WHAT ARE YOU REALLY GETTING WITH BULK GASES?
What Do You Really Pay For When You Buy Bulk Nitrogen?
Let’s go through the process of filling a liquid nitrogen tank. There are no flow meters for liquid nitrogen—it is an inert gas. There are no tank gauges for bulk liquid gases, period.
A bulk liquid tanker truck is filled at the cryogenic air separation plant and weighed. The weight of the truck determines the number of gallons of liquid in the truck. The customer is charged based on this figure.
Because the DOT has strict guidelines about the pressure allowed in the truck, it starts releasing nitrogen into the air as soon as it is filled. Estimates vary between 3% and 5% of a tanker’s volume is released during the commute to the consumer. If the truck stops at another customer’s location before yours, a “best-guess estimate” is made as to how much nitrogen goes to one customer and how much goes to the next, and so on. In fact, for the gas company, the best-case scenario is that the customer gets about 90-95% of what they are charged for into their bulk tank—then tank evaporation begins.
The Truth About Bulk Liquid Nitrogen Tanks:
Because nitrogen boils at approximately -320 degrees Fahrenheit, the nitrogen in a bulk liquid tank is always boiling. The standard bulk tank has a relief valve, not unlike a pressure cooker, which releases evaporated (boiled off) liquid nitrogen as the pressure builds (which it is always doing). This means that if untouched for a period of time, all of the liquid nitrogen will boil, vaporize, and release into the atmosphere. The speed with which your nitrogen is used has very little effect on the amount of boil-off.
Imagine a pressure cooker from which we draw off water at the bottom. Evaporation and pressure buildup comes from the surface and fills the space above the liquid. Since pressurization is fairly rapid and vaporous, if the cooker is nearly full or nearly empty, it has little effect on the speed of buildup of vapor above the surface of the liquid. Subsequently, the nitrogen is boiling at such a low temperature it has very little effect on the process regardless of whether the tank is located in Alaska or Florida—ambient temperature plays essentially no role.
The following figure comes directly from a bulk gas specialist at AirGas: a bulk cryogenic tank boils off at a rate of 2% of the volume of the tank (not the volume of liquid in the tank) per day. So, if this is a 1500-gallon tank, 30 gallons per day (or 3000 SCFH) will vent into the atmosphere every 24 hours. This figure will be the same if the tank is full or nearly empty. We do not take this figure into account when we size a system. So what’s important for YOU to know? The gas company does not want anyone to know what they are paying to nitrogenate the air every day.
The Truth About Dewar’s:
Dewar’s are always venting boiled-off nitrogen, and as a result, their release valves degrade over time. So, Dewar’s, which ordinarily hold 4500 SCF of nitrogen, could hold less when they arrive at the plant/facility. Dewar’s will also vent off their gas at different rates. Unfortunately, no one gets all of the gas or evaporated liquid out of a Dewar or tank—in fact, laser cutters, which operate at an N2 pressure of up to 500 PSIG, may only get about one third of the gas out of the Dewar’s before the pressure is neutralized and the nitrogen will not flow.