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Vacuum Steam Troubleshooting - Failed traps suspected but need confirmation of vacuum fundamentals
I am having a very difficult time finding any good design information on vacuum steam systems. I'm not trying to design one, but rather I'm trying to troubleshoot one and don't know what is right or wrong. Having confirmation that I know the fundamentals would help. I appreciate any comments.
As I understand it, a vacuum condensate return removes air from the condensate lines and in turn the steam lines, thus drawing a vacuum. In my mind, the only way a vacuum would be drawn in the steam line is through any thermostatic traps or elements in F&T traps while those elements are open (i.e. before the hot steam closes them). Is that correct? The vacuum return system crosses over into the steam supply through open thermostatic vents? Or does the vacuum system connect to the steam supply some other way?
The few piping diagrams I have found show an "equalizing line" from the condensate to the steam header. There is a check valve in the line. I am assuming that check only opens if the steam header were to fall under a deeper vacuum than the condensate header. So in other words, it's a "deep vacuum breaker" to put the steam header under the same "more shallow vacuum" as the return header. This equalizing line also had a U-Trap in it. Why???
So if I am correct so far, I think I understand the basics. Now, let's say I have some failed traps in the system that are always open. So now, I am constantly sucking steam through the coils, traps, etc all the way back to the vacuum pump. The problem I am facing does have symptoms of steam discharging from the vacuum pump. This is combined with poor heat transfer from coils, so I am thinking that I may be pulling steam so fast through the coils that it isn't condensing. So the coils are essentially sensible heat transfer devices with hot vapor inside the tubes (that doesn't condense) and cold air outside that doesn't heat well. Sound possible?
The other possibility I wondered is at what temperature do the thermostatic elements close? Is it possible that I have 15" Hg vacuum steam at 180 degrees hitting the thermostatic elements, and the vents remaining open because they aren't hot enough to close?
The system is unique in that the boiler makes 10 psig steam. Some of that steam goes off on it's merry way like any other low pressure, gravity return system. But then there are a couple of zones with control valves right off the 10 psig steam header. These valves modulate to maintain 15" Hg vacuum on the other side. The vacuum return tank is around 20" Hg. Is this a normal way to control? Let's assume for now that the atmospheric and vacuum returns do not criss-cross.
The overall problem is that the steam header gets "sucked dry" under certain conditions, the heat of the steam doesn't get properly transferred at the coils, and steam comes back to the vacuum pump. That suggests to me that traps are failed open. Any thoughts on that or the overall system? 1-line diagram attached. multiple zones exist for both the 10 psig and vacuum zones, but i only showed one of each.