I'm looking at an oil export system that has two valves in parallel. Only one at a time should be controlling the flowrate in a cascade level onto flow control.Due to piping anomalies, I'm being asked whether the two valves can be operated with one in manual (@35% open) and the other in auto. I think this won't work because as the one valve closes, the flow will increase through the other one. (Which is now behaving like a fixed orifice)However, I can't find any formulae to describe the interaction so I'm struggling to convince anyone. Upstream of the two valves are a couple of BIG (2x7MW) centrifugal pumps so I don't want things to go wrong... The fixed open valve will only change flow if the inlet or outlet pressure changes when the other valve is closed or open. If it does and you can calculate the flow through the valve at this maximum condition, and this is not more than the required minimum flow, then it should be OK. When the flow through the fixed valve is reduced, the other control valve should pick up the slack.However, since these are both control valves, why not change the control technique to allow both valves to open (like split range?)I see no problem as long as one valve is in automatic. More interaction would occur if both were in automatic. If you already have two loop controllers, consider setting one in automatic and slowly open the second valve in manual and observe the affect for each 5% (or smaller) output change on the manual valve.

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