Combination air valve. how it works?

[akupikree]

Hi guys,

currently im in the middle of selecting ARV for firewater system. I believe at high points a combination air valve shall be used.

can someone explain to me on how this combination air valve works? im confuse with the small orifice. is this small orifice will be used to discharge large amount of air from the beginning? im talking about orifice with diameter of 3/32" (2.38 mm).

Appreciate if you guys can help.

Thanks and regards,

Fikree

 

[gerhardl]

The answer is probably yes, but no one can confirm this before you post detailed information (illustration) of valve innards.

(...and if you find this you will likely also find a description of the working principle, but OK - come on!)( ;-))

 

[cvg]

combination air / vacuum valve

the small orifice operates while the line is pressurized, releasing small pockets of air that collect at the high point. the large orifice opens to allow air into the pipeline during draining and allows air release during line filling

 

[akupikree]

Thank you guys,

Im agree with cvg, i think its called double orifice isnt it?

Ive read about it somewhere but i dont know whether this criteria is a typical for all type of combination air valves.

This is important for me since im doing the transient surge analysis. And at the moment, i use the small orifice as the venting line.

 

[cvg]

if your surge creates a vacuum then the large orifice will allow air into the line and allow the air to release when the pressure wave passes. the small orifice only functions to bleed off small amounts of trapped air

 

[akupikree]

Ok, i need to change the principle to use the large orifice as venting.

The only concern is, large orifice will cause the speed of water rise too fast and probability of surge happens is high. Do you know how to calculate the optimum orifice size during pipe filling?

 

[cvg]

maximum pressure drop is indicated in the valve literature or call your valve sales rep. for startup filling, you can also open fire hydrants to allow air to escape

 

[stanier]

Suggest you examin the technical papers at

http://www.ventomat.com.

The orifice size you mention appears very small.

Many ARV suppliers have copied designs from the 1930s (Glenfield & Kennedy). they have little understanding of how the ARV works and how it should be sized. Ventomat has done a great deal of research into ARV behaviour and provide a proven design.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[cvg]

3/32 is standard size for the small orifice on an APCO double orifice comb air/vac valve (CAV) for 2" through 4" nominal size valves. See attachment in my first post, it gives the standard sizes, pressures and flow rates. Without knowing the size of watermain, flow rate, length of main and pressure, it would be difficult to know if a 2" or 4" valve is too small or not.

 

[akupikree]

i have done my simulation, normal combination air valve will definitely gives high pressure surge in my system. this is due to the fast water speed filling up the pipeline.

i need to get this thing right, do you guys know something about a non-slam vacuum breaker?

 

[stanier]

The Ventomat air valve has multiple orifices and behaves a s a"non slam air valve". Readt he technical papers on their website.

Waterhammer software packages such as Hytran and Impulse both are able to model the Ventomat air release valve.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[MortenA]

In my appinion air valves can often do little good and some damage. Remember that the hose valve/nozzle/hydrant whatever is up there has a much higher capacity for air than for water. This means that the water may see very high velocities as you have notised too. One way around it is to include check valves at the bottom of all risers. The ventomat as mentioned by stanier may also do the trick.

Best regrads

Morten

 

[stanier]

For steep risers Prof. ARD Thorley (Fluid Transients in Pressure Pipelines) illustrates that a pressure control valve and air valves combination located before the check valve at the top of the riser works. I have used such an installation on a gas platform equipped with submersible pumps (1500kW).

On pump start up the riser is empty so the liquid accelerates rapidly. Air is expelled in a controlled manner so that the liquid column doesnt strike the closed check valve and create a high transient pressure. It goes without saying the system needs to be modelled to have any chance of selecting the correct oeprating criteria. It is useful to have the four quadrant & motor torque vs. speed data for the pump for use in the modelling.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/