One pipe parallel flow with dry returns and boiler feed tank.

rc8534

Im working on a old steam system with a wiel mclain lgb7(its next to the orginal boiler almost twice the size. The Boiler feed tank keeps overfilling due to slow condensate return. There is a master trap installed just before bolier feed tank and it's coming from a wet return that the pipng is basically a p trap before the master trap. I know this master trap is a bad idea but this is how boiler is. Currently my easiest solution is to just put a pump delay timer on boiler feed pump to let condensate back to tank. Ideally I would like to give owner a proposal to turn back to a true one pipe with wet returns and build up a condensate reservoir back into Hartford loop at boiler so condensate goes directly to boiler. One question I can't really find answer to is can I use boiler feed tank to pump water in to a wet return. If there is check valve and vent pipe is open to atmosphere above header. If not would have to replace boiler feed tank with a low water cutoff feeder. The boiler feed tank is a brand new replacement. I'm discovering the original contactor 20 years ago didn't install boiler and near boiler piping correctly.

rc8534

And the master trap is also improperly installed below boike water line

Jamie Hall

A potentially silly question: what controls the pump on that condensate tank? If the pump is being controlled by the boiler low water cutoff -- the high one -- then it's a boiler feed tank and boiler feed pump, and it should never overfill the boiler. However, if it's being controlled by a float or the level in the condensate tank, that's a problem.

So... can you rewire the thing so that the pump from the condensate tank to the boiler is controlled by the boiler only? Then for makeup water -- if any is required, control that with a float in the condensate tank and put the makeup feed into that tank -- not the boiler or a return. You will probably need to provide an overflow to a drain for the tank as well.

rc8534

It's a boiler feed tank hoffman 30 vbfs-b with a float in the tank to maintain water level in tank. The pump on the tank is controlled by a McDonnell miller 150s that controls boiler water level. All condensate returns to tank before going in boiler. So due to slow return of condensate boiler feed tank fills with water from float before all condensate comes back. And fills more water to boiler. Then feed tank becomes overfilled.

EBEBRATT-Ed

@rc8534

I would keep the tank. If the pump in controlled by the 150 pump control you should be ok. The condensate is being hung up by the master trap causing the system to be double trapped. Yo need to fix or rebuild all the traps and get rid of or gut the master trap.

You may have other issues but that is where to start

rc8534

There are no other traps in building. Just air vents at the ends of dry returns before they drop down to wet return then wet return is piped into master trap which is below water line(why lol!)I cut the copper wet return before master trap to be sure is clear. I want to remove the master trap but what will stop the steam from pushing out condensate? Move boiler feed tank to platform to create water seal or install 5 traps in building?

Jamie Hall

rc8534 said:

It's a boiler feed tank hoffman 30 vbfs-b with a float in the tank to maintain water level in tank. The pump on the tank is controlled by a McDonnell miller 150s that controls boiler water level. All condensate returns to tank before going in boiler. So due to slow return of condensate boiler feed tank fills with water from float before all condensate comes back. And fills more water to boiler. Then feed tank becomes overfilled.

Somethings amiss with that description -- or the system. If the McDonnel Miller 150 is the control for the pump from the tank to the boiler, and it's on the boiler water level, you can't overfill the boiler unless the tank is higher than the boiler. If, on the other hand, there's a float in the tank which switches on that pump, then you can and will. The float in the tank should have one purpose and one purpose only -- to refill the tank when it gets low. Which, given your slow condensate return, will probably result in the tank overfilling -- not the boiler -- which is why you need an overflow to a drain.

rc8534

Jamie please re read comment you highlighted I appreciate your help

Jamie Hall

I tried. What I read is "So due to slow return of condensate boiler feed tank fills with water from float before all condensate comes back. And fills more water to boiler". It's that last sentence that bothers me. Whether the feed tank is full or not should make no difference to whether there is a feed to the boiler. If that's really what it's doing, that's exactly what it shouldn't do.

rc8534

Correct because the 150s called for more water in boiler from float on boiler. Eventually boiler feed tank is overfilled with water.

Jamie Hall

OK. I'm confused then. If the float -- or whatever -- in the boiler is at the correct level, are you saying it overfills the boiler? Or that the boiler feed tank gets overfilled, but the boiler itself stays at the correct level? If it's the latter -- there shouldn't be a problem -- just let it overflow to somewhere handy (although it would be worthwhile to adjust the feed tank size and makeup water control settings to accommodate the slow returns). If it's the former, that's a control problem -- and that should never happen. Which is it?

Steamhead

The basic problem here is the location of the trap.

If you're going to keep the boiler-feed tank- which I don't recommend- then you need to install a properly sized F&T trap at the end of each steam main and any other place there is a drip (condensate drain from main to return). You can't get away with just one trap at the tank inlet.

Install an air vent on the outlet of each F&T so it can vent air coming thru the trap, and open to break a downstream vacuum.

EBEBRATT-Ed

What @Steamhead said

If you dont' need the boiler feed tank then you remove it take out the master trap and pipe the wet return to the boiler.

What may have happened (I am guessing) they replace an old large boiler witha smaller water content boiler. The system didn't hold enough water or the return was sluggish so the added the feed tank and the master trap.

But the problem is a master trap doesn't work well you need sepate traps as @Steamhead mentioned

rc8534

I proposed this rough sketch to my supervisor. To remove master trap and create a wet return to the boiler feed tank. 

Jamie Hall

I'm still not certain of the answer to my question, which is -- stated a little differently -- is the problem that the boiler gets overfilled, or that the feed tank gets overfilled?

If it's the former, something -- I don't know what -- is turning on the pump from that dang tank to the boiler when it doesn't need to, and whatever else you do, that needs to be fixed.

Pumpguy

@rc8534, can you show us some more pictures of the boiler feed unit, especially its float controls and vent pipe?

It sounds like there is some confusion here that these pictures might clear up.

EBEBRATT-Ed

The OP has stated that the MM #150 on the boiler controls the pump.

@rc8534

I like your digaram but a little confused by the trap above the condensate tank. Are you putting a steam equalizer from the boiler header to the submerged return lines or is that the trap outlet?

Jamie Hall

I think that that is what I read, too, @EBEBRATT-Ed -- in which case the boiler should never overfill, since that tank is well below the boiler water level.

Which means that the problem we are looking at isn't the boiler overfilling, but slow returns to the receiver. A different problem.

rc8534

  wet return behind boiler 2" copper is clear

Next 2 pictures are one of the steam mains drop from a tee to a return. 

Next picture is the boiler 150s lwco controlling  boiler feed tank pump

Fill with internal float on tank

Boiler  is a wiel mclain lgb7 running at 1psi with a vapor stat I just put on. I found boiler running almost to 4 psi from a pa404. Building Heats fine at 1 psi.

Boiler feed tank overfills eventually 

EBEBRATT-Ed

@rc8534

If the feed tank overfills it could be sluggish returns aka the master trap. Feeder fills tank becaue returns are slow eventually system gets too much water.Or the make up feeder to the tank could be leaking by. You might check your city water pressure. Some of the tank feeders can't take high psi may need a prv in the line

I used to size feed tank for a t least 20 min of steam time at full fire, 1/2hour is better. But that should be figured from where the tank overflows down to the level the water feed starts to make up......whatever those gallons are won't = the tank capacity