Hi JGS
I'm sorry but I really don't know anything about the built-in control in a Sinamics S120 drive. Knowing Siemens I bet there are all kinds of tricks you can get it to do, but I'm no expert and this isn't a drives forum. You may be lucky with someone else who can offer specific guidance on that.
Picking up on a couple of points in your post #12 -
Yes, you can set a minimum value for the output of the FB41 block. You'll find a couple of input parameters LMN_FAC and LMN_OFF. These allow you to apply a FACtor and an OFFset to the Lmn - the output. The factor is a multiplier to adjust the slope of the Lmn response, and the offset determines what the zero point is. The default values you will see in the help file and in the instance data block are 1.0 and 0.0 respectively. If you want the pumps never to run at less than 20% then change the offset to 0.2 etc
Simulation using PLC-SIM only simulates the CPU, not the process. You're right that you need to add some operation in the CPU which will simulate the process. There is another FB which can be used for this in a very simple way. (Oh, no. Not that word 'simple' again!!) In the STEP7 TI-S7 Library you will find an FB named LEAD_LAG - I think it's FB80. Add it to your OB35. You can feed the input of this block with the Lmn value, and send the output of the LEAD_LAG block back to the PID as the PV_IN. You can adjust the lead and lag time parameters so that this block gives something for the FB41 to exercise against. It's not designed for detailed process simulation so don't try to tune the FB41 assuming the same values will work on your tank. But at least you can experiment with some of the issues we've discussed.
Good luck
Ken