I think you need to have safety valve to protect the piping from damage that would lead to significant damage.
Sizing the safety valve depends on the gas properties (expansion coefficient and others) that will be subjected to constant heat flux when the line is blocked.
scesare,
I agree with SmartEngineer about the need for a safety valve. Assuming you can block in the heater and still have it powered up, I think you should size the PSV as follows:
1) Heat enters the system at the capacity of the electric heater; i.e. if the heater's element is 1500 watts, then use 1500 watts as the heat input.
2) You have a confined volume. Allow the pressure to build to the heater's design pressure.
3) At that point, your safety valve will pop to vent the excessive pressure. It must be sized to vent gas at a rate rapid enough to avoid any additional pressure buildup.
Note that this is an unsteady state situation. The quantity of gas in your volume will change with time, as will the flowrate and conditions of the gas you will need to vent. You might perform calculations at the point where you initiate relief flow and at some time later when a portion of the gas has been discharged.
Perhaps a bigger problem will be one of overheating your equipment. The heater should always be deenergized when there is no flow.
