Mini Expansion Vessel/Arrestor Pressure Setting

[kevkdg]

Hi,

I have fitted an Altecnic mini expansion vessel/arrestor on my mains cold supply feed to the combi boiler (we have a water meter fitted).

Prior to fitting:
Static Pressure (no flow conditions): ~5 bar
Flow Pressure (one cold tap open): ~4 bar
Temporary spike when shutting tap quickly: ~7 bar

This clip from the company on youtube suggests setting the vessel pressure to be slightly higher than the system pressure (at 1 min 23 seconds into the below video). So, I've set it to just above 5 bar taking static pressure under no flow conditions to be "system pressure".

How to Remove Water Hammer? - YouTube

I've then re-tested with my gauge and am no longer getting temporary spikes when suddenly shutting off the tap, it just goes straight back to approx 5 bar very quickly.

However, the pipe banging (which was present before) has got louder (it is in the same location, so assume the pipes may not be securely fixed). This occurs when you next open a tap (as opposed to when closing a tap). I'm guessing it has something to do with the built up pressure suddenly being released, BUT I thought the mini expansion vessel was there to reduce pipe banging.

I did consider lowering the expansion vessel pressure, but have followed the manufacturers guidance, thinking that if I lowered it to less than the static (no flow) water pressure, the water pressure would overcome the air pressure and fill the vessel negating its effectiveness.

Any ideas greatly welcome as I'm worried I've not set the pressure correctly!

 

[Johntheo5]

I would suggest setting the vessel pressure to 3 bar, at a dynamic pressure of 4 bar the air end volume will be 80% of total volume, at a static pressure of 5 bar it will be 67% of the total vol and at 7 bar the air end volume will still be 50% of the total volume, no fear of filling the E.vessel with water, the lower charging pressure may help with the banging, give it a go.

 

[kevkdg]

@Johntheo5 - Thanks, I have let a little air out, now reads 58 psi (just under 4 bar), probably lost a little more air when removing the gauge from the schrader valve. Pipe banging completely eliminated now and water pressure behaving exactly the same on the water pressure gauge when shutting taps off quickly as it was with the e.vessel inflated at around 5 bar. So all seems good, not sure why the manufacturers state inflate to static pressure plus about half a bar!

How did you calculate the air to water ratios above?

 

[Johntheo5]

Allways work from the air end, you allways use absolute pressures when dealing with gases. absolute pressure = gauge pressure (bar) +1.
and volume is inversely proportional to pressure, V2=(P1xV1)/P2. Pressure P1 say 3 Bar.G = 4 Bar.A (3+1). Pressure P2 say 4 Bar.G = 5 Bar.A (4+1).
V2=(P1xV1)/P2, So V2= (4xV1)/5, = 4/5V1 or 80% of V1 and so on.