Visbreaker unit control valve stem failure

[gopalsankar]

We had frequent failure of Visbreaker feed line control valve stem(Breaking into two pieces).Stem metallurgy is SS321.Feed sulphur percentage is about 4-4.5% Wt and temperature is around 425-450 deg.C. After repeated failure the designer has changed the metallurgy to Nitronic grade and even after changing the metallurgy the failure is continuing.We expect some amount of vibration in the line during run and valve details are as below:-
Valve size : 6"-300#(body 9Cr material)
Seat ring material : SS321 Stellited
Valve plug : -- do--
Steam material : -- do--
Stem size : 3/4"
Port size : 5"
Port : Single port
Inlet pressure : 7.6 kg/sq.cm
pressure drop : 5.7 kg/sq.cm
Shut-off press : 12.0 kg/sq.cm
Liquid flow rate : 81 m3/hr
Inlet velocity : 5.61 m/s
Outlet velocity : 11.6 m/s
Upstream area : 79 sq.inch
Downstream are : 79 sq.in
Sonic velocity : 233 m/s
Balance : Unbalanced
Medium handled : Soaker effluent(hydrocarbon liquid plus vapor)

VALVE TYPE ANGLE/GLOBE DOWNWARD FLOW.

Mounting : Vertical

Type of failure: Britlle fracture.

Any member had similar failure?Any other suitable material to withstand the minimum amount of chattering ?

 

[metengr]

gopalsankar;
My advice to you is to have the valve stem failure analyzed by an independent metallurgical laboratory to determine the root cause of the failure. This is the only way you are going to get at the bottom of this re-occurring problem. Most firms like to blame material selection.

 

[gopalsankar]

We have analysed the failed piece in a reputed laboratory and the concluded with britle type fracture originated from the pre-existed slag.However,we have x-rayed the new material made of up nitronic grade and used a sound one.Then also the failure happens.

 

[metengr]

gopalsankar;
When you refer to "brittle type" fracture appearance is that what the laboratory analysis concluded or is that your terminology to describe a flat fracture appearance (no reduction in cross sectional area or shear lip or tearing) of the valve stem. "Brittle type fracture" can imply several failure mechanisms in this material - thermal or mechanical fatigue crack propagation and stress corrosion cracking.

I would go back to the laboratory and ask them to provide you with a detailed metallurgical report. The report should include a confirmation of the material composition, hardness, and photographs of the fracture surface and valve stem microstructure. Sounds like the laboratory is not that reputable or something is lost in translation.

 

[Scotsinst]

I assume the valve is from a reputable supplier of visbreaker valves. I have had good experience with both Masoneilan and Fisher valves in this service. Any stem subject to vibration such as this cantilevered design could fail if the vibration is excessive. Normally the stem is steam purged to prevent buildup in the packing box but I don't know of any stem failures.
Perhaps you have excessive vibration due to poor piping layout/support?

 

[0707]

this same subject was posted on thread483-173603.

Luis