Cracked sections

Darin(in Michigan)

Somewhere in the literature on multiple steam boilers serving a common header I read that if there is a pressure difference between the two boilers caused by staging teh hot fired boiler will force the water out of itself and flood the non functioning boiler which is at a lower pressure because the burner is off. This causes a dry firing effect. Hard to say without seeing a picture though. Are there protective devices between the boilers to insure the water in each boiler stays in each boiler?

Dan Law_4

Cracked sections

OK guys, like to think I can see the forest even with the trees, but would like a few outside opinions. Here's the skinny:

Recently had a cracked rear section on one of two replacement boilers (WM 788's) Saw no evidence of over firing the burner creating impingement on the rear wall. Had the WM rep out to look at the installation, and he saw nothing out of order with the piping. (correct take offs, plenty of rise from the tapings before turning horizontal, correct connection to the header, properly equalized, etc) WM gave me the section in warranty. We chalked it up to a bad casting and replaced the section. 1 month later now and we've have had a multiple section failure on the same boiler. The second boiler - no problems. All sections (3 of 5 intermediate and the fron end) all cracked along the bottom of the crown - bottom side in the burner chamber. The rear cracked vertically from the top 1/3 to the center. I know .... sounds like dry firing and thermal shock.

The boiler goes back on line tomorrow, after a THOROUGH wiring check, particularly the LWCO. Assuming the wiring is correct, if all else failed, the LWCO should have prevented a dry crown. Sooo, what else? A little sequence might help.

The boilers are piped a bit peculiar in terms of boiler feed per the owners insistence, as follows: the feed water from each of 2 boiler feed pumps is tee'd and feeds both boilers whenever one calls for water. Each boiler controls one of the pumps. Boilers can be both on or only one, depending on demand or selection by the owner through his DDC control system. I know this could lead to over filling in the "off" boiler, due to condensing, but not any low water condition I can see. So far as the over filling, the boilers are equalized between each other by the common feedwater line. I would think the disparity in water levels would "equalize" when the "on" boiler becomes satisfied and ceases firing. No steam production = no difference in pressure between boilers, ergo equal water lines. No?

So aside from the odd feedwater sequence, I can see no reason for the repeat failure, though there obviously is something amiss. My suspicion is either a wiring error in the LWCO control (which we will be going through wire by wire) or possibly a calibration problem with the LWCO switch settings (they are the new micro switch type rather than mercury bulbs) Any other thoughts?

will smith_4

Maybe my coffee hasn't kicked in yet-

But I don't understand why the rep wouldn't take issue with the common feed piping. I mean, why have two boilers, two feed pumps, two low water cutoffs/pump controls, and only one common water feed line? Unless the water there is smarter than the water here, I don't see how a proper level is maintained. Love to see some pics of the system!

Dan Law_4

Sections

I'll work on some pics - but a really tight mechanical room.
So far as the "why" have it that way, as I said, owner's insistance. Didn't want to have 1/2 steam capacity due to a feedwater pump failure. That having been said, I cannot see anything about the out of ordinary pumping sequence that could result in any type of low water conditon that would expose the crown. In fact if anything, I could see an overfilling situation. On that note, I just got off the phone with the rep again, and went over the feedwater arrangement. He felt as I did, the common feed line to the two hartford loops would equalize water levels on the off cycles. BTW the LWCO test confirmed the wiring was correct. LWCO chamber and float was clean as a whistle. The firing test showed the burner to be slightly underfired if anything. I'll be visitng the sight myself tomorrow, and will try to get some pics then.

will smith_4

I guess I still don't see it-Open our eyes Dan!

If each has a Hartford Loop, neither knows what the other is doing. I just can't see how this system can work properly with only one feed line. I'll be the first to admit- actually, I'm sure my dear wife will be happy to beat me to this-I'm not the sharpest tool in the shed. But two boilers, two low water cutoffs, feeds, one feed line can't cut it. Gotta see more!

brucewo1b

Will,

What Darin is saying is in order for the two boilers to equilize there has to be a common connection below the waterline, when one boiler fires it creates pressure forcing the water into the cold boiler, now either the LWCO isn't working and the empty boiler dry fires, or it feeds cold water into a hot dry boiler either way it cracks.

Bruce