Boiler goes off on low water at high fire

Zoso

I am troubleshooting a system that has boilers going off on low water when they go into high fire.  I have 3 Triad GMS1600 boilers that deliver low pressure (i think less than 5psi) steam.  Each produces 1,454 pounds per hour.  They are fed from a common feed pump.  According to the drawings, any boiler can call for the feed pump to start.  Each boiler has a solenoid that then opens.  The pump is rated for 22.5 GPM @ 20psi. 



I am told that the water level becomes erratic under high fire and usually trips out on low water.  I'm trying to offer solutions as to why.  I'm a little confused on the manufacturer's recommendation for feed water.  They recommend a a "loop" of 9 GPM @ 20 psi for the GMS1600 boiler.  By extension, does that mean I need a loop of 27 GPM @20 psi since i have 3 boilers?  Just calculating the pounds per hour and converting to GPM doesn't make sense.  I'm coming up with about 3 GPM per boiler.  What am I missing?  What is meant by "loop"?  The pipe just goes straight from the pump and tees off at each boiler with solenoids at each.



One other note that might be important.  I just read Dan's Hartford Loop article and think that my equalizer lines might be undersized.  It sounded like that might not be so important with pumped feedwater, but my equalizer lines are only 2".



Any suggestions would be greatly appreciated.    





 

sreja

re: Boiler goes off on low water at high fire

There are much more knowledgeable people on this forum than me, so I'm sure they will have better advice, but a quick comment.



Is your system set up so that the boiler can call for water BEFORE it gets to the point where the LWCO (low water cutoff) turns off the boiler, or is it the case that they call for water simultaneously with shutting off on low water?



The reason I ask is this:



It's a known phenomena that if a condensate pump delivers cold water too quickly it can collapses the steam in a boiler and trigger a low-water cutoff, and the solution to that is to slow down the rate that water is fed into the boiler.



But your description of your system seems to suggest that, unlike a standard condensate pump, which delivers water whenever it wants -- you have a system where the boiler explicitly ASKS for water when it needs it.



That suggests that in your system, the only time a boiler asks for water is when it is shutting down from low water -- in which case your problem cannot be that water is being fed in too quickly.



If that's indeed your situation, then perhaps the problem is simply that when the boiler is low on water, it asks for and gets JUST BARELY enough to trigger the water feeder, which then inevitably triggers soon again.  In such a case your solution would be to have the water feeder always open for some preset time to ensure that additional water is added each time there is a call for water.



One of the well known water feeder devices used by people on the forum (VXT) has an adjustable feed timer that can do this.

Joe V_2

more troubleshooting questions

has this been a problem from installation?



is the pump

pressure at least 10 psi when ALL boilers are calling for water? are the feedwater valves actually opening 100%? Have you observed low water in site glass when when burners shutdown? do all three boilers trip on low water?