Co2 Vent Valve

[Jim Cliff]

Hi,

I'm trying to design and make a Co2 vent valve which basically releases pressure from a larger pilot valve causing actuation. I am using the small 16g Co2 cartridges which provide 55 bar of pressure. My vent valve is basically the same as a direct acting pressure relief with a manual pull shank. I'm hoping i'll be able to trigger using a small 12vdc solenoid.

It consists of an alloy block, with a drilled 1mm orifice (see calculations below). Above this orifice sits a ball bearing, cupped shank, spring and bolt (see design attached). The idea being that the spring is set at just the right amount of compression to allow the shank to pulled up with minimal force. Co2 is vented through the loose hole in the bolt in which the shank sits.

@ 55 bar with 1mm orifice force against ball = 4.3 N.
Therefore with a spring force of 7 N I should be able to pull the shank with 2.7 N of force?

Can anyone confirm the calcs above are correct, and I'm on the right track here?

Many thanks,
Jim

 

[Jim Cliff]

Any thoughts?

 

[LittleInch]

Well kind of.

This might be better in the Safety Relief Valve forum as it looks correct in theory, but in practice?

The seat arrangement of the ball bearing on the hole looks very out of size, i.e. if the hole is 1mm the ball looks like 15mm.

You would be much better off with a plug and some sort of resilient seal pressing against a 1 mm ID tube or nozzle.

If you look at most ball type valves the seat is about 50% of the diameter of the ball and is a machined seat.

CO2 gets very cold when you do this so freezing will be an issue plus velocity will wear the hole edge away.

So in theory fine, but practical - no. It will just hiss away until your cartridge runs out.

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[Jim Cliff]

LittleInch,

Many thanks for the reply... Much appreciated!

Apologies for the poor diagram, this was meant as a rough model to show my thoughts. I quickly machined the valve this morning on the lathe. The ball I'm using is 8mm and sits well over the orifice. Having looked a few hydraulic check valves, you are right the seat is roughly half the size of the ball. My test valve does leak slightly if the spring pressure isn't set right. Tightening it up stops the weeping, but it's still taking more force to pull the shank than I would like. I agree, it works better in theory than in practice. I didn't have much time today, but will have another go to see if I can refine it tomorrow.

With regards to the plug, what about replacing the ball bearing with a spool (cylindrical plug) that hugs the spring chamber wall and seals against the bottom with a point? Like below:

This may work better in terms of sealing and control, but would still be susceptible to corrosion.

Thanks again.

 

[LittleInch]

Might be better but you will need to allow the plug to move a little inside the cylinder to effect good sealing.

Note that most spring type relief valves don't seal well above about 90% of set pressure.

Your less than ideal system might be closer to 80% so your set pressure might need to be closer to 70 bar and hence need more spring force.

all the small relief valves are susceptible to grit, dirt etc especially if the seal is metal to metal.

You also might get erosion from the sonic velocity of the gas coming out at the sealing point.



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