MIL-STD-1411B
(see 5.1.1.2). Prefix, specification, and service pressure (ICC 3AA 2015 or DOT 3AA 2015)
will be marked in accordance with the current issue of 49 CFR, as applicable.
3.15 Disposable cylinders. Disposable cylinders are fabricated for a specific gas distribution. They are fabricated and used in accordance with the DOT 39 specification. Disposable cylinders are low-pressure containers and are tested at the vapor pressure of the gas of intended use at 130 °F or at 180 psig, whichever is greater. Cylinders of each lot are pressure tested to destruction, which cannot occur at less than two times the test pressure in any application. Disposable cylinders are fabricated for one-trip use from the commodity supplier to utilization by the user.
3.16 Dissolved gas. A dissolved gas is defined as a compressed gas that will be absorbed or will be dissolved in a liquefied medium. Acetylene gas dissolved in acetone is a system that will dissolve as much as 400 volumes of acetylene gas in one volume of acetone liquid under 250 psig of gas pressure at 70 °F.
3.17 Drawn seamless cylinders. Drawn seamless cylinders are made by heating a billet
of steel and piercing it with a mandrel. The cylinder is progressively drawn by hydraulic press to the desired shell size. Drawn seamless cylinders are also made from pre-cut steel disks and
drawn in hydraulic presses to the desired size and shape. The wall thickness is controlled in accordance with the DOT 3A or DOT 3AA specification for a given service pressure application. The shell is again heated and spun as a swinging arbor shapes the shoulder and neck for closure of the upper end. The neck is drilled and tapped and fitted with a valve as applicable for
intended use. Low-pressure seamless cylinders may have a foot ring welded directly to the bottom of the pressure bearing portion of the cylinder for protection against wear and for stability. High-pressure cylinders cannot be welded or heated after fabrication at any time, except by the manufacturer of similar cylinders. Neck rings are pressed and peened in place and are maintained without the use of heat.
3.18 Filling density. The term "filling density" should designate the percent ratio of the weight of gas in a container to the weight of water (water capacity) the container will hold at
60 °F. Percent filling density for a specific liquefied gas should be in accordance with 49 CFR. To determine the actual filling density (amount of gas that will safely fill a specific cylinder), the water capacity of the cylinder is multiplied by the decimal equivalent of the percent filling density for the gas of intended use. This filling density, correctly applied, will protect a cylinder against the excessive pressures of thermal expansion up to 130 °F, either from the increased pressure from the gaseous state or from the fluid expansion of the liquid state, as applicable.
For example, the critical temperature for carbon dioxide with a filling density of 68 percent is
87.8 °F. Below 87.8 °F carbon dioxide will liquefy, but above 87.8 °F a liquid state cannot exist regardless of pressure. The filling density has been carefully selected for each liquefied gas to take into account potential safety hazards.
3.19 Hydrostatic testing. The hydrostatic pressure tests are performed on cylinders every five years for most services to determine fitness for further use. During the hydrostatic
test, a cylinder is stressed with water pressure to a value determined by the cylinder specification and the marked service pressure. The total expansion of the cylinder and the value of permanent expansion after the pressure is released are recorded. The permanent expansion is subtracted
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