Failed thermal relief

[rocketscientist]

I am investigating the failure of a differential thermal relief valve on a twin-seated plug valve used in an outlet from butane sphere. The relief valve failed to open and the back (bottom) of the valve popped open; a semi-circle of bolts were punched out; one was severely bent, a couple were broken. A black residue was discovered plugging the relief valve and its inlet line. The residue came out as iron sulfide. The valve manufacturer claims that the high pressure should have forced its way through the valve – what do you think? He recommends periodically opening the vent on the line to the relief valve as a workaround. I am think of taking a look at the upstream filters. This is a buried butane line for most of the run to the blenders. Has anyone else had similar problems with their valves regarding plugged thermal reliefs?