CO₂ Storage Units Overview

Storage units have made TOMCO₂ the worldwide leader in CO₂ Equipment. Right from our very beginning, TOMCO₂ has produced highly innovative, reliable and efficient CO₂ storage units. Many of the storage units that we manufactured more than thirty years ago are still in use today. Today we offer several different models and sizes of storage units. You can choose from urethane insulated and vacuum insulated, horizontal and vertical units depending on your particular needs. Please find out more details about the differences between vacuum insulated and urethane insulated storage units, and about TOMCO₂ storage units in general.

> Read more about Urethane Insulated VS. Vacuum Insulated Storage Units

Carbon Dioxide Storage Units

Our durable storage units consist of a welded steel pressure vessel designed, constructed, and tested in accordance with the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII. Div. I, current at the time of storage unit construction, for Unfired Pressure Vessels.

The storage unit is constructed using high strength, fine grain carbon steel. TOMCO₂'s storage units are constructed with SA-612, which has been normalized to improve the toughness of the material. The storage unit is designed for a maximum allowable working pressure (MAWP) of 350 psig (2.41 mPa) at the minimum designed metal temperature (MDMT). The standard MDMT for a TOMCO₂ liquid carbon dioxide storage unit is -20 F (-28.9 C), or colder.

Liquid carbon dioxide storage units are designed, constructed and tested for specific operating pressures and fill rates. Liquid carbon dioxide storage units must never be allowed to become completely full at the pressure relief device setting. The design pressure of a liquid carbon dioxide storage unit must be at least 10% above the normal operating pressure.

Capacity & Fill Rate

Capacity is defined as the amount of liquid carbon dioxide a storage unit will hold in gallons or liters, kilograms or pounds, under equilibrium conditions at normal operating pressure and temperature, when filled with liquid to the designed level. When a storage unit is filled with colder liquid carbon dioxide, it will hold more product. All TOMCO₂ liquid carbon dioxide storage units are designed to limit normal liquid charge to 91% of the volumetric capacity of the storage unit.

Insulation

The purpose of insulation is to reduce heat gain into the contents of the storage unit, and to prevent frost build up on the outer surface. TOMCO₂'s tough and durable storage units are insulated with 4" (10.16 cm) urethane insulation.

TOMCO₂'s FlexFlow^™ storage units have 6" (150 mm) of insulation with a refrigeration unit as an optional piece of equipment. All insulation is covered with .063" (1.66 mm) pre-painted white aluminum outer jacket. Both ends of the liquid carbon storage unit are covered with 091" (2.3 mm) aluminum pre-formed flanged and dished heads. Since TOMCO₂ began using aluminum jackets in 1980, there is no record of any TOMCO₂ liquid carbon dioxide storage unit needing re-insulation.

Instrumentation

Instrumentation supplied with all liquid carbon dioxide storage units shall include the following:

1. 0 to 600 psi (4100 kPa) pressure gauge with a 6" (150 mm) dial face. This gauge gives the operating pressure of the storage unit at all times.
2. A gauge is used to indicate capacity in the storage unit. The differential liquid level gauge has a 6" (150 mm) diameter dial face, calibrated to read in kilograms and pounds of liquid carbon dioxide.
3. High and low audible pressure alarm. This alarm monitors the pressure of the storage unit. If the pressure fluctuates outside of normal operating conditions, an alarm will sound.

Piping & Valves as a Part of the Storage Unit

Liquid carbon dioxide is compatible with most metallic piping materials. There are piping hazards associated with liquid carbon dioxide storage units such as elevated pressure. Corrosion is typically a problem only on the exterior of piping that is subjected to moisture condensations from temperature cycling.

All piping and fittings furnished as a part of the liquid carbon dioxide storage unit will be schedule 80 black. Fittings are screw type 2,000 class forged steel. All nozzles and valves that penetrate the outer jacket are stainless steel.

Manway

TOMCO₂'s urethane insulated liquid carbon dioxide storage units are built with a manway as standard equipment. Manways are optional on vacuum insulated units. The manway serves two purposes:

1. Internal inspection.
2. Cleaning inside of the storage unit.

There has been discussion within certain regulatory agencies in the United States that may require periodic hydrostatic testing and internal inspections of all storage units. If these regulations become mandatory, then the manway will play an important role in compliance with these regulations.

Refrigeration Evaporator Coil

A refrigeration coil is installed as standard equipment in the urethane insulated storage units. Refrigeration coils are optional on vacuum insulated and Flex-Flow storage units. As carbon dioxide is withdrawn from the storage unit, a self refrigeration effect occurs, and pressure in the storage unit is reduced. This self refrigeration effect is more pronounced in vapor withdrawal. For every pound of vapor withdrawn, approximately 120 BTU's must be added back to the CO₂ to offset the drop in pressure. If the withdrawal is at rate large enough to require more BTU's to be introduced into the product than is available from normal heat entry through the insulation, an outside source of heat must be introduced.

Example: A storage unit has a normal heat entry of 1955 BTU/ hr at 70 F (21 C) ambient temperature. Product is withdrawn as a vapor, at a rate of 300 Lbs./ hr (.136 Kg/hr).  Heat must be introduced into the product at a rate of 300 Lbs./ hr x 120 BTU/Lbs. = 36,000 BTU/hr.

An outside source such as an electric vaporizer must be provided with a capacity of 36,000 - 1955 = 34.045 BTU/hr to maintain pressure in the storage unit. In the case of an electric vaporizer, this would be equivalent to 10 KW.

A refrigeration unit is not required as long as vapor is being is being withdrawn. In fact, as long as vapor is being withdrawn at a rate as small as (1955 BTU/hr) / (120 BTU/hr) = 16.3 Lbs./ hr (7.39 Kg/ hr), no refrigeration is required. The only time product would be vented would be during withdrawal rates less than 16.3 Lbs/ hr (7.39 Kg/hr) on during total shutdown periods greater than 6 to 10 days. Urethane insulation is adequate for the storage of relatively warm liquid carbon dioxide at normal storage temperatures.

There are certain applications that require refrigeration to prevent product loss. This is true regardless of the type of storage unit used as an insulation system, urethane insulated or vacuum insulated. One example of this type of application is a loop system which circulates liquid through the process and returns back to the storage unit. Another application would include cylinder filling operations where liquid is continuously returned back the storage unit.

Vaporizers (Optional)

A pressure building vaporizer is required to maintain the operating pressure in the storage unit during periods of high liquid or vapor withdrawal. There is a cooling effect when product is withdrawn from the storage unit, resulting in decreasing pressure and temperature. For every pound of vapor withdrawn, approximately 120 BTU's are pulled from the liquid CO₂ in the vessel. (To fill the space left by with drawn vapor, a pound of liquid is vaporized. The BTU's are required to vaporize the liquid into vapor.) TOMCO₂ offers both a steam and an electric vaporizer. Electric vaporizer capacities range from 9 KW to 56 KW. TOMCO₂ steam vaporizers have output capacities based on 40 psig steam pressure. All units are capable of operating between 15 and 250 psig steam pressure.

Refrigeration Unit Horizontal & Vertical

Depending on your CO₂ usage, a refrigeration unit may be necessary for some applications. The storage unit can be equipped with a low-temperature, air cooled refrigerant R-404A (HP-62) condensing unit. TOMCO₂ uses Copeland compressor units as our standard. The system automatically maintains the carbon dioxide in the storage unit at 0 F (- 18 C ) and 300 psig. The standard units have 230/ 460 volts, 60 cycle electrical requirements. TOMCO₂ routinely supplies units with the voltages/cycles that are used around the world. The condensing unit includes a filter dryer, refrigerant line, expansion valve, and a refrigeration coil mounted internally in the storage unit. Automatic controls start and stop the compressor. The automatic controls monitor the temperature of the liquid carbon dioxide to maintain proper operating pressures.

Area Considerations

Carbon dioxide storage units are generally filled from large transports. The storage unit location should provide access for transports and/or smaller D.O.T. delivery units. The storage unit should not be located in an area where it will be subjected to temperature above 100 F (43 C). At these high temperatures, there may be a loss of efficiency. When requested, units can be designed for higher ambient temperatures.

A dry, well ventilated area is the best place to set a liquid carbon dioxide storage unit. Avoid locations where the storage unit may be struck by heavy or moving objects.