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.

corkgsxr

Thinking about it prv shouldn't leave air in. But they seem to

Wearb

shane0007 wrote: »

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.

I have reread the above lots of times and am having difficulty understanding it. I sometimes hit a brick wall like this when trying to understand some things. I will reread it later after thinking about it for a while.

Here is where my line of thought is at present. If you fill a metal can (lets say a jerry can) with hot water and cap it. Then let it cool. Is there not negative pressure in the can when it cools?

Pete67

corkgsxr wrote: »

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

I didn't know this, it seems counter intuitive to me - the safety valve should not open until there is more than 3 bar (say, depends on the rating) positive pressure difference between the heating system and the atmosphere.

Under the vacuum conditions described the pressure difference would be less than 1 bar, and acting in a direction that would tend to close the PRV rather than open it?

Large industrial systems often have vacuum breakers installed, which open when the internal pressure drops below atmospheric pressure to protect components from collapse. Maybe some domestic PRVs have a built-in vacuum breaker, but the few that I've dismantled did not have any sign of this.

If the expansion vessel has failed, and the PRV opened to release water to lower the pressure due to thermal expansion, then without some sort of vacuum breaker device, the system will go into negative pressure as it cools down, and there will be small pockets at the tops of radiators and other high points that contain no water. They will contain a low pressure mixture of water vapour, and any gases that come out of solution - oxygen, nitrogen, possibly also corrosion by-products such as hydrogen sulphide.

The radiators and piping will not collapse as they are strong enough to withstand the external pressure. Pex will probably compress a little, but being slightly elastic it will recover when the pressure rises.

Most systems are not 100% gas tight, so over time air will slowly be drawn in through valve stems etc and the pressure will slowly equalise. If the situation continues, as more and more water is expelled each time the boiler heats up, the system will gradually reach a point where the water quantity is low enough to trip out the low pressure switch (hopefully) or boiling will take place leading to boiler damage.

shane0007

Wearb wrote: »

I have reread the above lots of times and am having difficulty understanding it. I sometimes hit a brick wall like this when trying to understand some things. I will reread it later after thinking about it for a while.

Here is where my line of thought is at present. If you fill a metal can (lets say a jerry can) with hot water and cap it. Then let it cool. Is there not negative pressure in the can when it cools?

No there is positive pressure of 0.1 bar per meter of head height so if the can was 1.0 meter high regardless of diameter, it would have a positive pressure of 0.1 bar.

Think of the system. The volume is 100 litres. It's the bungalow example. So the gauge pressure is 0.1 bar per meter of head height. The gauge reads 0.25 bar. If you unscrew a cap at the top of the system, no water would come out as this pressure would be 0 at this point but the system is full.
Now we raise the pressure to 1.0 bar. In order to do this we most add more water under force. So we bring the pressure to 1.0 bar. We have just added say 20 litres so the pressure now reads 1.0 bar.
But the system now contains 120 litres.
We heat the system & the prv lets out 4 litres. The system cools & the pressure gauge now reads 0.9 bar as the system now has 116 litres of water in it.

Now this continues every time we heat the system until let's say the gauge reads 0.25 bar. The system volume is now 100 litres. If we opened the prv nothing may come out due to air locking like holding a glad upside down but if we opened the top of the system it would empty. The exact same as emptying a radiator. We need to open the bleed screw to empty it.

Mods: can you add a "Donate" button beside my name. I should be paid for this :-)

Pete67

shane0007 wrote: »

No there is positive pressure of 0.1 bar per meter of head height so if the can was 1.0 meter high regardless of diameter, it would have a positive pressure of 0.1 bar.

Think of the system. The volume is 100 litres. It's the bungalow example. So the gauge pressure is 0.1 bar per meter of head height. The gauge reads 0.25 bar. If you unscrew a cap at the top of the system, no water would come out as this pressure would be 0 at this point but the system is full.
Now we raise the pressure to 1.0 bar. In order to do this we most add more water under force. So we bring the pressure to 1.0 bar. We have just added say 20 litres so the pressure now reads 1.0 bar.
But the system now contains 120 litres.
We heat the system & the prv lets out 4 litres. The system cools & the pressure gauge now reads 0.9 bar as the system now has 116 litres of water in it.

Now this continues every time we heat the system until let's say the gauge reads 0.25 bar. The system volume is now 100 litres. If we opened the prv nothing may come out due to air locking like holding a glad upside down but if we opened the top of the system it would empty. The exact same as emptying a radiator. We need to open the bleed screw to empty it.

Mods: can you add a "Donate" button beside my name. I should be paid for this :-)

Shane, water is virtually incompressible, you will not be able to put 120 litres into a 100 litre system without bursting something.

Back to the can analogy - as you say regardless of the diameter the pressure at the bottom of a 1 meter high container will be 0.1 bar. But it is still zero at the top. If you fill the can, seal it, and heat it up it will quickly burst as the water tries to expand but has nowhere to go. If you fill it with hot water, then seal it, and allow it to cool, the internal pressure will drop below atmospheric, and the can will tend to be crushed slightly. If it is strong enough, it will resist the greater external pressure, and maintain a slight internal vacuum (or negative pressure if you like) if not, it will deform and crumple visibly.

shane0007

Pete67 wrote: »

Shane, water is virtually incompressible, you will not be able to put 120 litres into a 100 litre system without bursting something.

Back to the can analogy - as you say regardless of the diameter the pressure at the bottom of a 1 meter high container will be 0.1 bar. But it is still zero at the top. If you fill the can, seal it, and heat it up it will quickly burst as the water tries to expand but has nowhere to go. If you fill it with hot water, then seal it, and allow it to cool, the internal pressure will drop below atmospheric, and the can will tend to be crushed slightly. If it is strong enough, it will resist the greater external pressure, and maintain a slight internal vacuum (or negative pressure if you like) if not, it will deform and crumple visibly.

Of course if you fill an open top can with water the pressure at the top is zero as it open to atmosphere. The pressure at the bottom would be 0.1 bar. It was an example to explain. I thought that would be a simple analogy to give but I did not know some people would not understand the simple principle.
The last time I checked there was not hot water coming out of my mains so I would not be filling by system with hot water.
If the mains is at 2 bar & left open, the system pressure will increase to 2 bar also as it is under compression. The amount of compression will be determined by the force upon it this trying to force more water into the system than it can hold thus an increase in pressure.
Commercial systems are competely different to domestic as they are far larger & then surface resistance plus pressure loss over the system must be taken into account. I am referring to pressure loss & not pressure drop as the two are very different.

Wearb

shane0007 wrote: »

No there is positive pressure of 0.1 bar per meter of head height so if the can was 1.0 meter high regardless of diameter, it would have a positive pressure of 0.1 bar.

Think of the system. The volume is 100 litres. It's the bungalow example. So the gauge pressure is 0.1 bar per meter of head height. The gauge reads 0.25 bar. If you unscrew a cap at the top of the system, no water would come out as this pressure would be 0 at this point but the system is full.
Now we raise the pressure to 1.0 bar. In order to do this we most add more water under force. So we bring the pressure to 1.0 bar. We have just added say 20 litres so the pressure now reads 1.0 bar.
But the system now contains 120 litres.
We heat the system & the prv lets out 4 litres. The system cools & the pressure gauge now reads 0.9 bar as the system now has 116 litres of water in it.

Now this continues every time we heat the system until let's say the gauge reads 0.25 bar. The system volume is now 100 litres. If we opened the prv nothing may come out due to air locking like holding a glad upside down but if we opened the top of the system it would empty. The exact same as emptying a radiator. We need to open the bleed screw to empty it.

Mods: can you add a "Donate" button beside my name. I should be paid for this :-)

Remember that we are talking about a system with no (effectively) EV so if it holds 100 litres cold at .1bar at the bottom and 0 at the top (assuming 1 meter head) you cannot get more than 100 litres (ignoring materials expansion) into it no matter how much pressure is applied. So now if we loose 4 litres as the water heats. What happens when the water cools back to the starting temperature.

And yes, and I do think you and a few others do deserve a donate button for all your help on this forum.
:pac::pac::pac::pac:

shane0007

I have not calculated the compression factors of water as that would be just anal. I used the 120litres as an amount that we can imagine. The punt is most likely far less, but the principle remains the same. There is no vacuum & the water is being compressed to raise the pressure.

I was referring to no exapanion vessel.

Re: donation, I do also accept your bank account number, sort code, your mother's maiden name & of course, the name of your first pet.
PM only for security reasons :-)

Wearb

shane0007 wrote: »

I have not calculated the compression factors of water as that would be just anal. I used the 120litres as an amount that we can imagine. The punt is most likely far less, but the principle remains the same. There is no vacuum & the water is being compressed to raise the pressure.

I was referring to no exapanion vessel.

Re: donation, I do also accept your bak account number, sort code, your mother's maiden name & of course, the name of your first pet.
PM only for security reasons :-)

OK, if you are still interested, lets take this back to the can. We have a 1 meter high uncapped (or sealed with a 3 bar PRV) can holding 100 litres (brimful) of cold water. We heat the water to just under 100C which will cause (lets say) 4 litres of water to spill out the top. We then cap it and let it cool down. We now have 96 litres trying to fill a 100 litre space. To my way of thinking this can only happen if gasses are released from the water to occupy the space, or the can collapses a little, or a vacuum forms.

To expand this further If you were to accept the above, the system would opperate next start-up without the need for an expansion vessel, because as the temperature rises, the system would reach the state it was in when we replaced the cap at near boiling point.


All this deserves a donation.
A/C 100-212, Sort code 373.15, Mother Catherine Boyle, Dog Gnasher.


Oppps just noticed the PM note.

shane0007

You can't use that example because too many factors are at play. As soon as you apply heat the water will expand. Once it's heated the volume will increase so as soon as you cap it the volume is the expanded volume & not the cold volume & therefore a weak vacuum would be caused. That is not related to your original question.
Secondly you are capping at atmospheric pressure (open to atmosphere) & not capped at 1.0 bar within.

.jacksparrow.

Pete67 wrote: »

Shane, water is virtually incompressible, you will not be able to put 120 litres into a 100 litre system without bursting something.

Back to the can analogy - as you say regardless of the diameter the pressure at the bottom of a 1 meter high container will be 0.1 bar. But it is still zero at the top. If you fill the can, seal it, and heat it up it will quickly burst as the water tries to expand but has nowhere to go. If you fill it with hot water, then seal it, and allow it to cool, the internal pressure will drop below atmospheric, and the can will tend to be crushed slightly. If it is strong enough, it will resist the greater external pressure, and maintain a slight internal vacuum (or negative pressure if you like) if not, it will deform and crumple visibly.

That's not really true, have you never seen a hydraulic test been carried out on a boiler?

Wearb

shane0007 wrote: »

You can't use that example because too many factors are at play. As soon as you apply heat the water will expand. Once it's heated the volume will increase so as soon as you cap it the volume is the expanded volume & not the cold volume & therefore a weak vacuum would be caused. That is not related to your original question.
Secondly you are capping at atmospheric pressure (open to atmosphere) & not capped at 1.0 bar within.

I don't see why both examples a not comparable. The materials are the only variables and we can easily assume that the can (for the sake of this discussion we can make it stronger) will react in the same way as the heating system, or boiler in particular.

We seem to be on two different tracks on this one and not understanding each other. At least I am not understanding everything you say. Perhaps I have an erroneous view of the behaviour of water under those different conditions. Must get an E-Book on it for to have on those shopping trips with "er indoors".

Wearb

.jacksparrow. wrote: »

That's not really true, have you never seen a hydraulic test been carried out on a boiler?

I assume the water in it is being pressurised and not compressed. I can't see how more water can be gotten in to it than was in there originally at the start of the test, assuming it was full to start with. Only way for more water would be for the boiler to expand before bursting.

Pete67

.jacksparrow. wrote: »

That's not really true, have you never seen a hydraulic test been carried out on a boiler?

I have, many times. Most recently a 5 MW gas fired LPHW boiler. It takes very little water to raise the pressure to test pressure, certainly orders of magnitude less than 20% of the system volume. And some of this water goes to fill the volume increase which arises as the boiler tubes expand slightly under the test pressure. Water is virtually incompressible - I can calculate the reduction in volume per cubic meter at any pressure you want - it is negligible.

Of course, if the expansion vessels are online during the pressure test, then considerably more water is required, as you have to compress the air above the bladders to get to the test pressure.

Anyway, all this is getting away from the the original question - domestic sealed heating systems can end up in negative pressure (vacuum) if the expansion vessel fails and water is discharged via the PRV. Think about it - how else could air be drawn INTO the system?

shane0007

Pete67 wrote: »

I have, many times. Most recently a 5 MW gas fired LPHW boiler. It takes very little water to raise the pressure to test pressure, certainly orders of magnitude less than 20% of the system volume. And some of this water goes to fill the volume increase which arises as the boiler tubes expand slightly under the test pressure. Water is virtually incompressible - I can calculate the reduction in volume per cubic meter at any pressure you want - it is negligible.

Of course, if the expansion vessels are online during the pressure test, then considerably more water is required, as you have to compress the air above the bladders to get to the test pressure.

Anyway, all this is getting away from the the original question - domestic sealed heating systems can end up in negative pressure (vacuum) if the expansion vessel fails and water is discharged via the PRV. Think about it - how else could air be drawn INTO the system?

As I outlined clearly before, I used the 20 litres as a more understandable volume. The compression factor would be more like 4 litres on a 100 litre system.

Show me a domestic system that the pressure is in vacuum, in that it will suck in a quantifiable amount upon opening.

Also hydraulic tests are hydraulic! There is no heat involved. I have had returned boilers to the manufacturer that leaked only when they were hot. When returned & tested by the manufacturer, they passed the hydraulic test as the heat was expanding the metal & opening the crack line which sealed when contracted.

.jacksparrow.

Pete67 wrote: »

I have, many times. Most recently a 5 MW gas fired LPHW boiler. It takes very little water to raise the pressure to test pressure, certainly orders of magnitude less than 20% of the system volume. And some of this water goes to fill the volume increase which arises as the boiler tubes expand slightly under the test pressure. Water is virtually incompressible - I can calculate the reduction in volume per cubic meter at any pressure you want - it is negligible.

Of course, if the expansion vessels are online during the pressure test, then considerably more water is required, as you have to compress the air above the bladders to get to the test pressure.

Anyway, all this is getting away from the the original question - domestic sealed heating systems can end up in negative pressure (vacuum) if the expansion vessel fails and water is discharged via the PRV. Think about it - how else could air be drawn INTO the system?

Ah right sorry I misunderstood what you were saying.

Pete67

OK, the compression factor for water is about 1% at 150 bar. So a 100 liter volume of water pressurized to 150 bar would end up at 99 litres. The same 100l pressurized to 1 bar would only would compress by 1/150 litre or 6.7ml so would end up at 99.993 liters.

This is an interesting discussion.

Wearb

Pete67 wrote: »

OK, the compression factor for water is about 1% at 150 bar. So a 100 liter volume of water pressurized to 150 bar would end up at 99 litres. The same 100l pressurized to 1 bar would only would compress by 1/150 litre or 6.7ml so would end up at 99.993 liters.

This is an interesting discussion.

Are there gasses forced out from between the water molecules at high pressure or what allows the compression. Maybe you shouldn't answer that yet as it may bring this discussion off on another tangent.

It is fascinating. Great forum. Thanks also for your input.

shane0007

Pete67 wrote: »

OK, the compression factor for water is about 1% at 150 bar. So a 100 liter volume of water pressurized to 150 bar would end up at 99 litres. The same 100l pressurized to 1 bar would only would compress by 1/150 litre or 6.7ml so would end up at 99.993 liters.

This is an interesting discussion.

I agree. The compression is really irrelevant though. It's the principle that is the factor.

shane0007

Wearb wrote: »

It is fascinating. Great forum. Thanks also for your input.

These are the type of threads I enjoy most. It's where I learn most when everything is thrown out there & bashed from all angles.

It's now even better since I now have an income generated from it. Thanks for the details.