Why doesn't boiler collapse?

Wearb

Here is something that I have never understood. Assuming (on a sealed system) that EV bladder has broken and the tank is now full of water and the air has all been expelled through auto air vents and the expanding water flows out through the pressure relief valve. Why doesn't the boiler, rads pipework, etc. collapse when the water cools? Is the auto air vent the only thing stopping this from happening?

agusta

My opinion is the water in the system contracts within itself.In basic terms the molecules in the water rearrange themselves.

corkgsxr

You'll suck air In the safety valve. Or air vent.

shane0007

Collapse from what?

scudo2

shane0007 wrote: »

Collapse from what?

Eh Shane,
We had this debate before. Remember, 1 bar atmospheric pressure at ground level.
If a vacume, as in lower pressure on the inside of a copper cylinder, it collapses inward. Due to 1 bar pressure constantly on the outside.




To Wearb's question.
PS. Boilers are strong + auto air vent will allow air back into a vacume situation on a sealed system.

shane0007

So explain it to me. For it to be a vacuum, it would have to go into negative pressure. Surely it would at best equalise with outside atmosphere.

I am genuinely curious.

scudo2

shane0007 wrote: »

So explain it to me. For it to be a vacuum, it would have to go into negative pressure. Surely it would at best equalise with outside atmosphere.

I am genuinely curious.

Wearb's question was, if I've got it right, boiler heats, water expands and blows out one way safety valve, boiler cools down, water contracts causing negative pressure/vacume, why doesn't boiler collapse ?
Pressure cannot equalize unless auto air vent allows air back into vacumed boiler.
Copper cylinders are weak from external pressure and so collapse like a crushed tin can.
Boilers are structural stronger.

Oil boiler walls do expand slightly when at 1.5 normal pressure and can also squeeze back in when at 0 pressure or negative pressure.



That is why I previously posted that if your having problems getting tight baffles out of a boiler, drop the pressure by draining some water, the boiler walls contract and baffles come out easy.
A tip I got from Joe @ Grant's, rather than an another of my ingenious ideas !

shane0007

You are only talking about a circa 4% increase in volume size through expansion. This would not warrant a vacuum upon reduction.

scudo2

shane0007 wrote: »

You are only talking about a circa 4% increase in volume size through expansion. This would not warrant a vacuum upon reduction.

A slight negative vacume won't affect an oil boiler.

Have you never seen what can happen a copper cylinder if attic has frozen cold feed + expansion pipe and somebody turns on the hot tap downstairs ?
Cylinder will crush in like somebody stud on an empty beer can.

Double check with "wearb" if this is what he's on about ?

I don't need beauty sleep, but I'm off to get some now.

Wearb

shane0007 wrote: »

Collapse from what?

Because the volume of water contracts when cool and therefore the system will be at less than atmospheric pressure and therefore will be subject to that pressure. It is the opposite of the reason that an expansion vessel is needed on a sealed system in the first place.

Wearb

scudo2 wrote: »

Eh Shane,
We had this debate before. Remember, 1 bar atmospheric pressure at ground level.
If a vacume, as in lower pressure on the inside of a copper cylinder, it collapses inward. Due to 1 bar pressure constantly on the outside.




To Wearb's question.
PS. Boilers are strong + auto air vent will allow air back into a vacume situation on a sealed system.

Auto air vents can be closed. Can the PRV also take in air?

sawdoubters

your thinking of a black hole

Wearb

shane0007 wrote: »

So explain it to me. For it to be a vacuum, it would have to go into negative pressure. Surely it would at best equalise with outside atmosphere.

I am genuinely curious.

The water molecules become excited under rising temperatures and expand the volume of a given amount. That same amount at lower temperatures has less excited molecules and that same volume contracts. Because the PVR has vented some water, then the system would be in negative pressure.

Wearb

shane0007 wrote: »

You are only talking about a circa 4% increase in volume size through expansion. This would not warrant a vacuum upon reduction.

4% of 25 litres is one litre. A lot of area to be compressed.

btw I don't mean that to be patronising Shane.

Wearb

scudo2 wrote: »

A slight negative vacume won't affect an oil boiler.

Have you never seen what can happen a copper cylinder if attic has frozen cold feed + expansion pipe and somebody turns on the hot tap downstairs ?
Cylinder will crush in like somebody stud on an empty beer can.

Double check with "wearb" if this is what he's on about ?

I don't need beauty sleep, but I'm off to get some now.

No Scudo, I am referring to the heating system only and wondering how the boiler, rads, piping, cylinder coil etc. can withstand this.

shane0007

scudo2 wrote: »

A slight negative vacume won't affect an oil boiler.

Have you never seen what can happen a copper cylinder if attic has frozen cold feed + expansion pipe and somebody turns on the hot tap downstairs ?
Cylinder will crush in like somebody stud on an empty beer can.

Double check with "wearb" if this is what he's on about ?

I don't need beauty sleep, but I'm off to get some now.

Yes I've seen it a few times during the big freeze we had a couple of years ago.

The reason was both vent & f & e froze. Total volume of water contained in a more contained space, i.e. the cylinder, so the surface area of the water is greatly reduced. Add that to the inherent weak material of copper.

An average of 4% increase in volume over a whole heating system is enough to increase the pressure as per The Pressure Law/Boyle's Law/Charle's Law but not enough to create any noticeable vacuum in a heating system.
The surface area spread over the whole of a heating system is greatly increased. Added to the stronger materials & differing expansions of those.

Wearb

shane0007 wrote: »

Yes I've seen it a few times during the big freeze we had a couple of years ago.

The reason was both vent & f & e froze. Total volume of water contained in a more contained space, i.e. the cylinder, so the surface area of the water is greatly reduced. Add that to the inherent weak material of copper.

An average of 4% increase in volume over a whole heating system is enough to increase the pressure as per The Pressure Law/Boyle's Law/Charle's Law but not enough to create any noticeable vacuum in a heating system.
The surface area spread over the whole of a heating system is greatly increased. Added to the stronger materials & differing expansions of those.

Boyle's Law I am familiar with, but not the other one. Would your explanation not then suggest that there isn't a need for an expansion tank in the first place? Just allow the system to expand and contract as temperatures demanded?

Ignore that if you want to. This thread may be going into the realms of advanced engineering.
Thanks for your take on it.

shane0007

Wearb wrote: »

Boyle's Law I am familiar with, but not the other one. Would your explanation not then suggest that there isn't a need for an expansion tank in the first place? Just allow the system to expand and contract as temperatures demanded?

Ignore that if you want to. This thread may be going into the realms of advanced engineering.
Thanks for your take on it.

No you must always allow for expansion, due to The Pressure Law in particular. As heat & pressure are directly related, as the water in the system is heated, the pressure will increase as positive pressure. As it decreases, it is not going into negative pressure, it is just going back to the previous set positive pressure it was previously set to.
If there was no expansion allowed for, the pressure would continue to rise in proportion to the heat being applied or until the safety valve opened which would be 3 bar.

For example, a small system of single rads would be circa 100 litres. This would expand under normal heating conditions to 104 litres, hence, we would install a vessel as good practice as 11% of the system volume giving 11 litres, so a 12 litres vessel is the available vessel.

Should the vessel fail, it will expand to a size of 104 litres until the 3 bar safety valve opens, expelling a volume of water until the pressure falls below 3 bar.
Let's say 1 litre expels & the pressure drops to below 3 bar. We now have 99 litres in the system & the process begins again, however our new 4% will be on the new total volume of 99 litres, giving an expanded volume of 102.9 litres. It would keep doing this until the volume was enough to raise the pressure enough to be above 3 bar or turn to steam if the temp increased to above its boil temp at the given pressure. Then it would expand to 1,600 times it's volume.

DGOBS

The simple answer here is material strength stops the collapse, pipework is small and round, hence strong, rads are steel with lots of bends that provide strength, boilers Heatexchangers are pretty robust, in comparison with a large copper cylinder (like a beer can)

Wearb

shane0007 wrote: »

No you must always allow for expansion, due to The Pressure Law in particular. As heat & pressure are directly related, as the water in the system is heated, the pressure will increase as positive pressure. As it decreases, it is not going into negative pressure, it is just going back to the previous set positive pressure it was previously set to.
If there was no expansion allowed for, the pressure would continue to rise in proportion to the heat being applied or until the safety valve opened which would be 3 bar.

For example, a small system of single rads would be circa 100 litres. This would expand under normal heating conditions to 104 litres, hence, we would install a vessel as good practice as 11% of the system volume giving 11 litres, so a 12 litres vessel is the available vessel.

Should the vessel fail, it will expand to a size of 104 litres until the 3 bar safety valve opens, expelling a volume of water until the pressure falls below 3 bar.
Let's say 1 litre expels & the pressure drops to below 3 bar. We now have 99 litres in the system & the process begins again, however our new 4% will be on the new total volume of 99 litres, giving an expanded volume of 102.9 litres. It would keep doing this until the volume was enough to raise the pressure enough to be above 3 bar or turn to steam if the temp increased to above its boil temp at the given pressure. Then it would expand to 1,600 times it's volume.

Ok. Lets say my system as in first post has 100 litres and expels 4 litres when hot, what occupies this 4 litre space when it cools?
Remembering that this space was comfortable holding 100 litres when cool before.

corkgsxr

Hes right. You would be minus 4 litres the system. Like if you had a 100 litre vessel with a safety valve. You heated it to 70 degrees lost 4 litres to expansion. The vessal now has 96 litres once cool. So its minus pressure. But you suck air in various places in the system

Wearb

DGOBS wrote: »

The simple answer here is material strength stops the collapse, pipework is small and round, hence strong, rads are steel with lots of bends that provide strength, boilers Heatexchangers are pretty robust, in comparison with a large copper cylinder (like a beer can)

That is what I suspected.

So the water must be kept at a higher temperature by the strength of the system minus some contractions in the system. Because as far as I understand it, that is the only way that a smaller volume of water could occupy the previous space. I suppose also that the water would give up some of its gas similar to the way that there is a maximum vacuum allowed on an oil line.

Thank you ALL for humouring me on this. I find it educational and facinating

Wearb

corkgsxr wrote: »

Hes right. You would be minus 4 litres the system. Like if you had a 100 litre vessel with a safety valve. You heated it to 70 degrees lost 4 litres to expansion. The vessal now has 96 litres once cool. So its minus pressure. But you suck air in various places in the system

Perhaps in practice air would be sucked in, but is there any where that is designed for this. Can't see how the PRV allows for this and AAVs can be closed.

shane0007

Wearb wrote: »

Ok. Lets say my system as in first post has 100 litres and expels 4 litres when hot, what occupies this 4 litre space when it cools?
Remembering that this space was comfortable holding 100 litres when cool before.

Think of larger surface area of the system.
You have to remember that metal expands also when heated, therefore when the metal within the system heats, i.e. rads, pipework, etc. it will expand thus increasing the volume it can hold within it.

shane0007

Wearb wrote: »

I suppose also that the water would give up some of its gas similar to the way that there is a maximum vacuum allowed on an oil line.

That's a different discussion as it due to cavitation of the oil.

Wearb

shane0007 wrote: »

Think of larger surface area of the system.
You have to remember that metal expands also when heated, therefore when the metal within the system heats, i.e. rads, pipework, etc. it will expand thus increasing the volume it can hold within it.

Ok I get all of that. My problem is understanding what replaces my 4 expelled litres when system cools.
System contraction would account for some, gasses released from water maybe acounr for some, but if all that doesn't account for all, the only way for the smaller volume of water to occupy the original area seems for it to be kept at a higher temp.
I know I am missing something here, because in spite of everything, the water would eventually reach ambient temperature. Does it boil a little under estreme negative pressure? There must be something at work that allows a system under those conditions to operate without a reversed PRV.
Hope I am not trying your (dare I suggest slightly limited:) ) patience too much.

corkgsxr

A normal safety valve will let in air in a vacuum situation.

Wearb

shane0007 wrote: »

That's a different discussion as it due to cavitation of the oil.

I don't agree. My understanding is that cavitation is caused by by the oil gassing under too low pressure.

Wearb

corkgsxr wrote: »

A normal safety valve will let in air in a vacuum situation.

Well that sorts it out then. The PRV works in both directions.

shane0007

You are missing the point. There is no vacuum.
There would be a vacuum if you go from positive pressure to negative pressure. You cannot have both in the same pipework.

In the first instance you have brought the pressure to let's say 1.0 bar positive pressure. So let's say its a bungalow with 2.5m head height above the gauge.
This means that when the system is full of water under its own weight the gauge reading is 0.25 bar pressure.
We now overfill the system by raising it to 1.0 bar. We have no expansion & we heat it. The safety valve releases 4 litres water & it is now 96 litres when cold & showing a pressure of 0.8 bar.
The original fill volume would only be about 80 litres when cold if you were to open the highest point & see water with none spilling out.

shane0007

Wearb wrote: »

I don't agree. My understanding is that cavitation is caused by by the oil gassing under too low pressure.

No, cavitation is caused by over-sized oil supply pipes or too high a head. The excess oxygen in the fuel gathers in the fuel pump & causes cavitation.