Fill plant personnel must understand the basic operations and regulatory requirements when distributing liquid cylinder containers for the safety of all.
The distribution of cryogenic liquid cylinders over the past few decades has not changed much since the design was upgraded for durability and efficient filling practices were adopted back in the ‘80s. 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 fleet.
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 one can find the DOT- 4L code for transportable liquid cylinders. This code is relatively short and efficient, with clear requirements for manufacturing, repairing, and the service of cryogenic liquid cylinders for over-the-road distribution.
One of the most important requirements in the 4L code is the marking and the 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 Gases Association (CGA) code, pamphlet S1.1 (CFR49 section 173.316). As a user of liquid cylinders, you need to be aware that 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 that they are installed on. Confirming this requirement during the procurement of replacement relief valves and rupture disks from a reputable supplier will help ensure device compliance.
Based 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, like Chart have adopted a model and color coding system to safely communicate this information.
|Model Suffix||Model Color||Service Pressure (psig)||Maximum Relief Valve (psig)||Maximim Rupture Disk (psig)|
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.
The second most important requirement of operating DOT- 4L liquid cylinders is following safe filling practices. As we all know, there are many model cylinders in use over the years that have 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. The only way to safely determine the DOT- 4L filling limits is to use the manufacturers 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 below.
As one can see, it is important to fill all liquid cylinders by their published fill weights to comply with the DOT- 4L regulations. Although, there is one exception to the filling regulation (CFR49 section 173.320): If you are operating under the 25.3 psig (40 psig) 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 it is 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 personnel changes at the fill plant, it is imperative that these two DOT- 4L regulatory requirements are consistently followed for the safe handling of all personnel that come in contact with liquid cylinders on a daily basis.