Cryogenic Liquid Cylinder Operation- Part 2B- Gas Withdrawal

The last two posts covered 4L DOT Liquid Cylinder Liquid Withdrawal and Vent operation. Summarizing- When the liquid valve is opened, liquid is drawn up from the bottom of the vessel through a dip tube. When the vent valve is opened gas is the top head space of the vessel is released. And- it was determined that the gas use valve cannot withdraw gas in the same manner as the vent or pressure would rapidly drop below needed levels.

“If gas doesn’t just pull right off the top, then where does it come from?”

Would you believe “It depends?”

And, in this case, what does it depend on?

It depends on the pressure of the contents compared to the setting for the pressure builder and economizer regulators which will be discussed next time. For now- the economizer setting will typically be about 15psi higher than the pressure builder setting. So, if your pressure builder regulator is set at 125 psi the economizer regulator actuate at about 140 psi.

When the contents pressure is below the economizer set point then the gas coming out of the cryogenic liquid cylinder actually originates from the bottom of the liquid cylinder. It comes out of the inner vessel through a small copper tube that is brazed to and circles around the inside of the outer vessel. Confusing? Not too. The key here is that, up to the point of needing gas, the purpose of the cryogenic liquid cylinder has been to protect the liquid oxygen, nitrogen or argon from outside energy (heat) in order to keep it in liquid form.

When gas is needed, all we have to do is expose the liquid to energy (via direct exposure to the outside wall of the vessel) to bring it above its boiling point. Now we have gas. By withdrawing liquid from the bottom and converting it to gas the head pressure of the liquid cylinder contents can be maintained far more easily. This provides more consistent output and allows the supported process to run normally.

The other aids to maintaining pressure within a usable range are the pressure building (PB) regulator, PB coil and economizer regulator. Stay tuned for more…….