Carbon Dioxide Mixture

Carbon Dioxide (CO2) may combine with water to form carbonic acid. Carbonic acid is mildly corrosive and at higher pressures can condense into a liquid which will act as a solvent and dilute the cylinder lubrication.  The gas properties of the CO2 mixture should be reviewed at the required operating conditions.
 
See the chart below for recommended materials of construction for piston rods and valves based on suction pressure and CO2 concentration.

The recommendations for compressor piston rod and valve materials are:
 
For sweet natural gas with a carbon dioxide concentration less than 5%, or at 5% with a suction pressure less than 1200 psig, or with a carbon dioxide concentration up to 50% with a suction pressure less than 400 psig, or with any suction pressure lower than 400 psig, use standard piston rod and valve materials.
 
For sweet natural gas with a carbon dioxide concentration outside the above limits, use 17-4PH stainless steel piston rods and stainless steel compressor valves.  (It is recommended that standard valves be used for unit start-up.  Once the system is confirmed clean, the stainless steel valves should be installed).
 
CO2 is soluble in mineral oils, thereby reducing the oil / gas mixture viscosity.  Mineral oils are completely miscible into CO2, which reduces the effectiveness of the oil.  Compounding mineral oils or PAG (poly-alkaline-glycol) synthetic lubricants are commonly used.
 
See Cylinder and Packing Lubrication Requirements--6.5 in the Ariel Packager's Standards  and contact Ariel in Mount Vernon for a recommendation.
 
 

Ariel Corporation Application Manual  
6-Sept-2001