The subject sounds like a noob question, but after 18 years in industry, I still haven't had a decent explanation regarding sudden and catastrophic failure of packing materials under pressure... I've seen the aftermath a few times though - thankfully never associated with an injury.

We just experienced a gland packing blowout on an 8" 600# gate valve in a 600psi steam service. The valve had been closed for several years. As the operator was opening it, the packing material extruded (in an explosive manner) out of the stuffing box, from between the gland follower and stem. The packing material appeared to be a graphite based sealing material of some kind, with a metal filament braid... hard to tell exactly since it was fairly disintegrated - suffice to say, it doesn't look like the expanded graphite flexible yarn that we would use today. They only have inconel wire strands on the outside.

I'm thinking that the older packings aren't as good as the current stuff, and that they "lose their nature" over time, due to temperature and age, especially if the gate valve is fully backseated so that there's no process pressure energising the stuffing box.

Any thoughts, comments, experiences, lessons, education to pass on??

Gate valves tend to be left in position for long periods of time.  Graphite packing can aggravate galvanic corrosion on the surface of the valve stem.  The stationary stem pits, the graphite conforms to the irregularities and sticks to the stem.  When somebody finally actuates the valve( monster cheater bars being likely tools), the packing shears and the small pieces can blow out.  Most name-brand packing is passivated with little zinc chips for the same reason that trash cans are galvanised-the zinc plates over preferentially and sacrificially to prevent corrosion of the base metal.  That only works as long as the zinc supply is not depleted.    

Packings are generally a blend of fibers, particles and fluid materials. They must be deformable to a certain extent to pack into a cavity and conform tightly to the walls to form a seal. This requires plastic type of flow. But they must then act like a solid material so that they won't flow out of a gap when under pressure. When a valve stem turns it is supposed to slide against the packing which contains lubricants to allow sliding. If the packing adheres to the moving stem it will be sheared and the fibers in it will be ground up into small pieces which cannot prevent extrusion of the packing material out of the packing gland. Fibers can also degrade from chemical attack and oxidation. The same can happen to rubber o-rings if they stick to a valve stem rather than slide.

It's sort of like a log jam in a river breaking loose.

Depending on the Valve Type, the stem may turn (Globe Valve) as it rises or simply move through the packing gland (Gate Valve).  In either case, issue is back seating.  The proper practice would be to open the valve fully and then turn back down about a half turn on the handwheel to take the valve off the back seat during operation.  This allows the steam to pressure the packing and wet it as well.  A valve should never be left on the back seat during operation for the safety of the operators.  If the valve is back seated and the packing dries out and disintergrates as described, the the exact problem occurs with the potential to injure the operator.  The back seat should be held in reserve in case of a packing failure from one of the other causes described.  

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