5 replies to this topic

[florentj]

Greetings,

 

I am a junior process control engineer with no chemical engineering degree who would like some help to understand what is the better choice for controlling the fuel gas to a heater: pressure control or flow control. Attached a simple flow diagram of the process where the location of all instruments drawn corresponds to the locations in the field. It involves a natural draft heater that is part of a hydrotreating unit. The objective is tight control of the coil outlet temperature. In this particular case, there is a cascade control configuration where the master is a (coil outlet) temperature controller and the slave is a (fuel gas) pressure controller.

 

I have heard and read, without much detail as to why, that it is better to control the flow of fuel gas instead of the pressure of the fuel gas. The only thing that I have heard as a supporting note to this assertion is that pressure would be affected by the number of burners being on (this particular unit has low NOx burners by the way)

 

Note that there is a density analyser available. The chart attached corresponds to a week of readings. I thought about the possibility of using it to provide some feed forward action to the temperature controller. Unfortunately, the effect seems to be very small plus the signal comes too late to be used. What about using the density to compensate flow or pressure (probably the same issue of late signal reading and small effect)?

 

Could anyone share any experience or knowledge as to which option is best for effective control of the temperature and why in general and/or in this particular case?

 

Any explanation, ideas or suggestions would be greatly appreciated!

Attached Files

•  Heater Flow diagram.pdf   46.48KB   124 downloads

[upcoming]

Fuel gas flow has faster response than pressure change. Cascade control is used so that master controller temperature can be maintained with changing disturbances in the fuel gas header.Hence when the fuel gas composition is changing then it is better to temperature (COT ) cascaded with fuel gas flow than pressure. More over using the density correction would get even better control of temperature

 

Normally temperature cascaded with pressure can be used if the header pressure is steady with no composition change( like natural gas) 

[thorium90]

Wow, your fuel gas density has a high of 0.69 and a low of 0.43. That's quite a large change of composition, what are you burning? There are also what appears to be large spikes and plenty of fluctuations.

I would also look at the heater. You wont want to overpressure your heater.

You might want to mention the air too, im sure your burner needs air... More fuel needs more air. More cold air reduces the response time to heat up. It might help to understand how the air is introduced and what temperature it is introduced at.

Your drawing shows another pressure control valve before the flow and density meters. Might help to understand them too. Any increase in flow to the burner caused by the second valve will impact the first valve which then affects the second valve and may cause some positive feedback loop resulting in deteriorating control.

Edited by thorium90, 23 November 2013 - 12:07 PM.

[curious_cat]

What's the purpose of the additional PC? Is that redundant?

 

 

 

Wow, your fuel gas density has a high of 0.69 and a low of 0.43. That's quite a large change of composition, what are you burning? There are also what appears to be large spikes and plenty of fluctuations.

 

How accurate are these instruments?Just to make sure we aren't chasing noise. 

[Babu Prasad]

Flow controller will be better choice instead of pressure controller. Flow can be compensated by pressure, temp ,density meter to get more accurate flow. Pressure controller can be used in parallel to flow controller (thro high selector) to avoid flame out conditions during low fuel gas flow. In fact you can put off 20% burners if you got multiple burners to maintain minimum fuel gas pressure by configuring logic to close fuel gas XV of first burner in each row. (if you got 25 burners, one burner n each row can be put off automatically if it is provided with shutoff valves when low flow or low flue gas pressure alarm triggers to avoid flame out conditions during low firing) .

Feed forward controllers will be fine with distillation columns not for furnace which will be more complicated. NOX can be maintained with guidelines of Oxygen analyzer in the flue gas to control the excess air which is manual operation by the flame conditions.

Attached Files

•  dig.pdf   266.16KB   86 downloads

Edited by Babu Prasad, 24 November 2013 - 08:07 AM.

[florentj]

_{Thanks everyone for the input. From what I read, flow seems to be in general the better option for the slave controller although many things have to be taken into account for a really successful implementation (e.g. extra pressure protection, automatic switch off of burners, compensation, etc.)}

 

_{Here some answer to some of the questions asked by other posters:}

_{- The fuel gas is offgas mostly from the amine absorbers from the same hydrotreater unit so besides light hydrocarbons, you could find traces of H2 and H2S for instance.}

_{- As to how air is introduced: as I mentioned, the heater is a natural draft and there is a pressure controller moving a stack damper. Operators change the setpoint of the pressure controller frequently to try to get a desired oxigen %, which varies between 1.5 and 6 %.}

_{- The first pressure controller in the drawing is actually to regulat the supply itself. Besides the offgas from the absorbers I mentioned before, there is another line (not drawn) of fuel gas which can actually supply or take gas.}

_{- It is difficult to assess the accuracy of the density analyser. The lab only performs tests on the density every other day or so. I can only say that you see similar variations within a similar range. We think the analyser works well.}

 

_{Thanks again and regards}

Edited by florentj, 26 November 2013 - 07:42 AM.