fuel/air ratio control

Hello!
I am upgrading a furnace control system that will be using natural gas and combustion air ratio control. This will be done on a plc5/20e using rslogix5. I have done alot of programming on plc/5s, but have never actually gotten into this type of control scheme. I am hoping one of you experts can guide me generally in how to set this up. Of course Im not looking for a specific bunch of logic, just a general feel on how to proceed. I suppose the best way would be to use P.I.D. instructions, just not sure exactly how to set this up. My fuel range is 0-35000 scfh, and air is 0-350,000 scfh, both 4-20ma. The fuel will always get its setpoint from the air, (fuel follows air) controlled by temperature. Thank you, Jim

You can use a PID loop or floating control (thermostat type) for the temperature to fuel control. If you know the mass air flow of both (temparture and pressure corrected) you can simply use a ratio controller to provide a setpoint to the air flow control. The air flow control can also be PID or floating control.

Or You could find a UL listed Honeywell controller and be SAFE....

Using the PID functions for your actual control is the easy part, IMO. From experience the more critical- and safety related- aspects of this type of process is the use of limit select blocks to make sure that air flow leads fuel on an increase in demand and air lags fuel on a decrease in demand. Also it will be more precise to characterize the non-linear flow signals- especially air- through the use of custom lookup tables to ensure an optimum fuel/air ratio. Otherwise that expensive fuel tends to just rise out of the stack without creating maximum heat!
There should also be limits built in so that air flow can never be cutoff while the furnace is running- without halting fuel flow.

Done all this stuff in Modicon Quantums so certainly A-B can do the functions. Just be careful, as David suggested, safety is key!
mike

Boiler controls

I think I would check with my boiler insurance carrier and the local (state or city) boiler inspector before I started using PLC to do boiler safety controls.

Now if the boiler safety controls are all UL listed and compliant with who knows how many agencies then it MAY be OK with the above guys to use PLC for operation controls.

Dan Bentler

You haven’t given us very much to comment on but I’ll launch in anyway.

I’ve done combustion controls on a number of boilers with PLC 5/20; the application featured fully metered flows with cross-limiting and oxygen feedback trim of the fuel to air ratio.

The NFPA probably has a standard (see http://www.nfpa.org) for your furnace.

Aside from doing the ratio controls properly, you’ll probably have to write sequences to coordinate with the burner management system (BMS). (The BMS is typically provided by the burner manufacturer and is independent of the combustion control system). These sequences probably include at least some of the following: pre-purge, light-off, release to modulate, post-purge. You will need to understand / define the necessary BMS interface; at a minimum, it will send your plc inputs which should trigger your sequences and act as permissives. If your upgrade includes a new burner, get in touch with the guy that’s doing the BMS.

Back to combustion controls… the fuel and air PID setpoints are cascaded from a temperature controller and cross limited in a way that ensures you’ve excess air during load changes. This means air increases lead fuel increases and conversely, fuel decreases lead air decreases. This prevents accumulation of carbon monoxide (and unburned fuel); accumulate enough CO and you can quickly run into dangerous and expensive problems.

The proper fuel to air ratio is typically determined by onsite tests. A burner representative will have instruments to measure stack O2 and CO. You’ll run the unit up and down and determine light-ff conditions and a set of air and fuel data points that result in good combustion over its operating range; you’ll then normalize the air flow (with a lookup table / characterizer) such that a unit of normalized air flow corresponds to a unit of fuel flow. The output from your temperature controller then goes through the cross limiters and into the setpoints of your fuel and normalized air controller.

Hopefully, some of what I said will make sense and be applicable. There will be many other very important considerations such as output tracking, setpoint tracking, unit warmup, among others.

You’ve a lot of homework to do. I’d recommend you have someone experienced do the combustion controls or have such a person assist very closely with your project.

Good luck and have fun!

Jim

Combustion control or burner management?

Burner Management:
I think some of us are confused ... initially thought that you were interested in getting some ideas about how to create a combustion control algorithm but after reading the last 2 replies I am not quite sure ...
Burner management (some recipes for package boilers in NFPA 85x) is serious business. I wouldn't design ant system without a proper assessment (most people these days follow IEC61508 & IEC61511) and participation of an independent party (consultant) to corroborate that every thing is been look after as per IEC standards (compliance).

Combustion Control:
There are several strategies that can be used:
(1) Use 2 different control loops for air and fuel (that is a PID loop for air and PID loop for fuel) and cross limit the outputs to the final control elements. The main object of the combustion control is to get the right mixture of air/fuel into the furnace and make sure that at all time the percentage of air is higher than the percentage of fuel (otherwise the percentage of fuel in excess will not be burned and will remain in the furnace creating an explosion hazard). The cross limit algorithm will select the higher of the 2 demands between air and fuel and will use it to control the air damper. The lower of the 2 demands will control fuel valve.
(2) The other method more common for package boilers is to use a ratio control. Basically, the algorithm uses one PID controller whose output controls directly the fuel valve. The PID output is also pass through a transfer function f(x) (common in DCS systems or more recently available in PLC function block). The way that this algorithm is tune is as follow:
(a) Insert gas analyser in the boiler stack.
(b) Set PID controller to manual.
(c) Set air damper to manual.
(d) Open fuel valve a set percentage (small increments).
(e) Open/Close damper until the analyser reading of CO is minimum.
(f) Record points.

Steps (d), (e) and (f) are repeated until the fuel valve is 100% open. The recorded points are used to build the graph fuel vs. air that will be used to configure the f(x) function. The f(x) function can be easily created in ladder logic by using indirect addressing (indexing) and an array. The indexing register will be the output to the fuel valve and the indexed register the output to the air damper.

My final suggestion is to research and get accredited 3rd party advise. Beware that this type of industry is highly regulated.

Cheers,
LJAM

Jim,

Exactly what is this furnace going to be used for? I dont mean to be critical, however this may not be a good application to learn on. If your new to this type of application altogeather, and are not working with someone else with expirence in it; then I'd suggest that your already in trouble. You may want to do some research on locating a prepackaged burner control system that would include the gas valve, sensors, blower, and controller. Time is money, and you might find that your gonna spend a lot of time doing this from scratch.

With all do respect..

Sorry guys...flu got me good over the past few days. This application is for heating steel slabs, sometimes just right up to temperature and sometimes step heats. The present system has an old computer and monitor system supplying the mmi and cycle selections, and depending on what mode it is in, supplying a temperature setpoint to a moore 352 controller being used for fuel/air ratio control. It is this moore that I would like to remove from the system, but the most important part is removing the old computer system. I will be replacing that with a AB PLC5/20e and panelview 1400e. The moore 352 controller does'nt really HAVE to be replaced...and after reading these posts, Im a little inclined to leave it in. Anyway..thats a little more info on this project. It isnt a boiler. Thanks Jim

Jim,

Because you are using natural gas in a burner, your burner controller has to meet state gas codes and standards. If you write a PLC program, then it has to be submitted to and approved by the authorities to act as a burner controller. That is why most of us just buy an off-the-shelf burner relay or burner controller that HAS already been approved for the burner usage, and then we hitch our PLC to the Inputs and Outputs on the controller, so that our PLC can Start/Stop, monitor the cycle, and can even set the fuel/air valve in order to control the firing rate, AFTER the burner has been purged and lighted. Our PLC is NOT allowed, in any way whatsoever, to control the purging, iginition, flame detection, and so on..because it has not been approved for such use. It is possible, to set up PLC monitioring of the controller so that a remote operator can fully monitor the burner cycles.

So that Moore 352 controller that you want to get rid of: If you do then I recommend replacing it with another one, maybe a newer, different brand, but still that is the safest and most cost-effective way, unless you want to get into the business of developing burner controlers!!

Jim,
As I am sure you are aware, the Moore 352 series is just a single-loop digital controller so it would be relatively easy to replace it and the old computer system.
That said, I would strongly advise you to heed the previous several post. Having built several systems (Control and BMS) and having dealt with FM, NFPA, and various local and state regulatory groups, I personally would recommend that you contact a few of the burner automation and control groups that have the experience to upgrade the system for you.

That's just my two cents worth...

Thanks for all the helpful advice, people! I really appreciate it. The Moore Controller is stayin in. I'll write the program to update the heating step cycles, etc, but the existing fuel/air ratio is something I dont think I wanna attempt. Anyway..thanks again! Jim