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Hammer valve in GGDT

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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Hammer valve in GGDT

Unread postAuthor: john bunsenburner » Thu Jan 22, 2009 1:11 pm

Title says it all i guess. What is the flowcoefficiancy of a hammer valve and what exactly is the seat diameter, ow can i mesure/ calculate both. I made the valve my self, i will upload some pictures of it if needed, i will start a new threa don it too seeing many people are askign about hammer valves. Thank you in advance for your help.
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Unread postAuthor: daberno123 » Thu Jan 22, 2009 8:55 pm

The seat diameter refers to the ID of the piece of pipe that the piston is "seated" on. Its the one that leads to the barrel.

As for the flow coefficient of the valve GGDT has a valve efficiency calculator under the Tools menu in it that allows you to enter a string of pressure and measured velocities of the valve and it calculates the efficiency of it.

However, this method requires you to build the gun, fire it, and measure the velocity with a chronograph. This isn't helpful if your only designing a gun because you can't get velocity figures from a nonexistant gun or if you want accurate velocity figures from GGDT for your already built gun because you can already get them with your chronograph. :wink:
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Unread postAuthor: john bunsenburner » Fri Jan 23, 2009 1:28 am

Are you telling me that no one really knows the valve co efficiency of a hammer valve and that the only way to find out totally defeats the point of using GGDT... Maybe some one tried before me... Hopefully!

P.S.if any one is intrested the seat diameter is 8mm, so atleas ti have one thing down. Thank you for your help Daberno, it helped me alot. Pictures should be here sunday(its friday mornign now), Ill be snowboarding until then, hope to have some answers when i get back.
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Unread postAuthor: btrettel » Fri Jan 23, 2009 8:22 am

Any simulation isn't magic. To get accurate outputs you'll need accurate inputs. Until then anything you get is just a guess.

I believe the point of the flow efficiency (the percentage coefficient) in GGDT is that it'll scale well with different sized valves. Typical values for QEVs, piston valves, burst disks, etc., can be found with a little bit of search. Hammer valves don't seem to be very common however, so your guess is as good as mine. My quick searches haven't turned anything up but I might be using the wrong keywords. Try a typical value for a piston valve as a guess.

If you don't want to build your desired gun yet you could build a smaller gun just for the purpose of getting some test numbers from.

Edit: The Cv formula on this page might be helpful too: http://www.spudfiles.com/spud_wiki/index.php?title=GGDT
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