I work for ASRC Aerospace at the Kennedy Space Center (KSC). We support NASA in ground support equipment (GSE) design. I have a question concerning the type of multilayer insulation (MLI) to use in LOX vacuum Jacketed (VJ) lines and flex hoses.
In the past here at KSC we have only used LOX compatible materials in the annulus of LOX VJ lines. We have always wrapped the inner line with alternate layers of aluminum foil and thin fiberglass paper for the MLI radiant heat shield.
In discussions with some of the VJ piping manufacturers we are now told that the standard they use for MLI is Aluminized Mylar®. Of course to expose the annulus to oxygen the inner line would have to have leakage. Also there would need to be an ignition source for the material to become a problem. We do though have VJ hoses connected to the space vehicle that are withdrawn prior to liftoff. Those LOX lines would be exposed to heat and vibration at this point.
The issue is that Aluminized Mylar is easier to work with and therefore somewhat cheaper to use in manufacturing than aluminum and fiberglass. Do you have any thoughts here, or, know the extent of industrial use of Aluminized Mylar in LOX systems? Also do you know of any case where there has been an incident with the use of this material?
1 Comment
The Cryogenic Society of America
May 15, 2009CSA received a lot of good responses to this question. The following are excerpts from some of them:
1.) I wrote about this subject in Cold Facts a couple of issues ago. I referenced ADR Volume II, Paragraph 6.7.4.2.6 which says “Portable tanks intended for carriage of refrigerated liquefied gases having a boiling point below minus 182 C (91.15 K) at atmospheric pressure shall not include materials which may react with oxygen or oxygen enriched atmospheres in a dangerous manner, when located in parts of the thermal insulation when there is risk of contact with oxygen or with oxygen enriched fluid.”
This rules out use of aluminized Mylar for tanks containing oxygen and colder fluids.
I am comfortable using glass paper and aluminum foil in equipment, including oxygen, which runs at 90 K or colder.
Glen McIntosh
2.) When we know that a flex line or VJ piping is used for LOX or LH we absolutely use aluminum and glass insulation. We concur that non flammable materials should be used.
-Anonymous
3.) There is a long forgotten (by me) MIL Spec. for oxygen compatibility. We faced the problem of LOX compatibility about 45 years ago when we started manufacturing oxygen dewars for the Air Force and wanted to use low heat leak piping spacers.
After some preliminary tests, it appeared that NEMA G-11 might pass the MIL Spec. test. We performed the test successfully for the DCAS inspector and have used G-11 ever since. As I recall, the G-11 sample got a little “sun tanned” during the pure oxygen test but it never charred or burned. (It can be made to burn with an oxy/acetylene torch but it won’t burn on its own.)
This is a little short of gospel so I recommend that you find either the MIL or NASA Spec. for LOX compatibility and perform the test for yourself with appropriate witnesses or a video camera.
Glen McIntosh
4.) Here is a synopsis of the use of aluminized Mylar MLI in LOX service vacuum insulated transfer lines.
The use of aluminum foil as the reflective surface in Multi-layer Insulation (MLI) for oxygen service is currently required by only one of the major industrial gas companies to the best of my knowledge. Their decision is based on the fact, as you mentioned, that Mylar can ignite at a relatively low temperature. However, since the Mylar in the annular space of VIP is well controlled from an ignition source, ourselves and everyone else that I know of involved with either manufacturing or using VIP (including LOX service) uses aluminized Mylar for the reflective service as a standard. As you mentioned, aluminized Mylar is much easier to work with in the manufacturing process but in addition it has a lower emissivity value than the aluminum foil. This can be significant in overall system heat leak performance over long distances and time.
I spoke with John Urbin about an incident where the determined ignition source in the annular space was a small amount (approximately 1 gram) of palladium oxide (PDO) that is used as a hydrogen converter to help maintain the high vacuum over a long period of time. The oxygen molecule in palladium oxide has a weak bond to the palladium and when it comes in contact with hydrogen molecules will detach from the palladium and attach to the hydrogen thus converting it to water vapor. This is then absorbed by molecular sieve installed in the vacuum space. Over time as the oxygen molecules detach from the palladium the palladium oxide will become palladium hydrate. If the palladium hydrate then becomes introduced to a sudden rush of highly concentrated oxygen, such as from an inner line leak, a chemical reaction can cause the palladium hydrate to rapidly heat. If this happens when in contact with Mylar it could reach a high enough temperature to cause it to combust. In this incident, it was determined that the palladium oxide was installed by the manufacturer in direct contact with the MLI.
PHPK uses a cindered metal capsule to house the palladium oxide and attaches it to the evacuation valve plug away from the MLI. This assures it can never be installed improperly and keeps it from ever coming in contact with the MLI should the palladium ever be introduced to a high concentration of oxygen.
In conclusion, aluminized Mylar MLI used in transfer lines is industry standard, even for LOX service. The precautions taken to remove the ignition source from the Mylar has produced no more incidents to my knowledge.
I hope this helps.
Brad Kitzel
PHPK Technologies
In response to that from Irby:
Brad, the incident you are referring to is probably the Air Products Al/Mylar MLI line that burned in the annulus without external damage in Laport Texas around the mid 1980’s. The thought is that it just smoldered due to combustion gases generated limiting the burning. Sort of like a dry powder or halon fire extinguisher preventing the “fire triangle” from working.
5.) It is much more complicated than I first imagined. What is the ADR Volume referred to? What does ADR stand for? It appears to me that the event which started all this is not typical, and was used to form a rule that may not apply. The fire that got it all going was caused by a vacuum getter-catalyst put in an inappropriate place in a LOX transfer line. There was an air leak in the vacuum space. The getter’s job was to soak up the air leak and maintain the vacuum. The getter was overwhelmed by the air leak. These getters, which are designed to catch nitrogen are so active that one never opens the can of getter in air, but only under an inert gas. Because exposing the getter to air causes a spectacular flaming reaction. Its hard to react chemically anything with N2 because N2 is usually considered chemically inert. O2 is extremely reactive with the getter. Hence Glen is 100% correct (as usual ) but the situation under study now is for a huge surface area of Mylar and Al sheet exposed to a tiny leak. Mylar has other problems, such as melting in sunlight, which changes the game entirely. Irby and I are making sense of all this, but at this time, I do not know if the ADR derived code is really applicable or not, since it was derived fro a solid insulator, not thin films. There is no harm in following the ‘code’, except expense. We shall see.
Tom Flynn
6.) ADR is a two volume publication of the United Nations (I have them) with the subtitle of “European Agreement Concerning the International Carriage of Dangerous Goods by Road.”
These volumes have very broad coverage for moving all kinds of things by road. Cryogenic vessels fall into one of their categories. Any vessel, including liquid helium transport dewars, which may be used in Europe must comply with these regulations. It has been our experience that the Japanese also want their dewars designed and built to comply with the ADR regulations.
The ADR regulations are not unrealistic and, generally, I recommend compliance with them.
Glen McIntosh