We have a double pipe HEX with C2H4 at 1500bar in the tube and Water(4 bar) or steam(5Bar) in the annulus. The RD on the annulus set at 12 Bar is susceptible to rupture due to the thermal shocks.

Is it wise to replace this with a PSV.gas flow of 40 t/hr.

AFD

Without looking the details, my first response is no. PSV has a response time for full flow to occur. In fact, Rupture Disks are prefered devices for protecting heat exchangers for tube failure cases. Your controlling case looks to be thermal expansion, but the effect could be the same as in a tube failure.

Further thoughts:

For double pipe exchangers, if the tubes are made from pipe material, API does not recomend any protection device. The reason is that the probabality of pipe failure is remote.

I see you have ethane at 1500 bar. If you depressurize that to 12 bar, I am fairly sure you will be freezing up any water on the low pressure side. Without doing the dynamics, it is difficult to tell.

Could you add add more details? This case looks interesting. Relieving supercritical fluid thorugh a pin hole in a dynamic scenario is a rare case. I am pretty sure you will need to analyze this through a CFD study.
Regards