What pressure to charge HW expansion vessel?

[mc0911]

Afternoon,
this is purely a query and learning benefit. I am not a G3 qualified engineer, however am just using the down time during COVID to take on some extra learning. Would anyone know if there is calculation or something to determine what pressure and size the vessel should be on an unvented cyclinder?
thanks

 

[SJB060685]

Expansion vessel size and pre charge depends on system volume and initial pressure. When I did my G3 many moons ago Ariston told me to charge vessel to .5 bar under system static pressure, also most unvented kits comes with an already properly sized vessel. I have an excel spreadsheet on my phone which allows me to input certain parameters to calculate required vessel size and resulting figures ie. final pressure, water volume and air volume etc but I have no idea how to share with you. JohnG will be able to help with sharing that with you
[automerge]1585868552[/automerge]
@John.g

 

[king of pipes]

I like to fit flamco products if possible use their website tables for your calculations it will vary for different situations pressure, height, and temperature will all have a factor . Kop

 

[John.g]

Afternoon,
this is purely a query and learning benefit. I am not a G3 qualified engineer, however am just using the down time during COVID to take on some extra learning. Would anyone know if there is calculation or something to determine what pressure and size the vessel should be on an unvented cyclinder?
thanks

You will get very involved calculations on the various websites, Flamco, Caleffi, etc.
I made up my own one decades ago which is, I think, reasonably accurate and certainly good enough to confirm that the installed EV is not undersized. I havn't taken static head differences between the EV and the pressure vessel into account but if you add or subtract the head (depending on EV/pressure vessel height differences) in bar to the final pressure you will be reasonably correct. You can do your own calcs on this spreadsheet to see the effect different parameters have on the final pressure.
All these calculations are based on two very simple facts, water expands when it gets hot and in the case of a gas (Air), Pressure (absolute) is inversely proportional to Volume. (absolute pressure is gauge pressure+1 (bar).)

 

[SJB060685]

Another helpful chart John.g
May not be 100% accurate but good enough

 

[mc0911]

This has been really helpful, thanks and there is a lot to digest there. One question and may be a simple one, but when you say 'pre pressure'. What is this referring to?

 

[SJB060685]

This has been really helpful, thanks and there is a lot to digest there. One question and may be a simple one, but when you say 'pre pressure'. What is this referring to?

Pre pressure of a vessel is the charge of bladder when cold, not in contact with water, this pressure obviously increases as the water expands.
Pre pressure of a central heating system or unvented cylinder is again the static pressure before heating takes place

 

[John.g]

It might help to think of the pre (charge) pressure as a adjustable spring with a piston (the diaphragm) between it and the water end, when cold water is admitted then depending on the pre charge pressure and the cold water pressure; the piston will move towards the air end compressing this (air) spring until the pressure at both ends are the same, if the pre charge pressure and the water pressure are the same then the piston will stay up against the water end, As the water in the HWC starts expanding due to being heated, it will start entering the EV and will keep pushing back the piston until all the expanded water is contained within the EV. If you look at the first set of calcs you will see that with a pre charge pressure of 2.7 bar and a cold water (filling) pressure of 3 bar will give a final pressure after expansion of 3.79 bar, if for some reason the spring tension started decreasing (ie air leak at the schrader valve) then the final pressure will start rising, if the pre charge pressure fell to 1 bar then the final pressure would be 4.76 bar, if it fell to 0.23 bar then the final pressure would be practically 7 bar and would lift the PRV. Excessive pre charge pressure can have the same effect and that's why it is very important to check the pre pressure in the EV before fitting it despite what's written on the label and ideally it should also be checked at service time.
EV's are (or should be) pre charged with nitrogen as air (which is permeable) can leak and loose its pressure through the diaphragm.

 

[mc0911]

Thanks for the detailed reply and is making sense now. This will put in a better position getting my head around this now before doing hopefully doing my G3 In the future.