from the Technical department...
 Posted by ESKAY on 25 July, 2010 - 3:51 am
Dear All,

We have Fr-5 Mk6 control GT. Our machine is a dual fuel system. When the turbine is running in fuel gas mode, there is NO problem in exh. temp. spread. But in Distillate fuel mode, we have slight spread ,close to allowable spread.

But , I am interested to calculate/verify the allowable spread shown in the print out of the turbine parameters while the turbine was running in fuel gas mode at 17.1 MW.without any spread problem.{spread Normal}

The values noted are as follows:-

TTXM 491 deg.C
Max. CTDA 328 deg.C
TTXSPL 62 deg.diff
TTXSP1 23 deg.diff
TTXSP2 20 deg.diff
TTXSP3 18 deg.C

As per the GT manual, Mk6 algorithm TTXSPV4 is used for spread calculation and TTXSPL calculation is defined as:

TTXSPL = TTKSPL4* TTXM - TTKSPL3*CTDA + TTKSPL5

where TTXSPL4 = 0.145PU {comb.monitor TX gain constant}
TTXSPL3= 0.080PU {CTD limit gain}
TTXSPL5= 30 deg.F{CTD limited Exh.temp.offset}

When I substitute the a/m values in the TTXSPL formula, I don't get the result of 62 deg{TTXSPL} as shown in the print out.

Kindly someone guide me for the correct way of calculating the Allowable spread.

 Posted by otised on 25 July, 2010 - 1:42 pm
You made a common conversion mistake when dealing with temperature differentials. You likely converted TTXSPL5 of 30 degF to -1.1 degC.
If you just multiply 30 by 5/9 you get 16.7 degC, which will give you the 62 degC result your were expecting. DO NOT USE THE 32 degF OFFSET WHEN CONVERTING TEMPERATURE DIFFERENTIALS!

 Posted by WAC on 25 July, 2010 - 5:25 pm
you are using different temperature scales, maybe that's the problem. You must be careful to make this calculation and move through the two "scale line"

TTXSPL = TTKSPL4* TTXM - TTKSPL3*CTDA + TTKSPL5

TTXSPL = 0.145* 491 - 0.080*328 + TTKSPL5

TTXSPL = 44.95 C (almost 45 C) + TTKSPL5

TTXSPL = 113 F + 30 F = 143 F

Back to Celcius Line

TTXSPL = 61.6 C (almost 62)

Mistake is in convert all value at same scale in the same time.

 Posted by ESKAY on 26 July, 2010 - 2:38 am
Dear WAC,

Thanx a lot for explaining the calculation in a spread sheet manner, for the allowable spread. It is simply understood now.

Thanx again....

 Posted by ESKAY on 8 August, 2010 - 2:58 am
Dear WAC & CSA
For the combustion monitoring, the allowable spread and TTXM are used to create an allowable band for Exhaust temperatures.

ie. TTHA,High alarm = TTXM+TTKSP2*TTXSPL
TTLA,Low Alarm = TTXM +TTKSP4*TTXSPL

If,two adjacent Exh.TC falls below TTLA, there will be a combustion Alarm; A temp.above TTHA and another below TTLA will also cause the Comb.Trbl Alarm.

TTKSP2 = 0.400 PU {spread scaling}
TTKSP4 = 0.400 PU {scaling for spread}

Our GT running in Gas fuel at 17.0MW,without any spread, reads as follows:- TTXM=491deg.C TTXSPL=62deg.C.{TTXSP1=23deg.}

When I compute these values in TTHA and TTLA, I don't get the correct allowable band for the temperature, so as to find the Min. temp.band allowed between TTXSPL and TTXSP1.

Pls help solve the allowable band.

Thanx

 Posted by CSA on 25 July, 2010 - 5:35 pm
In addition to otised's fine advice, if you have no spread on gas fuel (or relatively low spread) and you have a high spread on liquid fuel it is likely easily found.

Since the spread is only present during liq fuel operation, you can likely rule out any problem with combustion liners, transition pieces, nozzle segments, etc. it's most likely something to do with the liquid fuel or purge air system.

Use the selector valve at the Liq. Fuel Flow Divider. The general rule is that all the fuel nozzle pressures should be within 10% of each other, and any pressure(s) that is(are) outside of that window are indicative of some kind of problem.

A low pressure could be indicative of a failed Liq. Fuel Check Valve, or a Liq. Fuel Purge Check Valve. If it's the latter, you will see fuel dribbling or flowing out of the Telltale Leak-off (see the Liq. Fuel and/or the Liq Fuel Purge Piping Schematics (P&IDs) to see the Telltale Leakoff is downstream of Liq. Fuel Purge solenoid.

A low fuel nozzle pressure could also be indicative of an improperly assembled liquid fuel nozzle cartridge/assembly, causing the internals to be loose and the clearances too large resulting in low pressure and poor pressure atomization.

A high fuel nozzle pressure could be indicative of a blockage of some sort in the Liq. Fuel Check Valve, or in the Liq. Fuel Nozzle.

May also be some blockage in one or more of the atomizing tips of the fuel nozzles (would would not result in a pressure differential at the selector valve).

My general rule for finding a specific combustor with a problem is to work in the reverse direction of rotation of the unit (which is anti-clockwise looking at the inlet to the axial compressor) and start by examining the combustor/fuel nozzle of the combustion can which is 90 degrees from the cold spot. If that's not the problem can, pull the nozzle on either side of that can, and if they're not the problem, continuing pulling the nozzles on either side of those cans until you find the problem. It's very important to examine both the liquid fuel check valve and the purge air check valve when removing the fuel nozzle. (Some people have access to or have developed their own "swirl angle charts"; I find that they are good for some of the newer machines, but not so good for many of the older machines.)

If a machine has an exhaust temperature spread, the exact magnitude of the calculated spreads shouldn't matter all that much. If the calculation is off by 2 or 8 or 12 degrees. Especially if there's no spread on one fuel and there is a spread on the other fuel, then there is some kind of problem. If there's no spread on one fuel and there is a spread on the other fuel, then it's most likely not a control system problem.

It's also always a common mistake to blame the control system when a spread occurs; and a lot of time and energy is spent by the controls and instrumentation group proving it's not a control system problem.

The Speedtronic doesn't control the amount of fuel going to each combustor; only the total amount of fuel going to the liq fuel flow divider, which is a mechanical device for achieving a uniform split of fuel flow-rates to each individual combustor. Also, the Speedtronic doesn't not control each individual liq fuel check valve, nor each individual liq fuel purge check valve. In a liquid fuel system, there is a surprising number of components the Speedtronic does not have any direct or even indirect control of! And they all have to work together in order for the liquid fuel system to function properly.

 Posted by ESKAY on 26 July, 2010 - 2:30 am
Mr.OTISED,

You are correct. I was calculating 30degF into Deg.C, using normal formulea C= {F-32}*5/9. As 30 deg.F is a temp. differential, I should use 30*5/9 direct.

Thanks a lot... for ur kind advice.

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