7 (mark 6), one of them de rated to 62 mw due to temperature control, whereas the other one temperature control takes place at 70 mw.

please help to find the reason of temperature control takes place at 62 mw for unit#1 whereas takes place at 70 mw for unit#2.<pre>
data as following
unit#1 unit#2
mw 58.4 58.6
exh. average 515 c 476 c
min/max exh. temp 494/532 459/496
FSR 52% 51.3%
IGV 84% 84%
TTIB 165 C 155 C
LCV 4kg/sec,53% 4kg/sec,53%</pre>

 

What do you mean by "de-rated"? De-rating usually means someone, the owner/operator or the OEM or some other reputable engineering company, has lowered the rated power output for some reason, usually mechanical or because of the available fuel characteristics or if there is a boiler on the exhaust perhaps there is some limitation (temporary or permanent) on the upper limit of exhaust heat or exhaust flow-rate.

The "full load" (Base Load) power output of most GE-design heavy duty gas turbines is usually a function of the CPD- (or CPR-)biased exhaust temperature control function. The two most important inputs to this calculation are the axial compressor discharge pressure transmitter(s) (there may be more than one--redundant sensors) and the exhaust temperature thermocouples. If the Mark IV uses CPR-biased exhaust temperature control, then another critical input is the ambient pressure transmitter(s) (again, there may be more than one) which is used, along with the CPD transmitter(s) to calculate CPR (Compressor Pressure Ratio).

It is presumed that Base Load is selected and active on <b>both</b> units--please confirm this, and if not, please tell us why not.

One of the most important questions to be asked when troubleshooting any problem on a machine that has been running for some time is: When did this problem start? What changed or what occurred at about the time the problem started?

We need to know what Process Alarms are annunciated on <b>both</b> units--every one of them.

Also, what Diagnostic Alarms are being annunciated on <b>both</b> units--every one of them.

Also, provide the following information for both units when running with Base Load selected and active:
<pre> UNIT 1 UNIT 2
TTRXP
TTRXS
TTXM
CPD
CPR
TTXSP1
TTXSP2
TTXSP3
TTK_C[0]
TTK_I[0]
TTK_M[0]
TTK_S[0]
TTK_C[1]
TTK_I[1]
TTK_M[1]
TTK_S[1]
TTK_C[2]
TTK_I[2]
TTK_M[2]
TTK_S[2]
TTK_C[3]
TTK_I[3]
TTK_M[3]
TTK_S[3]
TTK_C[4]
TTK_I[4]
TTK_M[4]
TTK_S[4]
TTK_C[5]
TTK_I[5]
TTK_M[5]
TTK_S[5]</pre>
What are the Turbine Inlet Air Filter Differential Pressures of <b>both</b> units at Base Load?

When the last time <b>each</b> of these units had an off-line compressor water wash?

Are these units burning crude oil or some other heavy fuel oil? What fuel is <b>each</b> of these units burning?

Is there any additive required for the fuel(s) being burned? If so, has the additive been injected at the proper flow-rate for the time the machine has been in operation (since the last maintenance outage (hot gas path or major inspection)? For <b>both</b> units?

Failure to provide answers to <b>all</b> of the above questions will result in a failure to reply to any answers/information provided.

 

You say "derated", do you mean physically derated or you mean that it reaches Temperature Cont. at the lower load? You say one unit makes 70 MW and the other makes only 62. Can you show us data at those loads i.e. when the machine is on Temp. Control. Can you give us the CPD for both machines, this may be a starting point to understand why the low output. Any relevant history on the two machines would be useful, how many hours run since last overhaul, any major work done recently which could be contributing to this problem? How long has this problem existed for? Has it been a progressive or sudden drop in output?
If you can provide this data, we can maybe move forward.

 

As Mr CSA mentioned that the main parameter for the Base Load Control (temp control) is CPD (Compressor Pressure Discharge)96 CD. If the Unit #1 is showing higher CPD than Unit#2 that means the unit #1 will generate lower output power, hence you can cross check this high value by comparing with Compressor Pressure Discharge Local Pressure Indicator.

If your Turbine is equipped with only one 96 CD, means it is not possible to calibrate it.

This may due to:
1- Axial Compressor fouling (Online/ Off Line cleaning or abrasive cleaning)
2- Inlet Filter high DP
3- 96 CD calibration

 

> ... If the Unit #1 is showing higher CPD than Unit#2 that means the unit #1 will generate lower output power, ....

Mustapha,

Higher CPD at Base Load will generally result in higher load. Lower CPD will generally result in lower load (under normal operating conditions).

We don't know enough, and the original poster did not provide proper information nor has he responded with requested information. So, it's likely we will never know the circumstances.

 

Dear

some of the required information not available in mark 6 toolbox. please find below the information collected from our toolbox. At base load and temp control mode is active.<pre>
UNIT#1 UNIT#2
TTRXP 1042.5 F 1030 F
TTRXS 1065 F 1046 F
TTK_K(8) 51.6 MW 51.6 MW
TTK_M(8) 2.58F/MW 2.58F/MW
TTK_C(8) 8.76 RATION 8.76RATIO
TTK_S(8) 25.05 F/RAT 25.05F/RAT.
TTK_I(8) 1100F 1100F
CTD 677F 680F
CPR 11 RATIO 11.5 RATIO
TTXM 561 C 554 C
Air filter 90MMH2O 100MMH2O
CPD 9KG/CM2 10KG/CM2
MW 65.5 MW 72 MW
Fuel flow 225L/M 230L/M
FSR 57.2% 61.3 %
IGV 84 % 84%
TTXSP1 37 C 47 C
TTXSP2 35 C 45 C
TTXSP3 32 C 41 C
Allowable spd 69 c 69 c.</pre>

i hope the above information enough to find a solution

 

The spreads are slightly higher on Unit 2 than unit 1. The inlet filter differential pressure is higher on Unit 2 than Unit 1.

Oddly, the CPD on Unit 2 is higher than on Unit 1.

This is not likely a Speedtronic-related issue (though you DID NOT include the alarms being annunciated while running at Base Load!). And, the TTK_x group of Control Constants is an array (an eight-element array) and you only provided the values for one of the array elements, and not likely the one(s) being used.

How long since the last maintenance outage for each of the units?

When was the last off-line water wash performed on each of the units?

Is there an HRSG (Heat Recovery Steam Generator, or "boiler") in the exhaust duct of the units? If so, is the exhaust flow directed into the HRSG?

I'm going to say the problem is likely the result of a fouled compressor and/or worn hot gas path components (combustion liners; transition pieces; nozzles; shrouds; etc.). Some of this is evident in the higher spreads, but it's likely just a worn and/or dirty machine which is running at very low efficiency for several reasons, all combining to cause the low output.

That's about all I can say based on the information provided. Hopefully, you will answer the above questions, so that others can learn from your experience and conditions.

 

> How long since the last maintenance outage for each of the units?
> When was the last off-line water wash performed on each of the units?

unit #1 finished major overhaul since 3 weeks with off line water wash
unit#2 finished major overhaul since 8 months with off line water wash

> Is there an HRSG (Heat Recovery Steam Generator, or "boiler") in the exhaust duct of the units? If so, is the exhaust flow directed into the HRSG?

there is no HRSG

 

The first question to always be asked when something like this occurs is:

When did this problem start?

Followed immediately by:

What changed at the time the problem started?

Something is amiss with the calibration of the IGVs or the CPD transmitter(s), or (not likely) the MW transducer(s). Usually, most sites "calibrate" field devices and instruments during major outages and if this was done, then it's certainly worth a check of the calibration to see if it was done accurately.

What does the indicator on the side of the compressor casing say the IGV angle is? (While they're not usually very accurate, they can be a decent indicator of position but shouldn't be trusted for calibration activity. And when actually measuring IGVs with a proper tool, the accuracy of the pointer on the side of the compressor casing should be checked, and adjusted, if necessary.)

You're running liquid fuel, so there may or may not be LVDTs on the Liquid Fuel Bypass Valve which could tell us the relative positions of the two LFBVs, but not much more than that.

But, for the amount of fuel being burned one would expect the two units to be producing approximately the same amount of power. And, for the unit which was most recently overhauled and had the most recent off-line water wash it would certainly be reasonable to believe it's performance should be better than the other unit.

So, either something was not reassembled correctly or some calibrations were not done correctly.

Another possibility is that one or more of the compressor bleed valves is leaking ("passing"). The difference in the two CPDs seems to suggest this, but the lack of accuracy of the information provided is disturbing.

You should be able to use an infrared heat "gun" to see if the piping of one or more of the extraction lines/bleed valve outlets is hotter than the other. If so, then it's a safe bet it's leaking. (There should be NO flow through the bleed valves while at Base Load. No flow means the temperature of the piping of all four valves will be approximately the same. Any valve which has a significantly higher temperature than the others is probably leaking.)

Some newer machines actually have limit switches to indicate compressor bleed valve closure (in addition to the ones that indicate open position). If you have these limit switches (usually signal names like L33CBnC, where n=1-4), what do they all indicate for the unit with the lowest power output at Base Load?

Lastly, on the subject of the data you provided, simply providing the integer value of readings in barg is not very helpful. Please use at least one decimal place with these readings (most of them, in fact, not just barg).

And, why are you providing TTRXP and TTRXS values in deg F and TTXM in deg C? Methinks someone is using CIMPLICITY displays to obtain some information, and Toolbox to obtain other information. Not a good practice. Be consistent with the units, and the method used to gather the data.

 

dear CSA, thanks for the correction as it was written by mistake.

 

I think you need to take a deeper look at why the CPD is so much lower on unit #1, particularly as you completed a Major 3 weeks ago. Are you sure IGV angle, bleed valves,etc are all OK? Do you have running records from before the overhaul, can you make any comparisons?