Scotsinst,
Thanks for your input, this is what I've always speculated and realized after pouring over formulas and methods in Crane 410 and ISA. It’s ironic you mention the Q-ball because the valve was sold by assuming a Cv curve similar to that obtained by a Neles Q-ball of the same NPS.
However, after deeper scrutiny I believe a better relation would be achieved in assuming my valve as having two of their "A-plates", one situated in both upstream and downstream positions.
This is because my valve actually has no ball trim, as the Q ball valve does.
I still have one main area of confusion with my design requirements: pressure. You mentioned that "downstream pressure is never determined by a valve but solely by the system hydraulics. The valve is simply a variable restriction in the overall system."
The way I see it, for a given flow rate, or velocity, at x angle of opening, there will be y pressure drop across the valve. As the ball is closed and a smaller opening is available for the fluid to flow through, to maintain a particular flow rate, the pressure must be increased on the upstream side.
Would the pressure drop not always be the same, regardless of upstream pressure, at that angle of opening?
In other words, and relating back to my initial post, how would system hydraulics affect (and thus determine) the downstream pressure if the valve’s angle of opening will only produce a certain pressure drop for a certain flow rate? How is this accomplished by the system?
My valve must pass a q= 19 gpm (not the 52 gpm as previously given), with input pressure available from 350 psig to 750 psig, and maintain an outlet pressure of 150 psig.
I have modeled the fluid into CFD software, using the given flow rate to determine a velocity for the inlet based on its diameter. The software has given me a magnitude of pressure drop across the valve of 163.5 psi, at my smallest angle of opening. If downstream pressure must be maintained at 150 psig, then the CFD would imply that the upstream pressure could be no more than 313.5 psig at 19 gpm. This would tell me that I must modify the valve geometry to produce a larger pressure drop because minimum inlet pressure is 350 psig. Would you agree with my thinking/methodology?
Thanks again for any input!