The Cryogenics Test Laboratory (CTL) at the Kennedy Space Center (KSC) routinely utilizes cryostat test hardware to evaluate comparative and absolute thermal conductivities of a wide array of insulation systems. The test method is based on measurement of the flow rate of gas evolved due to evaporative boil-off of a cryogenic liquid. The gas flow rate typically stabilizes after a period of a couple of hours to a couple of days, depending upon the test setup. The stable flow rate value is then used to calculate the thermal conductivity for the insulation system being tested. The latest set of identical cryostats, 1,000-L spherical tanks, exhibited different behavior. On a macro level, the flow rate did stabilize after a couple of days; however the stable flow rate was oscillatory with peak to peak amplitude of up to 25 percent of the nominal value. The period of the oscillation was consistently 12 hours. The source of the oscillation has been traced to variations in atmospheric pressure due to atmospheric tides similar to oceanic tides. This paper will present analysis of this phenomenon, including a calculation that explains why other cryostats are not affected by it

Frontier, C. R.

Sass, J. P.

NASA Technical Reports Server

DRAFT 
ATMOSPHERIC PRESSURE EFFECTS 
ON CRYOGENIC STORAGE TANK BOIL-OFF 
J. P. Sassa and C. R. Fortier1' 
aNASA Kennedy Space Center, KT-E 
Kennedy Space Center, FL, 32899, USA 
"NASA Kennedy Space Center, NIE-F7 
Kennedy Space Center, FL, 32899, USA 
ABSTRACT 
The Cryogenics Test Laboratory (CTL) at the Kennedy Space Center (KSC) routinely 
utilizes cryostat test hardware to evaluate comparative and absolute thermal 
conductivities of a wide array of insulation systems. The test method is based on 
measurement of the flow rate of gas evolved due to evaporative boil-off of a cryogenic 
liquid. The gas flow rate typically stabilizes after a period of a couple of hours to a 
couple of days, depending upon the test setup. The stable flow rate value is then used to 
calculate the thermal conductivity for the insulation system being tested. The latest set of 
identical cryostats, 1 000-L spherical tanks, exhibited different behavior. On a macro 
level, the flow rate did stabilize after a couple of days; however the stable flow rate was 
oscillatory with peak to peak amplitude of up to 25 percent of the nominal value. The 
period of the oscillation was consistently 12 hours. The source of the oscillation has been 
traced to variations in atmospheric pressure due to atmospheric tides similar to oceanic 
tides. This paper will present analysis of this phenomenon, including a calculation that 
explains why other cryostats are not affected by it. 
INTRODUCTION 
Cryostats are regularly used at the CTL to determine the thermal performance of 
various insulations. Tests are conducted in small 1O-L cylindrical cryostats or large 
1000-L spherical tanks. Both tanks are double walled and contain insulation in the 
annulus. The 1O-L cylindrical cryostats are used for initial material testing, such as 
thermal conductivity, vibrations, and boil-off, over a short time frame. The 1000-L tanks 
are used to determine boil-off rates and thermal cycling performance over a longer 
duration, providing a better demonstration of performance. The 1O-L and 1000-L tanks 
are shown in FIGURE 1. 
While undergoing a long duration test in a 1000-L tank an unforeseen change 
occurred in the tank's boil-off rate. The gas flow oscillated up to 25% of its nominal
https://ntrs.nasa.gov/search.jsp?R=20120000557 2019-08-30T18:32:56+00:00Z
III ''i' 4 ' '' 
Lv 
value. The peak to peak amplitude of the oscillation was 12-1/2 hours. These oscillations 
shared characteristics with barometric pressure changes and ocean tide levels. The 
average pressure change (AP) for Cape Canaveral, FL is +1- 3 millibars. The AP is 
concurrent with the local tides levels, which also change from high tide to high tide every 
12 hours and 25 minutes. 
FIGURE 1. (Left) The 1O-L cryostat tank is used for gatherin information on materials. 
(Right) The 1000-L is used for boil-off test and demonstration runs. 
EXPERIMENTAL 
Steady-state liquid nitrogen boil-off calorimeter methods developed by CTL were 
used during this experiment. Longer duration boil-off tests were performed in the 1 000-L 
tanks. These tanks are scaled versions of the propellant storage tanks used on the shuttle 
launch Pads 39 A/B for liquid hydrogen. The liquid level of the tank was 90 percent 
filled with liquid nitrogen. After filling, it takes several hours to a couple of days for the 
tanks boil-off to stabilize, as shown in FIGURE 2. The tank can be pulled to high, soft 
or no vacuum. For this experiment a high vacuum level of below 1 * i0 ton was used. 
Tank boil-off is measured inside the tanks vent line by two flow meters. One is for 
high volume and the other for low flow. Only the high volume flow meter can be used at 
high vacuum. The 1 000-L tanks also have a series of sensors and multiple scales to 
determine the tank's liquid conditions. Equipped inside are 12 liquid level/temperature 
sensors at different depth levels, shown in FIGURE 3. The tank also sits on three scales, 
determining the mass of the remaining liquid. Knowing the mass and internal conditions 
helps calculate the volume of liquid left in the tank. By comparing the liquid volumes 
from day to day and using the flow meters the boil-off of liquid nitrogen is determined.
Boil-off Flow Rate 
E Flow Rate (sccm) 
12000	 - - - 
10000 
8000-
E
6000 
4000-
2000
Significant variation in flow (20-25%)
requires long test runs (1-3 days minimum)
to obtain a good average flow measurement 
N
Refill Operation and 
Stabilization Period
Steady Boil-Off 
CAT\AAAAAJA1AAAJ\A A AA& 
Three Days
(Two cycles per day) 
275	 278	 281	 284
	
287	 290 
Xth Day of the Year 
FIGURE 2. Graph showing typical data collected during a boil-off test. 
Color Liquid Level % 
• 100.0% 
99.9% 
84.4% 
• 82.0% 
• 15.6% 
• 13.4% 
• 0.1% 
0.0%
FIGURE 3. Diagram of l000-L tank liquid level sensor locations. 
nIl 
alasKa
RESULTS 
The boil-off test was run for 30 days in the 1000-L tank. During testing the flow rate 
oscillation was discovered. It was also observed that liquid nitrogen temperatures inside 
the tank were fluctuating in a similar pattern. Both tanks are in an air conditioned 
environment, reducing the ambient temperature affecting the liquid. Further research lead 
to the collection of barometric pressure data from NOAA buoy #41009, 20 nautical miles 
off the coast of Cape Canaveral, Florida. Graphing the buoy's data showed a pressure 
oscillation occurring every 12-1/2 hours peak to peak. This shared characteristics with 
the liquid temperature and flow oscillations, see FIGURE 4. 
The data gathered shows a relationship between the pressure, temperature and gas 
flow rate. The increase in barometric pressure raised the liquid temperature. The liquid 
temperature increase lead to a higher flow rate, after the liquid nitrogen turned from 
liquid to gas. The state change naturally affects the boil-off since the ratio of liquid to gas 
is 1:694 at 68°F. This also explains the small lag in time between the temperature and 
flow. The temperature of liquid nitrogen is -320°F, the gas slowly increases in both 
temperature and volume as it evaporates. This change takes time and the graphs show the 
delay from the nitrogen changing state. 
	
Liqu.d Te4,p a)dBaolnet,Ic Press	 Aow Rate and Ba'ometic Pressure 
It 54* K	 7€ 5*454	 -N- IwFiss	 5* IC 
LJqud Te,r43 and Paow Rate 	 Oscillation Conpason 
FIGURE 4. (Upper Left) Tank Liquid Temperatures Barometric Pressure. (Upper Right) Flow 
Rate vs. Barometric Pressure. (Bottom Left) Tank Liquid Temperature vs. Flow Rate. (Bottom 
Right) Pressure, Temp and Flow oscillations. 
The oscillations on the graph are similar to each other. The flow of gaseous nitrogen has 
a small phase shift compared to the liquid temperature. Even with these shifts the plotted 
flow of the nitrogen closely resembles the plotted temperature in several locations. With 
an increase in cryogenic liquid temperature the amount of boil-off will naturally increase 
as the element changes states from liquid to gas. 
One justification for a change in liquid temperature is pressure. Any temperature can 
be directly affected by pressure. A pressure change might affect temperatures enough to 
fluctuate flow; consequently creating an oscillation in boil-off. 
The barometric pressure is directly affecting the temperature in of the liquid. 
Constantly the liquid temperature and pressure rise and fall together without a phase shift. 
This is most apparent toward the end of the graph at JCD 240 (2.04E+06 seconds). Here 
there is a steady increase in both temperature and pressure, followed by a decline, 
increase and a rapid decline towards the end. Keep in mind that these measurements were 
taken from transducers over 20 nautical miles away and have stunning consistency. 
With the plots of temperature and pressure being similar, it can be expected that the plot 
of pressure and flow will be much like the plot in FIGURE 4. Barometric pressure and 
the nitrogen flow from the storage tank have the same peak to peak amplitude of 12 and a 
half hours. Similar to the plot of temperature and flow, the flow peaks do not occur at the 
same time as the pressure peaks; however they are not opposite of each other. The pattern 
with the pressure and flow indicates a small phase shift. A few hours after the pressure 
reaches a high the flow will peak before the pressure again comes to a low. This can be 
seen in FIGURE 6. This can be explained as the time it take for the barometric pressure 
to effect the liquid nitrogen, warm it and have it boil-off from a liquid to a gas. 
The consistency with the amplitude show that the flow rate oscillation is directly 
associated with the liquid temperature and the barometric pressure. However, it has not 
been explained as to why this oscillation has been seen in the 1000-L tanks and not the 
10-L or even the LH2 tanks on the shuttle launch pads. 
DISCUSSION 
Cryogenic boil-off has been largely attributed to ambient temperatures, none steady 
state storage tanks conditions, pour insulation, and low tank liquid levels. While 
examining the problem none of the suspected causes showed a relationship to the 
oscillation. 
Tides Effects on Barometric Pressure 
Atmospheric pressure can change from rain storms, hurricanes, tropical depressions 
and so forth. Although the pressure can be changed by several things only one entity has 
the ability to adjust the pressure on a regular and stable time interval. The moon
30.06 
30.04 
3002 
30 
2998 
2996
Tides 
Pressure 
2994 3. 
29 92 
299 
29.88 
2986
Pressure and Tides 
1 .68E+06
	
1.75E+06	 1.82E+06	 1.90E+06	 1 97E+06	 2.04E+06 
2.5 
1.5 
5, 
E 
5, 
I 
5, 
I-
05 
continuously orbits the earth and takes over 27 earth days to complete one lunar day. 
Despite its long elliptical orbit, its gravitational forces continue to pull on earth. 
Gravity affects all substances differently. The moons gravity continually pulls on 
earth deforming the planets solid core, but its major effects are shown in the oceans. The 
tides that are experienced occur daily and from high to high tide is 12 hours and 25 
minutes. Although tides overall height can change depending on location on earth and 
where the moon is in its orbit, the interval that effects the tides never changes. Tides are 
always different from day to day by 50 minutes. 
Along with the effect the moon has on the earth's oceans the moon also affects the 
atmospheric pressure. This occurrence occurs with the same amplitude as the tides. The 
barometric pressure and oceans tides for Cape Canaveral, FL can be seen in FIGURE 7. 
Viewing the graph it can be seen that the rise and fall of the tides coincides with the rise 
and fall of the barometric pressure. When the pressure reaches a high the ocean is at a 
high tide. When the pressure is at a low the ocean is at a low tide. 
1 68E+O6
	
175E*06	 1 82E*06	 189E+06	 1 97E+06	 204E+06 
Time (Seconds) 
FIGURE 7. The graph shows the tide height and barometric pressure. (X-axis: JCD 236 at 
0:00:00 GMT to 241 0:00:00 GMT) 
ANALYSIS 
A model was completed in MathCad representing the oscillation for the storage tanks. 
The model addressed at three scenarios. 
1. Liquid nitrogen in the 1000-L tank. 
2. Liquid nitrogen in the l0-L tank.
AT 
!?	
'M 
PtLS5JRL TEMI 
tn
EZAT 
FLUX 
(Q) 
Q*C
N) 
3. Liquid hydrogen in the shuttle launch Pad 39-A/B tanks. 
The scenarios were solved by using steady state conditions of thermodynamics. Each 
tank was evaluated using two different conditions. The conditions are shown below in 
FIGURE 8. With the conditions, the tanks calculated flow rates of sccm (standard cubic 
centimeters per minute) could be calculated for the gas escaping each tank through its 
vent line. After this the flow rate can be converted to a volume loss of liquid per day. 
This is accomplished by assuming the liquid in the tank is saturated and the state of the 
liquid is under the conditions seen from the barometric pressure change. 
The calculated and actual results determine whether the barometric pressure changes 
have an effect on a tanks liquid volume, inevitably leading to an increase of boil-off. If 
the calculated number is below the known number of boiloff we can conclude that the 
pressure change does affect the tank. If the calculated value is above what we know to be 
the tank boil-off, then obviously the pressure change does not affect the liquid volume in 
the tank. A table of these results can be seen in FIGURE 9. From this it can be seen that 
the barometric pressure change does not affect extremely large storage tanks. However 
the smaller cryostats of 1 O-L and 1 000-L are affected. The amount of calculated boil-off 
for the 1 O-L is small enough that, it is without the range of accuracy for the flow meters 
in the cryostat to detect. 
CONDITION I 
Assume Md0t=O 
Pressure of liquid is increasing. 
Temperature of liquid is warming 
. Solve for the Heat Flux Qd0t.
CONDITION 2 
Assume Qdot=O, Md0t^O. 
Pressure of liquid is decreasing 
Temperature of liquid is cooling. 
. Find flow in seem and liter/day. 
FIGURE 8. This gives a brief explanation of the assumed conditions used to calculate the rate 
of boil-off. 
Tank Size Known Boil-off 
Rate
Calculated Boil-off 
Rate
Does Press. 
Effect it? 
1000-L 3.8 liter/day .363 liter/day Yes
DRAFT 
l0-L 2.16 liter/day 4.4*103 liter/day Yes 
850-kgal 400 gal/day 800 gal/day No
FIGURE 9. Table of calculated values compared to known historical boil-off rates. 
CONCLUSION 
Testing on the 1 000-L tanks has shown that barometric pressure changes are effecting 
the boil-ff of cryogenic liquids. These rates vary depending on weather, global location 
and the moon's location in its orbit around the earth. Although this problem has not been 
seen in larger storage dewars, it does effect dewars with a lower liquid volume. A small 
change of pressure in these tanks will have a larger result because of the lesser liquid 
volume. In small dewars the oscillations are still present, but are very minor. This limits 
what can be recorded since the amount can be so slow that at is within the range of 
uncertainty for some transducers. 
The research and calculations performed will help personal in the future understand 
any detected variations in boil-off and reduce the uncertainty that is associated with the 
instruments used to collect the data. 
ACKNOWLEDGMENTS 
The authors wish to acknowledge the assistance of Sierra Lobo, Inc. personell for 
their help during testing. 
REFERENCES 
1. Fesmire, J. E. and Augustynowicz, S. D., "Thermal Performance Testing of Glass Microspheres Under 
Cryogenic Vacuum Conditions," in Cryogenic Engineering Conference, Anchorage, September 2003. 
2. Jiao, J.J. and Li H., "Tide-induced Air Pressure Fluctuation in Coastal Unsaturated Zones", Invited 
keynote speaker, in Proceedings of International Symposium on Water Resources and the Urban 
Environment, November 9-10, 2003, Wuhan, P. R. China, p10-13. 
3. Dekhtyarenko, L. A. and Zverev N. I., "Nonsteady-state Evaporation of a Cryogenic Liquid into the 
Atmosphere.", in Fizika Goreniya I Vziyva, Vol.24, No. 2, pp. 95-98, March-April, 1988. 


Atmospheric Pressure Effects on Cryogenic Storage Tank Boil-Off

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