Air Release Valves for potable water 2

[Baram]

Hello All,

I have potable water application on duty, pipe size 32 inches and this line 25 years old from ductile iron material.
Pipeline depends on gravity and there are 8 miles distances with negative elevations, the line intake from reservoir to feed the system and house connection.
Water temprature approx 40 C degree.
I face problems with this lines and submains as from time to time pipes breaks and so TEES and Elbows leaks as there are fluctuating in water demands, we control pressure by pressure reducing valves by 5 bars fixed downstream pressure.

My question : I have noticed that there are double orifices air release valves size 2 inches installed vertically on each peak point on the 32 inches line, I wander if this small sized air valved eleminate the air or I should install bigger size?
I also think that one of my breakage problem from these air valves, so could anybody help how can I calculate and select the right size air valves for my on duty application?

thanks,
Baram

 

[gerhardl]

There is a number of companies producing and selling air release valves and vacuum breakers that will be happy to help you with calculations.

All are however depending on a 'guesstimate' on how much air is needed out (to vent) or in (as vacuum breaker to avoid collapse). It is quite easy (especiially for vacuum breaking purposes) to guess too much, causing excessive and unessesary installations, or guessing too small amounts, causing problems.

A supplier can perhaps say something about what is 'commonly used', but actually the owner sits with the responsibillity of giving out dimensioning data (amounts).

A very rough approximation, helping to estimate could be:

1. Remember that water volume, in or out, equals air valume.
2. Air out, case a, when filling and venting valve is dry: to be calculated sufficient to be let out in same amount as filling with water from emptied pipeline, usually (practise varies), say by 0,5 to 1,5 m main pipeline per second. Eg. normal max flowing conditions.
3. Air out when pipeline pressurized and filled with water: use a combined airvalve, best types will release sufficiently (and separately) smaller amounts for this, by separately, integrated float and valves arrangement in the main air release valves.
4. Vacuum breakers: combined types is found, and mostly sufficient if sufficient for outlet (CAUTION!). Control by guessing leakage by broken pipeline. Without complicated pressure mathematics simply assume 30 - 40m/s (or other, 'reasonable' equal to gas speed in a normal air or gas pipeline), and calculate the necessary opening. Control going the other way: actual speed given by proposed or excisting valve.
5. Producers will give you 'capacities' with varying backgrounds from producer to producer, with different 'speeds'. Some will go to absolute limit (speed of sound), some considerably lower. Remember to compare and calculate for real net, actual smallest area, not normal.(Your 2"??)
6. Some producers will supply valves 'slamming' when air speed out is to high, others 'non slam'- check actual speed needed!

At a guess I would say that you probably would need larger valves, especially for vacuum breaking, if not already installed.

Any problems caused by excessive filling speed airvalve slamming or waterhammer?

Good luck!

 

[gerhardl]

Thank you for the star!

Additional comment: Please note that I think I remember filling speed (my point 2) to be normally set at 0,3 m/s - but as said, this could vary, but caution against speed that will give water hammer!

 

[stanier]

Suggest you go to

http://www.ventomat.com

and read their technical reports. You can also download their software and you can model your system. The output will be conservative in terms of the number of air valves required. After all these companies sell air valves. However i have found ventomat representatives helpful in providing guidance as to the practical needs of such pipelines.

Care should be taken as some of the legacy type air valves have reduced orifice sizes. e.g. A DN80 may have a DN32 orifice. whereas the Ventomat has a major orifice that is equivalent to the valve body size.

If you are getting line breaks i suggest you engage awaterhammer analyst to model your system for you. There will be mitigation measures you can put in place to stop the breaks and thus improve the pipeline availability.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

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

 

[hsbcn]

Although it requires registration in order to download, the following air-valve sizing guide would be really useful for your valve calculations:

http://www.wateronline.com/doc.mvc/Sizing-Manual-Pressure-Air-Release-Valve-0001

My focus is on valve stockists in the us and uk who supply duplex and super duplex valves, and inconel and hastelloy valves, plus titanium, alloy 20 and 904L valves.