Cryogenic Liquid Cylinders

Fill plant personnel must understand the basic operations and DOT regulatory requirements when distributing liquid cylinder containers.

The distribution of cryogenic liquid cylinders over the past few decades has not changed much since their design was upgraded for durability and efficient filling practices were adopted back in the 1980s. However, operators must not lose sight of the regulations for the safe distribution of this convenient packaged gas product. They must understand the operation, stamping and filling requirements, as there are likely various models and manufacturers of these cylinders in their fleets.

This article will focus primarily on the regulations governed by the Department of Transportation (DOT) under the Code of Federal Regulations (CFR49) section 178.57, where the DOT – 4L code for transportable liquid cylinders is found. This code is relatively short and efficient, with clear requirements for manufacturing, repairing and servicing cryogenic liquid cylinders for over-the-road distribution.

Safety Devices
One of the most important requirements in the 4L code is the marking and calculation of the safety devices used to protect the inner pressure vessel. Manufacturers use these guidelines to design and select the appropriate safety devices based on calculations derived from the Compressed Gas Association (CGA) code, pamphlet S1.1 (CFR49 section 173.316). These safety devices are marked and sized accordingly to meet the minimum requirements for the capacity, liquid service and maximum operating pressure of the liquid cylinder on which they are installed. Confirming this requirement during the procurement of replacement relief valves and rupture disks from a reputable supplier will help ensure device compliance.

Neeser table 1Based on the liquid cylinder stamping data, calculating the pressure settings on both of these safety devices is actually relatively simple. On a DOT – 4L coded cylinder, you can find the service pressure on the cylinder data plate. The service pressure (psig) is noted directly after the DOT marking: DOT – 4L SP. This will be a three-digit numeric value that dictates the design of the inner vessel thickness and the maximum allowable operating pressure calculation from the manufacturer.

To calculate the maximum allowable operating pressure or maximum relief valve setting, the following equation is used:
1.25 x Service Pressure – 15 psi (for vacuum)
= Maximum Relief Valve

For example, a liquid cylinder with a service pressure of 200 psig will yield a maximum relief valve setting of 235 psig. Some manufacturers have adopted a model and color coding system to safely communicate this information.

To determine the maximum rupture disk setting, the following equation is used:
2 x Service Pressure = Maximum Rupture Disk

In the example above, the maximum rupture disk of 400 psig is calculated. Incidentally, the maximum rupture disk pressure rating is the hydrostatic test pressure of the inner vessel during its manufacturing process.

Filling Regulations
The second most important requirement in operating DOT – 4L liquid cylinders is following safe filling practices. As we all know, there have been many model cylinders in use over the years that have had unique capacities and operating pressures. The DOT – 4L code references the CFR49 section 173.316 for the liquid cylinder manufacturer to establish proper safe fill levels. Because of the variables from service (gas densities), gross capacity and maximum allowable operating pressures, a 4L cylinder MUST be filled by weight. This process ensures the liquid cylinder has enough gas space after filling to prevent liquid from escaping through the safety relief valve in the event the cylinder remains idle and the liquid grows (expands) as heat enters the inner vessel.

Neeser table 2The only way to safely determine the DOT – 4L filling limits is to use the manufacturer’s published values. Do not interpret model numbers for filling volumes as these are only nominal values that have no accurate relevance to the filling weights. Additionally, you should use the actual relief valve setting on the liquid cylinder to determine the proper maximum fill weight as noted on the example table above.

It is important to fill all liquid cylinders by their published fill weights to comply with the DOT – 4L regulations. There is, however, one exception to the filling regulation (CFR49 section 173.320): If you are operating under the 25.3 psig (40 psia) relief valve setting (22 psig is typically used in liquid service), you are not required to fill by weight so the operator can fill the liquid cylinder to “vent full.” This is allowed because the liquid will not have a chance to grow as the relief valve is set very low and hence there is minimal risk of liquid dispensing out the safety relief valve.

The fill plant operating standards must have these practices incorporated into their liquid cylinder filling section so they are clearly communicated to the liquid cylinder repair and filling personnel. As liquid cylinders change service and/or operating pressures (frequently done to match the customer’s application requirements), and as personnel changes at the fill plant, it is imperative that these two DOT – 4L regulatory requirements are consistently followed to ensure safe handling by all personnel who come in contact with liquid cylinders on a daily basis.

Gases and Welding Distributors Association

Tim Neeser Meet the Author
Tim Neeser is director of marketing and new product development for Chart Inc.’s Distribution and Storage Group. Chart is headquartered in Garfield Heights, Ohio, and at