Effect of Relative Humidity on GT Output


Thread Starter


This is a 2+1 CCPP having Frame 6 GTs with Mark VIe control system. I want to understand. Since GT control system uses turbine exhaust temperature biased by compressor pressure ratio to approximate firing temperature, and also the control system is set to follow the inlet air temperature function, therefore, increase in humidity will cause to reduce GT output. Is this is done by reducing firing temperature if humidity goes up or some thing other than this?

please explain.
Unless the gas turbine control systems use MBC (Model-Based Control) or some unusual application code, the change in air density will affect mass-flow through the machine, and since gas turbines are mass flow machines the output will be affected. Typically, the turbine control system does not try to adjust output based on inlet humidity--it just happens.

Dathan Bishop

I am a process operator and familiar with humidity analyzers being used to compensate for lbs of water/lbs of air being brought into the suction of the air compressor. The water content being brought to combustion zone will be offset in the steam to gas ratio for NoX control. Although in most cases the scenario of a "flameout" isn't really a risk some sites will bias their steam control to reduce NoX even more and gain power to the turbine from the added energy of steam. This increase of steam flow, high humidity and an online wheel wash can be pushing the edges of flame control.

What I am saying is high humidity will reduce your steam inject thus reducing transfer of power. Additionally high humidity situations also mean higher temperature and less dense air which equates to lower flows through the machine, this is a double whammy. Some machines use suction air chillers to help increase the mass flow in hot climates by cooling the inlet flow with a mist of water being sprayed into the suction where trays collect the water and circulate it back for distribution, it is pretty effective.