PetroStreet

We have a GE gas-turbine coupled with ~ 25 MW generator and a HRSG unit. During a de-bottleneck process, we installed a Propane based chiller, which cools the air intake required by gas turbine. Its temperature is controlled by adjusting the levels in vertical tubes of chiller. But when this cascade loop works on auto, it humps sinusoidal. We have tried some simulations and tuning, but the effect is still there, which also effect the gas-control-valve, as it also hunts with inlet temperature changes.
Is there any one who can help me out.

ibrahim, cascade control loops with temperature are always tricky ones. You should use PID controller for temperature with a sluggish response initially. Add some derivative factor also. Keep in mind there are two controllers in series working to control temperature. Tune level controller separately with a quicker response, while temperature controller with slower response. Hopefully this will help you properly control the temperatures.

Note one more thing, when temperature decrease of a gas turbine, so does the fuel for same RPM (in general) due to increase of thermal efficiency. That's why fuel valve opening will be reduced to control RPM setpoint.

thanks qasim for such useful suggestions. I will try those shortly and tell if we succeeded.

we have faced more or less same issue at our site also.We have an ammonia based chiller at air intake of Solar gas turbine. During the analysis we have found following major factors;

• - We have Saturn-1202 (aero-derivative type) machine as driver of a compressor. At fixed RPM of driver, speed of compressor varies due to variance in air inlet temperature. This makes the dependent system to shift to another Plataea. So verify the whole system, including dependencies before tuning this cascade control. But that may not be your case as you are generation power.
  - Variance in back pressure make the system unstable as the evaporation temperature and pressure varies of the coolant. So try to regulate it, using some pressure regulators.
  - We have installed an accumulator vertically along the chilling tubes. This will damp the change in level of coolant due to evaporating and valve opening.
  - Valve (LRC >> TRC) must be properly sized, to avoid hunting in the system. Check the variations in valve opening command by the PID.
  - Tune the PIDs from bottom to top in cascade (LRC >> TRC). Qasim has adequately explained this point.

After analyzing above points, hopefully you will make your cooling system perfectly stable.