Boiler meltdown mystery

Dave Yates (PAH)

Brand new 600K copper fin-tube boiler. Series 60 2" pump pumping away. Coils slumped into burners and have ruptured.

All safety limit switches are working & were checked out on start-up. Flow switch checked out too. The boiler's high limit switch is one that requires manual reset & isn't tripped. The secondary loop has its own high limit switch to kill the master circuit(two more boilers join this loop).

Wiring checked and rechecked today. Nothing wrong there.

Gas pressures were dead on at 4" water column to the manifold (exact requirements for this model in case you're thinking 3.5"). 9" upstream of the gas valves (there are two)factory spec range is 5" to 10.5". Test fired at the factory.

Draft switches (one internal for the boiler's draft inducer & one external on the DV draft fan) both working properly.

Factory rep can't pin point anything either. New coil arrives on Friday, but I'm leery of firing this thing off and letting it go the weekend.

Combustion analysis was done at start-up too. Everything checked out on Friday. Come Monday, she was toast!





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Sure feels like

a flow issue.

I've seen plenty of small copper tube boilers do this when the tubes become plugged inside, or when the fins on the outside plug from extended condensation issues.

Make sure it's the correct flow switch, and that the paddle is not hung up.

One brand, years ago, used to have some packing strips on top of the HX that needed to be removed before firing.


It did have water in it, didn't it

hot rod

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Steve Whitbeck

What water pressure were you operating at??
The pump should have been on the inlet side of the boiler since the boiler is the biggest restriction to flow.
With the pump on the outlet side you are causing a pressure drop inside the boiler and the boiler may have boiled due to this low pressure. This may have cavitated the pump. If the boiler was under full load when this happened the steam flow may have kept the flow switch closed. ( guessing here )

hr

Tink

good point, most all low mass and copper boilers like the pump pushing into the HX. You may be onto something there.

hot rod

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Mark Eatherton1

The ONLY way...

that I can see that that could have happened would be a stuck gas valve, of which I've seen 2 in my 30 year career...If I were you (and the rep), I'd replace the gas valve too.

I still can't understand how the manual reset didn't trip though...

Maybe someone had a bad heat exchanger and switched it out when you weren't looking. Remember the story about the missing power burner on the old system in N.Y.??

Maybe yo need to leave a C note on the boiler...

One of them things that makes you go "HMMMmmmm..."

ME

Dave Yates (PAH)

me too

I keep coming back to the pump too. This model boiler wants 50 GPM, which the series 60 can more than provide. The rep indicated the PD through the boiler is just 5 ft of hd. I expected a higher number and like you, had thought perhaps there was a sufficient drop in pressure to produce steam. But that brings us right back to the high limit and flow switches.

We kicked a stuck gas valve around too and that's still an option, although I can't for the life of me understand why that wouldn't have kicked the limit switch & here's why. A little history please.....

Three humongous boilers used to reside in this apartment building. I was often called in by the owner when their maintenance men couldn't get things running - typically at the end of a day - arghhhhhhh. He was a tight-wad and wouldn't consider new boilers. He sells out and the new owners have their own outside mechanical contractor, so yours truly doesn't see the inside of the building for 15 years. We were called in a few years ago to correct a botched installation (plumbing) and that was all it took to get back in (third owner by now).

In the interum, the previous contractor had installed three copper fin-tube boilers. They tied all three boilers together in what could loosely be called P/S piping - but it's not - a common header is more like it. No pump in the S piping, so they installed three series 60 pumps on the outlet side of the P piping, after a standard air scoop (boy does that look odd at floor level(G).

The building supply lines are taken off of the S piping. Each apt has a zone vlv & stat to govern same. I quoted them a sweet revision to install new boilers and an energy management system two years ago, but you know how that goes. By now, they could have saved the cost of the installation! This system runs 24/7 maintaining temperature with constant circulation throughout the building. As a result of the previous installation, the DV Tjerland blowers and the series 60 pumps are on 24/7. The boilers cycle off of their limit switches with the German outdoor reset controller (always set to "manual" because the maint men don't like it, has a redundant high limit to kill all three burners if it sees the max setting of 190.

So - all three were pumping away. All three were CF tube boilers. Constant pumping of both water and combustion air. Boiler 2 takes a nose dive last week in bitter cold snap & with all three running - this system struggled to maintain 70 on a design day as it was. No time to quibble over designs, a new boiler was needed immediately and obtained the next morning. Our supplier jumped through their collective arses to get one.

With constant pumping & a relatively low head loss (by the rep's say so)- this doesn't make sense - unless the pump somehow failed while the pressure switch stuck and the high limit decided to take a holiday.

The thing is, on Friday all of these items worked like a champ. DT was holding at 20 (I forgot to mention the by-pass line, which was adjusted for 20 degrees DT). Cut off the circ - immediate safety shut down. Cut off the Tjerland - same result. Gas pressures were checked on both valves & right smack on target. Safety limit switches checked out. Combustion analysis proved everything in the sealed combustion chamber was within limits. The sealed combustion chamber's dual acting pressure switch performed like a champ. Pre-purge function as per normal. She was purring like a kitten when we left.
I keep going back to the pump & have a nagging feeling it somehow plays into this weird situation. BTW, we tossed in the possibility of sustained flue gas condensation too, but that doesn't add up either due to the by-pass having been set for 20 dDT. Here's a simple line drawing to illustrate the installation & design as it sits.

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Dave Yates (PAH)

drawings

The boiler manufacturer's recommended piping shows the pump pumping away on the supply side exactly as we have it on this job. They do show an alternate for low-temp systems where the pump is mounted on the return with a system by-pass.

Sign me - still stumped in PA

New coil is due here tomorrow. Design temps due here tonight.

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Joannie

Something to Check

Hi, Dave.

A longshot, but something to check:

I've seen an instance where in one of our (Laars) boilers, the water barrier between the inlet and outlet was out of whack, and was letting alot of return water go directly to the outlet side of the boiler without going through the heat exchanger. Since the high limit and the flow switch are just after that point on the inlet/outlet header, they were seeing mixed water temp, and full flow, so they "saw" everything as fine and dandy, but the heat exchanger was being starved.

As I said, it's a long shot, but since you're changing the coil, it's something to check.

Good luck,

Joannie

Dave Yates (PAH)

one smart cookie!

Thanks, that never even entered my mind, but it sure would answer the riddle this has become.

BTW, it's not a Laars.

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tombig

Laars

Way to be upfront Joannie! I've got a few JVS's out there (humming like watches I might add) and all info is good info!

carl_4

flow problem?

i know most copper finns should be primary secondaried....ive ran into this before and lochinvar for example will not warrant if its not...also does the pump run continuously or is it time delayed....is there a bypass....what flow would there be if all the zones were closed in the building...just a few thoughts

Joannie

JV Units

Hi, Tom.

The JV units don't have the same water barrier design that I was talking about. That was something that we found when we had a head scratcher that nobody could figure out, on a commercial unit. It was a rare one, for sure, but we altered the barrier, just in case.

If you need help with the JVs that are making your head scratch, let me know, and I'll hook you up with one of our Service Managers.

Joannie

Dave Yates (PAH)

Coming up empty

No dice Joannie. The interior partition is just fine. The new slab arrived today and it struck me that the galvanized baffles that lay over (between actually) the fin tubes either melted or were missing. The new coil slab has them - the old one does not. The damage is even more dramatic to view now that we have the slab out of the boiler.

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