HRSG Duct Burner FG flow/pressure control


Thread Starter

Mark Jones

I am having some trouble tuning both a flow and pressure control loop. I will give a quick rundown of equipment in the loop first:

* Plant gas header ~500PSI
* Unit pressure control valve (PCV - first device in line)
- operator SP (norm 35-55 PSI)
* Unit low pressure trip switches (25PSI dec)
* Unit Flow control valve (FCV)
- operator SP (norm 100-580 Kscfh)
* Unit Hi pressure trip switches (35PSI inc)
* PSV betw press valve and flow valve
* Pilot gas block valve (immediately after PCV)

There are of course all of the standard boiler safeties and block valves that are not listed.

My problem starts with the PCV - I have been trying to characterize the valve for tuning by manually setting the FCV valve position and changing setpoint on the PCV. Using the Z-N tuning method, I attempted to achieve ultimate gain oscillation. I plugged in the suggested values from the std Z-N equations and had problems during duct startup when the pilot valve opens up. It runs ok (not optimally) when the disturbances are less than in startup. There is much more to this question and I can supply all information needed to help, but the bottom line is that no matter what I do the PCV and the FCV fight each other and cause the system upset to increase gradually over time frequently tripping the ducts. I am beginning to think that the system would operate better with a cascade control loop controlling both pCV and FCV together. Any thoughts on corrective action? Please ask me questions if you need more info. I have spent SEVERAL hours tuning this loop and need some guidance since I am fairly novice in PID loops.