Cryogenic Storage Systems
By Joseph V. Messina, CPD
Cryogenic gas is any gas in a liquid form at or below -20°F (-29°C). The following discussion covers cryogenic liquids used in laboratories and light industrial applications, not cryogenic storage systems for gases used in healthcare facilities. Gases usually stored as cryogenics are nitrogen, argon and oxygen. Gases less commonly stored in bulk, but available, are carbon dioxide, hydrogen and helium.
Cryogenic storage systems have a few basic components. The storage tank contains the gas in a liquid form; the vaporizer turns the liquid into a gaseous form when necessary; and the piping conveys either the liquid or the gas to the point of use.
Codes and standards are very important to know and follow when designing a cryogenic storage system. The following codes and standards are most commonly used:
• UL 644: Standard for Safety for Container Assemblies for LP-Gas
• ASME Boiler and Pressure Vessel Code
• NFPA 55: Compressed Gases and Cryogenic Fluids Code
• NFPA 99: Health Care Facilities Code
• ASTM G88: Standard Guide for Designing Systems for Oxygen Service
• ASME B31.3: Process Piping Code
Two types of tanks are generally used to store cryogenic liquids: bulk storage tanks or dewars. Large bulk tanks are comprised of a tank inside of a tank, which allows them to be highly insulated with pearlite or another high-efficiency insulation in a vacuum. These tanks can be installed either horizontally or vertically. While horizontal tanks are slightly smaller and lighter, vertical tanks are the most common installation because they occupy less site area than a horizontal tank. Also, vertical tanks have less tank area in which the liquid can vaporize while filling, and it is very important to keep the stored liquid in that state as long as possible. All capacities are given in gallons of water. All tanks are ASME rated, and manufacturers provide similar sizes, with the smallest tank typically being 300 gallons. See Table 1 for common tank dimensions. Cryogenic storage tanks can be bought, but they are generally leased from the supplier.
Regarding the site location of the tank, there must be enough space for the delivery truck to get as close to the tank as possible. This is because the operator must vent gas from the hose connection of the truck to the storage tank by means of a manual vent before filling can start. While filling the tank, the operator must adjust the pressure in the tank. To be able to do this, it is important that the tank have two connections: one to the top of the tank in the vapor space and the other at the bottom of the tank in the liquid. Filling the tank from the bottom will compress the vapor at the top, increasing the pressure. Filling from the top of the tank will lower the pressure because some of the vapor will condense and turn back into a liquid, reducing the volume of vapor. A level gauge must be installed on the tank so the operator can see when the tank has reached a set level.
Dewers are used to store small amounts of cryogenic gases for individual laboratories. They are insulated tanks that can be used either in the lab or outdoors if space is an issue. Dewers can be manifolded together if larger storage capacities are required. Like the bulk storage tanks, dewers can be bought or leased.
The amount of liquid stored is based on the volume of the liquid used between the scheduled deliveries. The schedule represents the length of time between deliveries preferred by the supplier and the deliveries preferred by the client. The suggested time between deliveries can range from once every two weeks to once every month. The delivery truck can only hold a certain amount of liquid by law.
Sizing a Storage Tank
Let’s look at how to size a cryogenic storage tank. You must first know the amount of cryogenic liquid that will be used each day, shift or work week. If past information is not available, you can calculate the expected usage based on the total number of outlets and/or connected equipment, such as liquid nitrogen freezers or other lab equipment. You will need to know the amount of gas used by each and the duration each day that those pieces of equipment are used. Another way to get usage information is to contact the supplier to find out the delivery schedule and prices. They can also help you determine the size of the tank, but you should try to keep it as small as possible by maintaining a reasonable supply between deliveries. A common time between deliveries is usually two weeks, but once a month is not uncommon. Delivery drivers typically prefer to go back to their base office with an empty truck. If one customer didn’t take the total load, a driver often will go to the other customers to top off their tanks until the truck is empty.
The actual usable capacity of the storage tanks is calculated by multiplying the proposed usage per day by the number of days between deliveries. The conversion from gallons of liquid to cubic feet of gas is different based on the gas being used (see Table 2). Add 25 percent to the actual usable capacity to allow 15 percent for the empty volume used as vaporization space above the high allowable liquid level when the tank is full and 10 percent additional volume of liquid gas that is in the tank but not to be used (considered a reserve capacity). This 10 percent represents a two-day reserve supply of liquid in the tank after the low level alarm is reached to allow the supplier time to make a delivery. Therefore, select the volume of the storage tank and then add 25 percent.
Vertical tanks should be installed on a concrete foundation. The strength of the concrete should be a minimum of 3,000 pounds per square inch (psi), and the concrete should be reinforced with wire mesh for a small foundation or rebar (bottom and top) for a large foundation. The foundation should be a minimum of 6 inches larger than the tank diameter. If a vaporizer is included, the foundation should be able to handle its size as well. The foundation should sit on a 6-inch layer of crushed stone or gravel. For a quick estimate of the total weight the foundation must support, multiply the tank capacity in gallons by 15. This is a good rule of thumb. It is best to get a structural engineer involved in designing the foundation, but for a quick answer the tank foundation thickness should conform to the minimums in Table 3.
Another option for the plumbing engineer is to allow the supplier to size, design and install the tank based on the performance specifications. By doing this, it becomes the responsibility of the supplier to size the storage system. In many cases, the installation of the storage tank is paid for by the supplier, with the client being charged a rental fee. The supplier will be able to provide their requirements and the information needed to do the design.
For large horizontal tanks, a pier should be located 8 feet from each end of the tank. Smaller tanks have piers located 2 feet from each end. The piers should extend 2 feet below the frost area of the region where the tank is being installed. Vertical tanks are fastened to the foundation by bolts that secure the legs of the tank to the concrete. The bolts should be supplied by the manufacturer of the tank and installed by the contractor. One very important item to add to the specification is that the contractor shall install the tank per the manufacturer’s requirements and under their supervision.
Bulk gases that are stored as a liquid but need to be converted to gas prior to being used pass through a vaporizer. Most vaporizers don’t have any moving parts and use ambient air to warm the cryogenic liquid as it passes through the finned tubing. Aluminum is the most common material used to make vaporizers. If a high-purity gas is required, stainless steel should be used. As mentioned above, the vaporizer should be installed as close to the storage tank as possible, usually on the same pad. The size of the vaporizer is selected based on the standard cubic feet per hour required, the type of cryogenic liquid to be vaporized, and the lowest outside temperature for the region in which it is located. If a large volume of gas must pass through a vaporizer, the addition of heat obtained from steam, electricity or fuel gas may be required. Sizing charts can be obtained from the manufacturer, most of which have proprietary methods for sizing vaporizers.
The information provided in this article gives you the basics needed to size a cryogenic system and the terminology to discuss the system with a manufacturer. However, to make it easier and to ensure that the client is getting the best system, it is always good practice to consult with the manufacturer and let them review your calculations and design.
Michael Frankel, Pharmaceutical Facilities Handbook, American Society of Plumbing Engineers, 2004.
Joseph V. Messina, CPD, is the section manager of plumbing engineering for HDR Architects Inc. in Atlanta. He has more than 30 years of experience specializing in plumbing and fire protection system design for instructional, research and medical facilities.