Vacuum return systems

GusHerb

RQE

Vacuum return systems

I have a building that has been heating fine for 20+ years, and in the last couple of weeks the upper floors (has 4)have been complaining of very little heat. there is 30 # steam entering the building and the condensate goes back into a main that has a vacuum pump on it in the boiler house, approximately 3 blocks away. We have checked traps, and replaced the one on the steam main as it enters the building. Any ideas would be appreciated.

Brad White_9

Questions for you

Is the vacuum pump maintaining vacuum? How many inches Hg.? Different than spec?

Were any air vents or valve installed by mistake? Or on purpose? Any obvious or less obvious leaks? (You may want to go around with a compound gauge and check for vacuum decay. Hope you have tappings available.)

At what pressure is the steam delivered to the radiators? Reduced from 30 psig I would hope to less than a pound. Any other points of leak-by?

You can see I am surmising this to be a loss of vacuum but is there anything on the supply side obstructing things?

Just some thoughts. Others I am sure know much more.

RQE

No loss of vacuum found. This is a college campus and no other building is experieincing the probelm. the problem building is closer than most to the boiler house. We did have some steam going by a trap. Thought it was the trap where the main enters the building, that was replaced and still have the problem.

Wondering if it is TRV's. Any sure way to test them that is somewha easy?

RQE

Steam is reduced from 30 # down to 5 #.

Brad White_9

Here is an idea that jumped out at me when you said 5#

The TRV's are generally rated at 3 psig maximum. If they are seeing 5# they may be opening and blowing by to the return, reducing the vacuum.

Are there TRV's everywhere? I suspect that some are blowing by when attempting to close but cannot close. I know you probably have traps on the radiators and that should stop the madness. But just curious if you drop the pressure to oh, say 1.5 to 2 psig.

Anyway, a thought.

Brad White_9

The main trap was a good call

Is the vacuum pump dedicated to this building? Or is it in the main plant? (lots of opportunity to lose vacuum).

If there are TRV's everywhere are they calibrated to temperature? Are the seats temperature rated or did they melt to the bodies?

When was the last time the 4th floor heated and what changed between then and the problem? Those are things I ask myself.

RQE

You pose a good question. That I will have to check on. My gut tells me it is somethign with th eTRV, the pressure could be it.

5 PSIG is just after the PRV in the bldg MR.

RQE

I too wonder if the TRV's haven't melted.

The vacuum is a central system in the boiler house. Handles about 30 bldgs. Some about 5 blocks away. This particular bldg is probably the closest to the boiler house.

TRV's were part of the original bldg system, but I believe a few were replaced this past fall. that is the only thing I can thiknk of that has been changed, but I am still researching that.

Brad White_9

That is one heck of a vacuum system...

Given it is essentially sucking 5 blocks away and this building next door is suffering... lets look at that supply side again, shall we?

Where is the campus may I ask? I have done work at several campuses around but that kind of central vacuum return does not sound familiar to me.

RQE

It is in Northern South Dakota. I may have mis-spoke, there are some condensate return pumps out in the farther buildings that do take the condensate back some distance, but the main idea to convey here is that this building is considerably closer to the vacuum pump than the furthest building, and it appears to have a good vacuum, although we have not measured it scientifically yet.

The pumps were having trouble in that they were wearing out and two years ago they were completely rebuilt by the manufacturer. All has been working fine until about one month ago. We replaced the main trap (twice just in case a bad trap was received)and the problem continues. I am an enigineer and have been throwing ideas back and forth to the Physical Plant Director on things to check, but am running out of ideas. So if you can think of anything I should look at please throw it out htere. I am visiting the site tomorrow to walk through it.

Brad White_9

I am an engineer too

are you consulting to the University? Just curious.

The part about this being isolated to just one floor has me perplexed. In other words how does the temperature and/or pressure degrade from the 3rd. floor (which purportedly is heating properly) to the 4th?

Heading to my train, then home but I will continue to think on this one.

Brad

Christian Egli_2

Double crossed by a double trap

Before this monster pump you have comes after you to suck your brain out, here is a scenario we could suspect.

You mention you have a master trap, I assume that within your building, all your radiators are trapped, then all the returns lead back downstairs where they collect before going through this master trap on their final way back to the boiler house. Master traps here are a very common application in district systems, particularly if there is a condensate meter.

Master traps bring in a whole set of problem, and you say you've already changed it, so we can suspect some problem going on with what is brought to this master trap.

The master trap is a double trap on your system. Double trapping a return causes water hammer as soon as any one of the upstream radiator traps fails. I suspect you have this situation going on. It is an epidemic that is going to ravage most traps in this building.

I assume your building has a return main going around the basement and return risers going through each floor to the radiators. Water hammer going on in the horizontal parts would easily bust the low leveled traps. The upper floors are a little immune to this depending on the layout.

With even one of the lower floor trap busted and leaking steam, your returns are now pressurized like the steam lines. This equally applies within each riser individually. With the returns pressurized, why would both condensate and air care to flow that way anymore? The further up floors have a further disadvantage in that there is the extra pipe friction loss to get to them, why then would heat even think of wanting to get there?

How to eradicate the problem? Shut the whole building (or appropriate zones) and go after all the radiator traps and crossover traps. The most positive check is to open each one because as long as you have slugs of stuff not moving, you can't tell externally if the trap is good or bad. I am guessing your traps are 20 years old anyway. Install the traps you think are most hammer proof.

Alternate temporary patch: depressurize the building returns causing the trouble by trepanning a hole into them. This will relieve the pressure into the atmosphere, just like you would on an ordinary vented condensate accumulation tank. With bad traps, what will come out of the hole is steam, and if you operate the system this way, it would be a big energy waste.

Of course I am rooting for my idea, but it is just that, an idea. Wouldn't it be embarrassing if someone had simply shut the valve that feeds the upper floors?

Best of luck, like Brad, I think I am impressed by your pump. Any pictures of the vacuumizer? perhaps?

RQE

Yes, I am consulting to the University. I have to leave to head up there this morning. If you thinkof anything else I might look at or try, I'll check later.

Thnaks for all.

Brad White_9

Sorry no new info

Christian Egli has a good thread of ideas. I had thought of that (main trap issue) and the effect on one small area has me thinking the same as Christian, something simple and local like a closed valve....

Keep us posted. Helping is learning.

RQE

The trap is on the steam main as it enters the building. That trap then empties into the condensate main that leaves the building via the vacuum line.

However, you have me wondering if someone didn't put a new trap in the system and create the situation you describe. I am headed up there this orning to look at it.

Thanks.

Brad White_32

What did you find today?

GusHerb

RQE

Sorry, it has been a busy couple of days.

To follow up. We isolated the system within the building. we found there is steam leaking by a trap on the East side of the building. It turned out to be a trap that noone knew existed.

We then isolated the short piece of underground condensate that goes to the tunnel and found if we tried to put a pressure on it, it would not hold any pressure. It appears the pipe is bad. That would mean there is a leak in the vacuum pipe that runs from the building ot hte tunnel and the main condensate pipe.

Seems to make sense the vacuum was not working.

Anything we mihgt have overlooked?

Brad White_9

Sounds like

you nailed it or at least identified the likely suspect. (OJ did it by the way. )

GusHerb