Vacuum system

will597 · February 13

Good morning folks-got a good one…Dunkirk D249 1300S boiler partnered in a Dunham two pipe system, accumulator (no pump) in a pit with lift fitting piped to B&G vacuum tank, pumps to a receiver, boiler feed pump controlled by MM 150, with solenoid and check valve in the boiler feed line. Vacuum control is sensing at the vacuum tank factory location-not in the piping itself. Boiler replaced by others in '21, overall the near boiler piping was nicely done, (though more on that to follow), we replaced radiator traps recently after doing a full site survey. Thermostat enables the boiler, with an operating and high pressure Pressure Trol, MM 150 and a MM 67 in the mix.

Problem is we get more vacuum on the supply side than in the return. Vacuum system runs independent from the boiler because of the lift fitting, and works fine-cut-3/cut-out 8", average 5" at the tank. I see vacuum down to 10" at the boiler at times (we installed a vacuum guage at the boiler). At that level of vacuum the pressuretrols are irrelevant-the thermostat is the operator. Opening the blowdown on either MM at that point won't give up a drop of water, nor does it shut off the burner.

No insulation on the supply header, none on any supply piping we can see. (Finished ceilings/walls do a great job hiding stuff!)

Combustion air is directly from outside via 2) 10x8 ducts terminated down low only. Not enough for the gas equipment in this mechanical room-but that's another story. The ducts are about 10' from the boiler-so a ready (if inadequate) flow of outdoor Air is free to make itself known 24/7-and we're talking the north shore of Chicago-not Phoenix.

Wet returns are not plugged solid, but they aren't spring chickens either.

There is no equalizing line from the vacuum tank to the supply piping-though there may have been at some point- tape on the tank fitting makes me wonder.

So the lack of insulation is the main culprit as I see it. Would also rather have a transfer tank/pump instead of the accumulator, but I failed to mention this is also in a church-so the likelihood of that change happening is UN-likely!

One question I have is: without insulating the supply piping, would the addition of an equalizing line and relocating the vacuum sensing line cause even more problems? At some points during the cycle, you can hear the condensate being held up in the system piping, and I wonder what effect that 10" vacuum is having on the MM floats-like if there were a low water condition, HOW low would it go before the float/s dropped?

Here's the kicker: the church heats fine, with no maniacal hammer. Condensate is not blazing hot (it gets lots of time to cool off hovering in space!) So I guess another problem I have is that they don't know there's a problem!

Sorry to have made a short story long🙂-thanks in advance for any insights folks✌️

retiredguy · February 13

Does that accumulator tank in the pit collect all of the heating systems return lines or are there other return lines going to the vacuum return tank? Does the float switch on the accumulator tank operate and what does it control? It would normally control a solenoid valve in the discharge line between the lift fitting and the vacuum tank or control the operation of the vacuum pump depending on how the rest of the system is piped. Also, depending on how the returns are piped, there is normally an interlock between the vacuum system and the boilers that shuts off the vacuum system when there is no steam being produced in the boilers. This keeps the whole system from going into a vacuum which is what you seem to have. There may be other factors in play that I can not see just from pictures and a written explanation.

Another factor that may be in play here. Anytime there was a vacuum return system installed in one of these older heating systems, the boiler's were double or larger than the EDR of the heating system. These boilers were capable of producing a lot more steam than the system needed and the vacuum pumps would not be able to overwhelm the boilers and draw the whole system into a vacuum. When the old boilers are replaced, the new boilers are down sized closer to the actual,steam needs and the vacuum system will overwhelm the boilers ability to produce steam, which is probably your situation. I will bet that the problems started just after the new boilers were installed. If that is the case then there was poor engineering or none at all.. This can be fixed but it won't be a quick fix.

Dave in QCA · February 13

I have not had the time to thoroughly digest your detail post, but what jumped out at me is the fact you're getting higher levels of vacuum in the steam and boiler than in the return piping. This can happen when the boiler shuts off and all of that steam starts to collapse. Dunham ALWAYS included an equilizer line between the return and the steam mains. It was often a 1/2" line with a check valve that would allow flow in the direction ONLY going from return piping to steam mains. This will prevent the vacuum in the boiler and steam mains from being greater than the returns. I will include a Dunham drawing that shows and example, but they ALWAYS included this in all of their various setups. …… After looking in my folders, I found that I had several Dunham drawings scanned, so I included several.

Dunham boiler piping snapshot.jpg

Dunham Return piping.jpg

dunham return trap piping.jpg

Dunham Vapor Heating.jpg

Dunham vaport heating snapshot.jpg

Dave in QCA · February 13

Here is another Dunham publication that has some real good information on the Dunham Differential Vacuum system. While applying vacuum to the boiler and piping, the system still utilizes gravity return to the boiler.

Darn! The file size is too big to post here.

Pumpguy · February 14

What you have there is an auxiliary accumulator tank sometimes used on 2 pipe vacuum return systems. These are used when it is structurally impractical to pipe for a gravity flow of condensate into the vacuum pump's receiver tank.

The float switch on this tank is wired in parallel with the vacuum switch, so when either of these switches' contacts close, the vacuum pump runs until either (or both) the system vacuum switch or float switch on the auxiliary accumulator tank opens.

The pressure sensing line for the vacuum switch is pied to the top of the auxiliary accumulator tank. This is because the pressure is more stable there, whereas in the vacuum pump's receiver tank there might be some pulsating due the air and water slugs coming up the lift pipe.

The attached file goes into detail.

You'll notice this tank is fitted with an adjustable vacuum relief valve. Typically these are set to break at 12 - 15" Hg. vacuum. This is to allow air to enter the tank to push condensate up the lift pipe, and also prevent the induced vacuum from condensing steam exceeding the vacuum producing capacity of the vacuum pump.

Also attached is my file on equalizer lines. Their purpose is to equalize the pressure between the steam header and return lines so condensate can gravity flow back to the lowest point in the system, which in your case is the auxiliary accumulator tank. This would be true even if there was a vacuum on the steam header and boiler steam chest.

Many are tempted to put vacuum breakers on the steam header, but this defeats the purpose of having a vacuum pump. The vacuum allows you to make steam at a lower pressure and temperature, but, especially if you have a separate vented boiler feed pump set, you need motorized valve(s) on the boiler feed pumps to prevent atmospheric pressure from pushing feed water into and flooding the boiler(s) when condensing steam causes vacuum to occur.

Hope this is helpful. Any questions, contact me directly. I'll do all I can to help.

equalizer lines.pdf

AUX Ac Tank.pdf

will597 · February 14

Thanks for the responses and the excellent info-VERY much appreciated!

jumper · February 16

Tanks can improve steam heating. Worthwhile investment if there's enough room.

Vacuum system

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