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I have placed a Flow Control Valve (FCV) and I see a large headloss through it. Can I prevent this?
As long as the rest of the model is set up to reflect reality, the headloss you see through the FCV is real. If it's very high, it means that the valve would need to close almost all the way in order to restrict the flow to the desired value, while maintaining energy balance for the rest of the system.
Let's look at a simple example:
In both cases, we have 1000 feet of 10" pipe (same roughness), with a head difference of 50 ft, specified by the reservoir elevation. In case A, the model solves the pipe flow as about 3100 gpm, which is the flow necessary to have 50 feet of headloss in a 1000', 10" pipe.
In Case B, we've inserted a FCV with a setting of 1000 gpm. We see that it takes 44' of headloss to achieve this reduction from 3100 to 1000 gpm. Another way of viewing it is that 1000 gpm through 1000' of 10" pipe only results in 6' of headloss, so in order for there still to be a 50' head difference (as dictated by the reservoir boundary conditions), there must be an additional 44' from the FCV.
You could also think of it as an Orifice. At a given total head, in order to have a relatively low flow, the orifice size needs to be relatively small.
Now, if you expect that the FCV shouldn't have to close very much, you'll want to check your model setup, especially boundary conditions.
A similar explanation also applies to Pressure Reducing Valves (PRV) and Pressure Sustaining Valves (PSV.) In these cases, the headloss is what was necessary to achieved the desired upstream or downstream pressure while maintaining boundary conditions. In other words, if you see a large headloss through a PSV, then it means that the PSV would need to close nearly all the way in order to "push" the upstream pressure up to the setting (the more headloss through the valve, the less flow in the pipes to acheive the same total headloss. With less headloss in the upstream pipes, the upstream HGL will be higher)