HVAC-Loops - Pressure Differential Sensor Locations

Is the differential pressure measurement across a flow element, so that the DP is converted to flow rate?

If it's a flow meter, it just measures flow and tells you what the flow rate is, it doesn't pump the water. If this DP is part of a flow meter, the 'DP' doesn't need to be anything. The DP is a flow rate measurement, it is what it is. (the flow meter can be mis-configured, so the flow rate is incorrect)

If the requirement is to run a pump at partial speed for a given flow rate, then the VFD/motor/pump combination has to be of sufficient capacity to run at lower than maximum speed to achieve a less-than-maximum flow rate

If the VFD has to be overridden to run the pump at full speed in order to achieve the flow rate needed at the point at which the flow meter is installed, then it appears that the pump/motor is not sized large enough to run at partial speed, hence the VFD is superfluous.

I suspect that you are blaming the flow meter, when there are likely other issues coming into play.

Sorry if I put to many variables into the mix. I am concerned that the DP at the end of the loop is not the best way the control the VFD the controls the pump to a certain DP. The system uses two way modulating control valves.

Modulating valves are (typically) used to drop pressure created by a pump running at full speed in order to regulate flow.

VFD's are used to regulate motor/pump speed to regulate flow.

Each does so, typically with a PID loop using flow rate as the process variable.

Why would a given flow loop have both a modulating valve and a VFD? Which is controlling the flow?

sorry, modulating two way valves are on the individual coils off the loop. the VFD is being regulated by the DP signal from the pressure sensors. Pressure sensor location is the question not how it is being controlled.

Is the process variable pressure or flow?