Re: Heater outlet temperature control

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Posted by Rafael Gonzalez-Martin on July 03, 2000 at 10:10:41:

In Reply to: Re: Heater outlet temperature control posted by Mike McCarty on June 13, 2000 at 12:44:25:

: Like Tod, I also like the TC-FC set up. Again, the big
: advantage of this is that it automatically compensates for
: furnace burner changes. If, as you say, you don't want to neglect
: FG header pressure or composition changes, then I'd implement a
: TC - compensated FC cascade, or TC - fuel gas Q cascade (using flow).
: The PC cascade would not help you on composition changes.

: Also, if a furnace has a firing limit (maybe due to permit
: constraints), this limit will be based on fuel flow rates. So it
: is more straightforward to limit this in the DCS system when your
: temperature controller is set up as a TC-FC cascade.

: The big advantage of a TC-PC cascade is that most furnace safety
: shutdown systems are tied to FG burner pressure. Therefore, it
: is more straightforward to limit the range of the allowable
: burner pressure moves in the DCS system when your temperature
: controller is set up as a TC-PC cascade.

: I don't know about your set up with a circulating FO flow, but it
: sounds like TC-PC is indeed the best cascade for your system. The
: flow we're talking about in the TC-FC cascade is the fuel flow
: that is being burned. If this flow is not measured directly, then
: a FC cascade is probably not a good idea.

As far as my experience is concerned, TC-FC is always prefered from the control point of view.

TC-FC scheme is more linear so tuning of TC is much easyer because, it does not depend on the number of burners on service, where as the gain of TC-PC is highly correlated with the number of burners on service.

Using of TC-PC is mainly due to safety reasons, using TC-FC does not prevent the heater from unwanted shutdowns due to low pressure in the burners, therefore if you use TC-FC you should have a
local mounted minimum PC to always ensure that the safety minimum pressure in the burners is guaranteed.

TC-FC is less robust due to instrumentation problems. If using F.O. you should use a mass flow transmiter, coriolis based instruments work perfectly avoiding noise problems even if you compute the difference ( most common case) , where as volumetric flow transmiters often produces noise.
Using F.G. normaly does not have any measurement problem, but it is true that you should compensate for pressure and density changes in the header, remember that we want to have the gaing FC-->TC to be as constant as possible.
Even if you do not compensate the F.G. measurement the overall gain is usually more constant than the one using TC-PC.

Finally be very careful when compensating F.G. measurement, analyzers tend to fail very often, and pressure compensated F.G. flow controllers tend to be unstable depending on the pressure measurement used for the compensation.

This is all for the moment. Good luck.

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Comments:

: : Like Tod, I also like the TC-FC set up. Again, the big
: : advantage of this is that it automatically compensates for
: : furnace burner changes. If, as you say, you don't want to neglect
: : FG header pressure or composition changes, then I'd implement a
: : TC - compensated FC cascade, or TC - fuel gas Q cascade (using flow).
: : The PC cascade would not help you on composition changes.
: : Also, if a furnace has a firing limit (maybe due to permit
: : constraints), this limit will be based on fuel flow rates. So it
: : is more straightforward to limit this in the DCS system when your
: : temperature controller is set up as a TC-FC cascade.
: : The big advantage of a TC-PC cascade is that most furnace safety
: : shutdown systems are tied to FG burner pressure. Therefore, it
: : is more straightforward to limit the range of the allowable
: : burner pressure moves in the DCS system when your temperature
: : controller is set up as a TC-PC cascade.
: : I don't know about your set up with a circulating FO flow, but it
: : sounds like TC-PC is indeed the best cascade for your system. The
: : flow we're talking about in the TC-FC cascade is the fuel flow
: : that is being burned. If this flow is not measured directly, then
: : a FC cascade is probably not a good idea.
: As far as my experience is concerned, TC-FC is always prefered from the control point of view.
: TC-FC scheme is more linear so tuning of TC is much easyer because, it does not depend on the number of burners on service, where as the gain of TC-PC is highly correlated with the number of burners on service.
: Using of TC-PC is mainly due to safety reasons, using TC-FC does not prevent the heater from unwanted shutdowns due to low pressure in the burners, therefore if you use TC-FC you should have a
: local mounted minimum PC to always ensure that the safety minimum pressure in the burners is guaranteed.
: TC-FC is less robust due to instrumentation problems. If using F.O. you should use a mass flow transmiter, coriolis based instruments work perfectly avoiding noise problems even if you compute the difference ( most common case) , where as volumetric flow transmiters often produces noise.
: Using F.G. normaly does not have any measurement problem, but it is true that you should compensate for pressure and density changes in the header, remember that we want to have the gaing FC-->TC to be as constant as possible.
: Even if you do not compensate the F.G. measurement the overall gain is usually more constant than the one using TC-PC.
: Finally be very careful when compensating F.G. measurement, analyzers tend to fail very often, and pressure compensated F.G. flow controllers tend to be unstable depending on the pressure measurement used for the compensation.
: This is all for the moment. Good luck.

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