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Blade Path Temp
BPT Bias & Over Firing ?

1. How BPT Bias is determined in Gas Turbine and What is the Importance of it ?

2. Why Load rate has to be reduced when we reach close to full load , Is it just to prevent Over Firing ?

I would like to know about it in details if possible.

By Namatimangan08 on 31 January, 2011 - 2:15 am

I will answer the second one assuming your GT serves for power generation. Three reasons that I know: (1) To prevent temperature spread in combustion chambers (2) To prevent transient temperature rise, in particular at turbine inlet (3)To prevent governor oscillation.

Those three reasons directly or indirectly have something to do with frequency influence to your turbine output via speed droop response. See the discussion about the speed droop. I can't help you where to find it but I know it is here.

For (1) and (2) they have something to do with firing temperature or turbine inlet temperature limitation. Sudden change in fuel flow is bad for good temperature distribution. Slow rate of loading helps tom preserve uniform temperature.

For (3) above, apart from operator's set point your GT is also responding to its own governor speed droop. The droop biases your GT load based on system frequency deviation from its reference frequency and its percentage set point.

Assuming you load your GT at its maximum output of 130MW when frequency is at 49.95Hz and its reference frequency is at 50hz. Your GT's final load is 130 + Frequency bias. In this case the bias will be positive since system frequency is lower than the reference frequency. Otherwise it is negative. Then your GT output will be higher than 130MW. If high exhaust temperature trip is well calibrated up to equivalent to 130MW, the speed droop will load your GT higher than the maximum allowable temperature limit. Unless you operate your GT on temperature limiter.

CSA,

Can you help me in understanding correctly?

Does it mean that power output will be lower since my frequency has dropped? Frequency dropped because of inability of generator or not having enough thrust/power/energy/fuel from the engine? What's the correlation with the exhaust temperature in this case? I may be wrong but I am under impression that all GT has temperature controller to prevent over firing or to protect GT.

>I will answer the second one assuming your GT serves for
>power generation. Three reasons that I know: (1) To prevent
>temperature spread in combustion chambers (2) To prevent
>transient temperature rise, in particular at turbine inlet
>(3)To prevent governor oscillation.
>
>Those three reasons directly or indirectly have something to
>do with frequency influence to your turbine output via speed
>droop response. See the discussion about the speed droop. I
>can't help you where to find it but I know it is here.
>
>For (1) and (2) they have something to do with firing
>temperature or turbine inlet temperature limitation. Sudden
>change in fuel flow is bad for good temperature
>distribution. Slow rate of loading helps tom preserve
>uniform temperature.
>
>For (3) above, apart from operator's set point your GT is
>also responding to its own governor speed droop. The droop
>biases your GT load based on system frequency deviation from
>its reference frequency and its percentage set point.
>
>Assuming you load your GT at its maximum output of 130MW
>when frequency is at 49.95Hz and its reference frequency is
>at 50hz. Your GT's final load is 130 + Frequency bias. In
>this case the bias will be positive since system frequency
>is lower than the reference frequency.

Regards,
Daniel

0 out of 1 members thought this post was helpful...

Daniel,

I'm not sure I understand the nature of your enquiry. This is an old thread, and I disagree with most of Namatmangan08's assessment and reasoning.

In general,a generator-set synchronized with other generator-sets on a large or infinite grid has no control of it's speed/frequency. It is synchronized with other generator-sets and they all must rotate at speeds that results in the same frequency for all machines. Other than extreme transient conditions, all machines synchronized together produce the same frequency--one machine can't produce 50.8 Hz while another produces 49.5 Hz and another produces 51.27 Hz and another produces 48.89 Hz. They all have to produce the same frequency--and in fact, they all do under normal circumstances. (Magnetic forces at work in the generator ensure the rotor spins at a speed that is proportional to the frequency of the grid with which it is synchronized and since most prime movers are directly connected to the generator (even through a reduction gear) they also spin at speeds proportional to the frequency (called "synchronous speed").)

Multiple generator-sets synchronized together act as one big generator, just like all the motors and lights and computers and computer monitors and televisions all act as a single load. And, when the amount of generation--the amount of torque being produced by the prime movers driving the generators--is exactly equal to the amount of load from all the motors and lights and computers and computer monitors and televisions at the desired frequency then the frequency will be at rated.

However, if one generator trips off line, let's say it's a large generator (tens or hundreds of MW) and no other machine comes on line to take its place then the torque being produced by all the generator prime movers is not sufficient to maintain both the load AND the frequency at the same time, so some of the energy that was going into maintaining frequency goes into maintaining the load. The number of motors and lights and computers and computer monitors and televisions remains the same--but the generation has decreased. So the frequency decreases.

It's just like you riding a bicycle at a constant speed on a flat road, and suddenly someone throws you a heavy package. If you don't increase the pressure you apply to the pedals--or you can't, because you were already pedaling as hard as you could--then the speed of you and the bicycle and the package will decrease.

So, generator-sets can't directly control their speed and frequency once they are synchronized to a grid with other generator-sets. The frequency of the grid determines the speeds of the generator-sets. Full stop. Period.

Now, let's say all the generator sets are all producing the maximum amount of torque they can produce, and a large generator-set trips off line as above. Again--the number of motors and lights and computers and computer monitors and televisions isn't changing, it's remaining the same. But, some of the energy being produced by the prime movers and generators has been lost. And, if all the prime movers (turbines; reciprocating motors; etc.) are already producing maximum power, then the frequency will still decrease and there is no opportunity to increase the power of any of the machines still synchronized to the grid.

For a gas turbine prime mover, when the frequency of the grid with which it is synchronized decreases and the gas turbine is already operating at maximum power (maximum exhaust temperature for the operating conditions) and the speed of the gas turbine goes down because the generator frequency makes it go down the air flowing through the gas turbine's axial compressor also goes down. If the fuel were held constant the exhaust temperature would increase--but it's already at maximum so it can't be increased any more. And, in fact, because the air flow has decreased the exhaust temperature will tend to increase--so the turbine control system will decrease the fuel flow-rate to limit the exhaust temperature. And, this in turn causes the power output of the turbine--and the generator--to decrease also. It can't produce any more power--because the air flow through the machine has decreased because the speed of the axial compressor has decreased because the grid frequency has caused the machine speed to decrease because the generator is synchronized to the grid.

Droop speed control is NOT applicable when a gas turbine is operating at rated power output--it's all about limiting the exhaust temperature to protect the machine from over-firing. Droop speed control only applies when the gas turbine is operating at less than rated power output (below exhaust temperature control). And, even if the machine is operating at Part Load if the machine speed goes down because the grid frequency goes down and the exhaust temperature hits the exhaust temperature limit as fuel is increased the fuel flow-rate will be limited by exhaust temperature control to protect the machine.

This is the "dirty little secret" of gas turbines: When they are operating at or near rated power output (Base Load; Exhaust Temperature Control) and the grid frequency decreases the output of the gas turbine decreases and/or is limited. And, that's the exact opposite of what one wants to happen when the frequency decreases. It actually makes the grid frequency decrease worse when this happens. But, it's how gas turbines work.

And the opposite is true if the grid frequency increases above rated when a gas turbine is operating at or very near rated power output--the output goes up when it should go down. Again, this is just how gas turbines operate today.

Some gas turbine manufacturers are writing special code to allow machines to temporarily increase their power output under these conditions (Base Load with low frequency). But, it's not an indefinite period of time--not more than a few minutes. And, it's not very common--yet.

Other than this, you would have to provide a lot more information about your question and conditions of operation. And, remember, AC (Alternating Current) power generation is just like riding a bicycle at a constant speed while carrying packages. If you keep the force being applied to the pedals constant and change the weight of the packages, the speed of the bicycle and rider and packages has to change. If you don't, or can't, produce any more pressure on the pedals (torque on the crankshaft) the bicycle can't travel at the same rate of speed if you increase the weight of the packages. Or, if you decrease the weight of the packages and keep the same pressure on the pedals, the bicycle speed will increase. AC power generation is just like that--it's about torque and speed while "carrying" a load.

If you want to move a large load at a constant speed--larger than any single bicycle and rider could move--you could hitch a bunch of riders to the load and they would have to coordinate with each other to get to and maintain the desired speed. And, if the load changed, or the number of riders changed, then it takes a good deal of coordination to keep the load moving at the desired speed. (Fortunately, for AC power generation--it's easier than with a bunch of humans on bicycles to coordinate a steady speed (frequency)!)

Hope this helps!

Thanks CSA

This helps.
Would you mind explaining about BPT spread vs variance?

0 out of 1 members thought this post was helpful...

Daniel,

Thanks you for the feedback; I'm glad to have been of help.

GE-design heavy duty gas turbines don't use BPT (Blade Path Temperature) for control or monitoring, and I'm not really familiar with the concept. If you can tell us more about it, or if you can tell us which turbine you are working on, perhaps someone here can provide some information which may be helpful.

The concept of exhaust temperature spreads (particularly with regard to GE-design heavy duty gas turbines with can annular combustors) has been covered MANY times before on control.com. (There is a very fast and powerful 'Search' field cleverly hidden at the far right of the Menu bar at the top of every control.com page. Use the Search 'Help' because the syntax of the commands is not intuitive, but is fairly simple. Try the Search term:

     +"exhaust temperature spread"
with the plus sign and double quotes enclosing the term.)

Any combustion (gas) turbine with can annular combustors (individual combustors, as opposed to a single "silo" combustor, or a single annular combustor (as is used on a lot of aircraft-derivative gas turbines) will usually have spread detection--even if it's called something else. The concept is that if the combustion in each individual combustor isn't relatively equal, producing combustion gases of nearly equal temperature, then there's something amiss with the fuel delivery or the combustion components inside the combustor which is causing the hot gas temperature from that combustor (or combustors) to be much high or much lower than the adjacent combustors. This causes a problem for the rotating turbine buckets which pass by the hotter or colder area once per revolution and are heated or cooled, respectively, repeatedly. If the temperature differential ("spread") is high, then the turbine buckets (as GE calls gas turbine blades) will experience thermal stresses at a very high frequency which can lead to premature failure and usually catastrophic damage. Also, left unchecked for long, the combustion components in the combustor (nozzle(s), liners, transition pieces, etc.) can experience cracking and in the worst case, deformation and even melting--which can also be catastrophic.

Hope this helps--sorry I'm not familiar with the whole BPT concept, and "variance" either.

Blade Path Variance is the difference between the average blade path temperature and the low blade path temperature. Blade Path Spread is the difference between the average blade path temperature and the high blade path temperature. During startup, there are different set points for alarm and trip based on the speed of the gas turbine. Once you've reached rated speed the set points for alarm, unload, or trip are no longer variable. Typically, blade path spread is an issue during startup on a cold machine and ends up in a "fired abort".