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Snapvalve explained

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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Snapvalve explained

Unread postAuthor: Brian the brain » Sat Oct 27, 2012 5:48 am

At the request of wyz I decided to repost the inner workings of the Snapvalve.

The design is based on the regular pop-off valve but optimised for piloting purposes.

In a regular pop-off an inlet is held shut by a piston.
Incoming pressure pushes the piston back against the springpressure at a certain pressure and some of the pressure bleeds off.


The snapvalve is designed not to bleed off, but snap open at full force as the piloting pressure is reached.
It's either closed ...or very open... :D

The piston has a much larger surface area to generate more force at the moment the pressure overcomes the spring.

The venthole is placed further back so the pressure cannot escape without pushing the piston back fully.

This results in a valve that pops open really hard.
I could increase the pressure slowly with a manual pump and without any hissing the valve stayed shut intill it snapped open violently.

It's a very potent pilot valve for automatic applications..

As a main valve it would resemble a detent.
Makes me wonder what it would do if it was placed between a blowgun and a bbmg's bb reservoir... :D


I have included a QEV mod drawing because both valves work together very well.
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Last edited by Brian the brain on Sat Oct 27, 2012 5:55 am, edited 1 time in total.
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: wyz2285 » Sat Oct 27, 2012 5:55 am

Thanks mr. Brian!
Actually it's exactly the same as I thought, except I thought the piston needs to be oring sealed.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: archie370 » Sat Oct 27, 2012 8:25 am

Is this more efficient than a simple quick exhaust valve whats the different other than the hole, is it better ?
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Unread postAuthor: pneumaticcannons » Sat Oct 27, 2012 10:13 am

No, the hole just allows you to fill from the chamber side. It's not about efficiency either. The point of this setup is not be "better" than a normal QEV setup but instead to achieve a simple full auto valve.
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Unread postAuthor: Technician1002 » Sat Oct 27, 2012 12:28 pm

The OP mixed the terms pressure relief valve and pop off valve. A pressure relief valve behaves exactly as he described a pop off valve. Pressure relief valves are used on water heaters to prevent over pressure when heated water expands and it has no place to go. These valves dribble prevent over pressure.

A pop off valve on the other hand are used on air compressors and steam boilers. They remain seated until they reach a set point and due to the piston design, they pop open floating on the stream of air or steam until the pressure has dropped too low to hold it open at which time they snap back closed.
he first thing to determine is the type of pressure relief valve you are looking for. There are two common types of relief valves. One is the safety or pop off valve that opens completely when the set pressure is reached, diverting all, or almost all, of the flow out the discharge port of the valve. An example of this type of relief valve, made of metal, is found on hot water heaters to protect them from damage should steam form inside the heater and cause overpressurization.


http://www.haywardflowcontrol.com/webapp/wcs/stores/servlet/HaywardContent_10202_10053_-1_FC-SelPresRel
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Unread postAuthor: wyz2285 » Sat Oct 27, 2012 5:02 pm

But my safety valve is for air compressors, but if the flow is low (from a blow gun), it does not pop, it leaks :?
BTB did you do anything special to keep the piston airtight inside the housing? If you didn't, won't the air leak out around the piston once the air start coming out from the smaller port instead of build up and push it back?
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: archie370 » Sat Oct 27, 2012 5:13 pm

oh alright well i hope it works for you guys :)
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Unread postAuthor: Crna Legija » Sat Oct 27, 2012 7:14 pm

archie370 wrote:oh alright well i hope it works for you guys :)


it definitely does!

[youtube]http://www.youtube.com/watch?v=xQxthZnunAo[/youtube]
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 12:50 am

In my experience:

Image

If the vent is not accessible to the pressure source when the piston is even slightly open, then it will hang and become the proverbial farting machine. If you want to make it like Brian drew it, the piston cannot be airtight.

Some important rules I found when playing with these things:

- piston diameter significantly larger than piston seat.

- if the piston is airtight, then the exhaust port should be exposed even when the piston is closed.

- the exhaust porting diameter should be significantly smaller than the piston seat, in order to allow pressure buildup that causes the piston to "pop".

Some links to similar homemade pop-offs I've made which could be useful:

Modified pop-off valve

custom pop-off v1.0

custom pop-off v2.0

custom pop-off v3.0


This could be a way of converting a QEV into a "snap valve", never tried it though.

Image
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Unread postAuthor: wyz2285 » Sun Oct 28, 2012 5:44 am

Thanks jack :D
Soon from now I'll present you a 6mm full auto (won't be firing airsoft bb through)
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 7:17 am

Interesting, let's see what you can come up with :)
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Unread postAuthor: Brian the brain » Sun Oct 28, 2012 8:04 am

Technician is probably right..I wasn;t aware of the use of two terms..I just noticed that one type works a bit better than the other.
:D

The true pop-off valves I used still hissed when the pressure is increased slowly.
This indicated to me that it could be improved.

Comparing a snapvalve to a pop-off is like comparing a ballvalve to a piston valve.
A faster opening valve with higher flow does a better job.

Don't use O-rings on my design or you will get resetting problems like JSr explained.

although combining it with a QEV solves the problem.
The QEV efficiently dumps the pressure that would otherwise cause the pilot to hover.



The modified QEV helps you to get a compact system and achieve a more efficient ( in terms of airconsumption) semi-auto setup.

It also wastes less air than the usual pilotfilled QEV-popoff combo as the pilot is dumped without the issue of air rushing into the pilot continuously.
Although the difference isn't as huge as it would be when compared to the differnse with pilotside filled slidevalve QEV-combo launchers..


With the modded QEV you can continuously fill from the chamberside making the incoming air part of the volume that fires the projectile.

It seems illogical to me to dump pilot volume whilst using the same space as your filling point. :roll:
If you want a powerful shot..why would you dampen the effect by refilling the pilot area?
I have noticed a massive difference in performance in both designs when compared to the original.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2012 12:15 pm

Brian the brain wrote:It seems illogical to me to dump pilot volume whilst using the same space as your filling point. :roll:
If you want a powerful shot..why would you dampen the effect by refilling the pilot area?


Hear hear :)
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Unread postAuthor: Brian the brain » Sun Oct 28, 2012 12:35 pm

BTB did you do anything special to keep the piston airtight inside the housing? If you didn't, won't the air leak out around the piston once the air start coming out from the smaller port instead of build up and push it back?


It a good fit but it does not seal completely.
The large surface area generates a lot of force, even with a small leak and once the piston is exposed to this extra force it will start to move back and the "leak"turns to full flow in an instant.

at the moment it starts to leak the power of the spring is already overcome and all of a sudden this extra surface is hit with a lot of force so...

all hell breaks loose after that :D
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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