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QEV/Piston Valve Longevity

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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QEV/Piston Valve Longevity

Unread postAuthor: n05gr0th » Tue Sep 16, 2014 8:09 am

So I've been trawling through the Spudfiles archives, thoroughly researching my next pneumatic launcher, and have discovered that longevity can sometimes be an issue for QEV and piston valves. Currently, I'm weighing the pros and cons of commercially bought QEVs versus homemade piston valves. What I'd like to know is:

1.) What sort life expectancy can I expect to have with a typical commercial QEV when twice exceeding the intended operating pressure of about 150psi? Are these valves really that modifiable?

2.) If I were to instead build a piston valve, how should I construct it so that I get a good opening time yet also make it sturdy enough to last?
- Would a nuts, bolts, and washers design combine the best of both worlds?
- Would the addition of a spring behind the piston make any noticeable difference in decelerating it (and, unrelatedly, allow me to fill the gun via the chamber instead of through the pilot?)?
- From what I gather, neoprene sounds like a good material for the sealing face, but what about the piston bumper?

Unrelated question, but would epoxy be sufficiently strong enough to use internally in the chamber/valve - to sculpt a smoother path for the airflow? I don't have machining or welding equipment so those options are out of the question...

Many thanks!
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Re: QEV/Piston Valve Longevity

Unread postAuthor: Ianbuckwell » Tue Sep 16, 2014 4:09 pm

I use a 3/4" Parker QEV which is rated at around 100-150psi and have been using it at 300psi and a few times closer to 400psi (unintentional) with a slide valve pilot/fill, went to refill the gun and air was blowing straight through out the barrel. I thought I must have torn the diaphragm so took it apart and it still looks good as new, no damage at all, what had happened was a blob of solder had gotten into the valve stopping it from seating fully and sealing. I haven't shot thousands of shots, but maybe 300 or so and I am surprised and happy with how it is holding up.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: n05gr0th » Tue Sep 16, 2014 6:36 pm

Unfortunately, Parker valves are BSP. I'd be using NPT if I decide to go with a QEV. And a quick look at your model's spec sheet shows that its rated far below 500psi. It's good to know that those valves hold that high over their operating pressure, even after that many shots.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: Fnord » Tue Sep 16, 2014 8:08 pm

Most industrial valves from reputable companies have rated lifetimes.
These are normally in the tens of thousands of cycles, so the cost of ammo will almost certainly be more than the valve.

You should expect reduced life with higher pressures, but the largest issue is probably dry rot as the rubber components age. 300psi will probably not be a problem given how qevs are built.
I've got a 1.5" asco diaphragm valve cannon that's about six years old now, and it still shoots good as new.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: mark.f » Tue Sep 16, 2014 8:12 pm

My friend has had a cannon of mine at his house with a 3/4" QEV that regularly comes into contact with some nasty dinosaur distillates, and still works fine after several years. QEV's are pretty tough.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: Ianbuckwell » Wed Sep 17, 2014 1:10 am

Are you living in UK, as you have a UK flag under your name? If so just wondered why did you opt to use NPT as BSP fittings are cheaper and easier to find a larger selection here, well that's what I found when looking for fittings.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: n05gr0th » Wed Sep 17, 2014 5:26 am

@ Fnord - Ohhhhhh, so that's what cycles mean! I'll only have about 3-4 weeks a year to shoot it so something rated at 600 cycles sounds perfectly reasonable then. Would leaving the chamber pressurized after shooting help preserve the valves? I read that somewhere about commercial PCPs.

@ mark.f - That's great to know, you've really boosted my opinion of QEVs.

@ Ianbuckwell - I live here, yes, but I have a family cabin in the states. I'd much rather build it there because:
1) It's legal in my state!
2) Cheaper to build
3) Not many people wandering about where my cabin is
4) Large distances to shoot over
5) Don't want distractions at uni
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Re: QEV/Piston Valve Longevity

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Re: QEV/Piston Valve Longevity

Unread postAuthor: Fnord » Wed Sep 17, 2014 7:40 pm

n05gr0th wrote:@ Fnord - Ohhhhhh, so that's what cycles mean! I'll only have about 3-4 weeks a year to shoot it so something rated at 600 cycles sounds perfectly reasonable then. Would leaving the chamber pressurized after shooting help preserve the valves? I read that somewhere about commercial PCPs.


Personally I'd just spray a bit of lubricant into the valve before putting it away for long periods. Something silicone-based would probably work, petroleum should probably be avoided. Really depends on what the seals are made from.

Also, the valve rating for 600 cycles is probably cycles per minute, rather than expected lifetime. Even a mediocre homemade valve will last 600 cycles.
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Re: QEV/Piston Valve Longevity

Unread postAuthor: n05gr0th » Thu Sep 18, 2014 8:43 am

Brilliant, in that case I have a few ideas in mind for sourcing the right lubricant. Thanks again.
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