Temperture Matching


Thread Starter


We face problem when we tray to Temp matching in our plant
TNR changing rapidly
and spend many time to catch it to make matching
we need more details about TNR and relation to Temp matching
and why this important for matching
although we Siemins unit not applying TNR when we do Temp matching ( both units on the same grade )
When did this problem start? After a maintenance outage? After steam turbine control valve refurbishment? After some maintenance work was done in the DCS?

TNR stands for Turbine Speed Reference. When the gas turbine is operating at Part Load in Droop Speed Control, the unit is loaded and unloaded by increasing or decreasing (respectively) the Turbine Speed Reference.

When Temperature Matching is active, what's likely happening is that a 4-20 mA signal, which usually comes from a plant DCS (Distributed Control System) is used to increase or decrease TNR in order to control gas turbine exhaust in order to limit/control steam temperature and bowl metal temperatures to reduce thermal stresses on the steam turbine when it's being started "cold."

I would suggest that you have a look at the temperature matching setpoint input to the Speedtronic to see if it's stable. Or, have a look at the thermocouples (they are usually thermocouples) that provide the indication of steam and/or metal temperature input to the DCS.

As for why Temperature Matching "has" to occur at some particular speed reference, I don't think it does. I think it's mostly about limiting exhaust temperature to reduce thermal stresses, and that's usually done at lower loads.

And that means that loading the gas turbine usually takes longer than many people want it to take.

Another common problem with temperature matching is that there is(are) input(s) to the DCS, and then some "scaling" is done in the DCS to provide an output (usually a 4-20 mA signal) to the Speedtronic to provide the temperature matching reference that's used to adjust load to control exhaust temperature which affects steam temperature and metal temperatures.

Quite often the "scaling" of that DCS 4-20 mA output to the Speedtronic is not exactly what it should be, and so the temperature matching doesn't work exactly as it should. The process of adjusting and "tuning" this scaling is iterative, meaning that it takes time, and trial and error. We don't know how long the units have been running at your site, but it's not uncommon for the scaling to have never been properly adjusted to optimize steam turbine start-up times.

Another common problem is that after a steam turbine maintenance outage the thermocouples used to provide the signals to the DCS are not reconnected properly, or are not re-installed properly in the thermowells.

But, it's not likely there is a problem with the Speedtronic. You should be able to use the troubleshooting tools available for the Mark X Speedtronic turbine control panel to observe all of the inputs to the Turbine Speed Reference (TNR); there are usually several. There is usually a Minimum Select block or function that takes the minimum of all the inputs and writes that value to TNR. If one or more of the inputs is "dithering" (jumping up and down) then it could be causing TNR to jump around.

Please write back to let us know the results of your troubleshooting.

And, many turbine manufacturers use some kind of temperature matching for their units, but while the methods may be similar they are not usually identical.
You didn't say what the control system is. You also didn't say whether this is single shaft or multishaft combined cycle. If it is GE ICS (Mark VI or MarkVIe), then the temperature matching signal from the steam turbine is transferred over the UDH to one of the BOP or HRSG controllers.

For GE design, there may be two phases of temperature matching. The first phase would typically happen only on a cold start, and would limit the gas turbine exhaust temperature while the HRSG pressurizes. Once the HRSG is pressurized, the steam turbine inlet bowl (either HP or Reheat, depending on which section is used to start the steam turbine) temperature is read, 100 degF is added to it and that value is used as a setpoint for the gas turbine exhaust temperature (with some minimum, maybe 700F).

For the steam turbine, you want the incoming steam to be close to but above the steam turbine inlet metal temperature. Too cold is as bad as too hot.