Flash tank/drum Need a little help understanding

jacobsond · March 2020

Still learning but the end of the season is near. I got a call on steam coming out of a vent on campus. I need a little insite on this setup. I will describe as best as I can. 80lbs steam coming in close to the basement floor in one building. There is a TLV free float trap off the drip. That condensate line runs across the floor 10 ft or more then up 4ft to what I would assume is a flash tank. The 80lb steam line then runs across the ceiling over to the other side of the room where the pressure is reduced to 8. There is another drip on the high side with a Bestobell G10 disc or bi metal trap that looks original. That condensate line runs back to the flash tank. The tank is vented to atmosphere where the steam presents itself. The lowest output of the tank runs to a Hoffman 55 trap then to the common return then to the condensate pump. Now my way of looking at it is the Hoffman is running under 0 input pressure and 0 output pressure. I have monitored the output of the Hoffaman and condenaste just slowly dribbles out. Is this like a double trap situation?? Could a slight pressure on the common return cause the trap to fail to open since there is 0 pressure on the input of the trap? I have replaced the TLV on the high side. The ultrasound readings indicated a steam leak. I rebuilt the Hoffman even tho I think it was good. The Bestobell I'm not sure how to test. The output dB is 10dB higher than the upstream and downstream. I found a new one on the shelf and will likely replace because of the age of the original if anything. The venting is intermittent. The heat load is very low on the building now so testing difficult. Pictures would be difficult. I may be able to draw it out if I can figure out how.

jacobsond · March 2020

rough drawing of the setup

dopey27177 · March 2020

There is no double trap condition shown in the drawings.

The steam traps shown are used to remove water from the HPS side of the supply.

These trap remove condensate at temperatures as high as 323.8 degrees F (80 psi steam = 323.8 degree water). That water flashes into steam when it hits atmospheric pressure about 14.7 psi.

The flash tank is the space where the super heated water will relieve it's energy and become water at about 212 degrees and the pass thru the Hoffman trap as water with no steam in it.

It will then flow back to the boiler room. The vent on the flash tank should be piped to the outside of the building at a height where no one could get hurt or cause damage.

You stated that it is a small amount of steam. Where large amounts of steam occurs that can be piped to some type of coil that will recover the energy and be used to produce hot water or heat a space or other type of location. That's called energy recovery.

Drawings of those types of systems are in my book Steam the Perfect Fluid for Heating and Some of the Problems.

Jake

jacobsond · March 2020

Thanks for the reply. I understand better now. That tank is running about 208 now. There was some sediment in the bottom of the TLV free float that may have been causing issues. It sounds better now and the input temp is 300+ and the output less than 212.The temps on the Bestobell are better now also. About 305 on the input and only around 180 on the output after I changed it out. Before the output was in the 210 range. dB wise the downstream is still 6 or 7 dB lower than the output but I believe ok. I can hear the condensate running through. Ill have to look for your book. Im still learning.

EBEBRATT-Ed · March 2020

@jacosbond

Check these out

Steamhead · March 2020

What these guys said. Also, the pressure on the Hoffman 55 trap is only what the static height of the tank above the trap will produce. So there is a ΔP there but it's not much. If the trap was sized for a very low ΔP relative to the amount of condensate being produced, the condensate should not accumulate in the tank, and you should be OK.

jacobsond · March 2020

Thanks again. I have never been able to be the 1st on the repair when the venting call gets called in. The last time it happened they drained the flash tank then told me something is wrong with the traps. It was working pretty normal when I got there the next day of course. I hadn't been happy with the way the sound of the TLV since December and the Bestobell was very very old so I changed them. I rebuilt the Hoffman even tho I'm pretty sure it was fine. Not much time left in the heating season. Ill keep and eye on it. I hope I can be called 1st during the issue so I can diagnose the problem. I'm thinking maybe a trap close on the same return might stick and blow steam pressurizing the return causing the Hoffman not to work with such low input pressure. The very noisy large steam water heater comes to mind. Its only 6ft or less away. The way it sounds when under full load makes you think twice. I believe its the same size Hoffman on that. Oversized on the flash and maybe undersized on the heater. I will have to learn how to size traps. The school has been canceled now for the virus so the heater will not be under any heavy load until fall.

jacobsond · March 2020

Troubleshooting today. Pulled up and the vent was splashing condensate out with a little steam. Thermal image on the tank showed the condensate level where it was yesterday right at the output of the tank. So tank not overflowing like everyone thought. More investigation on the route of the vent. 3/4 pipe goes up than sideways then up again then sideways then up again a couple more 90s then increased to 2 1/2 out the building then up again about 3 ft. The pitch of the 3/4 is all jacked up. there was a 2x4 that was meant to push down on the 3/4 in one place. It was not doing its job. I worked at getting the pitch back to the tank. Better but not as good as I would like. I drained the tank and let everything wash back. only got about 1 gal out. Still need to work on it,but one of the AHUs was stuck wide open and a couple floors 100 degrees. Control valve failure. I hate this building.

EBEBRATT-Ed · April 2020

3/4 vent sounds way to small to me. Too much velocity in a small pipe is making the steam pull water to the vent. Also any condensate in the vent could drain back to the tank if the vent was sized larger instead of being blown out with the steam

jacobsond · April 2020

I agree.The 3/4 line is going to a 2 1/2 bell reducer 3 ft before it goes outside. Once outside it goes another 4ft then a 90 up about 3ft. There are puffs of steam are making it outside so it has to be condensing in the 2 1/2 pipe. Then not being able to make it back because of the reduction pressure builds up then poof it will shoot out. Now try to explain that to the supervisor when its been like that for years. I think I got put on another one of those problems that has been going on for years and have not been told that. The 2 1/2 is pitched up pretty good. Maybe I can reduce the pitch some and slow how fast the condensate is trying to go back from that larger pipe. Re-pipe with larger pipe would be a hard sell and a real pain. FYI the Hoffman on the output of the tank is less than 6in lower than the tank output. That don't help anything either.

jacobsond · April 2020

I took the insulation off the condensate lines from the high pressure traps. It cooled the condensate enough to really help on the flash tank venting. No more gurgling. The mech room is now 93 degrees instead of just 90. I hope they put the fresh air input in this summer like they talked about last year. We need to cool this room down.

EBEBRATT-Ed · April 2020

@jacobsond

Maybe you can cut a tee into the 3/4 riser at the botton and use that to drain the condensate back

Flash tank/drum Need a little help understanding

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