# VC3 Not Following Reference.

M

#### muz_sidhu

We are having two Frame 9E turbines with MK-VIe control system. During winter season, whenever temperature goes at its lowest point (around 3 to 5 DegC) specially in the night around 0300 to 0800 hrs this alarm appear on the Alarm server then we switch to liquid Fuel screen to check the "fqrout" and "fql1" values. These values are disturbed too. The value of "fqrout" goes gradually up to 100% while the value of "fql1" stays where it was. Suppose previously both have same values like 73.05%, then suddenly the value of fqrout start increasing up to approx. 100%, but fql1 stays around 73.00% and no any major change was observed.

Before this problem arise we are running at Base load (approx. 112MW) but when this problem arise we reduce the load 5 or 10 MW and it was stable again. We don't know where the problem is because during last week offline water wash we stocked VC3 and its operation was normal. We have Liquid Fuel Bypass valve manufactured of Fisher Company without LVDTs. Please suggest the cause of this problem.

Thanks and Regards,
Sindhu

#### CSA

muz_sidhu,

FQL1 should match FQROUT (sometimes the signal names are different--but the units (lb/sec or kg/sec) should match. If the Speedtronic is calling for 3.2 kg/sec, the fuel flow-rate feedback from the liquid fuel flow divider should be 3.2 kg/sec. At your site, it seems the flow reference and flow feedback are in percent, not engineering units, but the idea is the same--when the flow reference is 73.05% the LFBV should move to the position that allows flow feedback to go to 73.05%.

<b>Based on the information provided,</b> it's not clear how one can stroke the LFBV (Liquid Fuel Bypass Valve) using AutoCalibrate--because it doesn't have LVDTs. If a position reference is put into the Manual feature of AutoCalibrate and there is no position feedback the servo-valve output will just drive the device to the full flow (in this case, fully closed) position. So, the report of stroking the valve and proving it's working doesn't seem to make sense. The only way to move the LFBV to some position--and get it to stop moving--is to simulate a flow divider feedback. The problem with this method is that one can't put in a flow reference of 3.2 kg/sec (presuming this is, say, 25% of maximum flow) and have the valve stop at 25% of stroke, because as soon as the simulated flow feedback equals the flow reference the valve will stop moving--at whatever position the simulated flow feedback equals the flow reference.

You need to determine if the LFBV is fully closed when FQROUT is at 100%. To do this you're going to need to know what the full travel (stroke) of the LFBV is, and then measure the LFBV at the fully open and fully closed positions and determine if the valve has the proper stroke (range of travel).

If the LFBV is capable of traveling its full range, then you need to find out what is restricting the flow of liquid fuel. Is the liquid fuel supply flow/pressure restricted? Sometimes, there are strainers on the liquid fuel forwarding pumps which get clogged and restrict the flow of liquid fuel into the pump. Or, the low- or high-pressure liquid fuel filters may be plugged, restricting the flow of liquid fuel through the filters. (There should be alarms for high differential pressure across any element (strainer; filters) if this is the problem, but frequently the switches are broken or the switches aren't properly valved in.) It should be possible to check the pressures upstream and downstream of any filter element.

I hate to say this, but if it's not a liquid fuel supply problem and the LFBV is operating over it's full range of travel, then it's likely that the scaling of the liquid fuel flow divider feedback is very wrong. So, when it's reporting 73.05% it's really at 100%. The unfortunate problem with this is that if the scaling gets changed, it affects firing and acceleration FSR values--and that can mean a lot of "tuning" has to be done.

Another way to see if the scaling of the flow divider feedback might be wrong is to use the liquid fuel flow meter at the forwarding skid to see if the flow-rate is at or near the maximum expected liquid fuel flow-rate for the unit on a very cold ambient day. (See Sect. 05.01.nn of the Control Specification drawing, the section labeled 'Expected Fuel Characteristics' for this data.) If, in fact the actual liquid fuel flow rate is at or very near the maximum expected liquid fuel flow-rate for a very cold ambient day then it's pretty certain the liquid fuel flow divider feedback scaling is incorrect.

If the flow-rate feedback can't equal the reference, then something is wrong. Either the flow is restricted (mechanically) because the control valve can't close enough or doesn't have the proper range of travel (it might even be that the control valve is too small for the application--not likely, but stranger things have happened), or something is restricting liquid fuel flow to the nozzles (either on the low pressure side of the high pressure liquid fuel pump/control valve, or on the high pressure side of the high pressure liquid fuel pump). It's not likely that the liquid fuel check valves are incorrectly sized (but, again, stranger things have happened), or that the liquid fuel nozzles are undersized (that would be very unusual--unless some kind of low BTU liquid fuel is being used which require higher liquid fuel flow-rates). Or, the scaling of the liquid fuel flow divider feedback is incorrect.

Of the possibilities, the incorrect liquid fuel flow divider feedback scaling is the harder to resolve. It will likely involve a fair amount of trial-and-error to resolve--of several parameters, including firing, warm-up and acceleration FSRs.

Please write back to let us know what you find.

M

#### muz_sidhu

Firstly, I feel sorry if my English sentences are not up to the mark. I am just trying to convert what is whirling in my mind.

You are right that FQL1 should match FQROUT and at our site these are identified in percent.

We stroked the LFBV by keeping the value of FQL1 same at zero and increasing or decreasing the value of FQROUT, following the procedure written in Control Specification (Sect. 05.01.110). You are right we can't identify 25%, 50%, 75% or other positions of VC3 according to the flow rate feedback from flow divider. But with reference to the positions observed for VC3 full open and full close, we just stroked the valve for null biasing check toward middle position.

Now, coming toward the position of the valve during operation, I have observed that whenever this alarm came the position of the VC3 remains at full close position. This fully close position of VC3 is verified during shutdown when we stroked the valve to full open and full close position. This position is achieved when the value of FQROUT (and similarly FQL1) was just 73% or little more. In same time in online trends for GT parameters the value of FQL1 remains constant for this time period i.e. suppose if the flow is 76% just before the problem, it will remain approx. to this value and will never be more than this value in any other time too. All the other time when ever this alarm appear or this problem come, same behavior of VC3 and FQLM1 is observed i.e. flow remains constant and didn't increase from 76.25% (Flow may vary in right side of point but the value on left side remains constant). I have some graph related to this if you need the one.

We are having strainers installed in number of location like, upstream and downstream of Forwarding pumps, then another filter are installed upstream of the main liquid fuel Pump. All the filters are recently cleaned.

Now coming toward your point that the Flow divider scaling was wrong, I have no idea how to compare the reading for the flow divider and the settings defined in control setting document. But what I have observed from the value from FQL1 in trend is mention above.

Thank you,

#### CSA

muz_sidhu,

So far, your English is just fine. Please just try to be as precise as you can (as you would when discussing technical issues in ANY language), and provide as much information as you can and we should be just fine.

I am, however, having a very difficult time understanding how you can manually "position" the LFBV when there are no LVDTs when the turbine is not running. It's just not possible to equate FQROUT and valve position unless there are LVDTs. So, let's forget about that for the moment. All you need to do when off-line is measure the valve travel from full open to full closed (however you get the valve to travel from one position to the other) and then compare that to the valve specification sheet--and the LFBV travel is usually specified in Sect. 05.01.nn of the Control Specification. (I say 'nn' because I don't know which section, but 05.01 is the Liquid Fuel section, and you should be able to find valve spec's for the LFBV in that section.)

If the measured travel is less than the specified travel, then there's something mechanically wrong with the valve <b>OR</b> the actuator. Either the valve or the actuator is mechanically binding or sticking and preventing the valve from moving over its specified range of travel. That would cause the LFBV to restrict the flow of fuel to the turbine, which would result in an ability to make load. (I want to get back to that load issue in a minute.)

If the valve is traveling over its specified range then either something else is restricting the flow of liquid fuel, or there is a problem with liquid fuel flow divider feedback scaling. It's easier to eliminate all the other possible causes of this issue before delving into the scaling issue--trust me on this. And, if you go through all the hassle of changing flow divider feedback scaling and there was an issue with liquid fuel flow being restricted that eventually gets resolved it will be necessary to go back to the original flow divider feedback scaling. Best to eliminate all the other possible problems as causes of this problem before turning to working on the scaling issue.

I also want to ask a very important question I have been extremely remiss for not asking earlier: Did this problem just start, or has it be ongoing since commissioning? Because if it just started, then there's likely a mechanical cause for the problem--and it's NOT related to flow divider feedback scaling. If it's been ongoing since commissioning, then it's more likely it's a flow divider feedback scaling issue.

Now, to the load issue. When the problem occurs, what are the values of TTRX and TTXM? I'm presuming the unit is being operated with Base Load selected and active when this problem occurs, and if TTXM is less than TTRX when the problem occurs then the unit is being "load-limited." That is, not enough fuel is getting into the turbine to keep TTXM (the actual median (average) exhaust temperature) equal to TTRX (the CPD-biased Exhaust Temperature Control Reference). When Base Load is selected and active the Speedtronic is putting as much fuel into the unit as it can in order to make TTXM equal to TTRX at all times. If there is something that is causing the actual exhaust temperature (TTXM) to be less than TTRX when Base Load is selected and active then something is limiting the fuel flow to the turbine. Plain and simple. If the valve can't open to allow enough fuel into the turbine to allow TTXM to always be equal to TTRX when Base Load is selected and active, then either there's something wrong with the LFBV or its actuator, or there's some mechanical restriction in the liquid fuel system that's preventing enough fuel from getting through the LFBV.

I've rarely seen the wrong LFBV provided with a GE-design heavy duty gas turbine--in fact, I've only seen that happen one time in three decades. So, I'm not thinking the LFBV was sized improperly. Maybe it was refurbished recently, and the mechanical stops weren't set properly.

Finally, I want to stress that on a very cold ambient day the unit is going to burn the most fuel when operating at Base Load. And, if the ambient is less than the machine was rated/built for, then that could be a problem--because the actual fuel flow-rate might be higher than anticipated, though by 20+% seems a little off.

Another possible issue could be that the liquid fuel being burned is different from that specified when the unit was ordered and being built. The fuel make-up the unit was built to should be in Sect. 05.01.nn, 'Expected Fuel Characteristics' of the Control Specification. Compare the current fuel data/analysis to this to see if there has been a large change in BTU content.

So, the questions you need to answer are:

1) Does the valve travel the full specified travel (when testing off-line)?

2) Has this problem been ongoing since commissioning or has it recently started? (How long has this unit been running since commissioning, and how long have you been experiencing this problem?)

3) What are the values of TTRX and TTXM when the alarm is being annunciated? And, confirm that Base Load is selected and active when this alarm is being annunciated.

4) How does the current fuel analysis compare to the 'Expected Fuel Characteristics' in the Control Specification? Please note any deviations, and list the amount of the deviation(s) also.

The flow divider scaling is defined in the Control Specification. It specifies how many Hz correspond to how many kg/sec. And, there is a 100% value for kg/sec, also. There is usually a signal, FQLM1, that is the mass flow-rate (kg/sec). This is the signal that you need to try to correlate to the timed readings you take from the mechanical liquid fuel meter usually located on the Liquid Fuel Forwarding skid. It's usually "spinning" VERY fast when the unit is operating at Base Load. You need two people to time this meter; one to watch the timer/watch and say "START" and STOP, and one to write down (as best as possible) the readings when "START" and "STOP" are said. And, I would suggest taking several readings of about two or three minutes each, and averaging them (as long as they are relatively close to each other) to get a value of actual liquid fuel flow. It will probably be in liters or gallons, and you can use the specific gravity for the liquid fuel you are burning to convert that to kg or pounds, and then to kg/sec or lb/sec. And, then compare that average reading to FQLM1. They should be close--if not, then either the readings at the meter weren't taken very well (and that's difficult), or the flow divider feedback scaling is not very good.

If you want more help--we need more information, in the form of what's called "actionable data" (the answers to all of the questions above--detailed answers to all of the above questions). Help us to help you!

M

#### muz_sidhu

Below are the answers of your questions. I have tries my level best to share regarding your questions;

> 1) Does the valve travel the full specified travel (when testing off-line)?

Yes, the travel of valve is according to the specification in documents. I want to add here that whenever the alarm comes, or before the alarm starts (Means, we have noted that when load is approx. 110MW) valve is almost at full close position, then alarm appear, or FQROUT value starts increasing from 77% towards 100%, but no change was observed in valve position. These conditions are compared to the stock done during unit offline.

> 2) Has this problem been ongoing since commissioning or has it recently started? (How long has this unit been running since commissioning, and how long have you been experiencing this problem?)

No, this problem just appeared in winter season, when temperature was at its lowest point according to the atmosphere here in this site. The unit was running for about one and half year after commissioning. After commissioning unit was running for about 8 to 10 months with Light Distillate Oil, but after that Crude oil system was commissioned and from last 8 months we are running with crude oil. Operating temperature range for crude oil is about 40 to 60 Deg C. About 2 to three weeks during we are running with this problem and every time in night (around 01:00 hrs. till 07:00 hrs. in morning) this problem appear. But in morning when ambient temperature goes up we put GT on base load and it is operating smoothly with load around 100 to 110 MW.

>3) What are the values of TTRX and TTXM when the alarm is being annunciated? And, confirm that Base Load is selected and active when this alarm is being annunciated.

The value of TTXM and TTRX are varying with change in FSR. Below is the variation (the temperature values are in Degree F);<pre>
TTXM -- TTRX -- FSR
885 -- 909 -- 77%
890 -- 910 -- 80%
895 -- 912 -- 85%
897 -- 914 -- 95%
900 -- 916 -- ~100%
901 -- 920 -- ~100%
902 -- 925 -- ~100%
902 -- 930 -- ~100%</pre>
Above is the rough trend that I can sketch with values up here. I don't know how to upload picture here and paste the link, because I have trend for all these values and I sourly want you to see it if it can help regarding this. Anyhow in this trend values increase and after this start decreasing when we put GT on preselect and reduce the load, all the values then start decreasing and after having some low values they again come back to normal values as per requirement of preselect load.

As far base-load selection, before the alarm appears, unit was on base load, after this no base load selection (alarm "TNR too low to support" like that alarm appear). Every time we select Base load during alarm state when FSR was running up toward 100%, control system didn't accept the command. Then finally we have to put GT on preselect.

>4) How does the current fuel analysis compare to the 'Expected Fuel Characteristics' in the Control Specification?

Here we don't have any analysis facility on site for fuel. The analysis was just performed during commissioning by the vendor and now we don't have any analysis facility. I don't think the owner will perform this test from third party since other machine is running good with same fuel. We are running with two Frame 9E turbines and the other turbine wasn't having this particular problem. Every time some difference or any problem comes, they first give reference of other turbine (WHY NOT THIS PROBLEM COME ON OTHER TURBINE?).

I want to add some other problem of same turbine we are discussing here, when we are running our system, that after every 250 hours we perform Offline water wash for our turbines with detergent as per recommendations. After last offline washing we face the problem that our GT was not having any flame inside Combustion system. We have checked all the system but it doesn't appear for 2 days, every time we start no flame was detected. No flame percentage was detected on single flame scanner. While trouble shooting we performed number of jobs as listed;

1- Cleaning of flame scanners (They are in good condition)

2- Cleaning of Liquid fuel Check valves (some carbon was observed in them, all are pressure tested on test bench and they are operating on 8bar pressure)

3- Inspection of nozzles for any blockage. (no blockage was observed)

4- Testing of igniters (one igniter cable was not Ok, But may be this isn't the big problem as we kept on operating one of ours turbine with one igniter for about 6 months).

5- Stock and testing of VC3 (it is responding good as per procedure defined in control specifications 05.00.xx)

6- 20CF-1 coil resistance was good according to documents

7- Resistances of speed pickups of flow divider (resistance of both the pickups was almost equal and according to standard)

8- Breaker of igniter was normal.

During this whole activity, the problem was that the reference "FQL1" was just shown for 2 or 3 seconds and then sticks to zero for the rest of firing timer. Every time we give start command same behavior was observed.

During this whole activity machine got started two times after we completed the washing process. 1st one was successful after having, one Excess fuel Tripping, second one was No Flame detected and third time machine got started and we successfully changeover our machine to crude oil from Distillate Oil. Just after successful change over, "FD1-1 Flow divider feedback alarm appears and machine tripped. After this purge cycle confirmed and again started and every time the problem comes with "FQL1" value. Finally after having massive purging of all the liquid fuel system we are successful to start turbine and now machine was running. This was the second time we started turbine after offline water wash.

I have discussed this problem because may be this got some clue regarding the original problem.

Hope that this will help to solve the problem.

Thank you,

M

#### muz_sidhu

Dear CSA,

Hope that the information provided is sufficient and helpful to further actions. Waiting for your response.

Thank you,

#### CSA

muz_sidhu,

Based on the exhaust temperature data, it's pretty clear that something is restricting the flow of liquid fuel to the turbine. What's not clear is whether it is something in the liquid fuel supply line to the turbine, or a problem with the high-pressure liquid fuel pump, or the liquid fuel bypass valve, or the flow divider, or the liquid fuel check valves, or the liquid fuel nozzles.

If the high-pressure liquid fuel pump and LFBV on the unit experiencing the problem is the same as on the unit which isn't experiencing the problem, then it's not likely a problem with them being the wrong size/rating. There may be a problem with the internal clearances of the high pressure liquid fuel pump, or with the setting of the LFBV plug/stroke.

The liquid fuel pump clutch may be slipping--but I would expect that if it was slipping the load would be unstable (oscillating) because if it slips it usually does so intermittently until it finally seizes--usually.

But, it's pretty clear that if the LFBV is going fully closed that something is preventing more fuel from getting through the liquid fuel system to the combustors.

Please write back to let us know what you find.

It would also be good if you could compare the liquid fuel flow divider feedback scaling of the two units. It's a configuration parameter in the Speedtronic. If the two control systems are the same, I would expect the two values would also be the same. Especially if the flow dividers are also the same on both machines.

Hope this helps!

M

#### muz_sidhu

CSA!

With reference to the above post, recently we have completed our HGPI activities. And we have started our turbine successfully with some problem but now the startup problem was resolved and machine is having smooth starts and normal. During change over from LDO to Crude Oil once our machine got tripped due to "FD1-1 Feedback Trouble". After that we started machine again and now machine is smooth again.

Now the problem comes again that during both LDO and Crude operation, at around 104-MW "fqrout" starts increasing up-to 96.6% from normal value 66.7%, then limiting the load to 95-MW it is stable now. All this activity was observed during night hour's operation and also in the morning with temperature around 24-Deg C. But last time when I am leaving office and temperature was around 28-Deg C, we again put our GT on Base Load and every-thing found normal. "fqrout" and "fql1" values are normal with GT operating at Base Load 103-MW.

From the observation done when this alarm appear in afternoon at around 1300-hrs is that; "fqrout" value start increasing along with "FSR" (since both are same), "fql1" is limited to 66.7% during all this time when "fqrout" was disturbed. GT just before this alarm was operating at Base load, after this event, Base Load command didn't except any more. It stays in the same condition as long as we didn't reduce the load. So we put GT on Preselect 95-MW.
Again we are facing same problem although we have done HGPI of the machines. All the mechanical equipment we changed from fuel nozzles to turbine 3rd stage except Liners. Still there is some blockage in fuel flow and some thing is restricting the flow towards turbine.

What do you say about all this?

Thankyou,

#### CSA

muz_sidhu,

I say--basically--the same thing(s) as before. Either:

1) There's something wrong with the high-pressure liquid fuel pump, or the liquid fuel pump clutch, or the relief valve around the high-pressure liquid fuel pump;

OR

2) There's some flow restriction in the liquid fuel supply piping/system;

OR

3) There's something amiss with the heat content of the fuel(s);

OR

4) There's something wrong with the liquid fuel flow divider not allowing sufficient fuel to pass through the flow divider.

You talk about LDO <b>and</b> heavy fuel (HFO), but you're not specific if the <b>same</b> problem exists for <b>both</b> fuel<b>s</b> at high load. Because if it's the <b>same</b> problem for <b>both</b> fuel<b>s</b> at high load then it's something common to the liquid fuel system or the fuels.

Presuming it's the same problem for both fuels at high load, it seems that the liquid fuel flow divider scaling is okay--and that can be said because fql1 seems to be limited and it's not at or near or above 100%. So, it would seem that no matter how far closed the LFBV goes to try to force more fuel into the combustors that either the fuel can't get to the combustors (because of some restriction in the supply piping/system), or the high-pressure liquid fuel pump can't pump enough fuel either because the internal clearances/seals of the pump are worn, or there is a problem with the relief valve around the high-pressure liquid fuel pump leaking/passing, or the liquid fuel pump clutch is slipping (not likely), or the flow divider can't get enough fuel through it (maybe because it's the wrong flow divider for the application).

If it's not common to both fuels at high loads, then it just might be that the heating content of one fuel (most likely the heavy fuel oil, because that's probably the fuel being burned at high load; LDO is probably only used for starting and accelerating to FSNL or some light load before switching to HFO).

You need to use the Liquid Fuel System P&IDs (Liquid Fuel Supply and Liquid Fuel System) to eliminate every possible source of leaking or restriction. Check every manual valve to be sure that it's fully open. You might even close manual valves (when the unit isn't running, of course) to make sure they close fully and there's not blocking the valve (this is just a cursory check, but it's better than nothing). Again, you need to check the filter canisters to make sure they aren't blocked or passing (the transfer valves are sealing correctly; sometimes the o-rings can become unseated and cut if they are operated in mid-position for any length of time under high flow-rates and then moved to the proper position).

You could compare part numbers for the high-pressure liquid fuel pumps of a working and the non-working machine. You could use a photo-tachometer to monitor the coupling between the liquid fuel pump clutch and the high-pressure liquid fuel pump to see if it's slipping. You could compare the part numbers of the flow dividers of a working and the non-working machine. You could compare the liquid fuel check valves of the working and non-working machines.

Most LDO/HFO units have manual valves in the fuel lines between the flow divider and the liquid fuel check valves at the fuel nozzles to purge HFO if the unit trips while running on HFO to prevent the HFO from getting too thick when allowed to cool in the fuel lines too long. It's not likely that the valves are ALL not working (because if only one or two or three were leaking or open when they should be closed there would be high exhaust temperature spreads and probably problems lighting off), but they still have to be eliminated as the cause of the problem.

This is just a logical process of troubleshooting and elimination. You must use the P&IDs and simply eliminate every possible cause of the problem, one at a time, until you arrive at the root cause. It can take time, and it should be preceded by a plan for logically working through every component. Because if you only check/verify that a few components are working and you don't solve the problem, well, that leaves all the unchecked components to still verify.

In thinking about this while writing, it seems a good bet to check the relieve valve around the high-pressure liquid fuel pump. If it's leaking, then the high-pressure liquid fuel pump can't get enough fuel to the nozzles regardless of the position of the liquid fuel bypass valve.

And, you could compare the part numbers (the manufacturer's part numbers--not GE's or the packagers, the manufacturer's part numbers (for all the items you compare this way--it's possible a wrong part got substituted during manufacturing and the GE/packager's part number will likely be the same, presuming the units are all about the same age/rating, but the manufacturer's P/N wouldn't be the same if the wrong part got used during assembly). It could also be that the wrong manufacturer's part number got put on the part, and that's going to be a real problem if that happened. But, you could possible contact the manufacturer with the component serial number to see if there might have been a problem at the manufacturer's factory.

It's most likely a mechanical issue, as described above. The Speedtronic is trying to get more fuel into the unit--but either the fuel doesn't have enough heat content, or there's something mechanical preventing sufficient fuel from flowing into the unit. Again, use the P&IDs to eliminate every component--that's what this is going to take. You can't assume anything, because when you do it makes an a of u and me (assume is spelled ass-u-me). That's a way to remember <b>NOT</b> to assume things--not a criticism or a slight. I'm just trying to reinforce what happens when one assumes that a part of component is okay when troubleshooting a problem--especially a problem like this. You say you've replaced all the hot gas path stuff, including fuel nozzles and check valves. So, starting working backwards from there--again, remembering to check any valves between the flow divider and the liquid fuel check valves!

And--write back to let us know how you fare!