I am working on a failure investigation of a control valve in my plant.  The valve type is a Fisher 461 Sweep Flow Valve.  The valve has been cavitating badly and has caused damage to the valve and downstream piping.  
What I want to do is to determine at what feed conditions (rate, upstream pressure, downstream pressure, etc) flashing starts, ie when the initial pressure drop to the vena contracta drops below the vapour pressure before recovering to the downstream pressure.
The reading I have done allows you to calculate choke flow conditions etc, but this is based on upstream and downstream pressure, not really looking at the vena contracta.

The valve/(gate valve) you mention is specifically designed with low Fl factor to flash/cavitate/mix more than a typical angle valve.  It is used for dirty flashing liquids.
It is used as a visbreaker valve and is useful for medium pressure drop applications.
If your pressure drop is beyond the 461 capabilities then consider a Fisher DST-G or Masoneilan 77000 series which will reduce vibration and cavitation while handling the flashing.
You need to size upstream piping large to keep the liquid above bubble-point with a long reducer just upstream of the valve and 5 pipe diameter straight run upstream of the valve then swage up immediately downstream of the valve.

You did not state the Viscosity or flowrate, but I guessed at some reasonable values for those parameters and when I ran the numbers through the Flowserve sizing program I get severe cavitation. Yes cavitation CAN cause vibration.

The math in any sizing program assumes pure chemicals with well-defined properties.  With Bitumen or most distillates you have sixteen dozen specific chemicals and they all have slightly different vapor pressures.  When you get a little pocket of flash it forms a bubble and launches a blob of tar downstream.  This would be sluggy flow and that causes real shock to the piping.  

I wouldn't use an anticavitation valve with the tarry product.  I don't think you are seeing cavitation damage inside the valve. You seem just to be describing inertial effects from the sluggy flow.  

If you can put the valve directly on the nozzle for the vessel into which it discharges you may find that the vibration is attenuated.  I think a lot of what you are experiencing is because of the way the stuff flows in the downstream piping.  If you get rid of the downstream pipe you get rid of the impulse mechanism.  

Moving the valve seat closer to the nozzle would also help but it would mean you need to replace the valve.  I'm thinking an eccentric rotary valve such as the Valtek Maxflo3, Masoneilan Camflex, with the seat at the outlet end. Hardfaced trim, probably reduced capacity. Go for the cylinder actuator option as it is stiffer and would be more stable with this stuff that's flowing nonuniformly through the valve. 

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