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Frequent low water cutoff during warm-up
Dear Sirs: I have a 2-pipe steam system with approximately 140 radiators attached to it. I recently had the badly leaking 2-million BTU Weil McLain boiler replaced with a new Burnham of similar heat output. Before replacing the boiler, I limped through one heating season and noticed that about 1/4 of the building was not getting much (if any heat), but the old boiler seemed to perform well, reaching 2 psi or higher pressures quickly and uneventfully. When the boiler was replaced several steam strainers and traps which were clogged were replaced with new ones. Now the entire building's radiators receive steam. However, after it is turned on, the new boiler frequently shuts off the burner when the water level approaches the level that has been set for condensate feed pump activation, which causes the burner to cut off, perform a pre-purge, then reignite, until the low water cutoff is then tripped again 5 or 10 minutes later. This cycle may repeat for over an hour until the system is stable at it's set modulation pressure at 1.5 psi. There appears be only be 1-2 inches separation between the heights for feed pump activation and low water cutoff activation. The feed pump is the same as used on the old boiler. The water in the gauge glass does tend to bounce about one inch when the boiler is under the start-up load. The boiler is level. There is a typical pattern for feed pump activation before the low water cutoff engages -- the pump comes on for several seconds, then off for several; this happens again within several seconds, then the low water cutoff kicks in and the pump runs continuously for a while as the boiler pre-purges. The water level in the boiler rises visibly within about 10 seconds as the feed pump runs -- and then may rise dramatically as the pre-purge continues, even with the feed pump off.
The only time I ever witnessed the fluid in the gauge glass not bounce or the low water cutoff be activated was right after a cleaning agent was added to the boiler fluid -- which leads me to believe that I am getting foaming under high load that is tripping the low water cutoff. The dramatic rise in the gauge glass when the cutoff is activated, even when the feed pump has ceased operation also makes me wonder if foam within the boiler is collapsing when the flame as been cut off. I note that since the cleaning agent was added I have performed numerous blow-downs and alkalinized the boiler fluid to a pH of 10.5. I have never seen foam in the gauge glass (which I do see in my other boilers if I over-alkalinize the boiler fluid).
Thank-you in advance for your thoughts!