Pressure (relief) valve before expansion vessel

[PelleT]

Hi all,
My first post to this forum.
I'm about to install a vacuum tube solar system for my house. In the pump unit there's a "pressure valve" marked 0.6 Bar (=9 psi) between solar system and expansion vessel. As of my understanding it relives system pressure toward the vessel and will stop any fluid return from vessel back to the system. Does any of you know the reason for this little fearure? With my limited brain I don't really see why it should be there. At all. Shouldn't passage to the vessel be free in both directions?
Any idea?

Thanks, Per

[Art VanDelay]

Sounds like a check valve, but is there an Installation or Owner's Manual?

Most of the time we can review the installation manual online. To do that, we need more information, like mfr, model #, etc.

Many of us are happy to start guessing at your problem, but the more we know up front, the quicker we can figure out the right answer.

[PelleT]

Most of the time we can review the installation manual online. To do that, we need more information, like mfr, model #, etc.

Many of us are happy to start guessing at your problem, but the more we know up front, the quicker we can figure out the right answer.

Thanks Art for your response,

The manufacturer is GTC-solar (www.GTC-solar.com). I cannot find any installation info on their website but I have one on paper. 60 vacuum heat pipes 1800*58mm

Here's the system (in case I managed to upload it...)
System schema.JPG

The valve I'm talking about is #2 "Pressure Valve". I can provide photos of the actual valve if you want.

Per

[MikeSolar]

Hi all,
My first post to this forum.
I'm about to install a vacuum tube solar system for my house. In the pump unit there's a "pressure valve" marked 0.6 Bar (=9 psi) between solar system and expansion vessel. As of my understanding it relives system pressure toward the vessel and will stop any fluid return from vessel back to the system. Does any of you know the reason for this little fearure? With my limited brain I don't really see why it should be there. At all. Shouldn't passage to the vessel be free in both directions?
Any idea?

Thanks, Per

I will be a bit blunt here. It is a cheap enough system so Buy two as you will need them. The best part about the Chinese systems are the panels. You probably won't find any American recognized certifications for the pump stations. I have changed out more Chinese solar stations than I can remember and some have been sent with counterfit Wilo pumps. The same goes for the tanks.

That said, the controller looks to be a RESOL, which is good and the general layout is standard, so I hope the pump station is from Europe. Check valves are there to provide enough back pressure so that warm water will not thermosyphon up to the panels during the night and lose heat.

0.6 bar is ridiculously low. Most glycol systems are pressurized to about 2 bar and the European ones have PRVs around 6 bar. I hope there is a typo.

[PelleT]

Sorry mate,
you're right!
I made a reado leading to the typo.
When looking closer it is 0,6 MPa = 6 bar NOT 0.6 bar

But my question remains.

Why is there a restriction to the expansion tank? If you're afraid of over pressure (which you should be) the valve should be to the atmosphere. And in particular that valve seems to function as a check valve, blocking flow back to the system. What am I missing?

[MikeSolar]

Sorry mate,
you're right!
When looking closer it is 0,6 MPa = 6 bar NOT 0.6 bar

But my question remains.

Why is there a restriction to the expansion tank? If you're afraid of over pressure (which you should be) the valve should be to the atmosphere. And in particular that valve seems to function as a check valve, blocking flow back to the system. What am I missing?

I wish you had a picture of it. Simply, the PRV is the last of the protective devices that should be used and it does vent to atmosphere. It should actually have a pipe srcewed into the port going to a small tank to take the glycol volume.

The expansion tank is the first line of defense and it does not need a a big pipe for flow, and neither does the PRV. One thing I would do is upsize the expansion tank. People often try to save a buck with a slightly smaller one and it is money poorly saved.

Especially with vacuum tubes which can steam faster than flat panels, having a bigger expansion volume will keep the pressure stable. Also, extend the piping down from the junction. A longer pipe and a lower tank means it will see less heat and last longer.

[Art VanDelay]

I've seen this in my own floor heating radiant system

Let's say you prime the closed loop system to 15 psi.

The "pressure valve" is a pressure REDUCING valve, also called a makeup water valve. In a hydronic system, it replaces water lost by tiny leaks. Larger leaks have to get fixed obviously.

In a glycol solar system, you can't have city water water as the makeup water, because you might dilute the antifreeze mixture.

The pressure reducing valve is set to 15psi, but the expansion tank is pressurized to 60 psi. Now you got yourself an antifreeze makeup system.

[Art VanDelay]

I've seen this in my own floor heating radiant system

Well, here's my theory:

Let's say you prime the closed loop system to 1.5 Bar.

Item #2, the "pressure valve", is a pressure REDUCING valve, also called a makeup water valve. In a hydronic system, it replaces water lost by tiny leaks. Larger leaks have to get fixed obviously.

In a glycol solar system, you can't have city water water as the makeup water, because you might dilute the antifreeze mixture.

The pressure reducing valve is set to 1.5 bar, but the expansion tank is pressurized to 6 bar. Now you got yourself an antifreeze makeup system.


But if that's what they are trying to accomplish, Item #2 must allow flow to the left at 1.5 bar, and flow to the right at 6 bar. Note there is no pressure relief valve in the system which may raise an eyebrow with your inspector.

But solar systems aren't like boilers. They can't runaway and explode. Worst case is the collectors boil and push out all their fluid back into the rest of the system as steam. If the expansion tank is big enough (6 gallons is big enough), there is no problem, and later in the afternoon, the steam condenses and the system is ready for collection the next day. No glycol down the drain, no service call.

Upon close examination of #2, do you think it would allow flow in either direction?

[MikeSolar]

Let's say you prime the closed loop system to 15 psi.

The "pressure valve" is a pressure REDUCING valve, also called a makeup water valve. In a hydronic system, it replaces water lost by tiny leaks. Larger leaks have to get fixed obviously.

In a glycol solar system, you can't have city water water as the makeup water, because you might dilute the antifreeze mixture.

The pressure reducing valve is set to 15psi, but the expansion tank is pressurized to 60 psi. Now you got yourself an antifreeze makeup system.

That would be complicating it way too much. The PRV is "pressure relief valve" and it is in the normal place for European type systems. I wouldn't worry about it at all. When the pic comes...all will be revealed.

[MikeSolar]

Well, here's my theory:

Let's say you prime the closed loop system to 1.5 Bar.

Item #2, the "pressure valve", is a pressure REDUCING valve, also called a makeup water valve. In a hydronic system, it replaces water lost by tiny leaks. Larger leaks have to get fixed obviously.

In a glycol solar system, you can't have city water water as the makeup water, because you might dilute the antifreeze mixture.

The pressure reducing valve is set to 1.5 bar, but the expansion tank is pressurized to 6 bar. Now you got yourself an antifreeze makeup system.


But if that's what they are trying to accomplish, Item #2 must allow flow to the left at 1.5 bar, and flow to the right at 6 bar. Note there is no pressure relief valve in the system which may raise an eyebrow with your inspector.

But solar systems aren't like boilers. They can't runaway and explode. Worst case is the collectors boil and push out all their fluid back into the rest of the system as steam. If the expansion tank is big enough (6 gallons is big enough), there is no problem, and later in the afternoon, the steam condenses and the system is ready for collection the next day. No glycol down the drain, no service call.

Upon close examination of #2, do you think it would allow flow in either direction?

Naaaa, if you pressurized the tank to 6 bar, under almost all operating conditions it would have no liquid in it. You cannot have anything between the panels and the exp tank (code requirement and good practice) so all it is, is a tee with the pressure relief valve on it and that is OK.

Solar systems definitely can run away and explode. I saw a Schott vacuum tube system do just that. someone stole the controller on a system we installed (120 panels) and the glycol got up to 250C+ and over time gummed up the glycol to the point where some of the individual panels could not get rid of their pressure. Made a big BOOM, fire trucks came too but obviously no fire, just steam.

[PelleT]

Thank you guys for all your comments.

It's late night here in Sweden so I'll get back to you after a nap.

[Art VanDelay]

Naaaa, if you pressurized the tank to 6 bar, under almost all operating conditions it would have no liquid in it. You cannot have anything between the panels and the exp tank (code requirement and good practice) so all it is, is a tee with the pressure relief valve on it and that is OK.

Solar systems definitely can run away and explode. I saw a Schott vacuum tube system do just that. someone stole the controller on a system we installed (120 panels) and the glycol got up to 250C+ and over time gummed up the glycol to the point where some of the individual panels could not get rid of their pressure. Made a big BOOM, fire trucks came too but obviously no fire, just steam.

Mike,

Sorry, I meant pressurize the liquid side of the expansion tank. The air side pressure would start out much lower, and of course increase as it filled with glycol mixture. That extra fluid in the expansion tank becomes the reservoir that can accomodate tiny leaks for years. I have no idea if the sort two-way pressure valve I described above is even manufactured. But there is a patent for it: http://www.google.com/patents/US4317467?

I'd say that the Schott system blew up even though it had a PRV inside the building. A solar panel filled with fluid and then isolated from the system would blow up, which is what happened. A solar SYSTEM can't blow up if it has enough expansion volume. The collectors flash to steam and fill up the expansion tank. Once the fluid inside the collectors vaporizes, very little additional heat can be collected, so it doesn't run away.

But you're right, my explanation is rather complicated, and your theory "so all it is, is a tee with the pressure relief valve on it and that is OK." is more likely correct per Occam's Razor.