Environmental


Is Your Gas Delivery System As Green and Safe As It Can Be?


I


n many research and pharmaceutical facili- ties, there are requirements for delivering carbon dioxide gas for cell culture incu- bator applications or bioreactor operations. For large-scale operations, the gas is deliv- ered in bulk form and stored outside in large cryogenic tanks. In medium to even some larger operations, it is delivered in transport- able cryogenic liquid containers commonly referred to as liquid cylinders or dewars; for smaller operations it is delivered in 50-lb-con- tent high-pressure cylinders. Typically, liquid cylinders are installed inside of the building. As with their larger bulk units, they have relief valves that vent excesspressure. High-pressure CO2


cylinders are also normally located inside the facility.


Though the cylinders technically contain much of their 50-lb content in liquid form, it is maintained in that liquid state by its own vapor pressure of 830 lb per square inch (psig) at normal room temperature. This pressure in- creases or decreases in relation to the cylinders’ ambient temperature, but in general there is no risk of their venting gas when not in use. High- pressure cylinders do not have relief valves, but rather each cylinder valve has a burst disk that protects against extreme pressure that would only be seen if the cylinder were subjected to


Figure 1 – Vertical cross-section of a typical liquid cylinder.


elevated temperatures in a fire or similar event. This is in contrast to bulk or transportable liquid cylinders, where the liquid CO2


is actually stored


in a cryogenic state at –57 °C (–70 °F) inside the tank or container, insulated from the ambient outside temperature with a vacuum-insulated barrier that maintains it in that state with a vapor pressure that is much lower than high-pressure cylinders, typically less than 350 psig. Figure 1 shows the features and cross-section of a typical liquid cylinder.


However, though the initial pressure may be as low as 145 psig, that pressure increases over


Table 1 – Typical contents’ relief valve setting and NER for liquid cylinders Nominal liquid volume Lbs of CO2


160 L 375


Relief valve setting


NER new containers percent per day evaporation


NER average in service percent per day evaporation


Maximum flow rate cfh/hr


Flow liters per minute based on 40% duty cycle


180 L 414


230 L 516


time as more of the liquid is changed to gas inside the container at a rate referred to as the Normal Evaporation Rate (NER) of 1–2% per day of its contents, until the pressure inside reaches the relief valve setting of 350 psig, at which time the relief device opens and releases some of that pressure. Unless gas is withdrawn or used at or above that rate, the relief will periodically actuate to maintain the pressure below the relief valve setting of 350 psig. Table 1 shows the typical contents’ relief valve setting and NER for liquid cylinders, and points out that any con- tainer not in use or not installed consuming gas at that rate may be venting 1–2% of its contents every day.


Inasmuch as CO2 is the prime “greenhouse” gas,


such setups beg significant questions in terms of the laboratory environment:


1. What steps can be taken to avoid excess venting?


2. Since CO2 does not support life, what is the


safety risk of its presence in the laboratory? 3. What is the solution to mitigate that risk?


This article explores these issues and what can be done to optimize the “green” state of your facility’s CO2


system, as well as its safety. As with


any requirement for gases in a laboratory, this will begin by evaluating the amount of CO2


your


specific installation requires for its daily opera- tion and the options for supplying that demand. Once the total volume of CO2


for daily operation


260 L 615


350 psig 350 psig 350 psig 350 psig 0.5–0.75% 0.5–0.75% 0.5–0.75% 0.5–0.75%


1–2% 150 70 lpm 1–2% 150 70 lpm 1–2% 150 70 lpm 1–2% 150 70 lpm


has been determined, it will define what form bulk, portable liquid cylinders, high-pressure cylinders, or a combination of those options you would need to maintain an uninterrupted supply of CO2


delivery, space, and storage limitations.


The article will also define what type of gas de- livery system to consider and how to integrate systems that can reduce excess venting, and at the same time monitor and reduce the safety risk venting may present with regard to creating an oxygen deficiency in your facility.


AMERICAN LABORATORY • 22 • SEPTEMBER 2013


by Larry Gallagher


to the processes, given supplier


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