Startup LWCO trigger

NRT_Rob

Hey Folks!

Been awhile, I hope everyone is doing well in these crazy days we're all going through!

I'm posting because the company I work with now recently replaced an old steam boiler. After measuring the EDR in the building we established that the installed boiler was too big, and we replaced with a smaller unit with a condensate pump matched to the new boiler. I believe we are about 30% smaller than previous. Building is big and old and has some radiators with TRVs on them... quality and function of existing radiation is currently not entirely known (i.e. the radiation is "As is", we haven't done significant upgrades there). This is single pipe steam.

We've gone back a couple of times for LWCO lockouts. It appears that on a cold start, but only sometimes, the system will evaporate enough water to trip the LWCO before the fill valve or the condensate pump can correct the situation. We've seen it get to within an inch of the LWCO on our own startups from cold but we haven't seen it trip yet, and this is intermittent. Once it gets past the cold start it's nowhere near the danger zone after that.

My theory at this point is that the existing piping could be not pitched perfectly and more water is "hanging up" in the piping and not getting back to the condensate pump fast enough in some conditions. Piping is definitely big as well. It's a 15 gallon condensate pump on a 5 section boiler, which I believe is about right so I don't think the pump sizing is the problem.

Fill, condensate pump, and boiler are all brand new.

Anyone smarter than me have any insight or ideas or resources for me to consider?

Thanks and best wishes to everyone!

Jamie Hall

Is this puppy a condensate pump, or a boiler feed pump? If it's a condensate pump, I could easily see problems if the condensate return is slow. However, if it is a boiler feed pump, it should come on well above the low water cutoff level, as the boiler water level starts to drop. Assuming that its capacity is greater than the steaming capacity of the boiler (you can figure that out from the BTUh input of the boiler -- 100,000 BTUh is around a quart per minute) there shouldn't be a problem. Always assuming, that is, that the condensate receiver/boiler feed tank is big enough -- and that the makeup water feed to the tank (where it belongs, not to the boiler!) is adequate.

NRT_Rob

It's a condensate pump. We have a regular solenoid activated fill valve for the boiler feed. As far as I can tell the fill valve can't keep up with the evaporation rate of the boiler (and that's not really it's job, so I guess that's probably fine).

The condensate pump is 15 gallons, boiler manual says it should be 16, but I would think if it was too small it would discharge more quickly and this is not the issue we'd see (water level drop in the boiler on startup fire).

Also I lied. it's a 4 section boiler: we were misshipped a 5 section, but did not install a section and changed venting so it's a legit 4 section install.

And thanks for the response, BTW.

ethicalpaul

Are you sure it's just steam that's leaving the boiler and not boiler water carryover?

Over the last two years I keep reading about "slow condensate return" and "condensation getting hung up" and "clogs preventing water from returning to boiler" and it almost invariably is actually that the near boiler piping is causing water to be ejected from the boiler.

NRT_Rob

Interesting question. You mean, "is it possible the water is being pushed up into the near boiler piping"?

Heck how would you even figure that out?

Jamie Hall

The problem with a condensate pump is very simple, really: it's not smart enough to start pumping until the condensate tank is getting full. It doesn't know that the boiler water level is low, and doesn't care.

Rewire the thing so it comes on when the boiler water level drops to an inch or so below the normal running level, and shuts off half an inch higher than that. Then replumb the makeup water into the condensate tank with a float switch to tell it that the condensate tank is low. You may find that you need an overflow on the condensate tank -- in fact, I daresay you will -- but that's normal.

nicholas bonham-carter

Check that the pressure is not pushing water up into the dry returns, ESPECIALLY if there are any horizontal sections of the wet return now slightly above the waterline. Those can hide a lot of water quickly, whereas steaming water loss is much slower.--NBC

ethicalpaul

NRT_Rob said:

Interesting question. You mean, "is it possible the water is being pushed up into the near boiler piping"?

Heck how would you even figure that out?

I was thinking of "carryover" where the steam carries water up into the main with it, but yes, high pressure in the boiler can also push water out of the boiler and into return pipes. Both things can be a problem.

To determine the latter, make sure your pressure is low. To determine the former, you can often hear it as a "wooshing" kind of sound in the risers (water drops getting shot around). Or you can try something like this:

https://forum.heatinghelp.com/discussion/169320/see-wet-steam-in-the-wild

NRT_Rob

Thank you all very much, you've given me good things to think about!

EBEBRATT-Ed

@NRT_Rob

I would shut the boiler down and see what it does on a really stone cold start.

I had a job in a school where the boilers were borderline undersized. When the piping was warm or hot the system was fine.

When the piping was cold on a cold start the boiler would start to steam as steam started leaving the boiler and hit the cold piping the steam condensed (1700 times in volume) causing a vacuum in the steam supply piping. This vacuum for a small period of time causes the boiler to evaporate more water than normal and drives the water level crazy and causes LWCO trips. Once the piping is hot condensation slows, the waterline stabilizes and the system runs fine.

The fine print every boiler MFG puts in their manuals is " consult the manufacturer for systems with extensive piping" or words to that effect. Sometimes the 1.33 standard pick up factor is not enough

motoguy128

I think as mentioned, the correctly sized boiler with a lot of piping might have needed more pickup factored in. How is the bear boiler piping? Maybe it needs skimmed and is throwing water into the header on startup.

Could add some Rectorseal 8-way and see if that settles it down. May end up needing to add some storage capacity to the boiler. Some manuals show adding a section of 4 or 6” pipe at the water line as storage. Modern boilers have fairly low volume.