Steam Boilers on/off staging control

[remp]

Hi

I am currently commissioning a PLC controlling the staging on and off of 3 nr. Steam boilers based on the modulation rate of the boilers. All boilers are different sizes. Staging them on is no problem. When the running boilers are at more than 90% modulation for 3 minutes another boiler is added .This works fine.

The problem is how do I get the PLC to decide to switch the last boiler off again. Again I’m going to look at the burner modulation rate. Is there a correlation between steam capacity from the boiler and the boiler burner modulation rate? E.g. can you say a 10 ton boiler at 50% modulation is giving 5 tons, at 30% gives you 3 ton etc..? If so I can write a simple algorithm to do the job. Does anyone know where I can find a curve of burner modulation V's Boiler output for a typical steam boiler gas fired?

Regards
Remp

 

[MintJulep]

Well, the percent of modulation should be pretty close to the percent of capacity. The boiler manufacturer should be able to provide exact curves.

But.

How is the modulation of each boiler controlled? From you description it seems not via your PLC.

If each boiler is controlling it's firing rate independently then things are not going to work out the way you are probably hoping that they will.