Imagine that you are filling a cryogenic tank sitting on weight scales. You fill it to the proper fill density from a tank supplying very cold zero psi liquid and record the weight. You then use your Superman X-ray vision and make a mark on the outside of the tank at the level of the liquid inside the tank.
A few weeks later, and not having used any product from the tank, you notice that the pressure in the tank has increased. Using your X-ray vision again, you look inside the tank and see that, amazingly, the tank has more product in it than when you filled it. The level has risen! You look at the scales, however, and the weight is still the same as the original value you recorded. How can this be?
Most substances expand in length and volume as they warm, and contract as they cool (there are a few exceptions). The sidewalk in front of your home has expansion joints to permit the concrete sections to expand in the summertime and contract in the wintertime. When you fill your car’s tanks with gasoline the nozzle trips off before the tank is 100% full, ensuring a bit of volume for the gas to expand into as it warms. If you have kept topping off your car you may have seen gasoline later weeping out of the tank and onto the ground on a hot day.
Cryogenic liquids behave the same way. As liquid is stored, over time heat leaks into a cryogenic tank and warms the liquid. You will notice a corresponding increase on the pressure gauge. As the molecules warm, they move faster and farther apart from each other thus occupying more volumetric space. Given the number of molecules in a tank being constant, as the cryogen warms it occupies a greater volumetric space. This is called ‘liquid growth’. Because the molecules are moving faster and colliding harder with the tank walls the pressure gauge also reads higher pressure. Pressure and temperature are directly related; if one increases or decreases then you can be assured that the other increases or decreases, accordingly.
Outage is the volumetric vapor space reserved above the liquid when a tank is filled. A cryogenic tank is never filled 100% full of liquid. The outage ensures that there is space for the liquid to ‘grow’ or expand into as it warms.
Relatively low-pressure rated cryogenic transports for non-flammable cryogenic liquids are often designed and built to the Compressed Gas Association (CGA) industry standard CGA-341 or Canada’s TC-341. These are commonly used to transport liquid nitrogen, oxygen, or argon and are generally filled 95% full. Such cargo tanks are typically designed and built in the 33 to 45 psi MAWP range and are marked ‘Nonspecific cargo tank, per 49 CFR 173.320 ’. They must operate over the road under 25.3 psi. When at a customer location making a delivery, the pressure can be increased. Normally a Road Relief Regulator circuit is employed to make sure the tank pressure remains below 25.3 when operating over the road.
A cargo tank designed and built to comply with US DOT MC-338 is required when transporting any liquid cryogen over the road at pressures above 25.3, or when transporting flammable products such as liquid hydrogen, LNG, etc. US DOT MC-338 code tanks offered for transportation must have an outage of at least 2% below the inlet of the pressure relief device or pressure control valve, under conditions of incipient opening, with the tank in a level attitude (173.318 (f) (1)) (d).
This means that when you fill a tank, you can only fill it to such level that if it is filled with zero psi cold liquid and allowed to sit unused, even given the expected liquid growth over time, the tank will still only be 98% liquid full (leaving a 2% ullage/vapor space), at the moment the tank pressure has risen sufficiently to begin to crack open the pressure relief valve.
Cryogenic liquid should never vent out of a relief valve and rain down on, and harm, people or equipment. And if liquid were to be released to the atmosphere it would soon make a cloud six to nine hundred times larger than if vapor alone were vented!
Filling density / Weight lading requirements
In many states you are limited to operating at 80,000 pounds over the road without an over-weight permit. Given the weight of your tractor and the tare weight of your cryogenic transport trailer, you must limit the weight of the product you transport to comply. However, if you are operating a DOT MC-338 transport trailer (these can be operated over the road at pressures above 25.3 psi), you must also comply with DOT fill density weight lading requirements.
‘Filling density’ is defined as the percent ratio of the weight of lading in the tank to the weight of water that the tank will hold at the design service temperature.
Note, one pound of water = 27.737 cubic inches at 60 °F, or one gallon of water = 231 cubic inches at 60 °F. and weighs 8.32828 pounds (49 CFR.173.318 (f) (1)).
In laymen’s terms, fill density is expressed as the maximum percentage of cryogen that you can legally put into an MC-338 code trailer, compared to the weight of water it can hold. So instead of stating how many pounds of oxygen, hydrogen, or LNG etc., as all their densities, temperatures and expansion ratios vary greatly, tables in 49 Code of Federal Regulations 173.318 list filling densities based on the vessel’s full water capacity at the vent/relief system and the maximum set-to-discharge pressure of the primary relief device or pressure control valve. However, depending upon the weight of your tractor and the tare weight of your trailer, you still may have to under-fill below the maximum DOT allowed fill density.
Per 173.318 (c) Weight of lading requirements, the weight of a cryogenic liquid in the tank must be determined by weighing or by the use of an authorized liquid level gauging device, defined as a fixed length dip tube or trycock line (CGA-341 permits a fixed length dip tube or a differential pressure liquid level gauge ). Moreover, it may not exceed the lesser of: (1) The weight of lading in the tank, based on the water capacity stamped on the nameplate and the appropriate maximum permitted filling density specified in paragraph (f) of 173.318; or (2) The maximum weight of lading for which the cargo tank was designed, as marked on the specification plate.
Rule of thumb
A rule-of-thumb to remember is that the higher the Maximum Allowable Working Pressure (MAWP) rating for an MC-338 cryogenic tank, the lower its legal filling density.
A couple of examples will illustrate this. Imagine two MC-338 liquid oxygen trailers with the same capacity. One trailer is rated with a set-to-discharge pressure of 30 psig, the other 250 psig. Per 49 CFR 173.318 (f), the maximum permitted filling density (percent by weight) is 105% (liquid oxygen is denser than water at 9.528 pounds per gallon) for the 40-psi trailer, and only 87 percent for the 250-psi trailer.
One more example from 173.318 (f) is provided for liquid hydrogen. Note, liquid hydrogen’s density is only 0.5921 pounds per gallon. The filling density of a 50-psi MAWP trailer is 6.0% of the weight of water it could hold, while the filling density for a 150-psi MAWP trailer is only 4.5%.
This can be explained by realizing that a lot more heat will have to leak into the higher-pressure tank to warm the product and hence increase the pressure sufficiently to lift the relief valve. And we know that the warmer the liquid is the greater the spacing is between molecules (and their velocities are higher); thus, the liquid occupies more volume than at a lower product temperature/pressure.
In summary, you are only permitted to fill an MC-338 vessel to such a level they it will ultimately be no more than 98% full of liquid if the tank were filled with zero psi cold liquid and left to sit unused, with the product warming until the relief valve opens.
Thus, you cannot really fill a tank full!
About the author
Keith Hall is a member of the gasworld editorial board and Executive Vice-President of Engineering and New Product Development at Premier Cryogenic Services in La Porte, Texas. PCS inspects, repairs, and rehabilitates all types of cryogenic equipment, and is the largest such service provider of liquid hydrogen trailers and ISO containers in North America. PCS also sells and leases rehabilitated cryogenic transportation equipment. The content herein is limited to the author’s experience and may contain errors. You should never work with cryogens based solely on this article. Always follow your company’s guidelines and never do anything you are not properly trained and prepared for. The views expressed in this article are those of the author and do not necessarily express the views of his employer. Keith can be reached at email@example.com