What is the recommended way to ensure that supplied hydrogen meets the specified 99.9995% ultra-pure requirement? The project in question receives gaseous hydrogen in K-size hydrogen cylinders and liquifies it for use in the lab. Over the last 8 years the project has had three safety related situations where the inlets to the liquid hydrogen tanks nearly froze shut due impurities in the incoming gas stream.
Cylinders are required to be tested periodically to verify structural integrity. The most common test method is hydrotesting, which means water is the most likely impurity that solidified in the inlets. Drying the tanks is normally a requirement after performing a hydrotest. In the USA, CFR 49 CFR § 180.209 applies: https://www.law.cornell.edu/cfr/text/49/180.209. Paragraph (b)(1)(v) requires that the “cylinder is dried immediately after hydrostatic testing to remove all traces of water.” This requirement is vague since there is no definition of “all traces” and might be interpreted to simply mean no visible water rather than a small enough quantity to meet the 99.9995% purity requirement. If the problem is caused by the gas supplier not meeting the specified purity requirements, there are some approaches that can be used for quality assurance of gas purity:
- Discuss the issue with the supplier and put in contract language that a significant fraction or possibly all cylinders be tested and certified for purity.
- Use non-steel cylinders since steel is usually the worst material for this issue.
- Purchase gas from a supplier that verifies the moisture within cylinders after requalification.
- Use a liquid nitrogen cold trap to remove water from the gaseous hydrogen flowing into the process.
It's important to note that over long periods of time, even the impurities within high purity systems can accumulate. It’s also important to understand that many suppliers will guarantee the total purity spec (i.e., percentage) based upon the measured impurities. Other impurities may exist, but not be included in the total purity, if not included on the list of impurities to be tested. In hydrogen, one of the most frequent impurities is helium since it is difficult to detect and it can pass through many hydrogen production and purification processes. However, it’s usually considered to be benign to nearly all processes. Helium would not have been the impurity for this application since it would not freeze at liquid hydrogen temperatures.