CO2 gas regulator

[Refurb1950]

I have been designing and prototyping a small pressure regulator to handle CO2 from standard small gas cartridges. The regulator is made of GF nylon 6/6 and is currently of the balance piston style. I am looking to regulate the 850 psi cartridge pressure to values between 5 and 40 psi. I seem to be having inconsistent results. I will get it to regulate quite well through several cartridges and then suddenly it becomes erratic. I have recently printed a housing from a translucent material which now allows me to get an idea of what is happening internally. What I have noticed is that when the regulator becomes erratic (returning to a pressure higher than it was set at or continually climbing) that I will see CO2 ice (dry ice) appearing on the regulated side of the balance piston (I know it is dry ice as I have removed some and examined it). This suggests that the high pressure valve (for which i am using a 5.5 mm stainless ball seated on a 90D urethane oring) is open or leaking past the oring seal. Questions: Why does it work fine for a number of exhaust and re-pressurize cycles and then become erratic? Is the dry ice forming because the ball is off the seat thus allowing a constant flow of gas?

 

[zeusfaber]

Have you had a chance to take a close look at the surface of your O Ring "seat" after it has failed? It seems an unusually soft material for a reducer seat.

Two possibilities off the top of my head:

1. When the valve opens, you're getting a little dry ice entrained within the gas flow, and this is eroding the seal surface.

2. You're getting gas dissolved into the surface of the ring, followed by ablation during the periodic depressurisations when the valve opens.

Both would damage the surface, leading to leaks when the valve is next called upon to lock up.

A.

 

[Compositepro]

A plastic CO2 regulator does not sound like a very good idea at all. The pressure can be very high and the temperature of expanding CO2 is very low. Plastics get brittle at low temperature and their thermal expansion is high. Plastic can crack just from the stress of the thermal shock.

The critical temperature of CO2 is 31C. Above this temp. there is no gas or liquid phase, just what can be described asa low density fluid or high density vapor and below this temp. there is liquid CO2 in the bottom of the cartridge. Thus the feed to your regulator may be liquid or gas depending on the orientation of the cartridge.

Most CO2 regulators are two stage because it is almost impossible to accurately control to a low outlet pressure with such a high inlet pressure. They will also often have fins on the regulator to absorb heat from the air to vaporize the CO2. Expansion of CO2 in one step will create solid CO2 which will prevent the valve from sealing. This will require some small amount of time because initially the warm plastic contains enough heat to vaporize the dry ice.

 

[Refurb1950]

Zeusfaber, I have examined under low power magnification the oring. No damage is visible, but I am not ruling it out. I chose 90 durometer urethane for the seat as I did want something that was slightly conformal, but that would be tough enough to withstand the cold temps. Any other suggestions for this application?

Compositepro, agree with both your observations on the use of polymer here as well as the two stage approach. I originally had started down the two stage route and then was given by a co-worker two examples of single stage CO2 regulators which had me puzzled. One example is a machined aluminum regulator which uses a 1.5 mm thick urethane diaphragm instead of a balance piston. The second example is made by a company in Tucson and is molded from GF nylon. It is used in a product called Fizz-Gizz and is used for carbonating soft drinks. I have tested both and both function as advertised. Does the aluminum version perhaps function because of the improved heat transfer over a polymer version?

I plan on testing a single stage diaphragm version in the next few days. Two stage is in the design phase also.

Thank you both for your help.