Calorifier

[Guest TerryL]

This is complete nonsense. Copper pressure vessels do not explode under hydraulic testing, they split and leak. That's why boilers are tested this way. The danger that is faced here is that the calorifier will split leaking water at about 60 C. The risk is quite low (due to PRV, low pressure, low temperature, location of calorifier). Surely there are better things to focus your efforts onto.

 

Richard

You wrongly assume all calorifiers are copper, you wrongly assume all calorifiers run at 60 degrees and that temperature is not dangerous, you wrongly assume that copper cylinders will burst or explode in a controlled and non violent manner, you ignore the potential of a violent steam explosion especially with a faulty immersion heater thermostat, you wrongly assume that sufficient safety controls are always in place. Just because luckily it doesn't happen often does not mean you should drop your guard and accept or advocate low standards that will increase risks.

 

Open your eyes and pray tell me if the risks are so low as you say then why in domestic premises unvented cylinder safety is such an important issue that mandatory safety controls and specialist installations have to be demanded? My efforts seem to be wasted on some ignorant morons who try to convince others that safety doesn't matter! You don't have any regulatory control or guidance on these installations on boats so where are you going to get good information from if someone like me doesn't draw your attention to higher standards? There are differences with a pumped system that can make it a more intergrated unit but essentially the safety aspects are the same.

 

http://www.modus-uk.org/installer.php?ID=0000000004

[dor]

My efforts seem to be wasted

 

I can think of a very easy way in which you will no longer have to waste any effort. I'm sure you don't need me to explain.

[Guest TerryL]

I can think of a very easy way in which you will no longer have to waste any effort. I'm sure you don't need me to explain.

Fair enough, if the cap fits it's all I need to hear!

Edited November 10, 2008 by TerryL

[smileypete]

One of these?

 

http://www.screwfix.com/prods/52162/Plumbi...ion-Vessel-8Ltr

 

 

Yes, though again it's intended for non-potable water supply.

 

As an aside I came across fairly good expansion vessel calculator on the web:

 

http://www.lecad.uni-lj.si/~leon/software/...sel-system.html

 

cheers,

Pete.

[chris w]

Oh yes it may need to be - it all depends on the pressure range!

 

Here we go with Boyle's Law - again.....

 

To take your example - an 8 litre vessel, 30 psi (2 barg, 3bara) pump cut out pressure and 45 psi (3 barg, 4bara) TRV lift pressure....

 

When the pump cuts out and the water is at its low temperature, the PV product for the air in the accumulator is 24. The TRV will lift at 4 bara, at which point the volume of air will be 6 litres - giving a "working" volume of only 2 litres from your 8 litre vessel.

 

Chris G

 

I wasn't talking about an accumulator, I was talking solely about an EV sitting behind an NRV. The most that 60 litres of calorifier water is going to expand, even from zero C to 100C, is ~4% (ie: 2.5 litres) so if I have the EV charged to just below the PRV (say 35psi at the EV) there will always be enough volume in the EV to accomodate the expansion.

 

Chris

Edited November 10, 2008 by chris w

[chris w]

I look forward to Gibbo's comments re (i) and (ii). You didn't understand (iii) the first time I gave the answer so what makes you think you'll understand it a second time? Quit while you're ahead! You're an intellectual bore, pity it doesn't make you clever or help you understand practical and technical issues.

 

Well Gibbo does NOT agree with you. He and I do talk off-line and despite any pseudo forum bickering occasionally, in reality, we get on very well. I know what his answer is and he'll be along to answer you in his own time. I already have his answer in black and white but, out of respect for him, I'll allow him to answer personally when he's ready. I think he's just letting you dig a bigger and deeper hole

 

I will reveal one line only from his email to me "Disconnect the battery.. bang goes the alternator and lots of other stuff".

[Batavia]

I wasn't talking about an accumulator, I was talking solely about an EV sitting behind an NRV. The most that 60 litres of calorifier water is going to expand, even from zero C to 100C, is ~4% (ie: 2.5 litres) so if I have the EV charged to just below the PRV (say 35psi at the EV) there will always be enough volume in the EV to accomodate the expansion.

 

Chris

Apologies for using the terms "accumulator" and "expansion vessel" interchangeably - to me they are the same thing.

 

You argument is not correct.

 

If you have the pre-charge set to just below the TRV relief pressure, the pressure in the (relatively inelastic) calorifier system will zonk up to the pre-charge pressure as soon as the temperature starts to rise (there is nowhere for the expanding liquid to go) and you can then only compress the air in the accumulator between the pre-charge pressure and the relief set pressure before the TRV lifts, leaving you worse off than had you started to accomodate the expansion as soon as the temperature started to rise by having a lower pre-charge pressure.

 

I lost the will to live with this topic a while ago, briefly regained it, but it is now fading fast............

 

Chris G

[chris w]

Apologies for using the terms "accumulator" and "expansion vessel" interchangeably - to me they are the same thing.

 

You argument is not correct.

 

If you have the pre-charge set to just below the TRV relief pressure, the pressure in the (relatively inelastic) calorifier system will zonk up to the pre-charge pressure as soon as the temperature starts to rise (there is nowhere for the expanding liquid to go) and you can then only compress the air in the accumulator between the pre-charge pressure and the relief set pressure before the TRV lifts, leaving you worse off than had you started to accomodate the expansion as soon as the temperature started to rise by having a lower pre-charge pressure.

 

I lost the will to live with this topic a while ago, briefly regained it, but it is now fading fast............

 

Chris G

But the absolute maximum volume of the expanding water is still only 2.5 litres so an 8 litre EV will always accommodate it. Because the EV's pre-charge pressure is higher than the water pump's cut-out pressure, the water pump alone cannot compress the diaphragm.

 

Chris

Edited November 10, 2008 by chris w

[RLWP]

You wrongly assume all calorifiers are copper, you wrongly assume all calorifiers run at 60 degrees and that temperature is not dangerous, you wrongly assume that copper cylinders will burst or explode in a controlled and non violent manner, you ignore the potential of a violent steam explosion especially with a faulty immersion heater thermostat, you wrongly assume that sufficient safety controls are always in place. Just because luckily it doesn't happen often does not mean you should drop your guard and accept or advocate low standards that will increase risks.

 

Open your eyes and pray tell me if the risks are so low as you say then why in domestic premises unvented cylinder safety is such an important issue that mandatory safety controls and specialist installations have to be demanded? My efforts seem to be wasted on some ignorant morons who try to convince others that safety doesn't matter! You don't have any regulatory control or guidance on these installations on boats so where are you going to get good information from if someone like me doesn't draw your attention to higher standards? There are differences with a pumped system that can make it a more intergrated unit but essentially the safety aspects are the same.

 

http://www.modus-uk.org/installer.php?ID=0000000004

 

Hi Terry,

 

I think it is time you learned how to do risk assessment properly.

 

Yours sincerely, Ignorant Moron Richard.

[chris w]

You've just proved yourself wrong as usual. Don't try difficult things.

 

 

What are you on about and what an odd, nasty and sad person you are, live alone do you?

 

You know what I'm on about, you just don't want to answer. So is your "missus" a "forensic biologist" of 25 years standing as you stated above or is she just a health & safety operative as you stated previously. If she's 110, then maybe she has had both careers. She'd have to be that old to want to go to bed with you - apologies to any 110 year olds who have slept with him.

 

Sorry to disappoint you and to be late on this morning but, if you want to know, she was German and blonde!!!

 

Hey TerryLicense(sic), you stated in another thread that you have been running your own plumbing, heating and electrics business for years. If that's so, how come it was only registered in 2008

[Batavia]

But the absolute maximum volume of the expanding water is still only 2.5 litres so an 8 litre EV will always accommodate it.

 

yes, 2.5 into 8 will go, but what will the pressure be (rhetorical question)? - Much higher than the TRV relief pressure!

 

Because the EV's pre-charge pressure is higher than the water pump's cut-out pressure, the water pump alone cannot compress the diaphragm.

Obviously

 

Chris

 

Chris G

[chris w]

Chris G

 

When I first bought my boat it had no EV and the PRV dribbled all day whilst cruising. I fitted an 8 litre EV on the hot water outlet pipe and, unlike TerryLicense(sic), it has never dribbled since.

 

Chris

Edited November 10, 2008 by chris w

[smileypete]

I wasn't talking about an accumulator, I was talking solely about an EV sitting behind an NRV. The most that 60 litres of calorifier water is going to expand, even from zero C to 100C, is ~4% (ie: 2.5 litres) so if I have the EV charged to just below the PRV (say 35psi at the EV) there will always be enough volume in the EV to accomodate the expansion.

 

Give this calculator a try, Chris:

 

http://www.lecad.uni-lj.si/~leon/software/...sel-system.html

 

cheers,

Pete.

Edited November 10, 2008 by smileypete

[Batavia]

When I first bought my boat it had no EV and the PRV dribbled all day whilst cruising. I fitted an 8 litre EV on the hot water outlet pipe and, unlike TerryLicense(sic), it has never dribbled since.

 

Chris

 

As previously suggested, there are probably other factors at work as well as the undoubted benefit of having an accumulator - in particular the elasticity of the calorifier (still waiting for suggestions as to the wall thickness of these), the NRV passing a little (but not enough on its own to keep the pressure down), etc.

 

And from my trivial example earlier, at 8 litres and a sensible pre-charge perssure, you are not a million miles away from containing all the expansion in the accumulator!

 

 

Chris G

[chris w]

Give this calculator a try, Chris:

 

http://www.lecad.uni-lj.si/~leon/software/...sel-system.html

 

cheers,

Pete.

It's not clear on that page as to whether the expansion vessel is meant to be of the diaphragm-type or the air-type.

 

Chris

[dor]

When I first bought my boat it had no EV and the PRV dribbled all day whilst cruising. I fitted an 8 litre EV on the hot water outlet pipe and, unlike TerryLicense(sic), it has never dribbled since.

 

This was the situation I had with my last boat when I bought it, i.e. 60l calorifier & no expansion vessel. I also fitted an EV, only 5 litres, on the HW outlet which stopped all the dribbling. It was precharged to about 15 psi. It then occured to me that the EV would push hot water into the cold feed when a cold tap was turned on (there was no NRV) so I moved it to the cold side of the system, not far from the pump, and that was where it stayed. I had no further problems with the system.

[chris w]

As previously suggested, there are probably other factors at work as well as the undoubted benefit of having an accumulator - in particular the elasticity of the calorifier (still waiting for suggestions as to the wall thickness of these), the NRV passing a little (but not enough on its own to keep the pressure down), etc.

 

And from my trivial example earlier, at 8 litres and a sensible pre-charge perssure, you are not a million miles away from containing all the expansion in the accumulator!

 

 

Chris G

Chris

 

I do understand what you are trying to show with the Boyle's Law analysis and, on the face of it, your calculation shows that the EV cannot work set at above pump cut-out pressure.

 

However, it does work in practice and works perfectly (and it's also what the manufacturers recommend) which means that while the basic equation is of course correct, its particular application may be incorrect in that we are not carrying out the analysis correctly.

 

The analysis is further exacerbated by the fact that temperature has not been taken into account ie: P₁V₁/T₁ = P₂V₂/T₂ The difference in temperature is from say 288degK (15degC) rising to say 358degK (85degC). This would imply the pressure inside the EV would rise to around 60 psi which would see water p*ssing out of the PRV. This clearly doesn't happen so we are analysing this incorrectly.

 

I will have a think and come back to you.

 

Chris

Edited November 10, 2008 by chris w

[chris w]

I think I've spotted the flaw in the argument......

 

Let's say, for the sake of argument, that the 8 litre EV contains 4 litres of water when the calorifier is cold and is pre-charged to 35psi. If it now absorbs 2 litres of water due to expansion, the pressure of the air in the EV, above the diaphragm, will rise (by Boyle's Law) to 70 psi. With that much I agree.

 

But so what...... Boyle's Law does not apply on the other side of the diaphragm as the calorifier contains a liquid not a gas. The pressure in the calorifier is not now 70psi as well. In fact, the pressure in the calorifier should stay the same, viz: the water pump's cut-out pressure (say 30psi) because any increased pressure due to expansion has been absorbed.

 

So the EV's diaphragm pressure must be ABOVE the water pump's (WP) cut-out pressure or otherwise the WP will fill the EV with water and no room will be left for expansion. But the EV pressure must also be BELOW the PRV pressure or else the expanding water will simply blow the PRV first. Hence with a typical PRV of around 42psi and a WP cut-out pressure of 30psi we set the EV to about 33-35psi.

 

Chris

Edited November 10, 2008 by chris w

[blackrose]

As previously suggested, there are probably other factors at work as well as the undoubted benefit of having an accumulator - in particular the elasticity of the calorifier (still waiting for suggestions as to the wall thickness of these), the NRV passing a little (but not enough on its own to keep the pressure down), etc.

 

I think it was at Crick when a distributor of high quality stainless steel marine calorifiers told me that the lower end calorifiers that most of us use are essentially just cheap household cylinders modified with extra coils. I don't know how true that is or even if it really matters, but I have seen a fairly new copper calorifier that leaked from a seam in the tank. The boat owner removed the tank (which he'd only installed a year earlier), from the boat and we could see that the foam insulation in one area was saturated. After scraping off the foam and refilling the tank with water the leak became evident. I guess there are also examples of calorifiers lasting for decades, but some of them at least are plainly not up to the job.

Edited November 10, 2008 by blackrose

[alan_fincher]

I think it was at Crick when a distributor of high quality stainless steel marine calorifiers told me that the lower end calorifiers that most of us use are essentially just cheap household cylinders modified with extra coils.

That seems pretty unlikely, as few domestic hot water cylinders are made to dimensions as small as those used for many boat calorifiers. Also there not too many horizontal cylinders in houses.

 

No doubt quality can vary enormously, but generally speaking a calorifier may be expected to hold less water, but often heated to higher temperatures or with greater pressures.

 

It's quite likely that some manufacturers make both domestic and marine cylinders, though I'm doubtful it would necessarily be to the same spec.

[Gibbo]

Well Gibbo does NOT agree with you. He and I do talk off-line and despite any pseudo forum bickering occasionally, in reality, we get on very well. I know what his answer is and he'll be along to answer you in his own time. I already have his answer in black and white but, out of respect for him, I'll allow him to answer personally when he's ready. I think he's just letting you dig a bigger and deeper hole

 

I will reveal one line only from his email to me "Disconnect the battery.. bang goes the alternator and lots of other stuff".

 

Chris I'm happy for you to post the text of the full email here.

 

Gibbo

[smileypete]

I think I've spotted the flaw in the argument......

 

Let's say, for the sake of argument, that the 8 litre EV contains 4 litres of water when the calorifier is cold and is pre-charged to 35psi. If it now absorbs 2 litres of water due to expansion, the pressure of the air in the EV, above the diaphragm, will rise (by Boyle's Law) to 70 psi. With that much I agree.

 

But so what...... Boyle's Law does not apply on the other side of the diaphragm as the calorifier contains a liquid not a gas. The pressure in the calorifier is not now 70psi as well. In fact, the pressure in the calorifier should stay the same, viz: the water pump's cut-out pressure (say 30psi) because any increased pressure due to expansion has been absorbed.

 

You're suggesting you can pressurise an expansion vessel to 70psi with water that is at... 30psi???

 

Have a think about this one...

 

Surely it's the air in the expansion vessel pressurising the water in the calorifier, as the air in the expansion vessel tries to resist being compressed to a smaller volume, by the extra water coming into the expansion vessel.

 

cheers,

Pete.

Edited November 10, 2008 by smileypete

[chris w]

But the water's only at 30psi until it expands due to heat. Then once the expansion is taken up by the EV there is now no excess pressure in the calorifier. The pressure is back to where it was.

 

The point is that P₁V₁/T₁ = P₂V₂/T₂ only applies to gases not to liquids so it's like comparing apples to oranges.

 

A 60 litre calorifier will expand by around 2 litres when heated by the engine. Clearly the pressure in that liquid will increase but if an additional volume is available to hold the "excess" liquid then the original liquid pressure will be restored.

 

Chris

[smileypete]

But the water's only at 30psi until it expands due to heat. Then once the expansion is taken up by the EV there is now no excess pressure in the calorifier. The pressure is back to where it was.

 

The air in the expansion vessel is pushing back against the water.

 

As the air is further compressed, it will push back with greater force.

 

 

Think of the air like a spring being compressed,

 

As long as the spring it's being compressed, it will push back against the force compressing it.

 

If you compress the spring further, it will push back with greater force.

 

cheers,

Pete.

[chris w]

The air in the expansion vessel is pushing back against the water.

 

As the air is further compressed, it will push back with greater force.

 

 

Think of the air like a spring being compressed,

 

As long as the spring it's being compressed, it will push back against the force compressing it.

 

If you compress the spring further, it will push back with greater force.

 

cheers,

Pete.

But the increased pressure in the calorifier was due to the (incompressible) liquid's heating and trying to expand. If that expansion is allowed to happen (by providing additional space for it with the EV) then the calorifier's pressure will not increase.

 

Chris

Edited November 10, 2008 by chris w