Krish Nitronic 50 is generally a better corrosion resistant and tougher material than SS 321 for this service. We had occasions to use - not in this particular Visbreaker service - but quite successfully in a relatively corrosion and turbulent flow conditions without problems for years. This was not in refinery service but in fertilizer plants. Although it is difficutl to comprehend, in a discussion forum on this specific failure, without doing an analysis on the failed stem (Nitronic 50 or original SS 321), your description of fluid velocity may be a vital contributor for the frequent failure of the 3/4" stem (downstream velocity seems to be high - 11 m/sec.) Please check with valve manufacturer whether at this fluid velocity and with vibration in the line during throttling or effluent feed control is done the design had taken care of stem design and stem material with this flow and fluid velocity of 11.6 m/sec. This is high from preliminary look (according to me) Trust this will help you C.V.Srinivasan Nishi Engineers Pvt Ltd India Dec 20,2006 E-mail: nishi@vsnl.com >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?For suggestion please.

Thanks prakash.The subject valve is Pressure Control Valve operated thro air actuator mechanism.The vendor is in the opinion that due to high actuator pressure i.e 12 kg/sq.cm the valve stem/plug is experiencing high amount of vibration/chattering which leads to failure of stem.Will increasing the size & reducing the acutator pressure help to solve the above problem? regards,