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 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(Screw pumps).

Good point, Scott.  To elaborate: a sweep-angle valve is designed to avoid damage from FLASHING service, thus it has a large bowl upstream of the seat ring, and a venturi discharge. In some designs the venturi is superhard alloy or ceramic, extended the full length of the tailpiece.  If the valve is installed in a CAVITATING service the only part of the previously mentioned modifications that help are the superhard trim.

Cavitation is caused by excessive velocity: Bernoulli tells us that as the velocity increases the pressure decreases.  Decrease the pressure to Pv and the fluid flashes. Slow the fluid down a bit and the pressure recovers. The bubbles collapse.  This is cavitation.
Cavitation PREVENTION trim has many stages incorporating  changes of section and direction. These cause the fluid to be throttled WHILE CONTROLLING THE VELOCITY.   Your sweep angle valve does nothing to reduce velocity and has a single stage of pressure reduction between the plug and seat.

Side note: there are also cavitation CONTROL trims that actually allow cavitation but force the bubbles to implode in free volume where the possibility of damage is eliminated.  Drilled cage type trims with radial inward flow operate in this manner.  The bubbles crash into each other in the center of the cage and no damage is done.  

If you have a clean service, cage anticavitation trims work. Example: CCI DRAG(tm) trim.  However these make highly effective but difficult to clean strainers.  So if there are any solids in the stream, an anticavitation design such as the Masoneilan LincolnLog should be considered.  

So the moral of this story is: you probably have a valve that is misapplied.  Get all the information together  about your application and contact your most competent valve vendor for some applications help.

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