Boiler Sequencer ( Lead / Lag )

Hi Everyone,

Question regarding Boiler Controls , if anyone would like to chime in. I work for a Boiler OEM where we manufacturer our own Boilers we use siemens and honeywell burner management systems. Now occasionally we will provide a package of 2 or more boilers. If so we provide the customer with duplex boiler sequencer panel, primary boiler would call on secondary if the primary load draw is above a certain percent. Now the 2 boilers that we provide are sized the same so we can generate the same firing rate using header pressure in our PID loop in we output the Firing Rate via a 4-20mA connection to each boiler. We cycle the standby on as necessary based of the primary boilers load demand. One particular customer only wants one of our units but wants sequenced control with an existing boiler that he has on site. the problem is that our boiler is 200 BHP and the existing is 150 BHP. My question is can we proceed in a similar fashion that we do and send both boilers the same firing rate, or would I have to essentially ramp up the standby primary boiler to match the " btu output " of the primary?

In cases like that we usually rely on simply using different pressure setpoints and max. working pressures.

For example.

Boiler 1 works on a SP of 10 BAR, with max working pressure at 12 BAR, hysteresis 1 BAR.
Boiler 2 works on a SP of 8 BAR, with max working pressure at 10 BAR, hysteresis 1 BAR.

This way the stand by boiler will start when ever pressure drops below your SP. I have over exaggerated the setpoints, but you get what I mean. This way there is no need for any communication between the boilers or an additional controller.

If you want to go with your solution, I don't see how it would not work. Perhaps if you are determining demand and setting boiler load according to that. But I doubt that is the case.

Since the goal is constant pressure in the header does it really matter how you approach the problem. If it were me I would use the 200 BHP boiler as the lead and keep the 150 BHP as a hot standby. the point at which you ramp up the standby boiler is dependent on how fast the load changes.
Ultimately it is up to the customer.