Compressed Air Tool - Valve Query

[Blade4]

A query I could do with some help on (most likely basic fluid dynamics but struggling!)

Situation:
An industrial tool requires a small high pressure air chamber (1000 PSI+) an electrically controlled valve (currently a solenoid but could be something else) and an outlet pipe (1/4"). The valve needs to open extrmely quickly to allow the max amount of pressure in a short space of time to flow out through the outlet pipe (ideally expel all the air from the chamber at once).

a. The setup currently has a 1/2" steel pipe air chamber, 1/2" solenoid valve, a reducer and then 1/4" outlet pipe. The only part that has to remain a constant is the 1/4" outlet pipe. (see pic: air chamber on the right)

Larger aperture solenoid valves are limited in the max pressure they can handle. I tried a 1/4" solenoid (with 1/2" air chamber) but that struggled to expel sufficient air in one go - due to the restriction of going from 1/2 - 1/4 before the valve.

My queries are:

1. What would be the optimum setup to provide the maximum air flow (at high pressure) to the outlet pipe? Would keeping all elements at the same diameter (1/4") enable greater pressure at the outlet pipe, or is it better to go from a larger air chamber and valve (1" or 1/2") to the smaller outlet pipe (Venturi affect?).

2. Can anyone recommend a compact alternative to a solenoid valve that is both electrically operated and fast opening?

Any thoughts would be greatly appreciated!

 

[EdStainless]

What is the cv of that valve?
You really want a straight flow pattern valve.
There are pilot operated valves, where the solenoid opens a small valve which then uses the high-pressure side to open the main valve.
Is that all of the air that you need in one shot?
You need to make your 1/4" as short as possible, like nearly zero length.
And use a smooth reducer and not pipe fittings for the reduction.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Blade4]

EdStainless, thanks for your reply!

This particular valve was just an initial test piece, it's only rated to 12 Bar. But it is indirect so should be pilot assisted.

(From the Manufacturer: Kv-value of 2.05 m3/h, which translates to a flow rate of 34.17 l/min at a differential pressure of 1 bar (for water). In imperial units: Cv = 2.37, which means 2.37 GPM @ 1 psi pressure drop)

Noted on the 1/4". By smooth reducer, I presume you mean not threaded?

I'm hoping to have the whole thing machined/ extruded once the optimum dimensions are established.

 

[danw2]

What about a ball valve actuated by a solenoid and linkage? Ball valves routinely have a 1000 PSIG WOG rating.

 

[Blade4]

danw2, thank you. Yes a very good suggestion. The only issue been the linkage would be crucial, and need to be under significant tension to release the ball valve quick enough to allow max flow. For a hand tool it would also add considerable uneven bulk.

 

[danw2]

Here's and inexpensive on-off hydraulic valve, pressure rated, but what complements it - the block housing, is out of my league.
Link

 

[Blade4]

danw2, thanks - looks decent! though I'm not sure how I'd incorporate a cartridge valve into the design? Plus the flow rate is quite low compared to most traditional solenoids it seems

 

[EdStainless]

We used actuated plug valves for rapid service since they are full diameter flow.
Spring in the direction that you want fast and air or controlled fluid to operate the other way.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Blade4]

EdStainless, thanks I'll take a look.

 

[LittleInch]

Blade4,

I'm not really understanding your system - can you sketch it out a bit better as the description doesn't make sense to me.

"The valve needs to open extremely quickly to allow the max amount of pressure in a short space of time to flow out through the outlet pipe (ideally expel all the air from the chamber at once)"

Pressure doesn't flow out, air does.
The air mass will reduce, not be expelled.

How is the air locked into the small chamber?
Where is the 1000 psi air coming from to fill the chamber in the first place?
What's the volume of the chamber.

You're going to get critical flow here whatever you do going from 71 bara to 1 bara across the valve.
What is a "short space of time"? 1 msec, 10, 100, 2000??

But I would try looking more at Hydraulic fluid control valves. They handle 1000's of PSI.

Is this something like a nail gun?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Blade4]

LittleInch, thanks for taking the time to respond.

Completely legitimate questions – I’m sure you can see my lack of experience in this field. Due to its application, I’ve been intentionally vague on some details, which I know is unhelpful (IP related rather than sinister I’ll add). But if I could use the analogy of an industrial potatoe gun, that may help?

The intention is the small chamber will be pressurised with a high pressure pump, a one way valve (though currently an industrial grade presta valve) and the Normally Closed solenoid (or other valve type) will keep the air in place until voltage is applied. High pressure air flowing through the valve will then move an item obturated at the muzzle end of the outlet/ barrel. After which, the air chamber will need to be refilled with the pump.

The chamber is currently 100mm ½” steel pipe so approx. 36cm3. Though like the valve, this is a variable that can be changed.

I’m looking for the optimum combination of chamber diameter and size with valve type, size and specification in order to maintain max pressure through the outlet barrel that is 1/4” in diameter at the end. I.e the least amount of pressure drop for max flow rate. Perhaps I need CFD modelling for this? Following the comment by ‘EdStainless’ about the reducer, I could use a custom tapered barrel from ½” – ¼” (if a ½”valve is determined as the optimum size for max flow).

Trials will be conducted to determine the appropriate pressure of the chamber to achieve what is required at the open end of the system, but I suspect it will be in the region of 1000 – 2000 psi. To avoid paying for countless solenoids/ different valve types and varying sizes of stainless steel tubing I’m trying to get an understanding of the basic fluid dynamics of such an open system so I can purchase a more specific selection for trials.

I’ll certainly take a look at hydraulic fluid control valves. The only real valve specifications are that it is inline and therefore not too bulky (this item needs to be portable), electronically controlled and reasonably light weight. Coaxial valves for example, look ideal and have a very fast speed of opening, but most appear to be very heavy (several kg’s).

Every comment here is helpful so much appreciated!

 

[LittleInch]

Hmmm,

I calculate your internal volume at 11cm3.

So if I understand this right, you pump up the chamber to 1000psi, decouple it or isolate from the pump so there is no more air going in and then put something at the end of your 1/4" nozzle / tube which then fires out of it as you release the air in the chamber.

Your issue really is that there are so many things going on here in a very short space of time that it is difficult to analyse and really only trial and monitoring will do anything for you.

E.g. You have this thing at the end of the tube. How much pressure does it take for it to start moving?
Air flow through any valve or device will be sonic velocity / choked flow and may also set up some sort of air shockwave in the tube even for a micro second.

As the air in the chamber starts to flow out, its pressure will fall as it pressure sup the barrel. I think you might need a bit more volume there to avoid the pressure being reduced too much before the thing at the end flies out.

Some sort of solenoid valve I think is your best bet. Everything else is too slow.

But this thing is sooo fast and transient that I don't think you're going to be able to do much before trying a few ideas out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Rogue909]

This sounds like a pellet gun. (Sorry if I just stepped all over your IP.)

If it's not; It sounds like you should buy a pellet gun and open it up. They sell 2000 psi pellet guns for a thousand (way less than a CFD package that could analyze this) that shoots 1/4" pellets. As a bonus; if you're looking to outsource this then you may be able to leverage one of those companies in manufacturing.

 

[LittleInch]

If you look up air gun valve diagram there are a lot of options. Most seem to use some sort of poppet valve to suddenly move and allow the air / CO2 out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

I have a 3,000 psi 50 cal pellet rifle.
The velocity will be limited by choked flow, and the air will be very cold (maybe below freezing).
Firing my rifle on humid days results in large 'fog' clouds down range 10-15'.
And water dripping out of and off of the barrel.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Blade4]

LittleInch,

Yes, that’s right, and the pump will be completely decoupled.

I believe you’re right - that trialling maybe the only option. If the device was similar to a breach loaded weapon, standard ballistic mathematics could be used to determine the pressure required to move the object. But as it will be positioned at the muzzle end and obturation will not be exactly the same each time (degrading o-rings for example) this will be difficult. Plus the biggest determining factor will always be the valve.

I’ll certainly play around with the volume in the outlet as you mentioned.

Rogue909 – If only it were a pellet gun! Things would be much simpler. Fair point on contacting the manufacturer though. From my understanding most pellet/ BB / paintball guns utilise a QEV which only releases a fraction of the air in the camber thereby leaving air remaining.

The reasons I’ve not looked at these so far:
- The object has significantly more mass than a typical pellet or paintball (TBC but 50-100g)
- I really require the chamber to be completely emptied once the valve is opened
- The valve needs to be electronically controlled

However if there are off the shelf QEV’s that can achieve this, it is certainly an option to look into, so thanks!