I was wondering also in the case of two boilers on the same steam header, and feed by seperate return pumps from a common return tank. When turning off and valve down one of the two at the header main gate valve. Would the vacumn that is made draw and raise the level in the boiler that is cooling down? Is it neccessary to turn off the down return pump and valve off the return line. I would think...the negitive pressure would pull in return past its return line check and hold excess water. Help!!
Running boilers in parallel is not a simple control problem. Steam production on each boiler will be set by the firing rate. Therefore, the fuel valves must not only respond to the header steam pressure, but also to the steam flow for proper load distribution. *Changes to the firing rate are very detrimental to efficiency. Therefore, it is better to share load changes among several boilers to minimize losses. When taking one of the boilers from service, it is better to leave it under pressure from the steam header, instead of closing the header valve off. Otherwise, the boiler will cool down and it will take much longer to bring it back on line. The rise or drop in level will depend on how tight the feedwater and steam valves are shut. Either way, it is irrelevant, since you will have to go through the startup sequence, which may last several hours, before you can bring it back on line again. =============== However, from your question it appears that the boilers start firing at full tilt, and stop on high steam pressure, I guess. It is not a good idea to sequence them, since then they would have more time to cool down, which would be a useless waste of fuel. Sequence them once a week, or after they had time to cool down completely. .
I would say, put one boiler on fixed load (flow rate) and the other on variable load (flow reset by pressure) so the header pressure is maintained. That is the basic principle for load sharing.
