High-Pressure Systems

In the high-pressure system, high-pressure cylinders are used to store liquid carbon dioxide at ambient temperatures (fig. 8-30). Normal cylinder pressure is nominally 600 psi and varies with the ambient temperature of the storage area.

Storage area ambient temperatures should not exceed 130F or be less than 32F. For safety purposes, high-pressure cylinders have a frangible disk that will burst at 3,000 psi to prevent cylinder rupture as a result of overpressurization.

Low-Pressure Systems

Low-pressure systems have a pressure vessel maintained at 0F by insulation and refrigeration equipment (fig. 8-31). At this temperature, the pressure in the container is approximately 300 psi. Because the container is kept at a low temperature, the container can be filled to 90 to 95 percent of capacity. For safety purposes a relief

Figure 8-31.-Refrigerated low-pressure C0 2 storage tank.

valve is installed to bleed off pressure at 341 psi. Another relief valve operates at 357 psi for rapid release of excess pressure. There is also a frangible disk designed to burst at 600 psi should the relief valves fail to control pressure buildups.

Advantages/Disadvantages of COZ Systems

There are advantages and disadvantages to each type of carbon dioxide system. Low-pressure storage units have a liquid level gauge that continuously monitors the amount of carbon dioxide in storage. High-pressure systems require weighing the cylinders. High-pressure systems permit storage of almost the exact amount of carbon dioxide required to protect a hazard area because of the flexibility and selection of cylinders in 50-, 75-, or 100-pound sizes. The smallest low pressure is 750 pounds. High-pressure systems require refilling and hydrostatic testing every 12 years. Low-pressure systems have no such requirement. Pressures in high-pressure systems vary with the ambient temperature; this affects the discharge rate of the system. Low-pressure systems keep the liquid carbon dioxide at 0F and 300 psi at all times, assuring a uniform discharge rate. Another advantage of low-pressure systems is their ability to allow automatic, simultaneous discharge for more than one hazard area on an engineered basis. Hose reels can also be attached to these systems to operate simultaneously with hazard protection. A reserve supply can be provided by increasing the storage unit size of low-pressure systems. High-pressure systems require manifolding and valving arrangements to achieve a reserve supply.

Storage of the carbon dioxide is also a consideration in showing advantages or disadvantages of these systems. High-pressure systems require approximately 3 pounds of equipment for every

pound. Usually, low-pressure systems require less floor space for storage of equal amounts of carbon dioxide as compared with high-pressure systems. In many instances, low-pressure storage containers may be placed outside of the buildings. High-pressure systems allow flexibility in space requirements since multiple cylinder banks may be stored in several smaller locations. Low-pressure systems require one large, single area for the refrigerated storage unit.

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