2 Boilers in lead/lag

[dogbertcountry2]

Our plant has just leased a package boiler as a backup to our exisiting boiler for the winter. They are essentially identical (small - fire-tube boilers). We are wanting to run the boilers in a lead/lag situation with the rental being the lag. We want to keep both units running continuously as to not create a time we are not without steam.

Both boilers operate by maintaining their individual drum pressures. Both boilers dump into the same header before the Header Pressure Control Valve. Originally, the CSE thought that we could just operate the second boiler at a lower pressure than the first and everything would be fine. There are concerns now, that this would cause that boiler to cycle on/off frequently as to not overpressure.

I am not used to the lead/lag. Does anyone have suggestions?

 

[hacksaw]

it is difficult to imagine how you came up with a design that violates the basic operating principles of fired equipment being operated parallel. this is not rocket science: your CSE cannot define these operating requirements; and there are serious mechanical and safety issues are involved.

suggest that you get some qualifed assistence involved immediately.

 

[saxon]

One question, what is the BHP of each boiler and at what pressure are you operating the system? I take it these boilers are controlled by pressure switches installed in the steam drum and are used for process heat at lower pressure and are not used as power boilers at higer pressure.

Yes, the lag boiler will cycle on and off around the pressure set point. This cycle will be dependent on the dead band that is manually set up between the two pressure switches (this can be a real pain to get set up correctly). You may want to take a look at installing a single pressure switch with backup, installed in the header with dual adjustable outputs.

As hacksaw alluded to, you do have some system over pressure issues in regards to the system relief valves. Such that, now that you have incresed steam production capacity, the system relieving flow capacity must now also be increaed to match the output capacity of the two boilers. You need to take a close look at this.

Hope this helps.
saxon