Fast load Rate


Thread Starter


Is it merely changing the load rate to quickly load turbine, i.e., gas turbine and/or steam turbine? What kind of damage or impact can it have on the hardware? How one can calculate this damage? Also interested in knowing as we are a combined cycle plant and wondering, if we change the load rate to quickly to load turbine, however not sure impact on steam generation and hence on steam turbine?

Looking for your suggestion and thoughts.

I can only address fast loading as implemented on GE-design heavy duty gas turbines. Fast loading is simply a VERY fast loading ramp rate for the gas turbine. For some smaller machines, it can be as fast as 30 seconds from 0 MW to Base Load (25MW, 40 MW, or 80 MW). Larger (F-class) machines have fast load rates as fast as 4- or 6 minutes from no-load to Base Load--so, VERY fast.

As you noted, the gas turbine exhaust temperature is going to climb VERY fast when fast load is selected. The HRSG manufacturer will have published a maximum rate of inlet temperature rise, which will define the limit of the HRSG (boiler). Usually, most boilers can't take the temperature rise which will occur when fast load is selected.

As for the steam turbine, it, too, usually has a limit of steam temperature rate of increase that has to be observed. Many combined cycle plants have to pause ("hold") the GT loading to wait for steam temperature to catch up so as not to put cold steam or a slug of water in the steam turbine--it can be quite a balancing act, as I'm sure you know. If the steam piping is cold it can take some time for the steam piping temperature to rise sufficiently to prevent quenching (cooling) of the steam before it reaches the steam turbine. There are a lot of factors to consider. Some GE gas turbines have a temperature matching mode which can be used to try to help shorten these hold times.

Most steam turbines in combined cycle plants are what are called "follower" turbines--meaning the output of the steam turbine is a function of the exhaust gas temperature (and therefore load) of the gas turbine. Usually, when steam conditions reach a certain point (somewhere around 50% of rated steam turbine load or so) the steam turbine control system just opens the steam turbine control valve fully ("valve wide open") and the steam turbine load will vary as the steam production varies--which is a function of gas turbine exhaust flow and temperature, which is a function of gas turbine load. Generally, operators at a combined cycle plant don't really control the steam turbine load (generally) and just make as much power with the steam turbine as steam conditions will allow. The steam turbine is smaller (rating) than the GT, and it can't control frequency since the GT is much larger--hence the "follower turbine" designation. It's really just along for the ride. (Some combined cycle plants have some pretty sophisticated means for "controlling" steam turbine power output, and they may involve bypassing steam to the condenser--which is wasteful, since it's not producing any power. But, in the larger scheme of things it may be desirable, at least for periods of time, if not always. It all depends on a lot of factors.)

Also, although you didn't ask, some GE gas turbines also have a fast start function, that reduces the amount of time it takes for the unit to reach rated speed. This can take many forms, and often does, and can require some additional instrumentation, also. But, once the unit reaches rated speed, the loading rate after synchronization is usually (not always, but usually) the normal automatic loading rate (which is slower). The GT exhaust temperature also increases quicker when fast start is selected, but the exhaust flow-rate is usually much less than when at rated speed and many boilers can handle higher temperatures for short periods of time because the exhaust flow-rate is lower during starting. Not all boilers, but some. Again, it all depends....

Finally, the loading rate for a manually (operator-initiated) RAISE or LOWER SPEED/LOAD signal is usually faster than for an automatic RAISE or LOWER SPEED/LOAD signal. So, if the operator clicks on RAISE or LOWER SPEED/LOAD the rate of change will usually be faster than if the operator puts in a new load setpoint and enables some kind of load control (such as Pre-Selected Load Control), or initiates a STOP. "Automatic" loading/unloading rates are usually a little slower than manual loading/unloading rates (the reasoning being that if operators are changing load manually they may be privy to information the turbine control system is not and they may want the load to change more quickly).

Hope this helps! If your site uses something other than GE-design heavy duty gas turbines and steam turbines, I can't be of any help--but, in general, I have found the principles to be about the same for most manufacturers and control systems. The terminology might be (and usually is) different, but the principles are mostly the same.
Thank you CSA,

I agree with you completely. Let us say I change the load rate once GT reaches 50MW and from there to Base Load (around 170MW). I can have different loading rate, only when needed. It's MHI turbine however concept and principle will remain same with some variation here and there.

Your comments Please.

It's now what I am going to do but just brainstorming - discussion.

<b>Moderator's Note:</b> Could actually mean to read, "It's <b><i>not</b></i> what I am going to do...."

On a GE-design heavy duty gas turbine control system, it would be possible to select Fast Load at any load after the generator breaker closes. But, if it was enabled at 41% load (70 MW out of 170 MW) the time to Base Load would be reduced by 41%--so instead of taking, let's say 4 minutes to go from No Load to Base Load, it would only take 2.35 minutes to get to Base Load from 70 MW.

That's still pretty darned fast--and it's still questionable if the HRSG (boiler) and/or steam turbine can handle the sudden increase in exhaust temperature and exhaust flow-rate from the GT....

In a combined cycle power plant, the limiting factor is not the GT loading rate, it's the HRSG's and steam turbine's ability to handle fast increases in HRSG inlet temperature and flow and the steam turbine's ability to be loaded without exceeding differential expansion rates, as well as the ability to get superheated steam to the steam turbine through the piping. One has to remember: in the history of power plants, combined cycle power plants using gas turbines to supply heat to boilers is relatively new. It's very efficient (lowest fuel/MW), but it's not really what GTs or STs were designed for. HRSGs and steam turbines, primarily, are being designed now more for combined cycle applications and fast loading and unloading. But, as a complete system--combined cycle power plant design just isn't there yet. Combined cycle power plant design, and the designs of the major components of a combined cycle power plant, are getting better, but it's a slow, iterative process. AND, some GT manufacturers are claiming their units are capable of VERY fast starting times--and they don't really have a lot of operational data to back up those claims. So, even if the times can be programmed into the control systems, it remains to be seen if the gas turbines can actually perform over years and decades at these fast starting times without decreasing maintenance outage intervals and replacing hot gas path parts at a faster rate....

Hope this helps! One has to think "system" at power plants, which are made of many components which have different operating characteristics and capabilities and formulate an operations plan which takes into account all of the limitations to produce power reliably and efficiently--for the longest period.
Thank you very much CSA.

Since I didn't want to make other changes to go quicker on load without much (obviously impacting hardware some extent), was thinking to go to BL from ~40 -50%. It's not that we will operate that way on regular basis, but occasionally with the assumption of impact on hardware. Also wondering how it would impact tuning of the other controllers such as IGV and Temperature controllers.

The only thing which sometimes happens, and usually when the unit is starting from a "cold" condition and the ambient is also cold is that sometimes if one tries to load a unit with DLN combustors too quickly it doesn't make the transfers very well. Mostly happens with DLN-I units, but there have been reports of lean blow-outs on DLN 2.6 units also when trying to transfer combustion modes very quickly when loading the unit very quickly.

Shouldn't be any issues with the turbine control--and again, I only have experience with GE turbines and controls. There's really no tuning that can be done to improve this on GE machines, it's either going to happen or it's not--it does or it doesn't.

I'm not too familiar with what kind of combustion systems the Mitsubishi units have, but I've been told as far as dry low NOx emissions systems go they aren't very sophisticated--which, in my personal opinion, can be a big plus many times. I guess they use more emissions reductions methods in the exhaust (SCRs and ammonia injection and the like) to reduce NOx than trying to do it in the combustor--but that's all third- and fourth-hand hearsay.

Best of luck!