PID Loops for High Speed Centrifuge

Has anyone written a successful PID loop for a high speed centrifuge? The centrifuge involved is a Humboldt model used in a wastewater treatment plant. The diffential speed developed between the bowl and the scroll is a hydraulic motor that is driven by a hydraulic pump which in turn controls the scroll.(The bowl is at a constant speed). The PLC involved is a Siemens(ie TI) 545-1101 using TISOFT 6.3. To try and make a long story short, the manufacturer has set up two special functions programs that perform the function of a PID loop controlling hydraulic pressure from the pump to the hydraulic motor via a proportional valve. A value is entered by an operator via an HIM to V-memory which in turn develops a differential speed between the bowl and scroll. While this process is good to help develop a differential speed, it give not direct control over it.
The HIM does monitor differential speed, so we were of the mind that we could use that input for more direct control over differential speed. Anyway, I could keep babbling on about it...the bottom line is I would appreciate any feedback, but especially, a compelling answer to the question I opened with.

Can't control velocity using pressure a lone.

Danbb6, you need to install a feedback device that will provide velocity information. You may need two encoders or tachometers, one for the bowl and one for the scroll. The PLC can then monitor the bowl speed and use this to determine what the scroll speed should be.
The PID should use the calculated scroll speed as a SP for a PID.

Unless the load is constant the current way of controlling cannot be that accurate. The motor will slow down as the load is increased.

This should be easy unless there is something we are not being told.
I think the approach the manufacturer used was flawed.

I think you need to look closely at the mechanical system. With a positive displacement pump and a positive displacement motor speed control within fairly tight tolerances by monitoring and controlling flow rate. A PID can do this, but probably isn't neccessary. If the centrifuuge manufacturer is controlling pressure he may be interested in torque as an indication of solids loading on the scroll more than speed.