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CV, flow effeciency etc.

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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CV, flow effeciency etc.

Unread postAuthor: POLAND_SPUD » Sun Feb 03, 2008 12:57 pm

Is there any page that has a list of valves' flow coef. ?
what do you think what is the flow coef of 1/2" QEV ?
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Unread postAuthor: Ragnarok » Sun Feb 03, 2008 1:04 pm

There was a nice equation I remember occasionally using:

Cv = K * D<sup>2</sup>

Where K is:
- 1 for a sprinkler or ball valve
- 2 for a piston of QEV
- 3 for a burst disc.

D is the seat diameter in centimetres. It's not 100% precise, but it's easy to use/remember, and reasonably accurate.
That means a 1/2" QEV has a Cv of around 3.4
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Unread postAuthor: POLAND_SPUD » Sun Feb 03, 2008 5:11 pm

thx Ragnarok

and how it is really calculated ? manufacturers ussually provide other specs like - "flow 1000 liters/minute @ 10 bar.... " can it be used to calculate effeciency ?
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Unread postAuthor: Ragnarok » Sun Feb 03, 2008 5:42 pm

POLAND_SPUD wrote:and how it is really calculated ? manufacturers ussually provide other specs like - "flow 1000 liters/minute @ 10 bar.... " can it be used to calculate effeciency ?

You can convert between flow coefficient, flow rate, and efficiency quite easily if you know the constants for the right gas at the right temperature.

In general... see the number K in the equation I gave above. If you multiply that by 20, that's roughly the percentage efficiency of the valve.
You can work backwards from a Cv to a percentage if you know the seat diameter.

Converting between flow rate and efficiency is harder. It involves dividing it by the difference of the squares of the absolute pressures on each side of the valve, then dividing by the area of the valve seat, and multiplying by a coefficient to correct the units.
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Unread postAuthor: psycix » Sun Feb 03, 2008 6:24 pm

POLAND_SPUD wrote:thx Ragnarok

and how it is really calculated ? manufacturers ussually provide other specs like - "flow 1000 liters/minute @ 10 bar.... " can it be used to calculate effeciency ?

I think they diddnt calculate the 1000 liters/minute @ 10 bar, but measured it.
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Unread postAuthor: Ragnarok » Sun Feb 03, 2008 6:41 pm

psycix wrote:I think they diddnt calculate the 1000 liters/minute @ 10 bar, but measured it.

Yes, that's normal, but what we want is to calculate from that figure to get to a value that we can use in GGDT.
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Unread postAuthor: POLAND_SPUD » Sun Feb 03, 2008 7:17 pm

hmmmm I am suprised that there is no flow coef/CV calculator available...
or maybe there is ....?
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Unread postAuthor: psycix » Mon Feb 04, 2008 5:18 am

Ragnarok wrote:
psycix wrote:I think they diddnt calculate the 1000 liters/minute @ 10 bar, but measured it.

Yes, that's normal, but what we want is to calculate from that figure to get to a value that we can use in GGDT.


Well maybe its possible to measure the flow of your custom valve in the same way manufacturers do.
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Unread postAuthor: Ragnarok » Mon Feb 04, 2008 6:20 am

psycix wrote:Well maybe its possible to measure the flow of your custom valve in the same way manufacturers do.

Of course, but only if you want to spend large amounts of money on it. It needs pretty specialised equipment.

Frankly, it would be cheaper and more accurate to just get a chrony, and run some ammo across it. If you really want, GGDT does have a feature that means you can use actual chrony readings and work backwards to get a Cv.
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