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

Quote (br)

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.

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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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?

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:

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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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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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.

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

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

What "code"?

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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.

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.

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.

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.

@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.

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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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.

All you really need is a vertical section of a pipe about 4 times the diameter of the ones coming in and out and a line off then end cap or flange or reducer and then a valve on the end of a piece of pipe to hold the air. you can make this out of simple standard components.

You really don't need anything fancy for your "temporary" system.

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A design more like an 'air scoop', i.e. 2 eccentric reducers and an oversized section of pipe with a branch connection looking up for the auto vent, might work as well. As long as you can get your fluid velocity low enough through the oversized section the air should de-entrain.

Not sure what your sizes and pressures are like or if anyone is in a position to come at you about this but I am always wary of inadvertently making a "pressure vessel". With the air scoop type configuration I would be more comfortable arguing that its just a wide spot in the pipe meant to temporarily to lower the fluid velocity.

an air separator is designed to get micro-bubbles out of the system. Often it is combined with a dirt-separator.
The design includes a wider cross-section (for low velocity) some magical mesh inside to cause air to form bubbles, and are located at a low pressure/high temperature section of the system to facilitate air to become bubbles. Main reason is to remove oxygen.

An air vent just releases the air that collects at a high point. An air -separator typically has an air-vent as well. But you also have them in the piping system (at top) to let air out that collects. They don't actively dissolve air. Main function is to remove air that already is a gas, so the water can flow. There are auto-vents. You don't have to bleed them manually. In normal sealed systems we close the air-vent off with a valve after a time of operation to avoid any potential leaks. They should be opened up for a few days seasonally. In a system that constantly brings in new water (and oxygen), you need to constantly remove air.

Fair enough. Stick some metal gauze in there as well before you weld it up!

It's better than nothing and at that size with flow going through it doesn't become a PV IMHO, but I know others might not think that way.

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Most of the B&G rolairtrol separators we get in lack the mesh plate that serves as nucleation sites for the air to disentrain. Aside from the tangential connections, they are an empty vessel no different than a chunk of pipe. Create a low-velocity section like LittleInch describes and it will do the same function of allowing bulk separation of any air brought through the system. The accompanying B&G auto vents just have a calibrated ball check in them that will fall open when there's a sufficient amount of air below the check to no longer suspend the ball- that's why they spit and spurt during startup when they're actually working.

Your 6-month "temporary" setup includes seasonal changes that may impact usage of the system substantially, I'd be very hesitant to agree to "temporary" accommodations without seeing a year or two of thermal data to ensure with high confidence you'll only be seeing a small temperature change during the entire duration (and an additional 6 months of buffer time for project delays).

The safer bet vs. removing the tank would be to check that expected temperature change and size a tank at the absolute minimal to account for your corresponding volume change. You might be able to get by with a tank small enough to fit, and cheap enough to not run up the budget, which will avoid all of the issues presented in discussion already.

If you're taking sections of pipe offline for rework and putting them back into service an air separator should not be removed. The only way you'll get away with no air separator is if you can guarantee no leaks, no low pressure events, and no service valves being opened for the duration of the "temporary" work to ensure you're not introducing large amount of air into the system.

And I keep writing "temporary" in quotes because temporary solutions that work have a bad habit of being left in place, or worse, becoming normal practice.

The B&G types are just a large tube. You could build those yourself (and paint it the same red!). B&G also don't tell you the removal efficiency for air and dirt... Spirotherm does.
For a permanent system I wouldn't cheap out on air/dirt removal. For this system, where someone talks about not even using an expansion tank, i wouldn't spend any money.

But you are right, those "temporary" solutions can be in for decades. which is why JHA likely treats them the same way as permanent systems.

In my case temporary means temporary. It is for the rental boiler and chiller only. New permanent ones will be installed with the new equipment. I am just looking for solutions for the rental equipment.

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In my case temporary means temporary.

That's what they ALL say.

10 years later.....

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@littleInch I get it but this is for a roll up trailer mounted chiller and boiler to serve the building while we gut and rebuild the plant. Temporary. I just need to get by for the duration of the rentals

Isn't that how trailer parks became "legal"? They said they don't have to meet building code requirements, because they are just a camping trailer used temporarily. And then, 20 years later, the wheels are off, a little garden built around it and water and electricity hooked up permanently

In my opinion the word "temporary" should be banned from any serious engineering discussion or design as it encourages all these "it's only temporary" type arguments for not doing something properly or using inferior or lower pressure rated components.

One second, one minute, one hour, one day, one month, one year, one decade IT DOESN'T MATTER. Design it like it is going to be there for ever and very little will go wrong. IMHO.

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The air separator is all about minimizing corrosion and keeping you from having to burp the system. 6 months? Should be fine.

The expansion tank should not be ignored, even if you need a temporary location. If the system stays hot the whole time, all good. If the system cools down, might be good too. If make-up water is connected, and there is no expansion tank, just don’t heat the system back up again.