Visbreaker unit control valve stem failure 1

[gopalsankar]

We had frequent failure of Visbreaker feed line control valve stem .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 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

VALVE TYPE ANGLE/GLOBE DOWNWARD FLOW.

Any members had similar failure?Any other suitable material to overcome the above problem?

 

[0707]

Maybe you have an isometric design problem and perhaps you are having a turbulent flow inducing high frequency vibrations to your stem valve, if I were you I will check on that. My feeling is that you are not having a material problem.

regards

Luis Marques

 

[JJPellin]

If you send the broken ends of the stem to a metallurgist, they should be able to tell you if it failed in tension (incorrect actuator adjustment), bending stress (misalignment, machining error), fatigue (vibration) or corrosion/chemical attack. In any case, the characteristics of the fracture surface should clearly show the mode of failure.

 

[reena1957]

Dear Gopalsanker, we had a Fisher make Angle CV on our visbreaker furnace outlet that gave wonderful service but for occasional slamming close due to the very high shutoff pressure which was not considered in the design of the actuator. I remember the stem was thouroghly guided by a sleeve and the only unguided portion was the visible one showing the stem travel. Following is a link to a very useful Fisher handbook:

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

where you can read about any CV problems
Best wishes