Can I operate a control valve at low flow rate/valve opening for a short time (during startup for example ) even if I know at this point, flashing or cavitation may occur? I will be operating the valve at a value less than the minimum flow rate the valve is sized based on.
Also, does the valve datasheet show when flashing occurs or I have to do calculation again for that purpose ..
Your inputs please from experience
Hi John, This response is from Hank Penner, one of our Senior Instructors in Educational Services:
The minimum capacity specified for a valve and trim set are based on several considerations. First each trim set has a clearance flow that is a result of the trim design. Globe valves , for example, exhibit a clearance flow based on the clearance between the cage and the plug. When the controlling flow area is reduced to the point at which the clearance area becomes more significant, the flow across the valve becomes less and less a function of valve position. Valve gain is reduced to the point where it no longer controls and instability may result.
Secondly, control valves operating very low in travel experience accelerated trim wear. You mentioned cavitation. Trims designed to eliminate or minimize cavitation damage often use a staged or divided pressure drop in their design. If the throttling point becomes more restrictive than the staging, higher pressure drops across the plug and seat will concentrate the cavitation across the seating surfaces, leading to an increased damage potential.
Having said all that, control valve wear and damage mechanisms are all time-based. Problem is the definition of “short time”. All control valves have to be able to transition from fully closed to a position it is design to control. Lengthening that transition to include a plant startup may result in finding that point at which the accelerated wear and damage may become noticeable. Adding cavitation can significantly accelerate the damage. This point is highly process dependent.
I know of no field on the ISA standard control valve data sheet that is specific to flashing. The valve sizing report does indicate flashing. If the valve outlet pressure (P2) is less than the thermodynamic vapor pressure (Pv), the process is flashing. If you are dealing with a mixture, if P2 is below any constituent vapor pressure, it is flashing.
Thanks again Hank!
Greg Dawes | Director, Educational Services | Greg.Dawes@Emerson.com
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