Heating boilers cutting off on low water

John Goyer

I’m an engineer working on a small boiler project at a facility for a major university. The system was installed about 2 years ago and has not operated correctly ever since installation. The system consists of two 40 HP heating boilers running at about 12 psig. There is one common feedwater tank for both boilers with dedicated feedwater pumps for each boiler as well. The load consists of 2 air handlers with preheat coils for the 100% outside air system. There is also a steam to hot water converter for reheat water. Finally, there is a large hot water converter for domestic hot water. The water at the facility comes from 3 different wells.

The basic problem has been that the boilers will have episodes of shutting off on low water. The problem is intermittent. When it does occur, the burners cannot stay on long enough for the system to build pressure. The burner will come on for 30 seconds or so and then the float controller will shut the burner off on the first low water cutoff. The problem occurs at all times of the year but is worst when the load is high due to cold outside air temperatures. I have observed large amounts of bubbles and an erratic water level in the sight glass under heavy load conditions and even at other times. The bubbling and erratic water occurs within 30 seconds of the burner main flame ignition. After the burner shuts off on low water, the level in the site glass drops dramatically.

Please post up with any and every possible causes. Also let me know if anyone has any questions about system details. Thanks.

Dale

New on old = skim

Sounds like the system wasn't cleaned properly when the new boilers were installed, is the condensate tank old also? And, who is responsible for water chemistry? This seems to be a non process system so you should be able to spend a few man days skimming and then Nalco or whomever to set up a batch chemical process to cure your major ills.

Dana

Would need detailed sketches of near boiler piping with all deminsions, boiler manufacturer and model, pictures would be helpful. Why 12 PSIg. Sounds to me like near boiler piping problems and can probably be fixed. First thing I would try is to lower the operating pressure and see if I could get a longer run time and stabilize the water a bit. If it does re-post. Also Dan has a couple steam books on this site you can buy which will help you. Dana