Odd vaporlock in 2 pipe steam system

furnacefigher15

I have a job Im really scratching my head about at the moment. I have 2 boilers common piped together. The customer only runs one or the other, the off boiler has the header and the boiler feed valve closed.

He stated that at the time he started the boiler, the system worked fine. The next morning, he arrived to find the boiler feed pump running, the boiler off, the system calling for heat, and the gauge glass full of water.

The boiler has a vacuum breaker installed above the low water control. The supplys from each boiler feed up, then over a few feet, and drop into a common header. The various supplys tap from that common header, and the header has a 2” F+T trap, that was new last year.

He said he shut the pump off, and then opened the relief valve. When he did that, air rushed in, and water then drained back to the feed tank at that time.

After this happened, he switched to the other boiler, and has not had the issue since.

I tested the lwco and associated pump relay and all is functioning. Also - the boilers, tank, controls were washed out 1 week prior to the episode.

Now today I had run the boiler a good long while, and about 10 minutes after shut down, I could hear the sound of return water comming back, but in waves, as if vacuum were holding the water somehow.

Any thoughts, or similar experiences would helpful.

Thanks

furnacefigher15

The boilers are essentially piped like the picture here. Except they only have one steam take off per boiler, and no leveling traps, as they were never intended to run in tandem.

EBEBRATT-Ed

I would remove the vacuum breaker and install a good quality Y pattern check valve in it's place. and maybe increase the size to 1". Sounds like a vacuum issue pulling feed water from the boiler feed tank into the boiler and holding up condensate

furnacefigher15

Thanks. Do you think the vacuum would still drag in the water from the feed tank with the piping run overhead, cause thats how it is.

EBEBRATT-Ed

Not sure I understand but if you think feed water is syphoning into the boiler from overhead piping you could put a vacuum breaker on it

PMJ

If in the initial reported failure the boiler was really over full as shown by the gauge glass why was the feed pump running?

furnacefigher15

PMJ said:

If in the initial reported failure the boiler was really over full as shown by the gauge glass why was the feed pump running?

I dont know. I saw this phenomenon one time before. It was like an internal current in the boiler would cause the float to drag downward, forcing the pump on. But that was on a firetube process boiler, and that one would blow the relief valve too. Turned out that boiler had the feed water pressure set too high, and from time to time, the flow would drag the float downward.

This one is a cast iron sectional. Unfortunately these only have one steam outlet, so its possible the waterline in the boiler is at an angle during operation, which is creating low water on one side (pump control side) and high water on the other. I remember reading something about that in one of Dans books. Maybe I’ll have a looksie.