Tom, I think you may be confusing people, by referring to the pressure control loop as "controlling flow", which it will definitely not be doing.
The confusion is because most controls guys don’t understand how centrifugal pumps really work.
Pumps produce flow, not pressure. If you doubt this, run a pump with the discharge piping disconnected. Lots of flow, no pressure. The system’s resistance to flow creates pressure (head) that the pump must overcome to move water through the system.
The pump performance curve identifies the pump’s pressure capability at a given flow when operating at a given speed. Reducing pump speed creates a new performance curve down and to the left of the original curve in conformance with the affinity laws. Looking at a pump curve by itself you cannot tell what the flow or pressure will be.
The system curve identifies the flow vs. pressure characteristics of the piping and the filter – the friction losses from moving water through the system. For a given condition the losses are proportional to the square of the flow – a parabola. Superimposing the system curve on the pump curve identifies the actual operating point at a set of conditions. Now you can see the operating flow and pressure.
When the filter gets dirty the friction increases for a given flow and the system curve gets steeper – the parabola moves to the left and up. If you don’t have a control system, the flow will drop and the pressure will increase simultaneously. If you want to get back to the initial pressure YOU CAN ONLY CHANGE THE FLOW. A lower flow will move the operating point down and to the left along the steeper system curve, resulting in the original pressure but at a lower flow.
The method of choice for reducing flow is changing the pump speed. This shifts the pump curve until the new curve intersects the steeper system curve at the lower flow. The new flow creates the same pressure drop through the dirty filter that the higher flow created in the clean filter.
Control engineers may think they are changing pressure directly by changing speed, but they aren’t. They are changing flow, moving up and down the system curve.
The attached Excel file shows how it works.