Solution:

The NOC chose the ONE Series SIL 2 Safety Transmitter with Safety Relay Output (SRO) because it combined the 4-20 mA output that went to the VFD with an integral safety relay that could directly shut down the PC pump. With the analog signal connected to the VFD and the embedded logic solver and relay providing redundant safety shutdown of the PC pump, the external power supply, relay and added enclosure were eliminated. An additional digital signal could be inputted to the VFD to alert it in the event the pump was shut down .
The new control scheme provided simpler installation and programming at significantly reduced cost. Furthermore, the improvement in device reliability and diagnostic output of the SIL 2 certified device meant that the NOC could now calculate and achieve SIL 2 application protection.