Tuned PID Parameters not controlling PID

Hi All,

I'm new to Siemens, so please bear that in mind!  

Background: We recently upgraded our distillery still boiler control system from an OMEGA PID control system to a PID program running on a Siemens S7-200 CPU controlled by a Smart Line 1000 ie v3 HMI.   I'm reasonably familiar with tuning PIDs, and PID theory. 

Problem: I'm using Step 7 (v2.5) to tune the PID to control the heating elements in the boiler.  The tuning completes successfully, and the PV temperature appears to be controlled as expected during the tuning.  However, when I copy the PID parameters to the CPU and try to run the system, the temperature sails past the set point.  It never plateaus.  I've tried countless tests, including manual tuning, and guessing.   The PV temperature never halts at the set point, but just keeps increasing.  This process has a relatively slow cycle time, so I can only run a a couple of tests per day.

Details: a) I start the tuning program when I have already manually raised the PV temperature to approximately the SP value.  b) The still is a double boiler (Baine Marie) design, with the heating elements in the heat transfer fluid that surrounds the boiler.  The elements are not directly heating the fluid in the boiler.  

Attachment 1: Image showing tuned parameters from Step 7.

This is driving me berserk - please share some wisdom.

Thanks,

Rob