Technique for oil pumping using Torque control

Hi

We have 4 Nos Sinamics G120 15kW VFDs (PM-240 + CU240E -2PN) that are used for controlling positive displacement pumps that handle very viscous oils. These are very viscous. There are some strainers on the discharge lines of the pumps and also automated valves that are used for flow cut-off.

Now, when the strainers get choked up with sediment, OR some valve in the discharge line is closed, the VFD must automatically reduce its speed, sensing the torque/current so that high pressures in the pipe are avoided. This is because the pumps are positive displacement type. However, when the valves are open and the strainer(filter) is clean the pumps are to be run at the highest speed possible when maintaining a current/torque limit.

Please explain whether and how this can be done with a G-120 VFD. There are also some pressure transmitters (PTs) for each pump on the delivery side. Is it necessary(or preferrable) to use PID loop using this pressure as input so that we maintain a Pressure set-point?    

Or is it sufficient/preferable to use the drive in torque mode so that the VFD automatically regulates its speed by sensing resistance to flow that requires higher torque.