Boiler lead/lag & auto set-back strategies

[rguimond]

Just looking for some advice here...

We're just about to commission two new boilers. Both are equipped with Rielo burners and Siemens RWF40.001A97 controllers. Regretably, I failed to select controllers with Modbus connectability (RWF40.002B97), so operating pressures must be set directly at the burners.

Of course, I could swap out the controllers, but they're $1800 a pop.

As a cheaper, more versatile alternitive, I'm considering the use of PLC to manipulate the signal from the pressure sensor. By adding or subtracting a bit to the analog value before it reaches the controller, I can effectively control which burner is lead or lag. For instance, let's say 0V = 0PSI and 10V = 100PSI. If both boilers are set to fire between 80 and 90 PSI (8V and 9V), I could send a value of 7.5 volts to the lag boiler and 8.0 volts to the lead boiler at 80 PSI and 8.5 volts to the lag and 9.0 volts to the lead at 90 PSI.

The boilers are equipped with a high-limit switch for safety, and the controllers default to a safe condition if no analog signal is detected, so there shouldn't be any safety concerns.

I'd appreciate any opinions and, would be more than willing to swap controllers with anyone if they happen to have the Modbus models and have no need for the feature.

 

[kittydog42]

Do you mean that you want to control the modulating output? The best way to do that is to allow the burner to fire and monitor the voltage to the fuel valve as a PLC input. Then apply some small amount of delay (3 seconds or so) and then actuate a relay that we normally call a "modulation mode" relay, which just allows your 4-20mA or 0-10VDC signal to directly control the modulating motor. This is done by wiring the C pole of the SPDT relay to the mod motor, wiring the NC pole to the RWF40 output, and your signal to the NO pole. The commons can either go through another pole or just tie together. Your Riellos might actually be wired with a Siemens mod motor that accepts a "floating/bumping" signal, although your RWF40 model number is for the model with an analog output. You can also provide your own remote setpoint via analog input 2. It will follow your remote setpoint whenever binary input 2 is active (D2 to GND).

Make sure that the RWF40 is not the primary steam operating control, because a major flaw with using it for steam is that the sensors are 0-10V, and typically read 0V when failed. This usually results in continuous high-fire operation until the primary steam operating control trips or worse, the manual reset steam control trips. Some manufacturers go the cheap route and list the assembly (with UL, etc.) using the RWF40 as the primary operating control. Boiler inspectors usually don't like this but in cases where it is listed they usually have no recourse but if it isn't then they usually can demand that another control is added for safety.

I wouldn't manipulate the process variable reading when you have other options such as remote setpoint or remote modulation available.