Fired heaters temperature controller with fuel gas controllers

[Jone k]

The F-1701 heater outlet temperature is measured by the dual temperature sensing element TE-11 (both measured values shall be displayed on the DCS screen). The first transmitter is used for the F-1701 heater firing control (TC-16A), while the second one is used for indication purpose only (TI-16B).
The total heat demand (fuel flow rate) is determined by the master temperature controller TC-16A responding to the demands of the process. The master temperature controller 120-TC-0161A output is cascaded to the fuel gas controller -QY-101B.
The module QY-101B selects the minimum value between the incoming signals: heat demand from TC-16A, the maximum allowable fuel flow from PC-122B. The output signal is compared again in selector QY-101C.
The module QY-101C selects the maximum value between the incoming signals: heat demand output from QY-101B, the minimum allowable fuel pressure from PC122A. The output signal is used as a set point for fuel gas controller QC-101.
In the normal operation, the fuel is controlled primarily by means of QC-101. As a process variable for the fuel gas controller, the actual heater heat rate (QY-101A) will be used as exactly corresponding to the heater actual firing duty. The fuel must be controlled to provide the exact amount of energy that the process requires. This implies that the heat content of the fuel as well as its flow rate must be known.
In order to achieve stable firing control, when the fuel gas composition swings, the fuel gas flow rate shall be corrected, using a fuel gas specific gravity online analyzer AT-181 located at the fuel gas common header. The fuel gas flow rate compensation block FY1043shall be configured first to compensate the measured fuel gas flow to the actual pressure, temperature and gas composition. The P, T, SG compensated value shall be further corrected within QY-101A calculator block to the design fuel gas lower heating value.
Any can explain more about this logic and how it’s work and how we can break the logic and adjust the fuel gas pressure by pressure controller?

 

[georgeverghese]

"In the normal operation, the fuel is controlled primarily by means of QC-101. As a process variable for the fuel gas controller, the actual heater heat rate (QY-101A) will be used as exactly corresponding to the heater actual firing duty. The fuel must be controlled to provide the exact amount of energy that the process requires. This implies that the heat content of the fuel as well as its flow rate must be known.
In order to achieve stable firing control, when the fuel gas composition swings, the fuel gas flow rate shall be corrected, using a fuel gas specific gravity online analyzer AT-181 located at the fuel gas common header. The fuel gas flow rate compensation block FY1043shall be configured first to compensate the measured fuel gas flow to the actual pressure, temperature and gas composition. The P, T, SG compensated value shall be further corrected within QY-101A calculator block to the design fuel gas lower heating value."

Is it really necessary to incorporate this feedback from FG composition to the QC 101, since it is dependent on the not so reliable online analyser AT 181?
Why not leave the QC 101 to pick up the resulting deviation through the master controller TC 16A? That is how most fired heaters are operated. Otherwise can you explain why you need to install this additional loop, especially when this is a feedforward loop which depends on AT 181 and a complicated algorithm at QY 101A to compute the new setpoint for QC 101A. My prediction is that this extra loop is going to cause problems for these reasons.

The prior narrative seems to be configured in a manner similar to the Shell MF on fired heater controls, with a min and a max pressure controller included. In the Shell MF, there is no recommendation for this extra feedforward loop you are now contemplating.

If you must some how compensate for fluctuating fuel gas LHV, it may be because you have a poor response from the TE 11. Instead of this ambitious and tricky feedforward loop, why not (a) replace the RTD at TE11 with a thermocouple and (b) use a thin wall thermowell at TE 11. This should make the master TC 16A more responsive and react quicker to temp changes.

 

[Jone k]

Dear georgeverghese
I notic from analyser it self one soft signal going as lower heating value and another as specific gravity. Why two signals is given ?

 

[georgeverghese]

My guess is that the LHV output is used to make corrections to the required fuel gas rate at QY 101A as part of the feedforward control loop, while the sg is most likely used to correct / compensate the flow measurement reading from the flow meter-transmitter feeding QC 101A.

 

[Jone k]

Higeorgeverghese'
I am agree with your comments
As you said the QC101 should be pick up the resulting deviation with master controller TC16A.what is your expectations?which problems can be happened if Qc101 not linked with TC016A.Second question incase of QC handover for maintenance how we can control the firing inside heater ? We can control by low pressure control?
Thank

 

[georgeverghese]

If for some reason, the feedforward loop at QC 101 is offline due to maintenance of the AT, then the firing controls should work reasonably okay without feedforward control without any modification. But not as good as compared to when feedforward control with QY-101B and the AT working.

My expectation is that the dependence on feedforward control is not necessary - the controls should work okay without AT 181 and QY-101B. Pls check with your process controls engineer the reliability and security of AT 181.

 

[Jone k]

During AT under maintenance operator can take it in Manual and can enter the specific gravity manually.

 

[georgeverghese]

Entering a value for LHV and sg manually is possible provided the values are correct. If it is not known, it is better not to enter any value - wrong values will result in longer time for the controller to correct for the errors.

 

[Jone k]

Which is more functionality,more accurate and good for turning to connect temperature controller TC16A with fuel gas controller QC101 or with Low selector QY101B and Why?
Thanks dear for your fast response

 

[georgeverghese]

In your process control description, TC16A is the feedforward calculator block / controller for heat rate. Where is the normal temperature controller ( feedback loop), which should be getting its feedback signal from TE 11 also? - not described.