Calculating pre-charge in CW expansion vessel..

[Masood]

Anybody have a good way to calculate this please? Cold water expansion vessel on an unvented. Vessel is not ruptured, PRV is OK but every now and again it drips a bit. I suspect the vessel pressure is too high for the system...

 

[Stigster]

The vessel should be charged to the same pressure as the mains static pressure or to the pressure reducing valve setting (usually 3 bar), whichever is less.

 

[Masood]

to the pressure relief valve setting (usually 3 bar),

I don't think this is correct. Otherwise the PRV would open with any expansion....

 

[Stigster]

I spotted that and edited it as soon as I posted. Was hoping to change it before anyone saw it!

 

[chris watkins]

Charge it to 0.1 or 0.2 bar greater than the Pressure Reducing Valve, Mas

You can't use the static pressure as it fluctuates you could set it up for a 10M head to find that at night when the Economy 7 is on & water is heating the mains is at 20 or 30Metre.

Think about it, you want the bladder held flat with the air pressure when cold, only allowing water in as the pressure in the system starts to rise as it heats, thereby creating more volume in the system when hot that when cold = controlling pressure rise.

 

[Masood]

Charge it to 0.1 or 0.2 bar greater than the Pressure Reducing Valve, Mas

You can't use the static pressure as it fluctuates you could set it up for a 10M head to find that at night when the Economy 7 is on & water is heating the mains is at 20 or 30Metre.

Think about it, you want the bladder held flat with the air pressure when cold, only allowing water in as the pressure in the system starts to rise as it heats, thereby creating more volume in the system when hot that when cold = controlling pressure rise.

Thanks Chris! I had the figure of 0.2 floating around in the back of my head but I was getting mixed up with calculating expansion vessel size and got a mental block!

 

[Dotty]

I set them for system design pressure + 10%.

Same thing as Chris really!

 

[Stigster]

I've found it depends where you look it seems. I've seen to charge it to match incoming pressure, which does seem wrong as pointed out by Chris because the pressure fluctuates anyway. Also to charge it to .20 bar over the reducing valve pressure.

Reliance Water Controls says to charge the vessel to .20 bar under the reducing valve pressure.

So my conclusion is follow the manufacturers instructions (as always) or "about 3 bar!"



Tribune says to match pressure reducing valve in the manual I read (usually 3 bar),