Removing expansion tank after system is hot 2

[BronYrAur]

What do you think of this one? On a hot water heating system, what if you removed the expansion tank after the system is hot? Everything has expanded and the pressure is stable. worse case, if everything gets cooled down, the pressure would drop. Auto fill would have to be isolated so as to not allow additional water.

The reason for this would be to utilize the floor space where the existing expansion tank sits. This is a temporary (less than 6 months) scenario using a rental boiler while the building boilers are replaced.

Can the expansion tank be removed once the system is hot?

 

[LittleInch]

In theory, maybe, but the pressure can rise by 3-4 bar per deg C in a truly closed system so unless you have a lot of design margin left or you are very very confident that the pressure won't exceed the temp you decide is "hot" then maybe.

However when the system cools down you will be drawing a vacuum / less than atmospheric pressure. Not all seals on valves etc are designed for negative pressure so gas could easily enter the system or you could collapse some of the equipment like the HX or any vessel that is connected to it. Even if it was perfect, air and water vapour can leak out creating gas bubbles gathering in high places once the system warms up again.

I hate the word "temporary" as it starts people going down tracks which they otherwise wouldn't and increase risk dramatically. The time something is in operation is simply not relevant to this sort of discussion.

So a wholly bad decision IMHO.

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[GBTorpenhow]

Can you take the wheels off of a plane after it takes off?

worse case, if everything gets cooled down, the pressure would drop

...to the vapor pressure of water at your system temperature (e.g. ~1 psia @ 100F). Any pumps in the system will not be happy. Realistically you will probably just suck in air through joints or the air eliminator(s) and the system would no longer be liquid full.

 

[LittleInch]

Basically nothing will be happy at sub atmospheric pressures including any vessels or even radiators or HXs.

Do it properly or you will have endless issues and breakdowns with your system. IMHO.

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[nuuvox000]

This has me a little stumped. Would it really go to sub-atmospheric pressures if it was hot at 30 psi and then cooled? Seems like it would just lose pressure but more than 30? Can anyone explain?

 

[GBTorpenhow]

The density of water is so strongly dependent on temperature (vs pressure) that the starting pressure almost doesn't matter.

Using NIST REFPROP for pure water, keeping density constant when going from 120F at 30 psig to 119F, a mere one degree F of cooling, gives you a pressure of about -13 psig. Similarly, one degree of heating gives you ~114 psig, etc.

Comparing 30 psig to 120 psig, 3x the pressure, it takes another one degree or so to start boiling:

 

[LittleInch]

Yes it would, but that program prob ignores water compressive and pipe contraction.

The volumes to be fair will be very small but at the high point it will be sub atmospheric, so depends on elevation of the system to know what pressure is at any point.

But it's the expansion that will be either very high pressure or constant dripping of the relief valve.

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[LittleInch]

But why don't you just replace the tank with a relief valve piped somewhere and a pressure controlled fill line set 10 psi below the relief valve setting?

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[HVAC-Novice]

You can't just use a relief valve instead of an expansion tank. RV is an emergency device and not for normal operation. The valve would open every time the boiler fire turns on or modulates up. It isn't designed to open 1000 times a day.

And there has to be a relief valve y each boiler anyway. If not, you soon will be in the expulsive business.

 

[LittleInch]

There's a big difference between a valve designed to open regularly and a pressure relief valve designed for safety use.

I should though have referred to this valve as a pressure regulating valve which could be a spring type or a control valve type. And yes it would operate quite a lot which is why typically they are not used in this sort of scenario. But it's a lot better than what the OP was proposing.. .

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[HVAC-Novice]

Before coming up with more great ideas, I'd check code if removing an expansion tank is even allowed.

 

[3DDave]

Or, relocate the tank. There must be some other location it can be plumbed to.

 

[LittleInch]

What "code"?

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[PEDARRIN2]

For a hot water heating system, I would assume either the International Mechanical Code or the Uniform Mechanical Code may apply. IMC Section 1009.1 indicates an expansion tank is required. UMC Section 1004.1 states the same.

 

[HVAC-Novice]

What code is for the OP and their AHJ to determine. I don't know if OP is in Japan, or Greenland. I also don't know those local codes.

Where I live I would consult boiler code, IMC, and the local mechanical codes to begin with. I also would ask AHJ.

FWIW, I wouldn't do it, even if code allows it.

 

[BronYrAur]

OK, follow-up question. The air separator is just as problematic, if not more so. It will be replaced, but the new one will go in a different location. So, thoughts on operating the system without an air separator? I will leave the expansion tank connected....just piped to the suction side of the pumps somewhere, but my temporary system will not have an air separator. maybe I could put some auto vents in but not an air separator. Thoughts about that? Probably 6 month duration.

 

[nuuvox000]

Is the cost to install an air separator somewhere in the system really worth all of the potential maintenance issues for those 6 months? I could imagine coils without flow, pumps with chewed up impellers, valves getting stuck open with dirt. I apologize and I know it's annoying when someone doesn't just answer the question, but it seems like air separators don't take up that much space since they're inline; I would push for it.

 

[BronYrAur]

@nuuvox000

it's just not that simple. I need to keep the air separator where it is for existing operation while I am piping up the temporary. I don't have a temporary air sep, so I would have to find a way to use the existing as the temporary also without taking the system down. Twice as many tees and valves to make it work.

 

[LittleInch]

The difference between an auto vent and an air separator is??

What is this system? All AHUs or some radiators or other places that would trap air and let you vent it?

We can't see your system so have no idea how complex it is.

Or just fit an air separator with a long bit of pipe on the top and fit a manual vent and bleed it every day?

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[BronYrAur]

I have the same issue with the chilled water and hot water systems. Entire plant is being gutted and replaced. chilled water goes to AHUS. Hot water goes to everything - AHUS, unit heaters, fin tube etc.

Both systems have rol-air-trols that are coming out and new ones installed. It is the temporary that concerns me. I am connecting to supply and return mains that leave the plant. I am providing temp pumps, boilers and chillers. But as of now, no one thought about the air separators and expansion tank that won't exist in the system once we switch to the rentals. So I guess I need to provide rental ones. Not sure if I can even get rental rol-air-trols. I'm sure I can get the expansion tanks.