@8-ball,
In priciple, you can "add" the curves as Ben has suggested. Practically speaking, it doesn't work to perfection. As you likely know, the impact on flow from a given object doesn't begin and end at the object's physical boundaries. Fortunately, the very nature of a centrifugal's curve means that it minimizes the errors in your estimate. What I mean is that if you guess your resistance higher than it really is, the flow will not go up much because increasing the flow will increase the delta-P. Likewise, if you estimate your resistance lower than it really is, the corresponding drop in flowrate drops the system resistance a little. It's a sort of attenuated error, I guess one could say. Does that make sense?
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