Thanks for all the advice everyone,
I did confirm that the tank will be insulated. I hope it is done before I start up the system otherwise it will make the tuning very difficult.
I do have a Level Transmitter in the tank feed to the PLC. So I can turn off the heaters when the level gets too low. Also there is a Type J Thermocouple in each heater that is tied to a Transmitter back to the PLC. This is provided by the heater manufacture and is used to detect a heater overheating situation The SSR's are feed through a Contactor so I can open up the Contactor if the temp in the tank gets above a set point. If all hell breaks loose and the heaters themselves over heat I can then fire the Shunt Trip on the main breaker and kill the whole system.
Thanks again for the help!!