I am new to this site and have a few questions. I'm sorry if this post is a bit length and/or newbie.
I am working on the controls for a heat treating furnace. I'm curious, for one, to know how ya'll go about designing these circuits?
I have listed different inputs and outputs, and made logic equations. I'm taking these equations and then going to a relay diagram. Is this a popular method...I'm interested to learn other (better/more thorough perhaps) ways to create and verify a control circuit. Is there a good method to take all of the failure possibilities into account other than by observation...because I could very possibily forget something. It seems my method has a snag translating to relays, when sharing contacts can back-feed signals into other parts of the circuit and such.
A portion of the circuit I am working on is for the furnace atmosphere.
We are using Honeywell UDC controllers, with one for a high limit and one for the process temp. I am using the PLC for process timers, door/drive control, etc. but not for the combustion and atmosphere control.
--------------
The burner circuit looks something like this:
High Limit--Low Air Pressure--Low Gas Pressure--High Gas--Combustion Blower--On Switch-- Reset OR Valve Opened Contact -- Gas Valve
--------------
Endothermic gas goes into the furnace chamber. Then natural gas is injected into the chamber and adjusted to change the carbon content, and air is injected if the carbon content is too high.
The Endo circuit is:
NOT Low Temp--Endo On Switch--Endo Reset Button
'OR'
Endo Switch On--Endo Valve On--Endo Flow OK
The 'Low Temp' is an alarm from one of the Honeywell Controllers. The NFPA 86 allows for the temperature to drop below the 1400 'Low Temp' after the introduction of gas,
if the flow is maintained (Class C, Type 2 Furnace, NFPA 86-42, 11.3.5.1 exception).
If the endothermic gas stops for some reason when it is at a low temperature, then the furnace needs to purge with inert gas (nitrogen) to flush out the rest of the endo.
Nitrogen = Low Temp--NOT Endo Flow OK--Endo Switch ON
I have already used the Flow Switch contact for the endo circuit above. This same kind of situation may arise elsewhere... Is it okay to use an additional relay to indicate 'Endo Flow OK' that is energized by the Flow Switch?
I ask because...
The NFPA 86-22, 7.2.7 talks about combustion safety circuits. There is an exception about interposing relays being used. In my case, all of the conditions are not met. I realize that I'm working on the atmosphere and not the combustion, but the 7.2.7 seems to be a sensible precaution, because what if your interposing relay fails
closed (can you use two in series, like those master control relays)? Also, I couldn't find any
mention in the NFPA 86-22 about redundancy of high limit controllers, etc. What if the relay inside of the Honeywell fails closed for example? I do like the idea of using the high limit controller/process controller alarm in series. The two controllers could be reduant to each other
for high limit and low temperature.
How paranoid should one be?
Thanks everyone for all of the help,
Matt
I am working on the controls for a heat treating furnace. I'm curious, for one, to know how ya'll go about designing these circuits?
I have listed different inputs and outputs, and made logic equations. I'm taking these equations and then going to a relay diagram. Is this a popular method...I'm interested to learn other (better/more thorough perhaps) ways to create and verify a control circuit. Is there a good method to take all of the failure possibilities into account other than by observation...because I could very possibily forget something. It seems my method has a snag translating to relays, when sharing contacts can back-feed signals into other parts of the circuit and such.
A portion of the circuit I am working on is for the furnace atmosphere.
We are using Honeywell UDC controllers, with one for a high limit and one for the process temp. I am using the PLC for process timers, door/drive control, etc. but not for the combustion and atmosphere control.
--------------
The burner circuit looks something like this:
High Limit--Low Air Pressure--Low Gas Pressure--High Gas--Combustion Blower--On Switch-- Reset OR Valve Opened Contact -- Gas Valve
--------------
Endothermic gas goes into the furnace chamber. Then natural gas is injected into the chamber and adjusted to change the carbon content, and air is injected if the carbon content is too high.
The Endo circuit is:
NOT Low Temp--Endo On Switch--Endo Reset Button
'OR'
Endo Switch On--Endo Valve On--Endo Flow OK
The 'Low Temp' is an alarm from one of the Honeywell Controllers. The NFPA 86 allows for the temperature to drop below the 1400 'Low Temp' after the introduction of gas,
if the flow is maintained (Class C, Type 2 Furnace, NFPA 86-42, 11.3.5.1 exception).
If the endothermic gas stops for some reason when it is at a low temperature, then the furnace needs to purge with inert gas (nitrogen) to flush out the rest of the endo.
Nitrogen = Low Temp--NOT Endo Flow OK--Endo Switch ON
I have already used the Flow Switch contact for the endo circuit above. This same kind of situation may arise elsewhere... Is it okay to use an additional relay to indicate 'Endo Flow OK' that is energized by the Flow Switch?
I ask because...
The NFPA 86-22, 7.2.7 talks about combustion safety circuits. There is an exception about interposing relays being used. In my case, all of the conditions are not met. I realize that I'm working on the atmosphere and not the combustion, but the 7.2.7 seems to be a sensible precaution, because what if your interposing relay fails
closed (can you use two in series, like those master control relays)? Also, I couldn't find any
mention in the NFPA 86-22 about redundancy of high limit controllers, etc. What if the relay inside of the Honeywell fails closed for example? I do like the idea of using the high limit controller/process controller alarm in series. The two controllers could be reduant to each other
for high limit and low temperature.
How paranoid should one be?
Thanks everyone for all of the help,
Matt
