High Exhaust Temperature


Thread Starter


A two shaft gas turbine drives gas compressor equipped with Speedtronic markII control system experiencing a high exhaust temperature after after complete sequence and during manual raise command for increase NHP so that the exhaust temperature increases rapidly and the high pressure speed can't reach 95% of unit rated speed,which is 11140, due to Temp. Fuel
The unit is not equipped with inlet guide vane, there is only one bleed valve.

Before the new conditions take place, The unit was running at rated speed 7 full loaded with relatively high exhaust temperature about 525 0C, ALARM SET POINT IS 540 0C before cleaning axial compressor

so a clean for axial compressor was done but the result was negative and the unit condition became worst than before, as described earlier and couldn't reach rated speed due to temperature limit.

It was observed during a start up trial that intrvalve pressure, P2,down stream pressure of SRV exceeds 10.5 bar to reach 15 bar while fuel gas pressure down stream gas control valve, gcv, reaches 6 bar

The following were checked and replaced where necessary:

- Air inlet filters
- fuel gas transmitter, 96FG
- comp. discharge press. transmitter 96CD
- Servovalves for speed/ratio & gas control valves
- LVDT'S for SRV & GCV
- Bleed valve
- speed sensor 77HC
- fuel gas valve's trim (plug & seat)was inspected

nothing abnormal is found, null bias of servo valves are ok, (0.2-0.6)
LVDT feed back (0.74 min- 3.25max)are ok

No positive result has been giving and still in the same condition of high exhaust temp.

would you please help what the problem could be?
Hi Hibrahim,

The first question is from where this occurring (High Exhaust Temp.)?, was sudden or gradual?

Considering that the entire review of the instrumentation was done correctly and PCD value is ok vs. HP speed, We think that probably the problem lies inside of the turbine.

have you seen an increase in wheel space temp. during normal operation?

How many fire hours since last major maintenance?

According to HP Speed,when the bleed valve must close?

Best regards,

During start up, acceleration phase it is noticed that exhaust temperature is relatively rapidly increased in abnormal way to reach about (480) 0C until the bleed valve closed at about 80 % of rated speed for the Exhaust temp to decrease to (440) 0C.

After complete sequence and during raise command, the speed is increased slowly while exhaust temp is increased faster to reach the TEMP FUEL before rated speed.

The following data was recorded after today’s trial of start up
NHP 90%, PCD = 4.5 BAR, EXH TEMP = 534 0C, P2 = 18 BAR, GAS CONTROL VALVE DOWNSTREAM PRESSURE = 6 BAR, wheel space temp is ranged from (255 1st & 204 2nd) 0C. Temp of BRG # 2 is 86 0C, BRG# 3 is 144 0C (relatively high), BRG #4 is 146 0C (high than normal).

The unit had 40,000 hours inspection in October, 2009. The firing hours since 40,000 inspection is (29,120) hours.

As a historical record the wheel space reading during normal operation was as follow:
1st fwd 341, AFTR 356 0C
2nd FWD 273, AFTR 215 0C

Best Regards,
A P2 pressure of 18 bar seems excessive. The P2 pressure reference is directly proportional to HP speed, and you should be able to find the relationship in the 'Control Specification - System Settings' document provided with the Mark II, and necessary for any calibration procedure.

You listed the min and max LVDT voltages, but did you verify that the action during stroking of the valves is smooth? What position is the SRV going to during this loading problem? Is it full open or nearly full open?

What position is the GCV at during this loading problem?

Does the unit have variable second stage nozzles? Is the nozzle actuator operating properly?

Do you have any way of comparing the CPD at any load now to the CPD at the same load prior to the water wash?

For newer Speedtronic turbine control panels operating on exhaust temperature control (Base Load), the exhaust overtemperature alarm set point is usually about 25 deg F greater than the exhaust temperature reference at all times during Base Load operation (and the exhaust overtemperature trip is about 40 deg F greater than the exhaust temperature reference at all times during exhaust temperature reference). So I don't understand why it was felt that because the exhaust temperature was close to the alarm setpoint that a water wash was necessary, because the control system would have operated the same after the water wash as before. An off-line water wash won't change the relationship between the exhaust temperature and the exhaust overtemperature alarm/trip setpoints. An off-line water wash should improve compressor efficiency, which should result in a higher CPD for the same load/speed as compared to what was being experienced prior to the wash.

The biggest problems with off-line water washes are a result of improper valve line-up (positions) during and after the water wash. Water (and detergent) has a way of getting into tubing and piping that it shouldn't be in, especially if valves were closed that should have been closed. If water/detergent gets into the tubing line to the CPD transmitter, the transmitter can be calibrated properly but won't sense the CPD properly when the unit is running.

I'm wagering the problem is related to the seemingly excessive P2 pressure, which is causing an excessive fuel flow-rate which is resulting in the higher-than-normal exhaust temperatures. You should be able to monitor the P2 pressure reference (voltage) versus the P2 pressure feedback (voltage) to determine if the actual P2 pressure is following the P2 pressure reference or not.

But, there just isn't a lot of information to go on. And with a Mark II, getting troubleshooting data is difficult. We're not even sure of the condition of the unit prior to the decision to perform an off-line water wash, which is difficult to understand.

I also question the maths involved in achieving 29,000+ hours firing since October 2009.... Is that comma in the fired hours number a decimal point or a place holder? If a 40,000 hour inspection was done in October 2009 it's likely that a water wash was done either before or after the inspection. So, if the fired hours since October 2009 is 29+ (not 29000+, or 29,000+) it's difficult to understand why a water wash was deemed necessary.

If there is only 29+ hours since the inspection in October 2009, it's entirely possible that there might be some problem with the oil and/or servo-valves.
Just to back you up on the fired hours, October 2009 is only 9 months ago. There are only 8760 hours in a 365 day year! 29000 hours is 3.3 years.
Thank you CSA for your kind reply.

Based on control system setting the relationship between NHP Speed and P2 is 0.0133 psi/rpm, that indicates how 18 bar P2 is excessive when rated speed is 11140 rpm, SO, P2 should be about 10.2 bar.

in order to eliminate this problem the hydraulic actuator and servo valve for SRV were replaced and verified that they work smoothly with unit running but no positive response is givin.

one thing was observed during some checks when the unit was in crank, there was a fuel gas pressure leaks from SRV and built up to 5 bar in about 7 minutes.

Since SRV is not that kind of tight shut off valve but I don't feel right for that percent of leaks.

the valve action during stroking is smoothed and its position during the loading is about 26 % opening while GCV IS ABOUT 31% opening from full travel.

after unit complete sequence I tried to gradually minimized the P2 via manual fuel gas isolating valve upstream SRV to become 10 bar, it was noted that SRV went to the full opening position and its LVDT feedback (voltage) was at maximum , gradually opened the isolating valve, SRV responded and started to close to restore P2 to 18 bar as before.

The unit has a variable speed nozzle, but the current NHP speed is not yet reached the value at which the nozzle starts to move.

The operation of nozzle actuator was tested off line and simulated. (sevo valve, hydraulic actuator and LVDT). the nozzle travel is +12 degree full open, -5 degree closed.

PCD prior washing was 6 bar, after washing 5.5 bar for the same load.

It was decided to perform washing because the exhaust temp was relatively high compared to the other running existing units especially the firing hours of that unit is 5000+ hours since october, 2009.
the previous mentioned figure, 29000, was wrong, sorry for that.

the existing normally running units reported exhaust temp is 485 0C at full load while for this unit was 520 0C for the same load.

The nozzles and tubing for the cooling system and impulse line for 96 CD were purged by air to make sure that no blocking or restrictions that could affect eth cooling efficiency and to make sure good reading for 96CD, residual mixer of water and detergent was found in tubing and impuls line of 96CD.

Do you think excessive P2 pressure comes from SRV valve itself, ist needed to replace the fuel gas valve block ( SRV& GCV)?

You may have a problem with the P2 pressure setpoint calculation. From your description it looks like the SRV is controlling to a pressure setpoint that is too high. The setpoint is a function of HP speed. You should verify that the SRV control loop settings are all correct per the GE Control Specification. I believe you said you had a Mark II control, and I don't remember the details of that system (it has been at least 25 years since I have worked on one). The control card itself may have a component failure that is incorrectly biasing the setpoint, but I don't remember enough about it to give troubleshooting advice other than replacing the card.
You still haven't told us what speed the HP shaft is at when it is experiencing 18 bar P2 pressure, which is clearly in excess of the 0.0133 psi/rpm (which would produce approx. 148 psi at 11140 RPM).

There MUST be a way to measure the P2 pressure reference (a voltage) and compare it to the P2 pressure feedback (also a voltage).

Changing the SRV hydraulic actuator and servo isn't going to affect the P2 pressure. Because the SRV regulator is continually comparing actual P2 pressure to the P2 pressure reference and adjusting the servo output current to make the actual equal to the reference.

The typical decision to perform an off-line water wash is when the current compressor discharge pressure drops below some percentage of the compressor discharge pressure for a clean compressor at a comparable operating point. Which, means that it's necessary to keep records and make comparisons over time and to decide when the performance and/or compressor discharge pressure drops low enough to warrant taking the unit off line (loss of production) to perform an off-line water wash.

And, when performing an off-line water wash, it's very necessary to make some visual inspections of the compressor bellmouth and inlet guide vanes (fixed or variable) before and after the wash to be sure that the desired amount of cleanliness has been achieved.

Rinsing after the wash is also very important, and it's important to monitor the drains to ensure that all of the detergent has been removed and that clear water is observed to be coming from all of the drains.

After improper valve line-up (positioning) the biggest cause of problems with water washing is incomplete rinsing and lack of examination to determine if the compressor has been cleaned. The latter is more difficult past the first couple of stages of the compressor, but some site use the borescope inspection ports of the compressor case to check for cleanliness.

And, along with rinsing there is a huge propensity for most sites to use excessive amounts of detergent when performing a water wash. Remember: The manufacturer-recommended amount of detergent is basically chosen by the manufacturer to ensure that you will buy as much detergent as possible. In other words, it's already the maximum amount of detergent that should be used. Depending on the visual inspection of the compressor before the wash, it's sometimes only necessary to use one-quarter or one-third of the manufacturer's recommendation to achieve the desired results.

Using too much detergent usually results in very long rinses if performed correctly in order to obtain clear water from the various drains. And, that takes a lot of water, but more importantly, a lot of patience. And, most sites have a dearth of both, water for rinsing and patience for performing the rinse to proper conclusion.

I have seen and seen pictures of compressors that were allegedly washed (off-line) prior to a major inspection, and they were so gummed up with sticky residue that was a combination of both the contaminants in the compressor and the detergent that it was a wonder the units ran at all. The fact that compressor discharge pressure decreased after the wash (for the same operating condition?) indicates that there is something wrong with the efficiency of the compressor.

Water washing requires preparation (valve line-up and inspection), and patience (rinsing and inspection).

There's just too much we don't know about the conditions at your site (type of contaminants which can or have fouled the compressor) and how the water wash was performed. We are getting very sketchy information about the operating conditions, and it seems there is little or no information about the operating conditions prior to the decision to wash (compressor discharge pressure vs. speed and load and ambient temperature and exhaust temperature).

It's also telling that water/detergent was found in the tubing/impulse line of the compressor discharge pressure (if the post is understood correctly).

We also don't know what the LP shaft is doing during this time of high exhaust temperature.

And, I find it very difficult to understand how the same load can be achieved for an exhaust temperature differential of more than 40 deg C; that's a lot of degrees C (a big differential) which would indicate something more is amiss than just the exhaust temperature.

We have also neglected to ask about the exhaust temperature spreads. How much is the difference between the highest and lowest exhaust thermocouple readings prior to the high exhaust temperature trip? And are the highest and lowest exhaust thermocouples located next to, or very near to, each other? Can you record the individual exhaust temperatures just before the trip?

It's beginning to sound like there's more than one problem here. The P2 pressure seems excessive (given the spec value provided) and the exhaust temperature seems excessive and we don't have any idea about the exhaust temperature spreads. And, it would seem there's something amiss with the compressor discharge pressure.

Lastly, the SRV is supposed to be a Stop valve (the "S" in SRV), and it shouldn't be leaking by as much as you describe under normal conditions. I don't recall if it's supposed to be a Class IV shut-off valve, but, it should be pretty leak-free. It is a metal to metal seat, lapped, I believe. With a very large spring for closure. I also think that supply pressure is pressing down on the plug to help keep it closed, along with spring pressure.

If the SRV wasn't leaking before changing the actuator and servo, are you certain the proper gap exists between the actuator rod and the valve stem when the unit is shut down and hydraulic pressure is absent? (The typical gap for the combined SRV & GCV assemblies used on machines of that vintage is 0.030-0.050 inches, or, 0.040 inches +/- 0.010 inches, measured when the unit is shut down and hydraulic pressure is not present. Insert feeler gauges between the actuator rod and the valve stem. Sometimes it's necessary to use a block of wood to push the actuator stem down and then use that same block of wood to pry gently up on the LVDT bar to lift the valve stem in order to be able to see and measure the gap.)
You should check the clearance between the SRV rod and actuator stem, following CSA's recommendations, May need to adjust the position of the actuator.

Typical P2 formula is GAIN*NHP + OFFSET. Where Gain is 0.0133 psi/rpm (in your unit) and OFFSET is a negative constant, So likely P2 pressure should be less than 10.2 bar @ 11140 rpm.

Have you no ignition trouble?

when you tried to close the valve upstream to SRV valve, SRV opened to reach P2 reference (18 Bar), so there are P2 pressure problem either feedback or reference.
I already replaced the SRV drive card, calibrated it as per control system setting.

p2 range from 1.67 kg/cm2 to 10.2kg/cm2 through Offset NHP and span NHP potentiometers.

Nothing is mentioned about pressure reference in control system setting.
I am running out of ideas here. It's just been too long since I have dealt with the Mark II controls, and I don't remember what cards did what. I still think the problem is in the setpoint circuit, because it appears that the valve is controlling to the 18 bar pressure. Without the elementary diagram, I am at a loss. If you can scan the pages (should only be 2 or 3 pages) that deal with this control loop and e-mail them to me at:

edoengr at gmail dot com

I will try to give more suggestions.

What is the turbine frame size, just for curiosity? I think you said the HP speed was something over 11000 rpm, which seems higher than I remember for a frame 3. Is this a frame 1? Or maybe an aero derivative?
Hello CSA,

The NHP shaft speed is 94 % when P2 reaches 18 bar while exhaust temp reaches over 530 for the temperature fuel take control and hold the fuel , consequently, no further increase in NHP.

It is easy to monitor actual P2 pressure in (voltage) as well as LVDT feedback but for pressure reference, I can’t find something mentioned about it in control system setting.

The speed ratio drive card equipped with zero and gain potentiometers for LVDT adjustment, circuit stability potentiometer, potentiometer for NHP offset to be used for the adjustment of NHP offset at minimum speed when P2 is minimum 0.476 vdc equivalent to 1.67 bar , Span potentiometer to adjust maximum speed 11140 rpm when P2 is maximum 2.7 vdc which equivalent to 10.2 bar in addition to time constant potentiometer.

I’d like to thank you for the valuable information you have given here related water wash recommendation, it is really so great.

Here is operating conditions for the unit before washing:

NHP 98%, NLP 88%, NCE 54%, EXHT TEMP 520 0C, compressor inlet temp. 32 0C, Compressor outlet temp. 292 0CM wheel space 1st FWD 357 0C, AFT 288 0C, wheel space 2nd FWD 304 0C, AFT 233 0C.

Yes, a water detergent mixer was found in the impulse line of 96CD and it was purged by service air and lots of viscous fluid brown in color was getting out the tube.

During high exhaust temp. the mode of control is via LP as its light indicates at NLP speed 70% from its rated speed.

The unit went to a new condition that it is not reach complete sequence.

It reaches fire, then acceleration with NHP speed doesn't exceed 70 % while exhaust temperature differential takes place rapidly to trip the turbine, the temperature spread is not nearly homogeneous and there is a big difference between some T/Cs, temp differential trip activated so quickly.

The gap between the actuator rod and SRV stem was checked, it is within limits, as per system setting, it should be 0.05 inch with hydraulic pressure off.

Subsequent periscope inspection done today by mechanical staff revealed failure for HPT blades,they reported that some HPT blade airfoils were found broken away.
I did check the clearance between the actuator rod and SRV stem, it is within limits.

No ignition troubles were reported during start up.

the unit experienced HPT blade failure based on periscope inspection done by mechanical staff.
This appears to be an excellent example of mechanical problems being attributed to the control system. Maybe.

I don't believe we have all the information, <b>however</b> if the HP turbine blades aren't all there, that's definitely a problem. The HP turbine can't reach rated speed, extracting all the temperature/work from the combustion gases if all the turbine blades aren't there.

I'm not familiar with an SRV (Stop-Ratio Valve, or Speed Ratio Valve as it's sometimes called) that doesn't use HP speed as it's controlling reference. The "intervalve" pressure, or P2 pressure, is controlled by the SRV as a function of HP speed. In other words, the pressure between the SRV and GCV is a function of HP speed. In most digital Speedtronic systems, the formula is:


where FPRG = P2 Pressure Reference
FPKGNG = P2 Pressure Reference Gain (psi/% HP Speed)
FPKGNO = P2 Pressure Reference Offset (psi)

It's a simple y = mx + b, or f(x) = mx + b, formula, where the P2 pressure reference is a function of speed plus an offset; no more, and no less.

And the Speedtronic compares the actual P2 pressure feedback to the P2 pressure reference, and when they are not equal the servo current to the SRV changes to try to make the P2 pressure equal to the P2 pressure reference.

Even in the Mark II system, where the P2 pressure feedback is a voltage that is directly proportional to pressure, it would seem there would be a "comparator" or summer (summing junction) that would be comparing the P2 pressure reference voltage (which is directly proportional, using the same scaling as the P2 pressure feedback) to the P2 pressure feedback voltage. So, if the P2 pressure feedback is scaled 0-5 VDC for 0-300 psig, the P2 pressure reference would also be scaled 0-5 VDC for 0-300 psig, and the summing junction would be comparing the reference voltage to the feedback voltage and if there was an error the servo current would be adjusted accordingly to try to make the feedback equal to the reference.

The SRV regulator uses position feedback as a stability means. Think of the pressure control as an "inner loop" and the position control as an "outer loop". So, when the actual pressure is equal to the reference then the SRV is in the correct position and doesn't need to move. If the feedback differs from the reference, then the SRV is not in the correct position and it needs to move. So, the SRV regulator is basically a pressure control loop with position feedback for stability. The SRV regulator will put the SRV at whatever position is necessary to make the actual P2 pressure feedback equal to the P2 pressure reference.

For a Mark II, the P2 pressure reference gain would likely be expressed as psi/VDC and the offset would likely be expressed as VDC. And, the maximum VDC would be equal to the maximum value of the P2 pressure transmitter (usually 5 VDC).

However, since we don't know what kind of turbine this is (heavy duty or aeroderivative or ?) the SRV operation could be very different from the normal, typical SRV operation.

It would be nice to see the whole Cont. Spec. section/paragraph about the SRV, because if there's a gain pot and an offset pot it would seem that the above description and formulae are applicable.

The point, though, is probably moot, since it would appear the bigger problem is the missing bits and pieces of the HP turbine blades.
So why are you trying to run this machine with broken/missing airfoils? By the way, if the "blades" are on the turbine wheel, they are more properly called "buckets." "Blades" are on the compressor wheels.
The turbine is frame 1, with rated speed 11140 rpm.

The schematic of SRV is already sent to you.

Thank you for your kind offer to give more suggestion related P2 excessive pressure, 18 bar.
As per document, the description of the SRV drive card is to compare NHP speed with Pressure P2, then generate a control signal to be compared with SRV actual position and then generate a current signal to servo valve of SRV as a function of Speed NHP & Pressure P2.

Based on the schematic diagram, The Turbine speed signal,NHP, is amplified then compared to Pressure P2 feedback signal from pressure transducer 96FG,that monitor the intervalve pressure.

A control amplifier compares these two signals and generates a pressure control signal to reposition the valve if there is an error between the speed, NHP, and the pressure signal.

The pressure control signal then compared to the actual valve position signal detected by LVDT.

If there is an error between the pressure control signal and the actual position signal, the servo amplifier will provide the necessary current to the servo valve to reposition the stop/speed ratio valve cylinder.

Calibration of (P2) versus speed (NHP)curve is achieved via offset pot to adjust P2 at minimum speed, and Span pot to adjust P2 at 100% speed.
After periscope inspection, there was not any trial to start the machine.

The inspection revealed broken "blades" on the turbine wheel.

That's probably a very good decision.

I still question the P2 pressure calibration, but I don't believe that's the entire problem. Certainly, missing turbine buckets could account for the problem being described here.