How does a 8" Crane 465.5 Gate Valve Stem Separate ? (cast iron gate & manganese bronze ste

[yamoffathoo]

thread408-405652
The damaged valve was disposed of before inspectors could characterize the failure mode.
Turning the handle raised the stem but UT indicated the gate did not move.
To fail the stem in torsion and compression, calculations indicate very low yoke sleeve/nut friction and very large torque would be required.
Perhaps the cast iron gate cracked open to release the stem but that seems unlikely in the closed position since it would be held together by seat pressure.
Any experience with this gate/stem material combination out there?
Drawing is attached for reference.
Bamby

 

[gerhardl]

I have no experience with this particular valve.

With no informaation about use (media, prussure, age, operation cyclus, possibillity of orrosion and abration etc.) we are stuck with 'educated gussing'.

In my mind the most likely possibillities are:
1. Wrong use, installation or operation. (I have seen valve stem breakdowns caused by tryng to open valves with lever and truck to overcome sticking forces.)
2. Sticking caused by various reasons: medium, lack of cleaning or maintenance, or bad machining (cracks,slark etc.)
3. Faults caused by corrosion or abration.

Very often breakdowns have a combination of causes, in some cases from all of the groups suggested above.

Good luck!

 

[yamoffathoo]

Thanks gerhardl,

This is an isolation valve in a Service Water System (fresh water at 98 psig and 33-73 degF), with silt possibly preventing the gate from seating, so 'extra' torque may have been used.
I found this advice online:
"The T-head/T-slot designs have a T-head on the end of the stem that fits into a T-slot at the top
of the disk. This design is used primarily in solid and flexible wedge gate valves. This design is
relatively rugged, there are no connection devices (such as pins or keys) to fail, and the “loose”
connection between the stem and disk makes it unlikely to be susceptible to vibration-related
failures. However, many of the failures of this design are attributed to:
1) Corrosion of the material due to being in a raw untreated water system, thus reducing
the strength
2) Pressure locking or thermal binding (PL/TB)
3) Inter granular stress corrosion cracking, hydrogen embrittlement, forging flaws,
back seating, improper heat treatment, excessive closing force yielding excessive
unwedging force, manufacturing defects, steam erosion, and wear/out flow erosion."

 

[gerhardl]

I agree, seems likely that the start of the failure might by attributed to 'group three'.

My thought is that if the valve in addition is stuck, the T-connections upward moment could lead the stem's T to eventuallu to be stuck (not rotate) against top of the avaiable room. This again cold lead to excessive rotarional forces for the stem, in addition to the longitudal forces.

In case this would indicate a possible combination of several causes. A pity that the valve is not available for
inspection.

 

[yamoffathoo]

Gerhardl,

Examination of another failure of the same type of valve revealed that the manganese bronze had been severely fretted by the cast iron gate causing separation.
This valve and the previous failed one were both located downstream and very close to a V-Ball Control Valve resulting in exposure to excessive wake turbulence.
The "online" advice was quite wrong when stating:

"This design is relatively rugged, there are no connection devices (such as pins or keys) to fail, and the “loose”
connection between the stem and disk makes it unlikely to be susceptible to vibration-related failures."