Cryogenic Liquid Cylinder Operation- Part 2C- Gas Withdrawal

A typical customer service call:

“Hello. This is ___________ and there is frost on the outside of our cryogenic liquid oxygen cylinder. Is this a problem?”

How do we determine without a game of 21 questions?

Q: Is the ice or frost all over the vessel or is it more localized?

A: Oh no. It is just at the bottom and some up on the pipes?

Q: Does the unit frost up all the time or only when in use?

A: Only when in use. And, the more gas used the worse it gets.

Conclusion- In all likelihood this is normal…. Especially in humid environments like the Gulf Coast and Northwest U.S. .

Remember. If the pressure of the liquid cylinder contents is below the economizer set point then Gas used is originating from the bottom of the liquid cylinder as liquid. This liquid is being exposed to outside heat via a set of coils brazed to the outside wall of the vessel. Exposing it to heat raises the temperature of the liquid above the boiling point to make it gas.

This liquid is coming out of the bottom of the liquid cylinder at something around -300 degrees Fahrenheit. It is only normal for outside cold are to frost up on the outside of a container that is exchanging energy at this level. Also, It is completely reasonable that the more gas you use the more frost will grow.

Is there a limit to how much liquid can be vaporized by the gas use coil?

Manana….

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…….