The following video desribes 2005 fire/explosion in Praxair’s gas cylinder filling and distribution center. The incident occurred when gas from a pressurized propylene cylinder was released through the relif valve and got ignited.

httpv://www.youtube.com/watch?v=-_ZLQkn7X-k

This is one of those situation where the risk mitigation measure (i.e. relief valve) is creating a hazard. 

In situations where there is demand on the relief valve, the flammable material being released may catch fire.

There are other issues with relief valves on high pressure cylinders. The relief valve may start relieving if the cylinder gets heated.  A relief valve may not operate when needed or may leak or pass.

So…are high pressure cylinders for flammable gases better off without relief valves?