Steam Boilers staging On/Off based on modulation rate

[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

 

[TD2K]

The efficiency of a boiler depends on the load, at turndown the efficiency is reduced so your firing would have to be proportionally higher than at full load. That is, at 50% of design steam production you could be firing at 60% of design fuel gas consumption.

Do you have a meter on your existing fuel gas supply to each boiler and steam meters? You could use that data to supply what you are looking for (assuming you trust them). Alternatively, do the manuals or documentation for these boilers have any information on what you are looking for?

 

[rmw]

There are so many variables that go into the mix that there probably isn't a one size fits all curve. For example, depending on where your boiler combustion air comes from, if it is an outside source, and it is cold outside, then a certain amount of the firing going on has to do with heating the combustion air vs. warmer air on a hot day. But then again, the colder denser air acts differently in the burner than does hot (summer time) air. All that goes into the firing mix.

What modulates the firing demand signal? If it is header pressure, then that ought to tell you when you have too much steam production capacity on line.

rmw