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I'm sorry for this semi-noobish topic, but does anyone know (approximately) the flow coefficient of a .5" burst disc valve? I have searched multiple times and haven't found anything. Also, does anyone know of a material for a burst disc that will rupture at around 130-150 psi? Sorry if this is a noob topic.

I read somewhere that a 1 inch union has twice the flow of a 2 inch sprinkler.

For a burst disk, I usually enter a flow coefficient of 50 if it is opened by raising the pressure until it blows, and 55 if it is a triggered design like the SCTBDC. I don't know if that is right or not. Flow coefficient is independent of valve size. It is more closely related to valve type.
A .5" burst disk and a 4" burst disk would have the same flow coefficient.

For a 0.5" burst disk valve, 8 layers of alu. foil *should* burst at around 150 psi, but I haven't tested that. Why do you even need a valve that small?

jon_89 wrote:I read somewhere that a 1 inch union has twice the flow of a 2 inch sprinkler.

And.........

DYI wrote:For a burst disk, I usually enter a flow coefficient of 50 if it is opened by raising the pressure until it blows, and 55 if it is a triggered design like the SCTBDC. I don't know if that is right or not. Flow coefficient is independent of valve size. It is more closely related to valve type.
A .5" burst disk and a 4" burst disk would have the same flow coefficient.

For a 0.5" burst disk valve, 8 layers of alu. foil *should* burst at around 150 psi, but I haven't tested that. Why do you even need a valve that small?

Thanks a lot, I'm planning on building a burst disc airsoft "cannon".

EDIT: I just GGDT'd the cannon, and it said 1213 fps and 6 foot-pounds of muzzle energy with a .12 gram airsoft pellet, it probably won't be supersonic, but I think it will be close.

You may want to consult someone with more knowledge than myself before going and using those figures, especially with the low mass airsoft rounds. The performance could change dramatically with a flow coefficient even 5 off of what it really is.

I was going to build a handheld gun to shoot airsoft rounds at >1km/s, but I decided to use 1/4" tungsten carbide ball bearings @0.6km/s instead (wouldn't be very good for shooting at people though, unless you're wearing full ballistics armour). The low mass projectiles won't go very far, and accuracy will be sh*t.

Also, I assume you won't be exceeding Mach 1 by very much. At some point, the BBs will drop below the SOS, and their shockwave will catch up to them, making accuracy even worse.

DYI wrote:You may want to consult someone with more knowledge than myself before going and using those figures, especially with the low mass airsoft rounds. The performance could change dramatically with a flow coefficient even 5 off of what it really is.

I was going to build a handheld gun to shoot airsoft rounds at >1km/s, but I decided to use 1/4" tungsten carbide ball bearings @0.6km/s instead (wouldn't be very good for shooting at people though, unless you're wearing full ballistics armour). The low mass projectiles won't go very far, and accuracy will be sh*t.

Also, I assume you won't be exceeding Mach 1 by very much. At some point, the BBs will drop below the SOS, and their shockwave will catch up to them, making accuracy even worse.

The gun is just supposed to be a fun project, I'm just building it out of spare parts, all I have to buy is the union, so as long as it works I'm happy. I was just curious about potential velocities.

I built one of those last weekend... Except it had a 1" burst disk, a 3/4" barrel, and operated at 400 psi. Haven't got a video yet because I've been working a lot.

Well assuming that is the correct flow coefficient, that would be dangerous. Mach 1 = 1116.44 fps approximately (rounded). Most average airsoft guns are 280 fps or less or slightly more (i had a airsoft shoutgun that was 280 fps). So that would be probably skin penetrating. But now you know

I would say... a Cv value between 4 and 5, if I remember my old formula right. Haven't used it in ages.

LikimysCrotchus5 wrote:Well assuming that is the correct flow coefficient, that would be dangerous. Mach 1 = 1116.44 fps approximately (rounded). Most average airsoft guns are 280 fps or less or slightly more (i had a airsoft shoutgun that was 280 fps). So that would be probably skin penetrating. But now you know

I WILL NOT be shooting this at any living thing, this gun is strictly for target shooting (although accuracy probably won't be very great, even with a 60" barrel)

Someone knowledgable back on Spudtech made a topic about finding that value for GGDT. its (d^2)(some coefficient between 1-11, depending on the flow of the valve). A burst disk has very good flow, thus the value for GGDT would be 11.

So you're saying that I should just enter 11 as the flow coefficient for GGDT?

Wait a minute. 11??

The GGDT that D_Hall created and posted for his "Pipe Dream" project used a flow coefficient of 50 for a burst disk valve. Considering that Hall created the program, and the rather large difference between 50 and 11, I think someone has some faulty information here.

I think our problem here are units. 50 what Cv, Kv, % ??

If you are using Cv or Kv then it is going to vary with size by a large amount. % allows us to talk far more generally about the flow of a certain kind of valve. For a burst disk it will be probably 50-60% flow depending on the contours of the gun and valve.