Purging is important when dealing with hydrogen systems because hydrogen can ignite in the presence of relatively little air. A mixture of H2 and only 23% air (5% oxygen) can ignite, as opposed to (for example) propane which requires a minimum of 89% air to ignite. Also, the friction caused by gas being forced at high pressure through a valve could theoretically create enough heat to ignite an appropriate H2 \/ air mixture. It is thus vital that no air be allowed in any piping or storage tank containing hydrogen.<\/p>\n\n\n\n
Happily, it is a simple matter to get rid of the air. The technique is one of repeated dilution. Suppose you have a tank full of ambient air at 1 atmosphere (\u2248 15 psi). You add, say 5 atm nitrogen (bringing the tank up to \u224880 psi). The concentration of air is now 17%. You bleed this gas out of the tank until it is back to just over 1 atm. Repeat the process twice more, and the concentration of air is down to 17% of 17% of 17%, or 0.5% air. Obviously, during bleeding the tank pressure must stay high enough (>1 atm) to prevent air re-entering. Three purges are desirable, to add a margin of safety in case one purge goes awry.<\/p>\n\n\n\n
You must keep a tank of N2 on hand and make careful purging a habit, always. This is easy to do. It is clearly desirable to isolate the main storage tank from all pipe lines with valves, because while purging a line is a trivial matter, purging a large tank takes a lot of K-cylinders of N2. Also include a bleed valve and a gas inlet (eg compression fitting and valve) at opposite ends of each run of pipe.<\/p>\n\n\n\n
We found that residual N2 in the tank (from purging) slightly diluted the H2 concentration in the fuel cell feed, lowering output proportionally.<\/p>\n","protected":false},"excerpt":{"rendered":"
Purging is important when dealing with hydrogen systems because hydrogen can ignite in the presence of relatively little air. A mixture of H2 and only 23% air (5% oxygen) can ignite, as opposed to (for example) propane which requires a minimum of 89% air to ignite. Also, the friction caused by gas being forced at … <\/p>\n