Vacuum pump strange happening

Paul_11

I did a steam survey recently on a two pipe vacuum system.
I had them shut the boiler down for three hours before I arrived.
When I turned the boiler back on, the vacuum pump immediately
had steam coming out of the vent pipe when the temperature
of the tank and of the condensate line feeding it was 85 degrees.

I don't see how this is possible, but I saw it with my own eyes.

Paul

DanHolohan

The vacuum pump is usually equalized back to the pressure side of the system because it's possible to get a deeper vacuum on the radiator side of the radiator traps than the vacuum pump is producing at the time.. This often happens during the spring and fall when the system starts up and shuts down quickly. Without the equalizer, everything flows backwards when the radiator traps open. The equalizer breaks the vacuum and keeps this from happening. There's a check valve in that equalizer, and it's normally at the bottom of a U-tube pipe arrangement. If that's not in place, or if the check valve isn't holding, steam would have access from the boiler to the vacuum pump's receiver.

Paul_11

Is there a picture in either lost art or the companion to last art?
I'm trying to picture how this works, without getting steam into the vacuum pump which would destroy it.

DanHolohan

Lost Art on page 191 shows the equalizer used with a transfer pump and vacuum pump, but it would be the same with just the vacuum pump.

Paul_11

Got it thanks.
Now I just have to think it through.
It looks like the steam equalizer line has a check valve aimed toward the steam header side.
So the equalizer would allow the higher pressure in the vacuum system to go into the steam side if it was in a deeper vacuum.
However, if that check valve is broken, the steam could flow right into the vacuum pump at all points of the heating cycle.

That could cause the issue I'm seeing.

However, if that equalizer line is not even there, it could not be causing this issue, but may cause another one later on under the right circumstances.

Paul

DanHolohan

Exactly.

Pumpguy

To be technically correct, the equalizer line doesn't break the vacuum, but equalizes the pressure between the steam side and the return side, which usually is still a pressure below atmosphere, AKA vacuum. Once the pressure is equalized, condensate can flow by gravity back to the receiver.

Sometimes these equalizer lines are fitted with a 3/4" thermostatic radiator trap an the end of an uncovered cooling leg, rather than the more common check valve arrangement Dan described.

Many times vacuum condensate return pump packages will have both a pressure vent line and a vacuum pump air discharge line. If live steam is coming out the pressure vent line, then you have live steam in the returns due to leaking trap(s) or defective equalizer line check valve. The reading on the compound gauge will be at zero, or slightly on the pressure side, depending on what the pressure actually is.

On the other hand, if you have what looks like steam discharging from the vacuum pump's air discharge line when the vacuum pump is running, this is moisture condensing out of the saturated air due to the temperature change from the returns to ambient air. The higher the discharged air temperature, the more moisture condenses out. Under these conditions, there will usually be a vacuum reading on the compound gauge.

I have never seen this happen with 85 degree condensate temperature.

Paul_11

Pumpguy,
Thanks so much for your insight.
I will be going back next year and taking a closer look.
I will most likely get back to you then.

I'm trying to get a hold of an installation manual for it and not having much luck.
Maybe you have it even though you rep nash jennings.
It is a varivac ventrite installed by G S Dunham.
Model # 178957

It has two condensate pumps and two vacuum pumps.

paul

DanHolohan

Paul, check the Heating Museum. Lots there about the pumps: https://heatinghelp.com/assets/documents/dunham.pdf

It pays to wander.

Pumpguy

Paul,

I have good information on Dunham and Dunham - Bush / Mepco equipment, although this can get quite convoluted due to many changes with that product line over the years. Unfortunately the information you show does not come up in what I have, and may not now be tracable. This could have been a custom built unit. It would be best if I saw some pictures of this unit.

The information Dan posted shows dual purpose pumps that both pump condensate and also water through nozzle jets to produce the vacuum. Float controlled valves direct the water to the nozzle jets or the boiler as needed.

From your description, I suspect your unit has a pair of separate pumps for vacuum and another pair for condensate pumping purposes. I would need to see pictures to comment further.