Why does gland packing blowout? 1

[robsalv]

Ok, 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??

Thanks in advance.

Rob



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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[JLSeagull]

Search the web for "explosive decompression", especially if the service contains carbon dioxide.

 

[786392]

Was there any chance of Water/steam hammer to occur?

Best Regards
Qalander(Chem)

 

[JimCasey]

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.

 

[robsalv]

Thanks guys.

Water hammer is worth thinking about. Thanks for that suggestion 786392.

JLSeagull given that this was a steam service and steam quality is good, I'm not sure explosive decompression was involved in this case - or most of the examples in my industrial experience. Might be interesting to pursue for interests sake though, so how does opening a gate valve result in explosive decompression?


Jim, I had never thought about galvanic corrosion before. The stuffing box would need to be wet though - which it would be if steam was passing through the packing... that's really interesting. However, in my OP scenario, we had a virtual total evacuation of the stuffing box - not just a bit of particulate ejection and plus the stuffing box was dry - so I don't think GC can explain the situation.



I'm thinking that long term unpressurised exposure at highish temperatures is a hellish environment for packings and they just degrade over time. Any other takes to this notion?

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[786392]

Dear
Although I am not 100% sure about prevalent conditions in the case under discussion;but

I've come across abnormally destructive forces produced due to Water/Steam Hammer once in a while.

Only as a result of not following/sticking to gradual warming up instructions of Steam systems by the operating staff(sometimes in a hurry)

Best Regards
Qalander(Chem)

 

[btrueblood]

"long term unpressurised exposure at highish temperatures is a hellish environment for packings "

Yes. Graphite will oxidize over time, and faster at higher temperatures. Gasket manufacturers won't rate graphite over 800F in air for that reason. Steam will degrade graphite too (via water gas reaction), though it's not clear if this packing saw steam pressure (don't know why it wouldn't).

 

[Compositepro]

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.

 

[JAlton]

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.

JAC

 

[JimCasey]

>>>Jim, I had never thought about galvanic corrosion before. The stuffing box would need to be wet though - which it would be if steam was passing through the packing... that's really interesting. However, in my OP scenario, we had a virtual total evacuation of the stuffing box - not just a bit of particulate ejection and plus the stuffing box was dry - so I don't think GC can explain the situation.<<<

Embellishing just a bit: once the leak path/fracture line is created thru the packing, it's over. Sonic velocity flow of steam will scour out every little black speck of packing in short order. (see: Grand Canyon)

 

[robsalv]

Well, the investigation continues. Our gasket material supplier has added an additional element to this story.

At the time of the incident, we recovered what we beleived was some of the extruded stuffing material. Our gasket supplier couldn't identify the type of packing that it was, but pointed out that it was not an example of good stuffing box practice. I'll see if I can include a pic.

Basically, it appears that the box wasn't "stuffed" with concentric rings, but had a spiral packing, which goes some way to explaining why the stuffing box evactuated the way it did. Once the packing started unwinding, it was all over.



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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[786392]

Dear OP, Thanks indeed for sharing update!
this adds not only to overall learning,but clarifies

some more pertinent issues which must be looked into whenever reliability is to be ensured or 'is in question'

Best Regards
Qalander(Chem)

 

[DS3476]

Dear Rob

As the valve had been closed for several years, entrapped air or line compressible gas below the packing becomes an elastic extrusion mean for the packing material. That can happen mostly with a non lubricated, hardened and adherent gland packing.
When the operator opening the pressure underneath de packing that can really break the static friction and extrude in an explosive manner.
A good valve design should have a fully open position back seat to enable the gland repacking with the valve in Service. Hope you not dealing with hazard fluid conditions.
Good Luck.
Manuel

 

[robsalv]

DS3476, can I clarify something.

This was a gate valve on the bypass around a high pressure desuperheater station and the valve was closed. So it had pressure on both sides of the gate. If the gate was sealing properly the bonnet and stuffing box shouldn't have been pressurised.

Does that change your picture about trapped gas?

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."