Archive for category Burner Control

Honeywell 7800 flame safety controller fails to execute Modbus remote reset

This one qualifies as a “Page the manufacturer left out of the manual”, and was brought to my attention by a customer who was having problems with remote reset on his flame safeguard system.

A typical multi-burner furnace has Honeywell 7800 flame safety controls on each burner as shown below.  Modbus is used to fetch fault codes for the plant’s HMI system, and to allow the control system to remotely reset the flame safety controls.

The reported problem was that Modbus did everything as advertised except remote reset on the one control that had a S7800A keyboard display module attached (colored orange in the network image on the right, or as shown in the controller image below).  

That unit would only do a reset when an operator pressed the reset button on the RM7838C controller.   Modbus failed to get the controller to execute a remote reset. 

The problem wasn’t communications – all Modbus read functions worked and Modbus writes to all the other controls worked as expected.  But somehow, the write command to do a reset action was ignored by the S7800 keyboard display.  

So, what’s causing the problem?

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Which temperature sensor do I need? An RTD or a thermocouple?

Temperature is probably the most measured parameter across applications and industries we work with on a daily basis. But there’s still confusion as to what temperature sensor to use.

I very often hear the Lesman inside sales team talking a customer through the selection process, so I thought I’d provide a quick tutorial on temperature sensor selection.

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How do I tune a process control loop?

Industry surveys say that nearly half of all processes aren’t accurately tuned. If you read my post on accuracy, stability, and repeatability, you’ll know that a poorly tuned process can result in bad readings, downtime, and wasted materials.

If you use a Honeywell UDC2500, UDC3200, or UDC3500 1/4 DIN universal digital controller, there’s a great built-in function called Accutune that can help make sure your control process is properly tuned.

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How do you keep production going when a control thermocouple burns out?

There’s no such thing as a fail-proof thermocouple. Over time, thermocouples fail. To compensate for that, a temperature controller will normally go into upscale burnout mode, and drive the furnace burner to low fire or turn down the SCRs.  But then, you have to deal with the downtime, rework, or even the potential of losing product.

Not long ago, a plant operator called to see if there we had a way to work around this burnout mode, so he wasn’t wasting time and materials.

His heat treat load had almost finished its final soak when the control thermocouple broke open. The controller, as expected, drove the furnace burner to low fire. The operator then popped the controller into manual mode, so he could nurse the load through the remainder of its soak cycle. He used the temperature reading on a recorder, fed from a second, unbroken thermocouple in the protection tube as temperature indication for the load.

If the situation had happened in the middle of the night, it may not have been handled with the same attention the day-shift operator had provided.

So, he asked if there was any way to have the controller automatically “fail over” to a second thermocouple. 

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