We know how to calculate the temperature margins from the thermocouples. But How to calculate/convert those temperature margins to load margins, during the start up and rolling of turbine to avoid/minimize thermal stresses?
This is what the original company that makes the turbine spends lots of time figuring out! If it were very simple, everybody would do it.
The basis of loading and temperature change rates is nothing more than good old machine design. As the metal temperature changes, it's size changes. Obviously if certain parts of the turbine are bolted or attached to other parts and one side changes temperatures faster than the other, stresses are created.
In the case of a turbine, the rotor can change temperature faster than the shell and this difference in growth can cause metal parts to contact that shouldn't contact.
The OEM (GE, Westinghouse, Siemens, Alstom, etc) will supply a startup and loading chart or program that has guidelines to minimize stresses and rubbing on the unit. Rubs show up immediately, but stresses often take years and years to show up as cracks and other problems.
If the plant chooses to pay extra for a fancy startup option on the control system, the startup can often occur faster because algorithms calculate stress levels using temperature measurements. The startup chart given on paper is usually designed to be conservative.
Many combined cycle plants are coming on/offline daily--and this is affecting the turbine and HRSG greatly because many of these units were designed for baseload operation.
