desuper heater modification

currently we are facing problems with the desuper heater control valve, whenever the valve is put in AUT mode where the DCS will handle
the control it should reduce the flow tempreture coming from the furnace the PV will trace the SP, where the SPF0C. this is controlled by the PID controller and this should be the status in normal basis. but the control valve was giving fluctuation in OP 0%-17%. So, it will not give enough steam in order to reduce the temp. and hence the temp. will increase till the trip point. having said that and the process conditions are: Max. designed flow is 10,000t/h delta P and the actual is 40 the Q. is do we need to modify the nozzel part ? do we need more holes? this doubt was due to the over size data about the valve.
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You give us some steady state information, but just remember that PID controller response is primarily about dynamics. What PID tuning settings are you using?
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